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4.3 misfire

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Old 01-30-2012, 03:37 PM   #1
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4.3 misfire

Really needing some input on this. I have a 1999 s10 4.3 with a misfire on cyl. 4 PO304. Its had this problem since i bought the truck. The things i have done to it are, fuel pump, fuel filter, updated spider injection, distributor cap and rotor, lower intake,plenum and throttle body gaskets, map senor, pcv valve and throttle position sensor. Ive check the wires and done a vacuum check on everything. Right now when the truck is in park or neutral it idles about 1000 rpm. When you put it in gear rpm go down to between 600-700 then you can really fill the misfire. When your driving it idle is way to high, on flat ground its like cruise control is set about 40mph. And then when you get in the throttle it will miss really bad for just a few secs almost die sometimes then the rpm get up above 1800-2000 and runs fine. Idk i guess im gonna go up to the auto parts store and have them check the distributor module. But could it possibly be the crank sensor? Any help would be greatly appreciated, going broke replacing parts.
Old 01-30-2012, 03:54 PM   #2
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Re: 4.3 misfire

sound to me like the modual in the distributor. you didn't mention above anything about spark plugs. did you check for cracks?
Old 01-30-2012, 04:29 PM   #3
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Re: 4.3 misfire

Forgot to mention them, but its got new bosch platinum +4 plugs. And the module, gonna go get one and see if it fixes it.
Old 01-30-2012, 04:59 PM   #4
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Re: 4.3 misfire

how old are the spark plug wires, and brand
Old 01-30-2012, 11:51 PM   #5
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Re: 4.3 misfire

Quote: Originally Posted by md0516
Forgot to mention them, but its got new bosch platinum +4 plugs. And the module, gonna go get one and see if it fixes it.
bosch = sucky should be using ac delco or NKG
Old 01-31-2012, 11:00 AM   #6
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Re: 4.3 misfire

Do a full tuneup of the ignition. Everything under, and including, the cap, plugs and the wires. I've never used Bosches so i can't verify or deny pjg1173's claim, but from posts of his i've seen he seems to know what he's talking about. Personally I love E3 plugs, I've put them on my Blazer and countless F-Bodies and seen nothing but great things from them.

PS: I ALWAYS do a full tune up immediately after buying a used car, cause unless you personally know the person you're buying it from, you can't trust the seller.
Old 01-31-2012, 11:36 AM   #7
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Re: 4.3 misfire

Okay so i went and got the pick up module and put it on.......still runs like crap. Ive notice that when you first start the truck, it runs okay. Then when it starts to warm up the misfire gets worse. The thing about it is the code reader is saying random misfire cyl4. So i dont think its spark plugs or wires because id think that would be a consistent miss not random. And now when your setting in park and rev it up and down, every now and then you'll hear it backfire. And the idle is still to high, in gear and out. Ive replaced the distributor cap, rotor, pickup module, checked the wires,new spark plugs and i took the ignition control module and had it tested(it was fine. Now thats most of the ignition system. The only thing it may be that i havent done would be the crank sensor. And all the fuel system is new, fuel pump,filter,regulator, and updated injection, so think its still somewhere in the ignition. Any help let me know, starting to get frustrated.
Old 01-31-2012, 11:41 AM   #8
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Re: 4.3 misfire

guy (twotoo) in my thread had a cylinder 4 misfire as well.

http://www.s10forum.com/forum/f107/0...isfire-483396/
Old 01-31-2012, 11:54 AM   #9
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Re: 4.3 misfire

Quote: Originally Posted by black_ice_02
guy (twotoo) in my thread had a cylinder 4 misfire as well.

http://www.s10forum.com/forum/f107/0...isfire-483396/
Bent or stuck? Would a compression test show that. That is about the only thing i havent done. Kinda sucks if it is that(more work and money),but that may have been the problem from the begining.
Old 01-31-2012, 11:58 AM   #10
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Re: 4.3 misfire

yes, i was going to suggest a compression test next. bent valve or broke valve spring would misfire all the time and be quite noisy. a sticky valve or possibly a weak valve spring would be more random. sometimes a good engine flush before an oil change can help degunk the valve guides. I still think you issue is either fuel or ignition related.
Old 01-31-2012, 12:01 PM   #11
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Re: 4.3 misfire

compression check should help, not saying this is your case, but most problems with these trucks, like I'm seeing, is random cylinder not one in particular
Old 01-31-2012, 12:31 PM   #12
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Re: 4.3 misfire

How much compression should there be. Getting ready to go check it. And there is a slight tick on that side of the motor when its running.
Old 01-31-2012, 12:58 PM   #13
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Re: 4.3 misfire

Did the compression check on cyl 4. probably gonna start pulling the heads. cyl 4 only had barely 75psi. Did a comparison on cyl 2 it had about 130psi.
Thats probably the problem, wouldnt ya say.
Old 01-31-2012, 01:45 PM   #14
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Re: 4.3 misfire

Quote: Originally Posted by md0516
Did the compression check on cyl 4. probably gonna start pulling the heads. cyl 4 only had barely 75psi. Did a comparison on cyl 2 it had about 130psi.
Thats probably the problem, wouldnt ya say.
yes I would, time to pull the heads
Old 01-31-2012, 01:49 PM   #15
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Re: 4.3 misfire

eew... I'd take the valve cover off and look at the valves. spin the motor by hand and see if you can see one stick or not move properly. it could be a weak lifter or just out of adjustment. even a week spring can be replaced without pulling the head.
Old 01-31-2012, 02:34 PM   #16
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Re: 4.3 misfire

oof, maybe u'll get lucky and it's only a leaking head gasket. unlikely but one could hope
Old 01-31-2012, 02:54 PM   #17
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Re: 4.3 misfire

least you caught it before spending more money in the wrong direction, mine is still random, but debating on doing the compression testing as well
Old 01-31-2012, 04:00 PM   #18
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Re: 4.3 misfire

I pulled off the valve cover and turned the engine over. Everything looked to be moving freely. The rockers were not out of adjustment. Its probably the valves not seating like they should. I mean the truck has 225xxx miles on it.
Valve job will probably fit everything, probably get them resurfaced while im at it.
Old 02-24-2012, 12:35 PM   #19
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Re: 4.3 misfire

Ok finally got it all put back together. Put reman heads on, spark plugs, spark plug wires and a new distributor. But now when i try to start it its very hard to start and backfires. I can get it to start but it seems the backfire is constant while running.
Old 02-24-2012, 12:41 PM   #20
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Re: 4.3 misfire

double check the fire order. the cap is labled and they are not in order like it appears.
Old 02-24-2012, 12:48 PM   #21
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Re: 4.3 misfire

Im a total moron that was it thanks man
Old 02-24-2012, 12:51 PM   #22
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Re: 4.3 misfire

so hows it run?
Old 02-24-2012, 01:10 PM   #23
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Re: 4.3 misfire

Once again there is still a problem. Starts great now take i got the spark plug wires where they go. But then i took it for a drive. When you go to take off or get in the throttle it hesitates and spits and sputters(BAD) then once the rpm are above 2500 its fine. When in park or neutral cant get it to do it, only does it in Drive or Reverse. No Check Engine lights either.
Old 02-24-2012, 01:15 PM   #24
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Re: 4.3 misfire

did you put a new cap and rotor on when you put the engine back in? also check fuel pressure.
Old 02-24-2012, 01:24 PM   #25
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Re: 4.3 misfire

Distributor, Cap, Rotor and Gear are new. The Fuel Pump, Fuel filter, Fuel Pressure Regulator, and Injection(updated) are new. I dont have a way today to check the fuel pressure, have to go to my grandpa's house to do that. Could it possibly be the Crank Position Sensor?
Old 02-24-2012, 01:30 PM   #26
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Re: 4.3 misfire

IDK I've not had to mess with one yet. I'm sure someone will jump in any time drinking the crank relearn coolaid. you need to research reasons for doing a crank relearn. you may have done something during your rebuild that warrants doing it.
Old 02-25-2012, 02:41 PM   #27
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Re: 4.3 misfire

is the cel blinking? mine was somewhat similar, but the cel would blink under power ended up being an 02 sensor, it kept showing p300 at auto zone, finally put it in the shop and they found the right 02 sensor. sadly i paid 400 to have an 02 sensor changed, but runs fine now.
Old 02-27-2012, 06:49 PM   #28
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Re: 4.3 misfire

New problem now. When you start it its idles up to about 2800 rpm then will slowly go down to about 1500 rpm. When you put it into gear, it runs very rough almost about to die.
Old 02-28-2012, 10:03 AM   #29
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Re: 4.3 misfire

sounds like a vac leak?
Old 02-28-2012, 01:16 PM   #30
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Re: 4.3 misfire

When it warms up the rpm will go down to about 800. But when you put it in gear it has a rough shaky idle for a sec. Then when you go to take off it falls on its face the suddenly takes off, but if you give it throttle slowly its fine. Like if your needing to pull out on to the highway and need take off quick thats when does it. And i notice a random misfire while idle. Showing no codes.
Old 02-28-2012, 01:53 PM   #31
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Re: 4.3 misfire

anything coming out of exhaust? raw fuel or soot?
Old 02-28-2012, 03:18 PM   #32
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Re: 4.3 misfire

nothing coming out the exhaust. i did just take if for a drive and got it to throw a code. so im gonna get the scanner from my grandpa and see what its saying. ill let ya know
Old 02-28-2012, 08:19 PM   #33
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Re: 4.3 misfire

Its showing PO113 and PO113 Pd. IAT high input. The IAT is new, so i got to checking the wire to it. I guess the is no voltage getting to it....i guess it should have 5volts and im not getting any.PCM? Is there a way to test the PCM?
Old 02-28-2012, 11:08 PM   #34
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Re: 4.3 misfire

DTC P0113 - IAT SENSOR CIRCUIT HIGH VOLTAGE
NOTE: For circuit reference, see the L - WIRING DIAGRAMS article.
Circuit Description
Intake Air Temperature (IAT) sensor is a thermistor that varies resistance based on temperature. As temperature of sensor
increases, resistance decreases. Low temperature will result in a high
signal voltage. DTC will set when VCM sees an IAT sensor voltage of
greater than 5 volts.
Conditions required to set DTC are:
* No ECT sensor related DTCs.
* No VSS DTCs are set.
* No MAF sensor related DTCs.
* Vehicle speed is less than 2 MPH.
* MAF value is less than 250 grams per second.
* ECT is greater than 84.7 F (29.3 C).
* Engine run time is greater than 100 seconds.
* IAT voltage is greater than 4.9 volts.

Diagnostic Procedures
1) Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. See the
F - BASIC TESTING - 4.3L article. Retrieve and record FREEZE FRAME
and/or FAIL RECORDS data for each DTC set. Go to next step.
2) Start engine and allow to idle. Using scan tool, read IAT
sensor voltage. If voltage is greater than 4.9 volts, go to next step.
If voltage is not greater than 4.9 volts, go to step 6).
3) Turn engine off. Turn ignition on. Disconnect IAT sensor
harness connector. Connect a jumper wire across IAT sensor harness
connector. If voltage is less than 0.82 volt, go to step 7). If
voltage is not less than 0.82 volt, go to next step.
4) Connect jumper wire between IAT sensor harness connector
signal circuit and chassis ground. If voltage is less than 0.82 volt,
go to step 8). If voltage is not less than 0.82 volt, go to next step.
5) If DTC P0123 is also set, go to DTC P0123. If DTC is not
set, go to step 9).
6) DTC P0113 is intermittent. If any additional DTCs are set,
diagnose affected DTCs. If no additional DTCs are set, see DIAGNOSTIC
AIDS.
7) Inspect VCM and IAT sensor for proper connection. If a
problem is found, go to step 10). If connections are okay, go to step
11).
8) Check IAT sensor harness connector ground circuit for open
between VCM and IAT sensor. If open is found, go to step 10). If open
is not found, go to step 12).
9) Check IAT sensor harness connector signal circuit for open
between VCM and IAT sensor. If open is found, go to next step. If open
is not found, go to step 12).
10) Repair circuit as necessary. After repairs, go to step
13).
11) Replace IAT sensor. After replacing sensor, go to step
13).
12) Replace VCM. Program replacement VCM using required
equipment. After replacing VCM, go to next step.
13) Using scan tool, select DTC, CLEAR INFO function. Start
engine and allow it to reach operating temperature. Using scan tool,
select DTC, SPECIFIC function and enter DTC P0113. Operate vehicle
within conditions required to set DTC. If scan tool displays RAN AND
PASSED, go to next step. If scan tool does not display RAN AND PASSED,
return to step 2).
14) Using scan tool, select Read and Record INFO, REVIEW INFO
function. If any undiagnosed DTCs are displayed, diagnose affected
DTCs. If no DTCs are displayed, system is okay.
Old 02-29-2012, 12:59 AM   #35
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Re: 4.3 misfire

I will give it a try. But what does the other code PO113 Pd mean. It is showing that one as well as the PO113. Whats the difference?
Old 02-29-2012, 12:32 PM   #36
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Re: 4.3 misfire

Besides the po113. I started it and let run this morning until it got to temp. Then did the scan. Its now showing po122 tps a circuit low input. And is showing po123 tps a circuit high input. Why and how is it show both?
Old 02-29-2012, 01:40 PM   #37
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Re: 4.3 misfire

Quote: Originally Posted by md0516
Besides the po113. I started it and let run this morning until it got to temp. Then did the scan. Its now showing po122 tps a circuit low input. And is showing po123 tps a circuit high input. Why and how is it show both?
Wiring problem?
http://easyautodiagnostics.com/gm_tp..._van_tps_1.php
Old 05-23-2012, 07:24 PM   #38
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Re: 4.3 misfire

Quote: Originally Posted by burned

Something to consider. Wiring has been suggested, and I think this is wise to check. Sounds like maybe the wiring that is supposed to fire the #4 injector maybe not working or shorting out. If not, try a full computer swap. You could have a faulty PCM...

Also, think about the crank sensor too... The crank sensor can pickup misfire vibrations. If you had changed the crank sensor at any time, you might have to do a special sequence of events to reprogram the computer to accept it. Sometimes this is an issue too. Since you bought the vehicle like this, I would guess that it might be possible that the previous owner had changed it. Good luck.
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