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Old 03-26-2005, 08:25 PM   #1
mrmck
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Thumbs up Chevy Engine Mystery Ailment

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?

Last edited by mrmck : 03-27-2005 at 12:14 AM.
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Old 03-26-2005, 08:34 PM   #2
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Quote:
Originally Posted by mrmck
Okay, I just spent an hour detailing my truck's problems here and get kicked by my ISP -- ARGH!!!!!!!!!!!!!!!!!!!

Don't have another hour, so I'm pasting an email to a buddy of mine for info:

Truck's a '90 Chevy 'Burban, 5.7L 350 -- pretty straight forward and thankfully enough a LOT like my mom's Buick, which I grew up working on, but is fuel-injected (Chevy's "new" EFI-assist, actually, but danged if I can find a carbeurator in it anywhere ;-). The problem is that the guy that owned it for 12 years before I bought it just didn't drive it much. I bought it in '01 with 58K on it, and have put 100K on it in a little over 3 years. All the "normal" things that have to be replaced around 100K are needing to be replaced now, starting "small" last year with the alternator, starter, EGR valve & solenoid, all belts (did all those myself), heater core, U-joints & carrier bearing, driver's window motor (back window motor is "holding" with a thorough cleaning but it's only a matter of time), and within the last 6 months, the O2 sensor, intake gasket (my fault - didn't tighten down the bracket bolt on the EGR solenoid enough when I changed it -- combine that with the bent driveshaft (before my time) and it vibrated itself loose, sucking a hole into the intake gasket *sigh*), and the thermostat, dist. cap & rotor button we just replaced "while we were in there" with the intake gasket. Replaced the PCV valve the next day, as it literally broke about 5 miles on my way home from the mechanic. The next thing was the spark plugs, because with everything else going on with it, I FORGOT that I hadn't changed them since I bought it (nearly 100K miles ago!!!!)(what an idiot!!). I'm afraid to check the brakes -- I did those the day before the plugs & wires.

Dadgum - never should have listed all that in writing!! Now I'm even more depressed *L*. Anyway, just for fun add 2 sets of tires (note: NEVER even CONSIDER buying Dunlops!), a battery, and an exhaust & muffler.

BUT, Chuck (mech.) adjusted the timing - which has been running high for almost two years - and now instead of running it's head off when I start it, it cycles up and WAY down -- almost like it's got a vacuum leak somewhere, and it sounds like it's "missing" still, but everything appears to be in order. The problem now is that anything else that could be wrong is beyond my knowledge -- there's dozens of "sensors" that could be causing that. You got a guess????


Since I consulted with him, we've changed the map sensor, temp. sensor, TPS, 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 yesrday myself, along with the oil & added Slick 50, for what that's worth. It now runs worse than it ever has (it now runs very similarly to a '78 Dodge Omni my best friend in HS had -- right before the engine siezed up driving it down the highway) and I'm scared to even drive it to work, let alone one of the other 50+ mile trips I take every weekend. HELP! Also, doesn't appear to the naked eye to be a vacuum issue (all hooked up, none collapsed). 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!
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.
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Old 03-26-2005, 09:25 PM   #3
mrmck
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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.
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Old 03-26-2005, 09:28 PM   #4
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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.
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Old 03-26-2005, 09:45 PM   #5
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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?
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Old 03-27-2005, 12:15 AM   #6
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What are DTC's, BWAG, & IAC?
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Old 03-27-2005, 12:35 AM   #7
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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.

Hunting?

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.
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Old 03-27-2005, 12:51 AM   #8
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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.
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Old 03-27-2005, 12:55 AM   #9
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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?
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Old 03-27-2005, 01:02 AM   #10
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Quote:
Originally 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?

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.
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Old 03-27-2005, 01:06 AM   #11
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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
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Old 03-27-2005, 01:13 AM   #12
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Originally Posted by mrmck
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

It will probably go into limp mode, so you may find the fuel consumption goes up and you revs are limited.
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Old 03-27-2005, 01:16 AM   #13
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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?
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Old 03-27-2005, 01:21 AM   #14
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5.7 litre chev? I had a hunting LS1 in one of my cars and it turned out to be a split PCV hose.
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Old 03-27-2005, 01:30 AM   #15
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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

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