Chevrolet 4.3 Random Misfire.

[krismoriah72]

I picked up a 2001 GMC 1500 last week for either a flip or keep. Truck has a 4.3L V6. Code shows random misfire, runs like [censored] at low rpms..wont hardly idle..runs great at 2500Rpm and above.

What I have done- New Distributor and cap. New Spider. New plugs. Swapped coil and wires from another great running 4.3 with no help. Also was told that the temperature sensor could be it so i replaced that with no help.

Compression Test- 135-140 psi on all 6 cylinders.

Tried to rule out cats by pulling o2 sensors..no help.

any ideas?

 

[michaelluscher]

You don't have any form of scanner?
No CEL?

 

[krismoriah72]

Sure do...like I said random misfire. P300. Intake gaskets are my next guess but im getting tired of guessing. was hoping someone would chime in that has been thru this before.

 

[michaelluscher]

Originally Posted By: krismoriah72
Sure do...like I said random misfire. P300. Intake gaskets are my next guess but im getting tired of guessing. was hoping someone would chime in that has been thru this before.


IAC clean? Start checking for major vacuum leaks.

 

[Vikas]

Have you tried to run the manual power balance test by disabling one spark plug / injector at a time when the car is NOT running good? This will tell you which cylinder is causing problem.

 

[krismoriah72]

Originally Posted By: Vikas
Have you tried to run the manual power balance test by disabling one spark plug / injector at a time when the car is NOT running good? This will tell you which cylinder is causing problem.


yes i did this and cyl 1 and 5 seemed to make no difference in how it ran. This has the spider with poppets so idk if i can test the injectors like this. Those cyls had good compression and good fire..but idk how to test the fuel.

I have also talked to several local 4.3 gurus. They said it could be camshaft, timing chain, catalytic converter, intake gaskets, head gaskets, valve not seating etc.

internet says- distributor, spider, coil, wires, plugs.

Also supposedly could be the ECM

The p300 is very generic and just about anything can cause it which is hard to figure out without tearing down the whole top of the motor.

 

[sasha]

Get a can of starting fluid or soapy water and start spraying. Rule out vacuum leaks.

 

[Food Scientist]

I would also check out the MAP sensor. Sister has the 5.3L engine and cleaned that. Runs a lot better but the input voltage from the sensor is out of range at times, so the sensor itself will have to be replaced.

Codes were misfire on all cylinders and A/F ratio way out of range.

 

[krismoriah72]

Originally Posted By: SVTCobra
I would also check out the MAP sensor. Sister has the 5.3L engine and cleaned that. Runs a lot better but the input voltage from the sensor is out of range at times, so the sensor itself will have to be replaced.

Codes were misfire on all cylinders and A/F ratio way out of range.


oh yeah i forgot to mention that..swapped that with a good running 4.3 along with the coil and wires. no help

 

[3800Series]

Clean the throttle body, MAF, and idle air sensor if you have one. My Regal was running very rough for a few weeks and didn't want to start randomly died just really bad. Turns out it was a bad coolant temp sensor causing the car to run rich.

A lot of things can effect ignition it could even be a bad wire arching on the engine. Never really know.

 

[Kira]

Hello, My buddy has a 4.3 in a 2003 S-10 and it has a miss. He searched and found mention of a known weakness in the ECM.
He went to automotive school and he is at a crossroads. That means he can't decide to just swap in a new controller or what. I'll ask him.

We did plugs, secondaries, cap and rotor first thing and only got the general expected improvement.
I'll ask him specifically what his symptom is. Kira

 

[Doog]

Originally Posted By: sasha
Get a can of starting fluid or soapy water and start spraying. Rule out vacuum leaks.


+1 but I use an unlit propane torch.

 

[mulehead]

Egr valve clean it happens all time on my astro van's.

 

[xxch4osxx]

Could be a O2 sensor causing it. I had an upstream O2 sensor go bad and was over fueling the passenger side bank. Gave the PO300 code as well and was blowing black smoke. I replaced the sensor with an NTK sensor and the problem was gone.

 

[cb_13]

Originally Posted By: Doog
Originally Posted By: sasha
Get a can of starting fluid or soapy water and start spraying. Rule out vacuum leaks.


+1 but I use an unlit propane torch.

Another vote for Propane, it works very well. In a shop environment my favorite was an EVAP smoke machine. I would smoke the intake and look for it leaking out. That's not exactly something everyone has sitting in the home garage though.

 

[michaelluscher]

Originally Posted By: cb_13
Originally Posted By: Doog
Originally Posted By: sasha
Get a can of starting fluid or soapy water and start spraying. Rule out vacuum leaks.


+1 but I use an unlit propane torch.

Another vote for Propane, it works very well. In a shop environment my favorite was an EVAP smoke machine. I would smoke the intake and look for it leaking out. That's not exactly something everyone has sitting in the home garage though.


I've seen the prices on those, your right

....but a $29 Halloween smoke machine and some tubing/adapters from Home Depot gets you the same jist, for less.

 

[Scott_Tucker]

The problem is the injectors. That spider fuel injection was terrible and this is a known problem. Replace all the injectors, not just the ones you think are bad or you'll regret it the third time you do the job.

 

[Trav]

It probably has a bad spider. You can upgrade these old units to a modern electronic hybrid unit.
Don't bother with rebuilds or trying to get this one rebuilt, they were junk when they were made. You cant fix broke.

Let me know though PM if you need one.

 

[spasm3]

Originally Posted By: Trav
It probably has a bad spider. You can upgrade these old units to a modern electronic hybrid unit.
Don't bother with rebuilds or trying to get this one rebuilt, they were junk when they were made. You cant fix broke.

Let me know though PM if you need one.


Can he bolt on a 2002-07 unit? I think gm redesigned them a bit.

 

[The_Eric]

Originally Posted By: Scott_Tucker
The problem is the injectors. That spider fuel injection was terrible and this is a known problem. Replace all the injectors, not just the ones you think are bad or you'll regret it the third time you do the job.

Originally Posted By: Trav
It probably has a bad spider. You can upgrade these old units to a modern electronic hybrid unit.
Don't bother with rebuilds or trying to get this one rebuilt, they were junk when they were made. You cant fix broke.

Let me know though PM if you need one.


Hey guys- it looks like he already replaced it?

Originally Posted By: krismoriah72

What I have done- New Distributor and cap. New Spider. New plugs. Swapped coil and wires from another great running 4.3 with no help. Also was told that the temperature sensor could be it so i replaced that with no help.




O/P- you were talking about swapping parts with a good running 4.3- I saw mention of MAP sensor, did you mean MAF sensor? If you haven't swapped it yet, you can try unplugging it. They don't always kick a code out and can cause lots of problems.

Did you replace the coil wire too with the coil? Seen lots of those burn through.

Have also seen leaky intake gaskets (both from the top and bottom) cause similar issues. A scan tool can point you in the right direction fuel trims.

Read the barometric pressure with the scan tool too.

Checking the EGR is a good suggestion as well. This will likely show up in the fuel trims also.

If you did replace the distributor, you'll need to have the timing set- it's called cam retard. This done with the aid of a scan tool. The typical low dollar ones won't allow you to set it. You'll need a Tech 2 or tool truck unit to read it.