'05 4.3 misfire, stumble

[TheManInBlack]

Hello All,

2005 GMC Sierra 4.3 4x4 regular cab short box

Recently my check engine light started coming on while driving. I went and had it scanned and got coolant temp too low and random misfire. I replaced the thermostat and now it heats up properly. I also replaced the spark plugs and the spark plug wires (the plugs were extremely bad, the worst I've ever seen). It starts much better now.

The current symptoms are as follows when cold in the morning (10-20 degrees F) it stumbles and misfires terribly but runs good if I kick it down to wide open. Check engine light is on all the time and blinks randomly while driving.

After the engine heats up when I'm on the interstate it seems to be cutting in and out slightly as I can feel it surging. Once again if I kick it down it seems to run great.

Also around the same time it seemed to develop a vibration (not sure if this is related or not). it vibrates through the steering wheel and gas pedal mostly when at part throttle 20mph - 60mph, less when off the gas and seems gone when at wide open.

From what I have read on here I am starting to lean towards cat converter problems/oxygen sensor.

What do you all think?

[clinebarger]

Did you replace the Cap & Rotor?

[thomas_a_reed]

my 01 silverado had the same miss fire issue.. Turns out my distributer had a hair line crack in it.. replaced it and it ran fine..

[TheManInBlack]

I haven't done anything with the distributor cap yet. I ran a bottle of sea foam through it yesterday and it got considerably better but the same symptoms are still present.

The vibration has lessened a lot which really surprised me, originally I thought the vibration was a u-joint but I guess its all tied to this engine management system.

[TheManInBlack]

Put a distributor cap and rotor in the truck last night. It runs a lot better now, still stumbled when cold (starting to think thats just the way it is, maybe I should warm it up before I leave in the morning).

The vibration is almost completely gone and the missing while driving (warmed up) seems to be gone.

The check engine light did come on after about 10 minutes this morning but it came on at the same time the "Change Engine Oil" warning came up, so hopefully its just that.

BTW - This truck has 145,000 miles on it, and I have reason to believe it wasn't maintained or cared for real well before I got it.

[RWB-713]

Just another area to look at.. I had an '02 s-10 with a 4.3L that had a similiar, although not the same problem. It would juts have a hard time idling and stumble at low RPM, of course, opening the throttle slightly would comepletely cure the problem. Only threw a random misfire code once in a great while. Changed the Idle Air Control motor about 6 months ago, haven't had a problem since.

Also, the cold start issue can be cause by a poor seal between the cap & distributor, alowing moisture to build up inside the cap, I went through that problem too. Make sure there is a good seal there, and spraying an ever-so-small amount of wd-40 inside the cap & WIPING IT OFF (that's important, WD-40 is flammable, and there are sparks inside the cap), before re-assmebly can help too. That trick was curtious of my local Napa store.

Hope you get it all fixed up!

[gmclowsierra]

Check fuel pressure and the fuel filter.
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[TheManInBlack]

Thanks

Not too much has changed I did put a new dist. cap and rotor in it because they looked pretty crusty inside. Which also helped it start and run better.

It still has a miss though, particularly from off-idle to about 10% after that it clears out. The stupid change engine oil warning is still up so I haven't cleared the check engine light yet.

[TheManInBlack]

HELP!!!

The check engine light has remained on and the miss is still there. I drove the truck all over town yesterday running errands. Come out this morning and it turns over and a couple cylinders cough but it won't start.

You can hear the fuel pump run when I cycle the key and I pulled the plugs and they were soaked in gas. I can't test for spark because I'm alone.

Anyone have any ideas???? I'm missing work right now this sucks big time.

[RWB-713]

Can you scan it & get the code)s) it's throwing?

[TheManInBlack]

Actually I just had it scanned Friday after work and got the following:
P0174
P0171
P0300

[RWB-713]

Both banks lean & a random misfire code.. Sounds ignition related for sure.... I know you replaced the cap & rotor... how about the coil? And after it "coughs" does it keep turning over?

[TheManInBlack]

Haven't done anything with the coil. Yeah it will continue to turn over. Sometimes it kind of diesels after I quit cranking.

I just got a fuel pressure gauge so I'm going to go check that.

[TheManInBlack]

Fuel pressure looks good. About 62psi while cranking and then sits at 58psi for over 3 minutes, doesn't seem to be leaking down.

[RWB-713]

Yeah, that's pretty normal.

This is a tough one man. A guy that works for me had a very similar problem a few weeks back.... he had it towed to a local auto shop.... and it started up just fine when it got there... hasn't had a problem since... I HATE problems like this!

[TheManInBlack]

Pulled the plug on the #2 cyl and it has spark while cranking. I'm pretty much at a loss now

[TheManInBlack]

What about crank position sensor? It kind of acts like the timing is way off. Tries to run, SUPER rough back firing through the throttle body ect.

[bighaas79]

Check the filaments on the maf sensor. the lean codes would indidcate vacuum leak or incorrect airflow readings from maf sensor. If there is any residue on the filaments be very careful when cleaning them off. if no change then try looking for vacuum leaks.