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Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

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Old 08-10-2012, 10:51 PM   #1
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Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Truck Specs. 1996 Chevy Blazer, 4.3 V6 AWD 4spd Auto. 150,000 miles on it. Maintained well, used to be great on gas. Mid to high 20's on the highway with the cruise at 70mph.

OK so I am a newb here, but not a newb to working on cars. My personal daily driver ride is an 87 Pontiac Fiero with a Nitrous Fed 4.9 Aluminum Block Fuel Injected V8 5spd that I built myself. So working on cars I am fairly good at it.

We bought a 96 AWD Blazer. Kinda neat unusual to find an AWD version but we got a good deal on it. The miss started out minor and would only act up at part throttle. If you got up to 1/2 throttle it would be fine. Then slowly it got worse to the point that at any throttle it misses. This is not a soft miss, its almost like someone is flicking the ignition off and on. This was over the course of 20,000 miles.

So far my list of parts.

New Plugs E3 Diamond Fire

New Wires, cant remember brand but they were one step below the Accell 8.5mm wires.

New Ignition Coil

New Accell Cap and Rotor. Farking plastic dist shaft, stripped out so I used a larger screw on one side.

Replaced MAP, MAF, Blocked off EGR, unblocked it when it had no effect on the miss.

Known Good Used Ignition Module.

Replaced the cam pickup sensor in the dist 3 times. The reason we did it more than once is the first time I changed it, the miss went away and the truck ran like a champ, but we used a used sensor, and the miss came back. So we installed a Napa Cheap one and it did not fix the miss, we were told they were actually crap, so we went to the dealer and bought a Delco sensor. Installed it and no change.

Then on to the big stuff, I didn't want to believe it was the spider assembly, but as luck would have it we ran into someone that had a 2000 Blazer Motor. The truck was wrecked with low miles on it, and the motor was saved. They were converting to a Carb so they sold us the entire top end of the motor. Spider, Intake manifold, all sensors everything from the heads up including the Throttle Body for 60 bucks. So I tore the top end down, and replaced the spider assembly. I was excited that we would have our Blazer back. I jumped in fired it up and honestly believed it was fixed until I took it for a drive and realized that it didn't make a damn bit of difference.

The only code we would get on occasion was for the EVAP system.

We have a hand held OBDII Scanner but I actually prefer my much cheaper Torque App on my Android phone with the OBDII Bluetooth transmitter.

About the only thing I have not changed is the Dist Shaft itself. I have replaced the sensor, the cap, the rotor all of that.

We took it to the Dealer and they had guessed spider assembly or worn timing chain but they were not sure, and gave us a bill for 240 bucks. We then took it to a private shop, they said the coil was bad and charged us $390 for parts and labor to diagnose and replace the coil. I just about took the coil off and threw it through their window. However I just shook my head and paid it.

Unrelated parts that we have done. Front and rear brakes, pads/shoes and both rear pistons.
New front hub/bearing assembly's
All the O2 sensors. (3 I believe)
plus tons of other stuff. New seal in the front diff cause it was leaking. Transmission flush with filter.


When the truck was running good it was great, even got amazing gas mileage for a 4.3 we were getting 26-28mpg on the highway with the cruise at 70mph. Its a fully loaded truck, leather, AC, overhead temp display, power seats, power windows, for a 16 year old truck, it was really great. Its got 240,000 kms on it. IIRC thats about 150,000 miles.


Any help from anyone would be greatly appreciated. Just an odd question do these trucks have any problems with the computers going on. Honestly in all my GM cars I have never had an ECM go bad, but I guess there is a first time for everything. I tried shaking the ECM when the truck was running and the miss did not change.
Old 08-10-2012, 11:09 PM   #2
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

What is the TPS voltage during the miss ? What cylinder has the miss ? Can I assume you checked the compression to rule out a mechanical issue ?
Old 08-10-2012, 11:40 PM   #3
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Hi & welcome

I also use Torq

I would start by graphing some things. If you have an intermittent issue graphing will find it quick.

I always start by graphing the O2. It should swing from about .2 to .8 volts while just driving and idling. Under WOT it should, no MUST pin the voltage at .9, anything less and you are running lean, likely fuel supply. Be sure to graph both Bank1 sensor1 & Bank 2 sensor 1

Then I select a digital/dial gauge for fuel trim. You will get +/- numbers and a dial that swings both sides of zero.
Ideally short term fuel trim should be as close to zero as possible. Real world, less than double digit is acceptable.
A + number means it's adding fuel, a - means it's subtracting fuel.

This should be a good start. If you see normal readings here then start graphing the obscure.

You want to graph the MAF, MAP, TPS and anything else.

I had a Cavalier with an intermittent miss and it turned out to be the TPS. Of course you can graph this without the engine running, and I do, but in this case it showed a nice smooth line. I had to actually drive it and then I saw a drop out. TPS replaced and the miss was gone.

My current Blazer had a miss with a CEL for lean, turned out to be the MAF.

One thing that might throw you is the distributor cap & rotor phasing, a good thread on it here. Seems that many of our trucks have so much advance that the rotor can be a long way from the cap terminal and the spark may not reach it every time.

You can of course mist the plug wires and cap with a spray bottle in the dark. A sudden engine stumble or snapping or visual sparks is an indication of something needing to be replaced.

BTW, my son is building a 4.9 Fiero!

Hey, we are neighbors!! PM me if you need some help!
Old 08-11-2012, 12:06 AM   #4
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

@Johnniespeed, we graphed the original TPS to check for a dead spot or drop out, and it was fine. When I did the spider assembly I used the TPS from the other motor, and assuming (yes I know that is a bad thing to do) seeing as the miss did not change, that the TPS from the other motor is good as well.

Compression Test, I am really kicking my A$$ I have done 2 plug changes well actually 3 because the first time, I actually bought 95 plugs installed them drove it and realized they were wrong. I should have grabbed a helper and done a compression test at any one of the plug changes, but at the time I just never felt the need because the engine is very strong pulls like a train out of the hole and revs clean up to redline. I am going to do a compression test as my next thing on the truck.

@AWDV8 I have set up 2 different Torque home screens to graph as many sensors as possible and also have the Torque Raw Data add in. I have actually purchased several different OBDII apps to use on the truck and our 2000 GTP.

I have changed out the MAF sensor and the Grams are showing about the same. The MAP sensor has been swapped with 3 different ones, the TPS I addressed earlier. I'll give the misting of the wires a shot, and see if that comes up with anything. I would be really pissed off if it turns out it was just a bunk plug wire this entire time.

O/T Very Cool that your son is building a 4.9 Fiero, we have 2 N/A 4.9's and a 4.5 Turbo Chop top in the club as well as about 2 dozen more Fiero's. WE have a club meeting in Port Kells tomorrow, you should come out and have a visit. We are all really good guys and gals. Its 6pm at the Ricky's, roughly 88th Ave and 204th Street on the North Side of Trans Canada. Our website is www.westcoastfieros.com if he needs any info, feel free to contact me. david@westcoastfieros.com With any luck we will have several cars tomorrow, including at least one 4.9 and a 3800 S/C car.
Old 08-11-2012, 12:11 AM   #5
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Did you graph the TPS while you drove? O2 voltages? Fuel trims?

Don't know about tomorrow, I'll see if my son is available, might be fun for us.
Old 08-11-2012, 12:12 AM   #6
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

@AWD Jimmy. Did you come to the All Pontiac Show in June. They have a GMC Truck Class so we had a Typhoon and a Cyclone there. Plus a few other really nice small GMC trucks. At one time I worked as a salesman for a GMC Dealership and we had a Typhoon, man that was a cool little hotrod. I jokingly told my wife, hey we are 1/2way there being as this thing is AWD, slap on a Turbo with Alcohol injection and we could be off to the races. (Well until it spat out the stock transmission) There used to be a Street Racer guy out in the eastern Fraser Valley, had an AWD Jimmy with a built to the nuts 383 SBC in it. He took me for a ride and it was insanely fast.
Old 08-11-2012, 12:18 AM   #7
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

No I graphed the TPS while we were sitting. The truck failed AirScare so its parked at our place for the moment. I do fire it up and do quick tests in the alley behind the house but its only about 200feet long so not enough to get up to a cruise. We have mapped the O2 sensors and 2 of 3 were switching really well, fast and good voltage range. The 3rd was working but not nearly as wide of a range or switching as fast. Honestly I don't know about the fuel trims, that is one thing that has sorta never caught onto in my head. I know what it is, its the ECM adding or subtracting fuel, long term is a long term history and short tern is what the ECM has done since it was last fired up. (does the Blazer have long and short term fuel trims) I'll toss Torque on it tomorrow and see what it says. I'll even see if I can data log it and email the spread sheet to myself and post a link here.
Old 08-11-2012, 12:32 AM   #8
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

If the PCM is found to be adding fuel on a consistent basis the engine is staving for fuel. Things like fuel pressure. My 95 Jimmy had an occasional CEL and a hesitation when hot. I checked fuel pressures and they were right on spec. Of course most assume that standing beside the truck looking at the fuel pressure gauge is enough, no it's not! I had a gauge with a long hose and had it hanging in the passenger window. I found that under load, WOT up a long hill, the fuel pressure dropped to under 50 psi, should have been 60 psi. I changed the pump and it fixed it.

If I had had my Torq app at that time I could have simply watched the O2 voltages and not even got my hands dirty!

You say the misfire is at cruise, meaning light throttle. Remember that these trucks go very lean at cruise, and the timing advances a lot. These are both vulnerable conditions for a misfire.

A lean A/F ratio needs a very good ignition. Wrong plugs, cheap or bad wires, timing too advanced already can all cause a misfire. One injector, or a leaking fuel reg can cause several cylinders to go lean causing a misfire.

As mentioned, check out the Rotor phasing thread; http://www.s10forum.com/forum/f107/d...s-work-489642/

May or may not be an issue for you.
Old 11-13-2012, 08:19 PM   #9
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

AWD V8, my wife really wants the Blazer running right again for the winter, as our GTP while great in the summer is not so great in the winter, and since I posted in this thread last, our 2nd Daughter was born. Oct 18th at 1am, Erica Paige Bergquist 6lbs 12oz was born 4 weeks premature, but at 6lbs its a perfectly healthy weight. So Richard, could you give me an idea when you could give me a 2nd opinion on this truck. We got a notice from the City about having too many cars at the house that are not insured, (we have 4 Fiero's the Blazer, the GTP, and my Goldwing, but the bike is moving into the Shop now that its cleaned up enough) Are you by chance interested in 4cly 2.5L Iron Duke 5spd Fiero? LOL. http://captfiero.com/newproject/
Old 11-13-2012, 08:32 PM   #10
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

I had 2 Astro vans with the 4.3. (1995 and 1999) Had random misfires no one could figure out. Turned out to be simple, a worn distributor gear. Guess it was a common problem.

(I didn't read the whole thread, so don't bust my balls)
Old 11-13-2012, 08:32 PM   #11
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

I have some time Thursday or Saturday if you want to come over. We can have a look and see if we can find something. PM me if that works for you.
Old 11-13-2012, 10:45 PM   #12
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Quote: Originally Posted by AWD V8
I have some time Thursday or Saturday if you want to come over. We can have a look and see if we can find something. PM me if that works for you.
I will talk to my wife and see what works. Is there any chance that what Tex is saying could be the issue? We do have a spare dist shaft here, and I did want to put it in, but it looked like a nightmare from hell to do. I should have done it when I replaced the spider, but honestly thought that the new spider assembly would fix the problem so I didn't touch the dist shaft. We would probably be looking more at Saturday. However I'll drop you a PM as soon as I can get some coordination with her.
Old 11-13-2012, 10:49 PM   #13
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Quote: Originally Posted by Capt Fiero
Is there any chance that what Tex is saying could be the issue?
Absolutely, that would be one of the first things I'd check after confirming fuel pressure.

Already did that to fix an issue with my son's Blazer.
Old 11-13-2012, 10:53 PM   #14
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Is it possible to do the dist shaft without pulling the entire upper intake? If so would you mind giving me a hand with it?
Old 11-13-2012, 11:14 PM   #15
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

you can just pull the distributor without touching the intake. really its just mark the rotor position, remove 10MM bolt, and lift up.
Old 11-14-2012, 01:03 AM   #16
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Quote: Originally Posted by beandip
you can just pull the distributor without touching the intake. really its just mark the rotor position, remove 10MM bolt, and lift up.
We had 2 issues when we were thinking about taking it off,
1st was the 10mm bolt feels like it is seized. I contemplated a 10mm flex extension on my impact gun, but being as we were not sure if the dist shaft was the problem didn't want to risk making things worse by busted the 10mm bolt off. At the time we were not sure if it was anchored in the composite plastic or into steel or aluminum.

The 2nd issue and I feel stupid for admitting it, the rotor seemed like it could be mounted on the dist shaft 180 out. I snapped pics when changing the cap and rotor to make sure the rotor went back on right, but was concerned about pulling the shaft and getting something screwed up.

I can change a dist in a 2.8 Fiero on the side of the road with one hand tied behind my back. On long trips I actually carry an entire rebuilt dist shaft with me, just in case the pickup coil goes and/or module and its faster to replace an entire dist shaft than to diagnose the issue when you are broke down on the highway. (due to engine bay heat Fiero's go through module's and pickup coils on average 12-18 months)

Its really humbling coming here with hat in hand so to speak, having to learn something so simple all over again.
Old 11-14-2012, 08:21 AM   #17
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

The dist hold down bolt is going into the aluminum intake. I use a 10mm ratchet wrench, thing is invaluable for that. NEVER and I mean, NEVER volunteer to change the distributor out in a Suburban with a vortec 454!! You think the 4.3 is bad...

I'm running this distributor, can't understand how they got it so cheap, everything is new, even comes w/ cam sensor: http://www.ebay.com/itm/CHEVY-GM-V6-...86b97c&vxp=mtr

I'd try replacing the distributor, I was having problems under load / light cruising that the new distributor cleared up, but as you know, internet diagnosing is what it is.

One thing to look at, I've never seen a 4wd OR awd blazer get anything over 22mpg, you might be running seriously lean. Have you checked for vacuum leaks, especially the length of the rubber intake?
Old 11-14-2012, 10:04 AM   #18
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

If you failed air care then you may be needing a new cat.
I failed myself and as soon as I put on a new cat I went and got re tested. The guy said it was the lowest readings he's ever seen.
Of course the O2 sensors are going to indicate whether or not the cat is working.

I wonder if someone put lithium grease on that distributor bolt? It reacts with aluminum and causes a sticky powder to form. Maybe some PB blaster or a 50/50 mix of acetone and tranny fluid sprayed down there will break it free?

Good luck, AWD V8 has the tools and knowledge to get you fixed.
Old 11-15-2012, 01:36 AM   #19
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

My wife and I are going to see Richard "AWD V8" tomorrow and I am sure he can sort this out. I am bringing along all of the extra parts we have, and maybe he can let me know what is what. We have bought 3 of the sensors that go inside the dist shaft, thinking that was the problem, and it fixed it the first time, but its possible that me messing with the dist shaft itself is what fixed it and not the actual part. I did notice that even in the new shaft we have here there is a bit of slop in it. So when we pull the the in the truck we may need to steal a shim from it and add it to the new shaft. Being as its just a simple roll pin to pull the gear off and re-shim it, it might be a good idea. Does anyone know off the top of their head with the end play tolerance is suppose to be max on one of these disit shafts? I am assuming no more than about .030 and I don't have a feeler gauge handy to check this one.

As to someone suggesting this truck is running lean, that "might" be, as I noticed when I pulled the plugs they were not as dark as I am used to seeing in either my Caddy powered Fiero or our 2.8 V6 powered Fiero's. I was on the fence that maybe its just a really clean running engine. Its now got the E3 plugs in it and I have not checked them as we have not had enough run time on them to be able to verify anything. Done a few short trips from our place at 169th Street and 104th ave down to the Bastien Island Ferry. (roughly 180th Street and 104th Ave) One good note is that when I installed the new spider assembly, the truck actually did seem to run better. So at least I know that the used spider is better than the one I pulled out. I had found a guy with a 40,000km 4.3 that he was changing to carb and didn't need any of the fuel injection parts. I may have mentioned that earlier in the thread, but it was a long time ago and I am still on pain killers from my Hospital stay last week.
Old 11-15-2012, 06:17 AM   #20
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

Good luck. If you pull the dist gear be aware that the gear needs to go back on the way it came off. Note what side the dimple in the gear is on otherwise you'll get crank/cam correlation errors. Don't ask how I know..
Old 11-15-2012, 06:31 PM   #21
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

I just want to issue a HUGE THANK YOU, to AWD V8 aka Richard for his help with the Blazer today. You guys won't believe just how bad that gear really was. I have in the past 20 years of tinkering with cars never seen a dist gear this bad in my life. You can see in the pics instead of the gears being a nice thick wide cut, they were almost a triangle and worn down to a knife edge at some points. It was just crazy. On top of the fact that our Rotor and Cap had issues, even though they were brand new. The screws for the rotor had worked their way out and the rotor was bouncing around inside. (oops bad on me for not tightening them down far enough)

Well on with the pics.

The Dist Shaft Marked for removal.


The Bad Gear and a Good Gear.










As well as paying him, I have offered to do some tech support on his sons 4.9 V8 Fiero they are building.

So Again Richard my wife and I Thank You.

David
aka Capt Fiero.
Old 11-15-2012, 07:52 PM   #22
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Re: Random Misfire. Help No Codes Dealer is Stumped. 1996 AWD 4.3 Blazer

David, you are welcome. I was happy to get this truck of your working right after all the hassle you had trying to get it running right.

It was nice to not only meet a forum member face to face, but you and your wife are pretty OK too

And thank you for your generosity.

Happy truckin'
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