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Old 10-25-2005, 05:37 PM   #1
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engine misses while driving

I got 154k on my 95 blazer (auto/4x4). I just changed the tranny fluid/filter 3 weeks ago. Since last night, I noticed that around 50-55 mph, the rpms would kinda jump in the 2000-2500 range as though it wasn't applying power properly. if I let off the gas and then reapply softly, it kinda smooths out and lets me keep cruising. The motor feels fine and sounds good and smooth so I thought this was a tranny issue.

I thought it might just be an overdrive issue, but the same thing happens in drive (3rd). Since this happens in more than one gear, I thought it might be the torque convertor, or perhaps I need to add more fluid...I checked the level on the dipstick and it seems ok.

Last year (Feb04) when I was handed the blazer, the tranny shifted roughly, so I changed the fluid/filter and it has been fine ever since. There was a difference in what I did in that fluid change and this recent change.

Last time I had a whole night to let the tranny drain (I ended up cutting the Y pipe so I could just remove the section of header to get the pan/filter out - then got that clamped up by an exhaust shop for 'easy' access next time). Because it drained so long I added at least 4.5 quarts. This time I did not have time to drain overnight and therefore I only added 4 quarts and it has been showing full. I wonder if perhaps I should just add an extra half quart and see how that goes....or perhaps the tq. converter is dying....not sure....

I talked to a couple guys in the shop here at work - they both immediately suggested fuel filter - I replaced it - same problem.

I changed the spark plugs. Still have the same problem.

If there was a vacuum leak, wouldn't I notice other symptoms like weak brakes and bad idle?
What else should I check?

I checked the u-joints and they looked ok.

Could this be a fuel pressure regulator issue? check this out, could this be the culprit:

fuel pressure regulator replacement
http://www.1aauto.com/1A/Drivetrain/...2560000/295473

someone suggested to me that perhaps the throttle is sticking, and that I should remove the intake hose and drench the throttle body with carb cleaner and wipe it all up and continue to drench the intake manifold runners with the throttle open and then try to fire it up (expecting lots of black smoke via the exhaust) and then problem should go away...however that sounds a little sketchy to me....

Someone suggested looking into error code po1870 but that's not it. Finally took it to Chevy. the technician and i went for 2 drives. of course, i couldn't readily replicate the problem as much in front of him. I got it to jerk a little and so we went back and hooked up the diagnostic scanner and went for another drive. The tranny is code free. In fact the motor is code free too. He seems to think the tranny is fine but the motor might have a miss somewhere. I will change the spark plug wires and add a little tranny fluid and call it a day. (i checked with the motor running and the fluid did seem a little low)

I forgot to mention that after 2 rides with the chevy technician, and all things looking good, I drop him off, pay the $80 diagnostic fee and leave. As soon as I turn the corner, it starts acting up really bad.

I noticed this: It did not start happenening until the car had been driven (warmed up) and turn on/off several times within an hour. So it seems to be temperature related too. The technician says it is code free, but the behavior may be ignition related.

I'll change the plug wires tonight. What else in the ignition system can I troublshoot/replace?
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Old 10-25-2005, 05:41 PM   #2
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Re: engine misses while driving

try checking your coils, maybe something's acting up there
Old 10-25-2005, 06:54 PM   #3
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Re: engine misses while driving

Thanks. I'm somewhat mechanically inclined, but my specialty is twin cam hondas.

Can someone help me with how to test the coils? Where can I find a how-to?

Thanks!
Old 10-26-2005, 10:32 PM   #4
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Re: engine misses while driving

check out ur ignition switch. been reading alot. found a few posts connecting the switch to miss firing ruff idling and such.
Old 10-27-2005, 01:16 AM   #5
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Re: engine misses while driving

well i just ordered a coil, control module, and plug wires...should be here next week. hopefully this does the trick.
Old 11-04-2005, 02:44 AM   #6
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Re: engine misses while driving

i got back from DC on business this morning to find that the parts have arrived. unfortunately it was like 2 am and i wasn't interested in diving right in.

so tonight after work i went to swap the coil first. then i see the factory coil is riveted to the mounting bracket and i have to drill them out somehow.

so - any tips on this? should i take the unit out and remove the rivets outside the vehicle? should I just use a drill or a chisel or a grinder or what? thanks!

Last edited by ( o )( o ); 11-04-2005 at 02:46 AM.
Old 11-05-2005, 06:14 PM   #7
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Re: engine misses while driving

well it was much easier than i thought it would be. i just took a 3/16" drill bit and a 4.5" grinder and the brackets came off no problem. so i swapped the coil and the control module, and the 1 wire from the coil to the distributor thing. i did not yet swap the other 6 plug wires. i figure i'll just fill'er up with 91octane and go for a drive and see if this made a difference...
Old 11-05-2005, 08:59 PM   #8
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Re: engine misses while driving

ok so the coil and control module and a tank of premium did not help. i babied her from my house to a backroad and then opened her up to like 80mph and she pulled just fine. but when i kept driving around, it just got worse and worse. i had a buddy riding with me and he seemed to think it was behaving as though it was starving for fuel. i'm sitting here reading the haynes manual and from what i gather i have the CMFI setup which seems to have 1 injector and 6 lines coming out of it. what else should i test before i remove the intake manifold and start replacing things? fuel pressure? if so how? this manual isn't terribly clear. any tips greatly appreciated.
Old 11-06-2005, 02:00 AM   #9
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Re: engine misses while driving

how's your gas mileage?
Old 11-06-2005, 03:00 AM   #10
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Re: engine misses while driving

i've pretty much always get 15 mpg, maybe 17 on a long highway trip.

but now its hard to tell because its hard to drive very far with out the engine really acting up.
Old 11-06-2005, 03:21 AM   #11
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Re: engine misses while driving

well if it was doing this in conjunction with bad gas mileage, and hard starting I'd say check your injector/reg/nut kit since that's a very common problem area with the CPI motor. Does it feel like it's bogging out or do the RPMs just fluctuate?
Old 11-06-2005, 04:41 PM   #12
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Re: engine misses while driving

it starts up fine and it idles fine. it just seems to bog intermittently while driving.

i found that injector with the 6 lines coming out of it. it seems to be a $350 part pretty much everywhere you look although I imagine the nice folks at chevy would gladly charge twice that.

i mean, if i have to replace the injector, then i can live with that. it's just a whole lot of work to do to only find out that is not the problem. how can i test to be sure that this is the bad part? i think i found a valve that might be where you test the fuel pressure but i'm not sure. but if the fuel pressure is good up to the point of the injector, then it would seem the injector might be faulty - correct?
Old 11-06-2005, 04:55 PM   #13
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Re: engine misses while driving

an easy way to tell is pull the upper intake plenum, and when you look in there, it should be dirty from the EGR, carbon all over, if you can look down in there and see clean spots then fuel is puddling up in there, and that's your problem. without it being hard starting though, I wouldn't think this was the problem.
Old 11-06-2005, 05:20 PM   #14
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Re: engine misses while driving

the thing that i do not want to acknowledge - what if one of the little bastards running around my court put something in my fuel tank....(several months ago, someone somehow popped of one of my locking gas caps from my k15, so tampering is unfortunately not necessarily out of the question)

my buddy seemed to think it was behaving as though something was plugging the fuel pump intermittently, and when you let off the gas, the mystery object unplugs the line so it is no longer blocking, and drives fine, but then it would eventually get back into pump and bog again.
Old 11-06-2005, 11:22 PM   #15
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Re: engine misses while driving

could a faulty o2 sensor cause this behaviour?
Old 11-07-2005, 01:53 AM   #16
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Re: engine misses while driving

I had a problem like that but I ran an extra ground wire to the frame cause my lights would dim and now its fine no rpm jump.
Old 11-07-2005, 03:09 AM   #17
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Re: engine misses while driving

Are your spark plug wires touching anything metal or even close? I had a problem with missfires and it turned out that my plugs were arching against random metal things they came in contact with.
Old 11-07-2005, 03:12 AM   #18
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Re: engine misses while driving

Quote: Originally Posted by digital_xtreme
Are your spark plug wires touching anything metal or even close? I had a problem with missfires and it turned out that my plugs were arching against random metal things they came in contact with.
no, at least i dont think so. i took it for another drive tonight and while i was out i pulled over, left the motor running, killed the lights, and popped the hood to check for any sparks from any of the wires. it was completely dark in the engine bay.
Old 11-07-2005, 12:08 PM   #19
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Re: engine misses while driving

before i remove that intake plenum i want to test the fuel pressure.

also - i would really like to be able to test fuel pressure while driving.

in theory this could tell me several things. 1) if pressure is good up to the manifold, then the injector is probably bad. 2) if, while driving, the pressure is good, then when the motor bogs/misses, and the pressure drops, then there might be something in the tank plugging the pump. or 3) if , while driving, the pressure is good, and the motor bogs/misses, and the pressure remains stable, then perhaps the injector is bad.

ok so - how can i properly test fuel pressure? and is it possible to test this and view the gauge in the cab while driving?
Old 11-08-2005, 12:31 AM   #20
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Re: engine misses while driving

well its raining so i just barely had time to do a quick fuel pressure test with the new guage i got from autozone. fortunately it was quick and easy.

the pressure was within spec according to the haynes manual. 58psi with the ignition on, and dropped to 54 with the motor on/idling.

also - my wife drives the blazer around the neighborhood (kids/school/etc) and she notices with slight inclines is when it studders for her (i hadn't really noticed, although i drive a bit more aggressively than her granny style since i venture out further). on another sidenote a coworker suggested instead of running lean perhaps i'm bogging from too much fuel.

with the pressure fine up to the plenum, should i be looking at the injector or perhaps a fuel pressure regulator?

edit - i also have not changed the distributor cap - this one is different than the common round ones. it's 3x3 side by side and i'm not quite sure whats involved here. any thoughts there?

Last edited by ( o )( o ); 11-08-2005 at 12:33 AM.
Old 11-08-2005, 02:34 AM   #21
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Re: engine misses while driving

I read the first 2 paragraphs and skipped the rest of the replys, sounds like a torque converter lockup issue maybe. thats my shot in the dark.
Old 11-08-2005, 02:41 AM   #22
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Re: engine misses while driving

change yer cap and rotr and make sure they ar tight, mine was slightly loose and was arcing out and misfiring along with sluggishness, i also swapped out the TPS sensor
Old 11-08-2005, 03:33 AM   #23
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Re: engine misses while driving

actually tranny/torque converter was my first guess weeks ago but thats what i originally took it to the dealer for. we drove with a scanner hooked up and we focused specifically on the lockup. he was able to control it from the scanner while i drove. but the scanner found no error codes.

hopefully it doesn't rain tomorrow (got a n/a 1.6L 200whp project taking up the garage) i';ll look into the cap and maybe troubleshoot the tps....

edit - i'm a retard. i can't believe i changed a coil and control module and took it to the dealer before changing the cap. i am so ignorant when it comes to this blazer and it's ignition system - i didn't realize what kind of ignition i was dealing with. back to the parts store tomorrow...

Last edited by ( o )( o ); 11-08-2005 at 03:40 AM.
Old 11-08-2005, 11:08 AM   #24
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Re: engine misses while driving

ok ill say it, your username is sweet
Old 11-08-2005, 03:15 PM   #25
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Re: engine misses while driving

does the miss happen when in park/nuetral and you rev the engine to 2500 rpm? that will verify if it is the trans or just an ignition/fuel issue.
Old 11-08-2005, 03:30 PM   #26
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Re: engine misses while driving

hmm i haven't really revved to 2500 while parked. i did notice last night when I tested the fuel pressure that the engine idles but has a slight miss even at idle. for the most part it seems stable, but it does not seem 100% perfect. it's almost lunch time, i'm going to the parts store in a bit....get me a cap/rotor and put that along with wires on tonight. (still kicking myself for not doing this weeks ago)

at least when all is said and done, this truck will be pretty much completely tuned up lol talk about taking the scenic route to get there though...
Old 11-08-2005, 05:10 PM   #27
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Re: engine misses while driving

whats frustrating is when youve replaced practically everything and it still has a miss.

good luck with it.
Old 11-08-2005, 05:37 PM   #28
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Re: engine misses while driving

lol yeah when i was at autozone last night the guy behind the counter said he replaced his entire motor and still had a miss....he used the same computer and distributor though....gee....hmm....lol ok 4 more hours until i can leave, go vote, and then get back under the hood.
Old 11-09-2005, 02:02 PM   #29
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Re: engine misses while driving

ok so the cap and rotor was not the problem. i swapped those out and still have the same issue.

i got this fuel pressure tester. in the driveway the fuel pressure is fine. how safe would it be to hook it up and tape the guage to the windsheild and drive around so i can monitor the fuel pressure when the engine starts bucking/bogging//missing?

shit shit shit. sorry. i just dont know if this is the injector, the regulator, a foreign object in the tank, perhaps a bad relay, o2 sensor. not sure what to do next. i really want to test as much as possible to pinpoint the problem before i spend more money on parts that wont fix it.

a wise man once said, "if it has tits or tires, you are going to have problems with it."
Old 11-09-2005, 02:08 PM   #30
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Re: engine misses while driving

Distributor is lined up correctly?
Old 11-09-2005, 02:42 PM   #31
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Re: engine misses while driving

Quote: Originally Posted by digital_xtreme
Distributor is lined up correctly?
no clue. how can i check that?
Old 11-09-2005, 06:28 PM   #32
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Re: engine misses while driving

I just read this article and I must say it was very informative and quite possibly worth your time to read.

http://www.samscars.com/mage7.html

I dont share all the same symptoms as my idle and exhaust are fine, but it was educational none the less. Also the article and the haynes manual say the fuel pressure regulator is part of the CPI, which it is, but they say you have to replace teh whole CPI if the regulator fails. Apparently this changes all that:

http://www.1aauto.com/1A/Drivetrain/...2560000/295473

Just sharing some info while surfin at work. Ok back to work - I'll do more diagnosis tonight and possibly even pull that plenum....
Old 11-10-2005, 01:50 AM   #33
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Re: engine misses while driving

ok i have another update. i changed the wires finally but that did not fix it. damn.


i may decide to get an o2 sensor tomorrow and see if that clears things up. i picked up a gasket for the plenum so i'll probably pull that this weekend and see what's going on with the CPI.

can someone please tell me whats involved with cleaning the map sensor?





this isn't mine, i just found this pic somewhere, but is it as simple as removing the 2 bolts and popping it out and using electrical spray and a cloth or paper towel?
Old 11-10-2005, 11:49 AM   #34
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Re: engine misses while driving

Interesting thread.

I have a 93 and a 94 Blazer. Both exibit the exact same symptoms. Especially the one of letting off the pedal and hitting it again..sometimes the problem will straighten out.

What I have ruled out:


#1 A foreign object in the tank won't cause this unless it is some kind of soft, rubbery substance capable of covering the inlet screen. I.e. a ping-pong ball, gravel etc...won't do it...

#2 Only one time have I ever removed a gas filter to find even a remote bit of dirt in it.

#3 I have never had a coil go bad-never. I've replaced many only to have the same symptoms.

#4 Fuel pressure was not the cause in mine.

#5. I've replaced the CPI's and nut kits...only to have the vehicle run a lot better but not solve the miss problem.

#6. The distributor caps are crap in these vehicles. They are difficult to line up and I always seem to get them on skewed so that the rotor chews up a bit of the cap.

#7 O2 sensors will adjust for rich lean conditions up to set points. Once those are exceded it sets a code. A set code doesn't mean that the sensor is neccessarily bad and needs replaced, it is meant to detected a too rich or too lean condition.

#8 It wasn't the EGRs..

#9 Hi grade gas makes it run better, but the miss is still there.

#10. Crappy gas makes it run ten times as bad...

#11. I've yet to see an arc from any wire...Trying to see under the hood when the problem occurs is kind of difficult. (On the road doing 75)

My best guess is still electrical or gas related.


When someone figures it out please let this old guy know....
Old 11-10-2005, 12:18 PM   #35
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Re: engine misses while driving

wow. that's not too encouraging. oh yeah i noticed something else last night.

how does the electronic 4wd transfer case work? is it vaccuum related at all?

I had to use 4hi for about 1 minute last night to back up about 20 feet in loose dirt. the most angled the truck got was about that of 2 wheels on a curb and 2 on a street, not much. but the truck stalled out when i hit the brakes, twice in that position. fired right back up, but stalled none-the-less, making me wonder: what does the 4wd system do that would cause the motor to stall, but does not stall when in 2wd?

maybe they are related and maybe not. but i'm wondering if maybe i just have a vacuum leak and i am spending hundreds of dollars on parts when I might just need a rubber hose....
Old 11-10-2005, 12:25 PM   #36
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Re: engine misses while driving

yeah the 4wd is vacuum and have you tried the tps sensor?
Old 11-10-2005, 01:32 PM   #37
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Re: engine misses while driving

i'll look for vacuum leaks tonight. i'll even look into the TPS. My question about hte TPS: is it adjustable, and do you have to find the right position with a multimeter? or does it just bolt into place, and is not adjustable? (i know my civic's TPS is a PITA to get set right)

also - would the TPS, MAP, EGR sensors throw codes if they were faulty, or not?
Old 11-10-2005, 08:58 PM   #38
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Re: engine misses while driving

on my first gen the TPS is not adjustable, you buy it and slap it on. I did buy me a bad one though, they should read between 0-5 volts (5 @ full throttle) hope that helps
Old 11-11-2005, 11:29 PM   #39
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Re: engine misses while driving

Quote: Originally Posted by ( o )( o )
ok so the cap and rotor was not the problem. i swapped those out and still have the same issue.

i got this fuel pressure tester. in the driveway the fuel pressure is fine. how safe would it be to hook it up and tape the guage to the windsheild and drive around so i can monitor the fuel pressure when the engine starts bucking/bogging//missing?

shit shit shit. sorry. i just dont know if this is the injector, the regulator, a foreign object in the tank, perhaps a bad relay, o2 sensor. not sure what to do next. i really want to test as much as possible to pinpoint the problem before i spend more money on parts that wont fix it.

a wise man once said, "if it has tits or tires, you are going to have problems with it."
I have a 92 Bravada with the TPI, I have the exact same symptoms as yours. It started about two weeks ago, nothing different on the vehicle. I have not changed all of the parts as you have because they have all been done in the last 4-15k mi.
On mine two things come to mind O2 sensor (OE with 130k mi) and the TPS. leaning towards the TPS. I have checked everything, wires, cap, plugs, coil, fuel pressure, filter is new in the last year. I did replace all of the vaccum lines, didn't help.
Old 11-12-2005, 12:20 AM   #40
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Re: engine misses while driving

I didn't think we were permitted to have nips in our user block?!?!?!
Old 11-12-2005, 06:13 PM   #41
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Re: engine misses while driving

Funny, but my s-10's seem to start this crap every fall about this time....coincides with the month that the good ole epa says we gotta use this damned "oxygenated" gasoline...something to think about.

Check this out and see if your state is on the list.

http://www.epa.gov/otaq/oxygenate.htm
Old 11-12-2005, 10:01 PM   #42
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Re: engine misses while driving

well, i cant stop smiling.

i put in a new TPS and she drives great now!!!

i'll drive it for a few days and monitor it but i just got back from a drive out in the country and she felt strong and didn't miss a beat. thanks for all your help guys.
Old 11-14-2005, 01:16 AM   #43
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Re: engine misses while driving

wtf. i went on another more extensive test drive earlier to make sure i could make it to work tomorrow. everything felt great. i just went to the grocery store and suddenly on the way back the motor bucks as i'm breaking for a red light. then the motor stalls. i fire it back up when the light turned green and limped around the left turn of the intersection. the blazer was then bucking all the way home, stalling several times in less than 1 residential mile. when i would coast up to a stop, with my foot off both pedals, the rpms would jump from 500 to 1500 by itself! it would then stall at each stop light and around turns. oh also when it would stall, the dash had 2 lights: "CHECK GUAGES" and the battery light. would an alternator cause the motor to buck and rev by itself and then stall? what about a corroded battery terminal? (not that the terminals are corroded, but i'll go clean them again just to be sure)

i'm so frustrated with this truck right now.

maybe the new tps is bad already? maybe it's the CPI? maybe it's an o2 and/or the catalytic converter? i dont know.

well i guess i'm reviving this thread, any more tips are greatly appreaciated. where should I start next?
this is what i have already done:

fuel filter
spark plugs
spark plug wires
distributor cap and rotor
ignition coil
ignition control module
throttle position sensor
Old 11-14-2005, 02:22 AM   #44
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Re: engine misses while driving

I do not know if the 95s have a cam position sensor, but mine use to studder after driving for 30 minutes. Changed that it it worked great after.
Old 11-14-2005, 02:29 AM   #45
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Re: engine misses while driving

Quote: Originally Posted by frknlo.com
I do not know if the 95s have a cam position sensor, but mine use to studder after driving for 30 minutes. Changed that it it worked great after.
that's interesting. what year/model did you have that had this cam position sensor you speak of?
Old 11-14-2005, 02:45 AM   #46
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Re: engine misses while driving

2001 4.3 and it was within the distributor.
Old 11-14-2005, 02:54 AM   #47
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Re: engine misses while driving

just an add to conv..., my sister in law put sugar in my tank, ran like hell for a few miles...since i'd been threw this before..i knew where to check...pull your fuel filter ,..drain for a sec.. put fingers over both holes, shake it, remove fingers, see if there is any build up on fingers or in inlet of filter...,...you may have bought a faulty tps,..if you did have something wrong with fuel(sugar, water in gas, dirty gas),..it could semi plug your injectors...just a thought
Old 11-14-2005, 02:57 AM   #48
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Re: engine misses while driving

man i'd hate to think someone is messing with my vehicle.

oh and i just read the haynes manual regarding the camshaft sensor. "In the event of failure or damage, the camshaft sensor is not directly involved with driveability" so since i have driveability issues, does this rule the camshaft sensor out? then again the haynes manual isn't necessarily gospel...found a few typos in them over the years...

edit: lol i just read another section that contradicts. "in the event the reference signal is lost or interrupted, the PCM will continue to pulse the fuel injectors using a default value but the synchronizing effect may be slightly off (stumble or misfire) due to the lack of precision" it then mentions code 41 but the Check Engine light is not on...

Last edited by ( o )( o ); 11-14-2005 at 03:00 AM.
Old 11-14-2005, 02:57 AM   #49
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Re: engine misses while driving

oh, and no, the altinator can't cause a stutter
Old 11-14-2005, 02:58 AM   #50
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Re: engine misses while driving

it should rule out cam pos sensor, if that was bad, your veh. won't start
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