Chevy Engine Mystery Ailment

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Truck's a '90 Chevy 'Burban, 5.7L 350
It cycles up and WAY down.

Replaced EGR valve & solenoid, all belts, heater core, the O2 sensor, intake gasket, thermostat, dist. cap & rotor button, intake gasket. Replaced the PCV valve the next day, spark plugs & wires day after that.
We've changed the map sensor, temp. sensor, distributor, ignition coil & module, and two other sensors (that I would knew if I heard, but can't remember right off).
I changed the TPS yesterday, along with oil & added Slick 50 for what that's worth. It now runs worse than it ever has. Computer read O2 sensor as of before I changed the TPS, haven't had it back on a computer since. O2 sensor less than 6 months old.

I'd be eternally grateful if anyone could tell me what's wrong with my once-beloved 'Burban!

Better?

posted by  mrmck

Love to help ya, lot's of words, not much of a story reader. Not even really sure what the complaint is except that your "mechanic" is changing parts instead of diagnosing the problem.

If the engine passes a visual inspection it would really help to know what kind of DTC's are present and see the data stream. BWAG based on incomplete and convoluted information? IAC loose or cracked where it threads into throttle body.

posted by  vwhobo

throttle body gasket! That's the other gasket we replaced. Glad you said that. Sorry about the missing info; like I said, I put EVERYthing in there and then got kicked, so it was fresh in my head (but not on the screen). It's still good for what we've already checked/changed...

Problem. Truck runs like crap. More specifically, when started, idles in cycles: out of a 2 second cycle, 1.5s will be great, smooth, etc., then the other .5 seconds it drops WAY down, cuts off until a little warmer, runs really rough. Once it's been running for a few minutes (literally - like 2 or 3), it just cycles, but generally doesn't cut off, unless I try to put it in gear. After about 10 minutes it's safe to put in gear, then surges. It used to surge only until it got to 35-40mph, then only intermittently. As of a few days ago it's doing that at ANY speed, and pretty much won't go over 65 at all unless downhill. If uphill, won't make it over 50 or so (and that's with a running start). It's like something's literally sucking the power out of it.

I'm not really sure how else to describe it, other than it reminds me very much of a 4-cyl Sunbird I drove once that blew two cylinders. Does that help at all? My two mechanics and I - plus a half-dozen friends whose mechanical abilities are similar to mine - are completely stumped.

posted by  mrmck

No, I left out more:
Oil looks good (not white), radiator hasn't lost any antifreeze in quite a while, no other indicators of a blown head gasket other than feel/sound.
Combined with the idling cycle, it doesn't seem like that makes sense, but I'm certainly no expert.

posted by  mrmck

Last thing:
My mom told me before I let them do the O2 sensor that everything else would start going wrong if I didn't go to a junkyard and get another one out of a similar truck. Although she's quite knowledgeable about cars (Buicks & 18-wheelers at least), she's not always right, and THAT just didn't make any sense to me. I'm starting to wonder now if she wasn't on to something. Anyone's thoughts on that?

posted by  mrmck

What are DTC's, BWAG, & IAC?

posted by  mrmck

Hunting? :wink2:

I take it you changed the fuel filter? Howse compression? What is a vacuum gauge needle doing?

Did you calibrate the TPS? Engine water temp sensor will cause hunting if there is an air pocket around it. Bad oxygen sensor sensor can cause hunting at low revs, but performance should come right when open loop revs are achieved.

posted by  Wally

Marble thief! :-) Forgive my ignorance; I'm okay with the obvious, but when it gets this deep I'm literally lost.

Fuel filter 1 year old (replaced 14-yr-old original with it). Have not tested compression yet... but didn't give any indication it needed it until a few days ago. Last engine light kicked off O2 problems & until last few days, ran exactly as you described (bad at idle, fine while running). Is it possible I got a bad one when we replaced it?

How do you get an air pocket out from around the temp sensor? I have a new one (the one that's on it was a tester - used but good - put on Thursday night while engine was hot) but could not get the current one out yesterday. Lastly... calibrate??? Um, how do you do that? All I did was unsrew the bolts, take it off & swap it with the new one, rescrew bolts.

posted by  mrmck

More info, possibly useless:
At idle, when I unplug the TPS, it runs like crazy, then when it's plugged back in goes right back into that 3-up/1-dn cycle. Will it hurt it to drive it without it plugged in to see what it does?

posted by  mrmck

I don't know what engine you have, but the TPS can have both a potentiometer and switch. The switch is for open loop operation and the potentiometer is for the rest. You should be able to find a manual to explain the calibration procedure.

posted by  Wally

Thanks, Wally. I'll look in my Chilton's. Don't laugh; I did try to read the directions in the box but none of them seemed to apply (things like sliding bolts...mine had holes, only one way to put it in). Will it hurt to drive it with off?

P.S. 5.7L 350 - 1990 truck

posted by  mrmck

It will probably go into limp mode, so you may find the fuel consumption goes up and you revs are limited.

posted by  Wally

Sorry, Wally -- I didn't mean drive it that way permanently, only long enough to see if that's what the problem is. My poor truck is currently driving two mechanics (and me) nuts, so I'm trying to minimize the time I spend with them.

Sounds like I won't be able to tell whether that's it or not by driving it without it. Is that a correct interpretation?

posted by  mrmck

5.7 litre chev? I had a hunting LS1 in one of my cars and it turned out to be a split PCV hose.

posted by  Wally

Given the start time of the problems & cost of trying replacement, I'll give it a shot. The one on it is (new) one that one of my mechanics pulled off a vette in order to get me home the night the old one broke (after spending 10 hours doing the intake gasket). Thanks again.

If that turns out to be the problem, you can keep my marbles; odds are good there's not enough to power a normal brain, though.

Hmmm...how come it's getting persistently worse, I wonder. Back to diagnosis by process of elimination :laughing:

posted by  mrmck

Continuing problem: Turning over fine, and tried to catch twice (out of +/- 15 tries), but no start. Like it's just not getting gas at all.
A buddy of mine suggested fuel pump a few weeks ago, telling me that sometimes there's a gasket in there that goes instead of the bearing that goes on most Chevys. One of my mechanics tells me that doesn't apply to my fuel pump (again, 5.7L 350). Does anyone know for sure?
I hear "humming" inside my truck (sounds like it's coming from around the middle underneath, similar to a muffled version of the sound a diesel makes when waiting for the points to heat up) that starts when I turn the key, and generally goes for about 3-4 seconds after I remove the key. Is that the fuel pump?
Does anyone have any suggestions on how to get it started, and can anyone tell me for sure if it is/is not a fuel pump problem?

posted by  mrmck

Last-ditch effort to beg for help... can't get to work tomorrow if I can't get it to start... :ohcrap:

posted by  mrmck

ok you have a long list of parts. question did you or can you compare the old rotor looked up some of the parts and there's two rotor's listed from ac delco for a 350 ???(not likely but worth a check) next how's the timing chain(milage) might be throwing valve timing off enough to give the idling problem check the tbi(throttle body injection)do both injecter's squirt fuel by turning key on and while cranking engine as for fuel pump you should be able to hear it by listening at the tank or fill neck(gas cap) are you getting any spark at the plugs? check this and see

posted by  osborste

Bless you...
Hard to tell at this point if getting spark (misting rain & wet dist.'s do this too) but unlikely, as started and ran (poorly, but running) yesterday after I changed the TPS.
New rotor "looked" like old one, but once it's running again, I'll check this to make sure.
TBI - both sprayed fuel as of yesterday, unevenly, but I think that's related to the idle issue (even with one another, just less fuel at "down" part of cycle). This morning, in trying to start it, could see/smell gas in the ports.
My brother suggested unhooking the line leading from the f. filter and trying to start it to see if it was pumping gas. Line was dry at start & finish of this "test" BUT, I can now see/smell gas in the ports again. Odd... line did drip a little when I changed the filter last time. No drip at all today.
As far as checking spray while starting, can't teach dog to turn ignition, I'll have to wait until my neighbor gets home & ask her to help me.
?????? :banghead:

posted by  mrmck

how long has the truck sat with out running how much gas was in the tank is it a metal tank or plastic sound like maybe the pickup screen on the fuel pump might be clogged with rust and dirt from the tank(limiting fuel supply)

posted by  osborste

in an earlier post you were looking for this

posted by  osborste

Drove it all day Friday (poorly running, but running -- only made the mistake of shutting it off once, barely got it re-started). Changed oil & TPS yesterday. Started it, it ran, still 3-up-1-dn idling, shut it off (total run time maybe 15 minutes). This morning wouldn't start at all. Put old TPS back in for kicks, no improvement.
Roughly 3/4 tank; full before I drove it around but gas mileage has plumetted last week or so.
Metal tank.
Have only let it get very low 2-3x (in 3-1/2 years), not at all since I changed the fuel filter, and added fuel injector/carb cleaner (Slick 50) at the same time I changed the oil.
I'll ask my mech. to check the screen prior to replacing pump *sigh* I'm not set up to drain gas before removing tank to get to the pump, nor have enough daylight time, nor time to be without my only vehicle. :cry:

Thank you all for your help -- I'll post back what it is that's wrong once I get it back (assuming anyone can ever figure it out). Happy Easter, y'all!

OH... thank you, orbste, for the diagram -- and for the pointing to something else we can check (I'm about out of ideas, now). I still don't know what an IAC is :laughing: , but I'm guessing Air Idle Control, maybe?

posted by  mrmck

yes iac is idle air control you had the distributor out for the intake maybe recheck its timing to no1 cylinder might be out one tooth

posted by  osborste

Thanks again. I'm (shamefully) in over my head, now. "recheck timing to no1 cylinder" is definitely out of my range of knowledge of how to do, but these things will be appreciated by whichever of the two mechanics I decide to torture tomorrow. Looking on the bright side, this may finally be the year AAA pays for itself (in towing). :-)

posted by  mrmck

so fuel pump/timing/setup of the iac valve good luck hope they get it fixed

posted by  osborste

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