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Chevy 4.3 Engine Saga

I’ve posted about this before. It’s a 1995 S10 4.3 TBI Vin Z engine. 237k miles. I had a used engine installed over a year ago and around 10k miles on it so far (engine supposedly had 100k miles on it). It ran good, burned a little oil but no noticeable problems. A month ago it started bogging down badly when stepping on the gas, it was idling fine though. I took it back to the shop that put the engine in.

They replaced the fuel pump, ignition module, ignition coil and the spark plugs for $800.00. They had it for over a week. Picked it up and it idles very rough and has reduced power. Exhaust chugs like a race car. I took it back, they had it over a week again and they said they can’t get it to run any better.

So I plugged in my scanner, it’s running very rich. The long term fuel trim is 108, which is the lowest it will go, the normal is supposed to be 128. The short term is around 110. The numbers jump up when hitting the throttle then fall back at idle. The fuel trims looked fairly normal before I took it to the shop.

I checked the timing with a light and it’s advanced quite a bit.I tried to set the base timing back to zero and it barely runs. The timing mark is jumping around also. I have the computer advance unplugged and it makes no difference, the mark jumps back and forth. I checked the rotor position with the crank mark aligned and it seems to be lined up with no. 1 plug. I ended up advancing the timing back to where they put it.

I took the distributor out to check the gear and it doesn’t look worn. The distributor is about 2 years old, cap and wires are 1 year old. So what causes timing to be off this much and jump around like this? I’m thinking timing chain or distributor might be faulty or off a tooth in the gear? Can this be bad valves or low compression? The mechanic had practically zero communication with me. They said they ran diagnostics but I have no idea what that entailed. They pretty much just wanted my truck gone and don’t come back.

You put a used engine in a 1995 S10 and spent another $800.00 on it. Why?


One reply and it’s just a troll…great.

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I’d be guessing timing chain.

I tried watching the rotor turn with the crank. It has a little play, I didn’t measure it. I guess if it jumped a tooth I’d have to take the cover off to find out.

Unclear about what those fuel trim values you report mean. I presume your truck is OBD II, right? When folks here talk about fuel trim under OBD II, the units of fuel trim are usually %, and somewhere near 0% is normal; something in the range of -2% to 2 % is what you’d like to see ideally for fuel trim. Are you able to somehow convert your FT numbers to the more common %?

From what I can tell, here’s how to set the timing. Verify this conforms to the label in the engine compartment. With key off disconnect PCM set-timing connector, tan/ black wire; connect inductive timing light to # 1; run engine, trans neutral, verify 0°BTDC on balancer/pulley. Makes sense to check this with a warm engine.

It’s normal on engines with electronic timing for the mark to jump around in normal operation; if the timing mark jumps around when setting the base timing tho, that isn’t normal. hmmm … well, timing is related to the MAP sensor and coolant temp sensor readings, so there could be something wrong there. If you run at 2000 rpm, does the timing change if you disconnect the set-timing connector? It should. Does the timing advance noticeably when you bump the accelerator briefly from idle? It should.

Problems with the crankshaft or camshaft position sensing function could cause a timing problem. How is your engine configured for this function? If those functions are done inside the distributor, there’s a pickup coil inside the distributor that might be defective. Or the distributor shaft may be loose in its bushing. That could cause the magnet to dither from the pickup coil. Or maybe the coil spacing from the magnet isn’t adjusted correctly.

You may need to replace the distributor to solve all this. But take a look inside and outside, including the cap, make sure there are no cracks, no signs of spark caused carbon “tracks”, the rotor looks ok, and the plug wires are in good shape. Are the wires connected to the spark plug in order?: I think for your engine it is 165432

The timing chain affects the valve timing, normally not the ignition timing.

My guess, you need a new distributor, or have the existing one rebuilt.

The large advance the shop dialed in just to get it to run could mean there’s a problem with the crankshaft pulley position or timing marks too. The gadget that produces the sparks should be considered suspect also, called ESC I think. Do you see a good healthy blue-white spark at the spark plug electrodes?

It’s the early OBD II system GM had. It’s technically OBD I but has an OBD II connector. It shows limited stream data. That’s the trim values it uses, 128 is ideal and 108 is maximum negative value. I don’t know too much about it but that’s what I’ve read.

I disconnected the ESC when doing the timing. I didn’t actually rev the engine when checking it, only at idle. The timing looked the same at idle whether the ESC was in or not. I’ll rev the engine next time and see what happens. I don’t even see an ESC offered at Rockauto for this car.

This doesn’t have a cam or crank sensor. I can try a new distributor, they’re not too expensive. I only checked the spark with a light tester, i can try taking them out. I guess I’ll have to dig into the ignition more. Thanks for the suggestions.

Well my memory is fuzzy but from my Riviera days I think the fuel trim was just the pulses or the open time for the injectors. So as the car would lean out, you’d get a lower number. Just indicates how much fuel is being supplied to tell you if its lean or rich.

The only time I had a fuel mixture issue like too rich was a bad MAF sensor but I got no codes, just the fuel trim going nuts. The computer just used the input information to adjust the fuel trim. So the fuel trim was the result, not the problem.

That’s the extent of my knowledge so I’ll shut up except my experience with the 95 which was between OBD I and II, is that only the dealer had the right equipment to properly read the computer. Newer or older was no problem but 95 was.

Did you ever change the oxygen sensor? I have this same engine except in a 2000. I had poor running and TERRIBLE fuel economy at about 80-90k. The exhaust smelled like a lawnmower. I gave it a normal tune-up and this barely helped. I had never changed the oxygen sensors so decided to do so even though there were no codes. The truck then ran like new. I have seen this in other vehicles. The oxygen sensors have to be REALLY out of whack to set a code. There must be a wide tolerance on them.

I do not know what the 1995 had. It may have one or two sensors on it and none downstream of the cat. If you have more than one only change the one(s) before the cat at first. 1995 was a transition year as mentioned so I don’t know how it was setup. Some cars from that era were more like an OBD1 than 2 setup. I don’t know about this one.

Also do some basic checks. Check engine compression to make sure the engine isn’t about shot. Poor compression on all cylinders may result if the engine jumped time. If all cylinders are consistently low I would start to suspect jumped timing. If some are high and the others not, that is something else like rings or valves. Look at the spark plugs too. Do some look OK and others look fouled?

You might also check fuel pressure and make sure any regulators are working OK.

This is a pretty tough engine and I would find it surprising if it jumped time. I hydrolocked mine in a creek once and you would never know if happened.

Are there any codes like a random misfire? I had to replace the stock coil pack at about 100K as well. It was getting random misfires and overall poor running again.

This reminds me of a magnetized distributor shaft. When a shaft gets magnetized it will cause the pickup signal to be askew which means the ignition module can’t do it’s job properly. the only way to prove it beside simply replacing the shaft is to scope the signal from the purple with white wire, if my memory serves me correctly. Back in the day we used to replace a dist shaft at the average of two a week to fix driveability problems similar to yours.

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The quickest way I’ve found to check for spark is to leave all the spark plugs installed & use another spark plug, properly gapped, as the tester.

I put a new distributor in today. It’s pretty much the same. The timing mark flutters around at idle. I noticed if I rev the engine the timing mark swings to the left and turns rock solid, no fluttering. Back to idle the mark jumps around. It’s the same with the esc on or off. Any ideas?

I already changed the o2 sensor, temp. sensor, IAC, TPS and swapped most of the emissions stuff.

The engine still runs poorly, after another distributor? hmm … the timing comes from coil pickups inside the distributor, from the distributor shaft turning, right? And there’s no mechanical advance in the distributor, it is all done electronically, is that also correct? hmm … do you feel any play if you twist the rotor a little? If so the problem might be the gear that drives the distributor, usually that is on the camshaft. It might be worn & not be meshing correctly.

The timing marks probably always jump around a little at idle, that’s the case w/my distributor timed Corolla anyway, but that jumping around effect should minimize greatly when you set the special test mode for setting the base timing. Well, next step is probably a compression test, checking the valve timing, and taking a look at the gear that drives the distributor.

Honestly, I’m not sure if the timing is the problem or not. Could be the fluttering isn’t the main problem, it’s a little hard to set it at zero though. From what I’ve read it should be steady with the esc computer advance disconnected. The new rotor doesn’t have any play, the old one had some vertical play.

I don’t have any pressure test equipment. I’ll have to buy it or pay another mechanic to test it. I guess I should get it checked before throwing more parts at it. I could try taking the timing cover off just to check the chain. I’ll have to look into the cam gear also. I don’t know how to do that…ugh.
Thanks for the suggestions.

Mine just has one o2 sensor and it’s fairly new. It’s not throwing any codes except for the ESC when I disconnect it. My truck has only thrown a code once in 22 years when the TPS failed. Would be nice if it was a little more sensitive, lol.

I think I figured it out today. I took out the fuel injectors. The regulator and pump are new but I never looked at the injectors. I took off the filter screens, sprayed them and the nozzles with carb cleaner. They didn’t look dirty but one had a big flake of plastic crud or something in it. Put them back in and it’s running way better. No shaking at idle and big power increase

It’s not quite 100% but it’s way better and this is the only thing that’s made a difference. I even tried setting the timing back to zero and it runs about the same, before it barely ran unless the timing was advanced.

The mechanic mentioned the injectors as a possibility once but later said the spray looked fine so probably not it. I’ll probably order new injectors. I guess I got too hung up on the timing. The mark still flutters around at idle but maybe it’s always done that.

Glad to hear something made a difference. I have the 2000 with the GM spider injectors so the entire upper intake must come off to replace or service the injectors. I do not like this design but it hasn’t failed me yet. The intake gasket leaks a tad as they are known for but it is very manageable to just keep adding coolant. I figure when I have to address the intake gasket that I will also replace the spider injectors for good measure.

If replacing the injectors solves the problem then you might want to also change the fuel filter again and then every 15,000 miles or so afterwards.

The compression tester can be rented as a loaner tool from most large auto parts chains. You basically pay for the tool and they allow you to return it without penalty when you are done. You can also buy them cheap from Harbor Freight although I hear they are not all that great. You get what you pay for. Some HF stuff is OK and some is not.

I wish I would’ve taken the injectors out before…few cents worth of carb cleaner…oh well. I’m pretty sure this is it but I’m not going to call it fixed yet.

I think your engine has more HP than mine with the multi injection. These are good trucks. Most of the issues are straight forward, the engine misfires have been a real chore to track down though. I fully admit I’m terrible at diagnostics.

I was gonna get the HF tester but I looked at the reviews and too many bad reviews. I bought their fuel pressure tester and that leaked.

Buy a used snap on, mac or matco compression tester kit on ebay

At least the quality will be good

I’d rather buy a used quality tool, versus new garbage