89 S10 serious problem

The vehicle in question is a 1989 Chevrolet S10 pickup, 2WD, 4.3L V6, automatic overdrive transmission (700R4 I think?), mileage estimated somewhere above 150K (odometer doesn't work).

A few months ago a problem began to develop where the engine would lose power and just sputter at different times, similar to fuel stavation. Usually the episode lasts no more than 2-3 seconds and then it resumes running normally.

It seems to be more frequent when the vehicle's torque converter is locked; in fact, on a number of occasions the sputter condition started immediately at TC lock, roughly 30-35 mph in third gear.

The vehicle has stalled completely a couple of times when the sputter condition occured at lower speeds; it has started up again immediately each time.

During this time, the ECM has reported no codes, so I've gone through the usual gamut of suspects: tune up (including new cap, rotor, and wires), ignition module, EGR valve (which was in fact not working, but replacing it did not make any difference in the problems), fuel pump. I even replaced the battery cables at the suggestion of a mechanic who noted they were moderately corroded and said that they might be affecting ECM voltage (it didn't help at all).

I've tested the ignition switch, and made sure that all electrical connections were good and tight. I've noticed no unusual smoke from the exhaust The exhaust isn't plugged as far as I can tell. Yet the problem persisted.

Yesterday, on the interstate, the engine began sputtering the way it had been, but this time it didn't sort itself out as it had in the past. Instead, it continued to lose power until even at wide-open throttle, the best the vehicle could manage was 5-10 miles per hour. I was fotunately able to limp the vehicle to an exit, but when I came to a stop and put it in neutral, the engine at idle was on the verge of stalling. It would not rev past about 2,000 rpm (and even at that speed it was really, really struggling). Disconnecting the EGR valve smoothed out the idle somewhat but did not help the poor-rev problem.

I had the vehicle towed home, and I was able to check the ECM for codes again; this time I got a 33. I've gotten conflicting information about code 33, but most often the answer seems to be MAP sensor - is this correct? Can a faulty or failed MAP sensor really cause all this grief? Is it possible to test a MAP sensor, or is it just some item you replace and just hope that was the problem? I really, really don't want to spend any more money than is absolutely necessary on this truck, and I'm tired of throwing parts at it without results.

Any assistance would be greatly appreciated.

Reply to
xmh31d_64
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Code 33 indicates high MAP signal voltage outside the normal operating range. Check all vacuum lines first, especially the OEM plastic ones which by now will likely be cracked at one place or another. The MAP line is one of these plastic lines. You can do a quick and dirty test using a DMM:

  1. Check MAP supply voltage from ECM by disconnecting the wire harness and probing the gray wire (+) and orange/black or black wire (-). You essentially are shorting the outer ports of the plug together with a DMM and taking the voltage reading. Turn ignition key to ON. You should have around 5 volts. If you don't, check all of the wiring connections clear back to the ECM. If they still check out, the ECM is likely pooched.
  2. Reconnect MAP sensor and backprobe the signal wire (light green wire, middle port on plug) with your DMM. Postive lead of DMM to signal wire, negative lead of DMM to good ground. Turn the key to ON. You should have between 4.0-5.0 volts. Fire the truck up. You should have between 0.5-2.0 volts at ilde speeds. If your voltages are different, MAP might be shot.

Doc

Reply to
"Doc"

Doc, thanks for your help. I tested the MAP sensor as you indicated and it was indeed smoked. I'd love to be able to report that this fixed the problem, but unfortunately improvements are only minor. It does run a little better than before (in that I can actually drive it at about 35 miles per hour now, but that is pretty much the limit of the vehicle's capabilities at the moment, and it absolutely *DESPISES* hills). The idle is now actually pretty good, but I've discovered something that I didn't notice yesterday - a hissing noise from under the vehicle, which now has me thinking the bulk of my woes are located in the catalytic converter. A friend of mine thinks that perhaps the old MAP sensor had the engine in a seriously-rich condition, resulting in the catalytic converter becoming clogged. I'll admit I don't know a whole lot about pollution control systems (most of my mechanical endeavors involve pre-1970 iron), but to me that doesn't sound right. Is it possible for a catalytic converter to become clogged that quickly, in that it runs pretty good at 6:15 and barely runs at all at 6:30? Or do I have that backwards, the catalytic converter became clogged and the condition caused the MAP sensor to say bye-bye?

Reply to
xmh31d_64

A rich engine can cause a cat to become plugged, but not within 15 minutes. A clogged cat on the other hand cannot affect the MAP in any way. To rule it out, just disconnect the y-pipe from the cat (if it's welded on, pop the exhaust loose where the y-pipe attaches to the manifolds) and run it with open manifolds. It'll be painfully loud, but you'll be able to tell if it runs better or not.

Have you checked the mechanics of the engine via a compression test? How about ignition timing? Poor compression and/or greatly retarded timing can both cause symptoms you have described.

Also, your ECM might be toasted and running in "limp-home mode" via the CALPAK and not the PROM, which will also cause symptoms similar to what you've described.

Doc

Reply to
"Doc"

I did something similar. It was suggested that I try removing an O2 sensor temporarily, the theory being that if the exhaust was indeed plugged up, the release of back pressure would allow the engine to operate marginally better. Obviously, this would not provide anywhere near the capacity of a fully-functional exhaust system, but might give me some indication of what's going on.

I cannot easily access the 02 sensor, so remembering that EGR circulates exhaust gases back into the intake, I removed the EGR valve, plugged the inlet to the intake manifold (since it would be a huge vacuum leak) and left exhaust-gas port open. The result was the engine did start easily and seemed to rev more readily, though the noise was incredible. I only operated it this way for maybe 30 seconds, and did not attempt to drive it like that. Once the EGR valve was reinstalled, it reverted back to its barely-running condition.

That, and the hissing noise under the truck, kinda has me thinking that the cat's junk.

I have not checked compression - I already know I've got some blow-by since the motor has such high miles, and getting to some of the spark plug holes on this truck is an adventure in double-jointedness. Ignition timing was checked during the truck's tune-up earlier this year.

This has been suggested to me as well - and I'm really, really hoping that isn't the problem. According to the books, i have to tear out the dash to get at it - and the prospect of that (and the cost of replacement) is kind of making me cringe. But, wouldn't the ECM throw a code if it's in limp-home mode?

Reply to
xmh31d_64

Inline......................

Whoever suggested removing an 02 sensor is an idiot. The 02 sensor helps determine the amount of fuel the injectors dump in the engine based on how much oxygen is in the exhaust stream. Yank an 02 and run the truck with the sensor sitting in atmospheric air and you'll have a motor that will run so rich you'll practically have fuel dripping out the exhaust as the sensor is telling the computer "lots of air in the exhaust stream, gimme more fuel!" So even if exhaust backpressure was an issue, running without an 02 will make it run so badly you'd never be able to tell either way.

Could very well be. I'd disconnect the exhaust pre-cat and see what happens.

Nope. A shot ECM won't spit codes it's shot. It's supposed to, but it won't. A dealership might be able to test the ECM using their scan equipment, but for something that old I dunno if they'd have the software to do it. ECM's are pretty cheap, about $100. Located behind the glove box.............not too hard to remove.

Rule out exhaust/cat blockage and engine mechanical failure via compression test before going that route.

Doc

Reply to
"Doc"

Hey Doc,

I noticed nothing was mentioned about the fuel filter. Could it be the root cause?

RJ in WV

Reply to
RJ in WV

Both fuel pump and filter were recently replaced.

Reply to
xmh31d_64

Not really.

That's the old belief, SAE paper #920289 says otherwise.

You're not thinking very well here Doc, who said anything about leaving it connected, and beside, an 89 S-10 uses a single wire O2 sensor, so even if it were left connected, it would never get to operating temperature, so the system would merely stay in open loop.

Um... No.

So easy to measure the back pressure, test, don't guess.

Or; put a 3.9K ohm resistor between pins "A" and "B" in the ALDL, this puts the ECM in backup fuel mode, if it runs the same, it's a safe bet that the ECM -was- in backup fuel mode, if it runs different, then it wasn't.

But do not drop the Y pipe off the manifolds as suggested, unless of course you want to start the vehicle on fire. Watched a mechanic do this against strong advice not to back in

1984, the test drive lasted 2 blocks before the vehicle (85 G van) was totally engulfed in flames. Total loss.
Reply to
aarcuda69062

The dealer, believe it or not. I voiced those very concerns to the guy who suggested it, and his response was that the 02 sensor is open-loop for a short time after cold startup anyway, and the computer ignores it until it hits a certain temperature. It sounded reasonable to me, but then, my grasp of pollution control is limited, to say the least.

The very next step, once I find the time again, and get my floor jack and jack stands back from my brother-in-law. Hate loaning tools out; it never fails that as soon as I let someone use them, I come up with a pressing need for those very tools.

Okay, that makes sense. Shoulda been able to figure that one out - maybe I'm just overly-aggravated with this whole mess. But I still cannot figure out how to get behind the glove box without tearing out the dash. Hopefully I won't have to.

Reply to
xmh31d_64

BESIDE the glovebox !!! closest to the Right side door !

Reply to
no one

Reply to
xmh31d_64

I figured that out ...

I just Restored a s10 89 and used a 88 cab and Gutted the wiring ,, too pictures of of the DASH when I had it out , so incase I had a problem rewiring the TRUCK

Reply to
no one

Okay, here's the recap.

89 S10 pickup, 2WD, 4.3L V6, auto trans (700R4 I think)

The problem began to develop a few months ago, in which the engine would sputter and lose power for a few seconds, then would resume running normally. The problem seems utterly random at highway speeds, but at lower speeds, it appears to be associated with torque converter lockup; in fact, on a number of occasions I've noticed that the instant the TC locks, the sputter problem arises. It has occurred pre-TC lock a couple of times as well, causing the vehicle to stall, but it always starts immediately back up.

On Friday, Nov 11th, the vehicle was on the freeway operating at about 60-65 mph, and as I began climbing a long moderate hill, the engine began to lose power - at first it appeared to the the intermittent problem, but this time it did not clear itself and the vehicle eventually slowed to about 5-10 mph, which was its maximum speed. I was able to limp the vehicle to an onramp, where it did idle, but very poorly. When I tried to give it gas, it felt like the engine was working against a load; it would reach about 2,000 rpm (estimated by ear) but would require approximately 1/2 throttle to do so (in neutral), and would not rev further. Attempting to drive it was out of the question, the vehicle could not exceed 10 mph.

Prior to the events of Nov 11th, the ECM threw no trouble codes, and in an effort to locate the problem, the fuel pump, fuel filter, EGR valve, and ignition module were all replaced. The vehicle was tuned up with new cap, rotor, and ignition wires.

After the vehicle was towed home Friday, I was able to pull code 33 out of the ECM - and based upon that, and a testing procedure Doc recommended, determined the MAP sensor was sayonara and replaced it.

The engine does run better now, but it still is not quite right. I noticed a hissing noise from under the vehicle; it wasn't particularly loud at idle but became very prominent when the engine was revved. Again the engine will not rev past about 2,000-2,500 rpm (slightly better than before). The vehicle can now actually be driven, but max speed is about 30 mph on level ground. On hills, the vehicle struggles terribly, and there is significant pinging - likewise if anything more than very gradual acceleration or higher speeds are attempted, and again, you can hear the hiss from under the vehicle quite clearly. Idle is pretty much normal, though.

Thinking the exhaust system (specifically the catalytic converter) may be plugged, I asked around and was told by a dealer's service writer that I could test for a plugged exhaust by removing the 02 sensor temporarily. The 02 sensor would have been difficult for me to access, but the EGR valve is right on top of the engine, so I removed it, plugged the inlet port of the manifold to stop up the vacuum leak, and left the exhaust-gas port open. I started the engine and noticed that it did start much more easily than before, and did seem to rev a bit more easily. I did not attempt to drive it like this, and only allowed it to run this way for about 30 seconds. Reinstalling the EGR valve returned the engine to its former poorly-functioning state.

It has been suggested that the ECM is toast and/or that serious mechanical problems within the engine may be to blame. The current working theory is that the MAP sensor failed, placing the engine in a seriously-rich condition, which lead to the failure and blockage of the catalytic converter. Doc seems to think that it could not possibly become blocked in such a short period of time, but I should point out that we aren't entirly certain that the converter wasn't partially clogged in the first place, as the map sensor may have failed an hour or perhaps even days before.

At this point, I'm not even entirely certain the catalytic converter is causing my current grief, but the symptoms seem to point to it, and once I'm able to get under the truck again, I plan to disconnect the exhaust pre-cat and see if the vehicle behaves any differently.

Reply to
xmh31d_64

pull the pcv valve out of the rocker cover and see if u get lots of STEAM... If so the converter may be plugging up!

Also BAD AC spark plugs are common even tho they are NEW They donnot Fire Directly to the tip. They will fire off to the side when put under Compression..

xmh31d snipped-for-privacy@yahoo.com wrote:

Reply to
no one

I suppose I could pull a "Chucky" and argue to my wits end that YOU are wrong..................but that would be foolish for two reasons:

  1. I was wrong
  2. You are right.

Dunno what the hell I was thinking when I wrote that post. Thanks for setting me straight Neil. Guess I've been spending too much time "underhood" on my 1/8 nitro trucks and not enough on the big ones lately!

Doc

Reply to
"Doc"

Okay, I've finally found time to get under the vehicle. I disconnected everything aft of the Y-pipe and hit the switch - the engine fired up instantly and revved up like it's supposed to. I took it for a short drive and it has power again. I ran it down to the muffler shop and had them install a new exhaust system, including catalytic converter. It runs MUCH better now, but there are still a few problems.

  1. The vehicle still exhibits the intermittent loss-of-power problem; it randomly sputters and loses power for a few seconds, then sorts itself out. It does not occur as often as before, but the left-hand lane of I-75 at rush hour is no place to be when this occurs.

  1. The engine's idle has changed. Since I've owned the vehicle, the idle has been a normal speed but slightly rough. Now, the idle is actually kind of smooth but the RPMs are very, very low in gear - only slightly above stalling. If it stalls it starts right back up (in fact, it starts easier now than it ever has!), but the idle speed is so low that the CHECK GAUGES light goes on, and the oil pressure gauge indicates little or no oil pressure. I can hold the RPMs up by holding the brake with my left foot and goosing the accelerator a little with my right. I'm told there is no way to adjust idle speed on these trucks - is that correct?

Right now, I'm thinking that the oxygen sensor got fouled during all the recent difficulties; the exhaust shop was supposed to put a new one in but somehow it got left off the work order. I wasn't charged for it, so I'll just chalk that up to miscommunication somewhere. I'm guessing my question here is, could a fouled oxygen sensor cause the low idle and intermittent power-loss problems?

Finally, am I correct in assuming the ECM controls ignition timing? I don't see a vacuum advance cannister on the distributor.

Reply to
xmh31d_64

Inline.....................

Great!

During the intermittant power loss, is it a complete loss of power (like the engine is shutting off and then firing back up), or does it just lose it's oomph?

The idle speed is controlled by the ECM via the IAC (idle air control) on your TBI unit. If the IAC is shot, the ECM will be unable to raise RPM's to the proper level. If the port in the TBI through which air is diverted thru the IAC is clogged this could also cause the same problem.

It could cause low idle, but not the intermittant issues. It would be more of a constant detriment than anything else. If it's older than 75k miles it could use replaced anyhow, so for $15 it won't hurt anything.

Yeah, you're correct. ECM controls ignition timing via ESC module, knock sensor and a few other components. I have experienced the EXACT same thing you describe on my truck (88' C1500, 305V8 TBI), with ridiculously low idle and stalling out. It sorted itself out within a day or so so I'll never know what caused it.

I'd check ignition timing (with ESC wire disconnected!) to see where it is as improper ignition timing can cause a low idle.

Doc

Reply to
"Doc"

I don't think the engine is shutting off, but it's hard to be absolutely certain. The problem seems most severe now during acceleration, either from a stop light or on the freeway. If I back off the accelerator, the engine smoothes out and runs normally, though of course now I'm not accelerating. If I play this game a couple of times per episode, it seems to figure itself out. It does NOT do this all the time, and the problem NEVER occurs when the engine is cold.

I also noticed (just today) that if the stumble occurs, I can stomp on the gas hard and it seems to clear up immediately.

If I had to describe the way it acts, I'd say fuel starvation - I don't know that's the case, but that sure is how it acts/feels. When this problem occurred in my old carbureted Fords, I'd usually discover the accelerator-pump linkage was misadjusted or had unassed itself.

Okay, looks like I've got some work to do tomorrow - at the very least I need to make sure that port is clear. Is there a way to check an IAC without replacing it?

(regarding oxygen sensor)

Yeah, I guess that's true. Looks like I'm gonna have to get under the truck again (oh,...joy.)

(regarding ignition timing control)

Okay, I'm gonna play with it some more. I'll probably be back with more questions, Doc, but first I'd like to thank you for all your help with this. This truck certainly is proving to be an education.

Reply to
xmh31d_64

Kinda of. A scan tool is needed for a full dx on the IAC, but there are a few tests you can do at home w/o one:

  1. Check to see if idle stays steady or slightly increases when putting a load on the engine at idle (such as A/C or power steering). Do this when the engine is at operating temp. When the load is placed on the engine, the IAC should compensate and keep the idle steady.
  2. Disconnect the IAC electrical connector. On the IAC, using the plastic tang where the connector holds on (cheesy diagram below)

plastic tang

D C B A

IAC valve

Using a DMM, test resistence between terminals A and B and also C and D. The resistence should be close to the same on both terminal sets, around

40-80ohms. If either set of terminals indicated an open circuit (infinite resistence) the valve is shot.

If you remove the IAC itself, you can check to see if the pintle is all gummed up and if the air port is gummed up as well. Basically the way it works: with throttle plates closed, the IAC allows air to bypass the throttle plates thru the IAC valve, the opening of which is controlled by the IAC pintle.

No problem bud.

Doc

Reply to
"Doc"

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