Chevy S-10 Mystery Ignition Problem & Lousy Mechanics

Hi, I have a 1995 Chevy s-10 4x4 with 4.3 V6 TBII Z engine…228,000 miles.
The problem it has is when applying light throttle, it cuts out or bogs down, sometimes backfires through the throttle body.
It idles fine and revs fine at higher RPMs. It only does it when first applying gas from idle.

I have been to 3 mechanics so far…
First guy cleaned the fuel injectors and declared it fixed. I pick it up and it’s exactly the same
Second guy replaced the distributor and declared it fixed. I pick it up and it’s exactly the same.
I returned it to the second guy again. He had it for a week and didn’t know what’s wrong with it.
It’s now at a third mechanic for over a week and he still hasn’t looked at it.

I changed the spark plugs after the 2nd mechanic gave up. The old ones only had 22k on them. They were all very dirty and worn, 2 plugs had big blobs of crust on the electrodes. With new plugs, it actually ran worse, same problem only more pronounced.
The fuel filter is new, the wires are only a few years old. I also changed the TPS and made no difference.

The truck is still in nice shape. I haven’t been able to drive it for 2 months, this is my only vehicle… I don’t want to scrap it over something that could be fixed but I can’t waste much more time and money on it. Can somebody offer some guidance? Like what the problem might be? Or the name of a decent mechanic in southern NH who knows how to do diagnostics instead of guessing.

Check engine light?

There’s no CEL. It does work because it comes on if I unplug a sensor.

Does it do this when the engine is cold and/or when the engine is warmed up? ( open loop vs closed loop)

It might be time to run a compression test. The engine may be getting tired at 228k miles.

The fact that new plugs are crudding up so quickly can also point to a tired engine.

Just my 2 cents, but verifying compression is a given when dealing with an engine performance problem even if the engine is question doesn’t have many miles on it.

It does it cold and warmed up. Seems worse warmed up though. I’ll ask this guy to check compression. I was afraid of the engine being worn out. I thought I would have smoke or noises if the engine was going.

Popping back through the throttle body and hesitating when stepping on the accelerator usually means a lean mixture condition. The throttle body may need a good cleaning. I don’t believe this engine uses an MAF so we can rule that out. What color were the “blobs” on the spark plugs, black or tan?.

The blobs were like a grayish color, not black anyway. I gave the plugs to the mechanic. I always changed the plugs myself and they’ve never looked this bad even at 50k. The rest of the plugs looked like overdue worn out plugs, a little white crust on the electrodes.

The performance problem and the rapid deterioration of the plugs and the color of their deposits are pointing to an excessively lean condition. I’m wondering if there might be a problem with an oxygen sensor approaching an out of range condition even though there is no CEL.

The problem you have is called a ‘tip in stumble’. Google ‘TBI tip in stumble’ for hours of entertaining reading. My personal experience would have me checking the egr first…

With the popping and backfiring, I wonder if the timing chain is stretched too far, throwing the valve timing off. 228k on a 16 year old engine is a strong possibility. Do modern mechanics know how to diagnose a loose timing chain? There is no code for it.

Check manifold vacuum at idle and as the throttle is opened quickly and again slowly.

Test fuel pump pressure. Low pressure is a greater problem when accelerating from a stop than when crusing at highway speed and accelerating.

And you might run the engine until it is fully warmed and unplug the MAP sensor, then check for an improvement in acceleration.

Do modern mechanics know how to diagnose a loose timing chain? There is no code for it.

It seems that alot of “mechanics” these days are of the impression that if there is no code, there is no problem. Or on the other hand maybe the customer does not want to authorize any diagnosis time, or thinks if the light aint on it aint broke, just wants a quick answer. And how much quicker can you be “no code, no problem,… next customer please”

The technology in that truck is fast fading from the scene as are the mechanics who are familiar with it and also the old scan tools that could communicate with the ECU. Younger mechanics are all about OBD II and few have access to a scanner that will function on OBD I or any experience with it. And for sure a great many young mechanics these days are too dependant on computers to diagnose a problem.

And a worn timing chain might be the basis for the OP’s problems but how many young mechanics are familiar with the rattle of a worn chain at start up?

On the high mileage truck…loose distributor shaft?

If the distributor seems loose, it’s probably due to a worn distributor drive gear

db4690 I meant a radial looseness-not rotary.

Thank you all for the responses. I heard back from the mechanic. He says the engine is mechanically sound. He said the timing is off. He thinks it’s either in the distributor or the ECM. He’s putting in a used ECM 1st, if that doesn’t work, he’s taking apart the distributor.

I’ve never touched anything beyond the distributor cap & rotor. It looked like the previous mechanic had changed the whole distributor with the shaft. The whole shaft looked new. I have no idea if that included the drive gear or not.

A quality rebuilt distributor is cheaper than replacing the pick up and module in the old one and they come complete with the drive gear. If the distributor was replaced without properly setting the timing your problem might be corrected by simply getting the basic timing set.

Okay thanks. The 2nd mechanic said he set the timing at 0, which is the spec.
This current guy also said might have a pinched wire going to the distributor.