Can't find misfire

[Chevyman91350]

Hello, this is my first time posting so I hope I can get some help. . I have a 1991 chevy 2500. It has a 350 tbi engine and it is misfiring. It does it a lot when it's cold but still does it warm, I've replaced cts, o2 sensor, new fuel filter, new distributor cap, and rotor, timing is at 5 degrees advanced. Spark plugs were replaced last summer. Truck has 100,000 miles. Thanks

[speedygonzales]

Identify the cylinder first, then decide on what testing you want to do. Start by pulling one plug wire at a time. Then you can do things like swapping plug wires to see if the problem moves or not. Then.........

[Chevyman91350]

I'm pretty sure it's multiple cylinders. I'm getting plug wires tomorrow

[mountie]

Originally Posted by Chevyman91350

I'm pretty sure it's multiple cylinders. I'm getting plug wires tomorrow

Spark plugs a year old? Inspect them.
My '88 2500 5.7 timing is 0. ( disconnect the little wire above/left of distributor)

[Chevyman91350]

Put new plug wires on today. Still doin it. I'm thinking ignition module. I do want to get a fuel pressure test done as well. I just hate to keep throwing money at it

[Chevyman91350]

The plugs are gapped a .040. Did that a week ago

[Kudzupatch]

If I remember right, the gap should be .035, not .040.

But I feel your pain. Been chasing a problem on mine for couple of weeks. Read the FAQ section. Lots of good information on how to test your sensors and trouble shoot your problem. I haven't found mine yet but I have eliminated a lot of stuff without spending money replacing good parts.

[Chevyman91350]

Ok sweet. I'll look. Any ideas on what it may be?

[mountie]

Double check the center coil - contact under the distributor cap. Make sure it still has a spring load against the rotor center.
I solved a miss after finding / replacing.

[Kudzupatch]

Originally Posted by mountie

Double check the center coil - contact under the distributor cap. Make sure it still has a spring load against the rotor center.
I solved a miss after finding / replacing.

That solved my misfire. Found I had about 4 million ohms resistance (EXTREMELY high) in the coil tower in the distributor cap. This was a cap that was only 6 month old too. Now mine no longer misfires and no more backfiring. Still looking for the that off idle stumble though.

[Chevyman91350]

I know about the piece under the rotor but how do I check if there's spring to it. That piece was rusted and I took skotch brite to it and cleaned It up. All though I don't think there is electrical contact in that piece. But could that be the problem?

[Kudzupatch]

I think you are misunderstanding. It is not the rotor or anything under it. It is the distributor cap we are talking about. In the center where the coil wire connects. Inside the cap the center post is spring loaded and presses on the rotor.

I used a multi-meter to check the resistance on mine and found out there was serious restriction. Meaning the electricity couldn't flow freely through it.

[Chevyman91350]

Ok yaw I was confused there. I know what your talking about. But the fact the my distributor cap is 1 month old with less than 50 miles on it makes me think that it is fine. It's a napa brand so it's not a cheap one

[Chevyman91350]

Yea^

[ChevyTech]

Don't be so sure a new part is good.

The quality of the parts everywhere seems to be going down.

Some NAPA parts are ok, but I have been very unhappy with the quality of several parts from NAPA lately. I always look to buy the better quality option from NAPA and still find their parts to be of questionable quality, to unacceptable.

[Chevyman91350]

Ok. So what is the best way to test that part of the cap? An ohms test would tell me. I'll do it tomorrow as I'm going to see a mechanic

[ChevyTech]

When looking for a misfire in the shop the fist thing we do is put the scope on it, and watch the scope while power braking. This way we can see the firing voltage of the individual cylinders. If a cylinder varies from the others is shows. A low firing voltage can be from, a lack of compression on a cylinder, or a secondary ignition leak. A high firing voltage and you look for a open, plug wire. High firing voltage on all cylinders and you check the coil wire / cap. Low firing voltage on all cylinders and you suspect a weak coil.

Bad coil wires are common on these trucks. The inner core burns away. Look at both ends of the coil wire.

With no scope:
I would watch and listen for a spark jumping.
If I hear nothing, and it has a dead misfire, I would check one cylinder at a time like Speedy told you to do.
For random misfiring I would use a spray bottle to mist water on it and watch for sparks.

Head gaskets failing between cylinders can start out as a misfire, can cause pinging, progress to backfiring, cause overheating...

I will not throw parts at a problem with the low quality of parts being sold. You could be adding more problems making it harder to fix.

There has not bee enough of a description for me to take a guess at what the problem is.

[Chevyman91350]

Hello sorry for the delay. Saw the mechanic today he is thinking it is a fuel problem. Possible injector. I just don't know anymore. Doing a compression test on fuel and cylinders. I will post what I find. Thanks for everyone helping

[Chevyman91350]

Just did the engine compression test. They were all 150 except cylinder 8 which was 160.

[Chevyman91350]

Fuel pressure test came out ok. Any ideas. Maybe distributor?

[ChevyTech]

Originally Posted by Chevyman91350

Fuel pressure test came out ok. Any ideas. Maybe distributor?

If the distributor shaft will wiggle sideways, replace the distributor.

On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

[Cabnchassis]

Is a misfiring or is cutting out like a vacuum leak? If it is cutting out when you first start it up and gets better as it gets warm I'd check the intake manifold gasket and TB base gasket for vacuum leaks as well as everything attached to the intake manifold. IAC valve failure can also cause stumbling.

[Kudzupatch]

Would this show up in WinALDL or some other software?

[Chevyman91350]

Well the truck surges in drive or reverse when hot and misfires. So I dont know. And I don't have an obd1 connector.

[Chevyman91350]

The distributor shaft does not move at all