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Old 08-26-2018, 04:48 PM   #1
firstoneman
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5.7L Timing Gear/Chain Problem?

Hello All.
I have a 1991 GMC K1500 with 5.7L engine. I've had a recent problem occur. The first time I wasn't sure what I heard. The second time I was next to a diesel at light and couldn't be sure it was my truck the third time I was able to hear it again. I'll do my best to describe it so someone can understand.

I was not at very high speeds when this occurs, the first two time right at takeoff from a light, the third time under acceleration in 2nd gear I think (I have 5spd stick).

Basically, when this happens the engine sounds very similar to the noise of diesel from the chain. It only happens for a short burst. To me, being inside the cab of my truck, it sounded just like that diesel engine noise. That's why, with Ford F-250 powerstroke next to me, I couldn't be sure if it was my engine. When I get the noise there is a slight hesitation in power/acceleration, like the chain slipped. I have only had one timing chain problem on a vehicle in all my years...a 1982 Grand Prix whose nylon timing chain shredded and got sucked up into my oil pump (another story). Anyhow, I have no experience with when a timing chain slips or something happens to the timing system through the ECU.

I have been getting, at higher speeds (65+mph), the CEL. I checked the code and it was a knock sensor code so I replaced the knock sensor and got the code again. The CEL doesn't always come on when I reach those speeds - it's intermittent. Is there a code reader that can give me more info - it's OBD-1. I read somewhere that someone put a scanner on a 93 and got a count for number of knocks per minute. What kind of scanner would that be?

A friend says that if the chain slipped my valves would be bent to heck. Or else, the obvious, if the chain failed it (of course) wouldn't run. He also said that maybe it could be an electrical system problem and that's where he'd start. What would I need to check if it's possibly electrical?

One other symptom is that there is nearly a raw gas smell out the exhaust. So i've been thinking that maybe there is a bad plug or plug wire.

I don't want to trash the motor...running very good til now. Has 230K miles on it. Not sure if previous owner ever installed a new timing set.

Thoughts greatly appreciated!
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1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-26-2018, 10:11 PM   #2
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Re: 5.7L Timing Gear/Chain Problem?

The code may be because the sensor is picking up the engine noise. Someone has to check this noise onsite, from here it is almost impossible to accurately offer any help. Obd1 has limited info available, see if there are any other codes.
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Old 08-27-2018, 12:26 AM   #3
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Re: 5.7L Timing Gear/Chain Problem?

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The code may be because the sensor is picking up the engine noise. Someone has to check this noise onsite, from here it is almost impossible to accurately offer any help. Obd1 has limited info available, see if there are any other codes.
No other codes...I'm hoping for a few ideas to troubleshoot. Haven't found clear trouble shooting info on what the knock sensor code could be telling me. I get that it's better in person. But hoping someone reads and recognizes something from what I've described. Can drive it for a week and not have it happen. So taking it to a mechanic seems useless.

For instance, on this engine can a chain jump and skip the a tooth (or a few teeth) and not trash the valves? Hoping someone can answer that because I've never been in a vehicle when this happened (excepting the 1982 Grand Prix - which had a nylon chain that shredded and no valves damaged).

Thanks much.
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1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-27-2018, 07:38 AM   #4
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Re: 5.7L Timing Gear/Chain Problem?

It is not an interference engine in stock form. There are valve reliefs in the piston tops to prevent this. A timing chain can fail completely and pistons will not hit valves with a stock valve train unless something catastrophic occurred with a rocker or spring, and even then would be unlikely.

Max is likely correct in that the KS is detecting mechanical noise from whatever the source may be. There are numerous possibilities, including ignition timing and fuel delivery, as well as mechanical damage, inadequate oil pressure, torque converter damage, and many others.

As for a scanner which can read a data stream from the ECM, a GM Tech II or Snap On MT-2500 should be able to read the 8192 baud serial data while the engine is running. Some shops would still have these scanners in their equipment inventory.

Incidentally, your '82 Poncho more likely had a timing sprocket with nylon teeth, and not a nylon chain. That was among the last years for that scheme, with GM abandoning that idea just about at that time. Your '91 350 should be steel/iron instead of plastic.
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Old 08-27-2018, 06:19 PM   #5
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Re: 5.7L Timing Gear/Chain Problem?

If the timing chain jumped a tooth or two you would notice an immediate and constant change in power!!
I personally doubt that this is a timing chain issue.

When the KS hears a noise, such as pinging, it's job is to retard the timing. This is probably what you're experiencing when you feel the "slight hesitation".

Try running "premium" fuel in it for a couple of weeks, see what it does.

Other than that do a complete tune-up. Plugs, wires, cap, rotor, 02 sensors. Check for vacuum and exhaust leaks. Check that the dist is not worn out. Make sure "base timing" is set correctly.

If you're not sure how to do this, find someone who does.
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Old 08-28-2018, 11:48 PM   #6
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Re: 5.7L Timing Gear/Chain Problem?

Quote:
Originally Posted by Blue Bowtie View Post
It is not an interference engine in stock form. There are valve reliefs in the piston tops to prevent this. A timing chain can fail completely and pistons will not hit valves with a stock valve train unless something catastrophic occurred with a rocker or spring, and even then would be unlikely.

Max is likely correct in that the KS is detecting mechanical noise from whatever the source may be. There are numerous possibilities, including ignition timing and fuel delivery, as well as mechanical damage, inadequate oil pressure, torque converter damage, and many others.

As for a scanner which can read a data stream from the ECM, a GM Tech II or Snap On MT-2500 should be able to read the 8192 baud serial data while the engine is running. Some shops would still have these scanners in their equipment inventory.

Incidentally, your '82 Poncho more likely had a timing sprocket with nylon teeth, and not a nylon chain. That was among the last years for that scheme, with GM abandoning that idea just about at that time. Your '91 350 should be steel/iron instead of plastic.
Great info...thanks much. This helps me move along. I've done a lot of wrenching on all other parts of my vehicle...but engines are rather new to me in regard to troubleshooting a problem like this. I've been fortunate to not have toasted any engines or had them toast on me in my 18 years of auto/truck wrenching. It's time to learn.

Thanks for clarification on the '82 sprocket...it's possible I have just remembered what was told to me incorrectly (that was before I ever really knew anything about working on vehicles). Now I can tell the factual truth if I tell the story again. What I do remember is being stranded in Houghton, MI two days before one of my best friend's wedding (in Michigan, 5 miles from the Ohio border, and if you don't know, that's nearly as far away as you can get from Houghton and still be in Michigan) and having virtually no cash, in middle of -30F storm, trying to find a way to make it in time for the wedding. According to the mechanic in Houghton who replaced my oil pump because it was clogged with nylon from the shredded sprocket, the guy who did the timing job was either lazy or probably looking forward to another repair when the nylon got sucked up into the oil pump. He didn't clean the oil pan of the shreds. Supposedly, when oil becomes thick in the deep cold of a Michigan winter the nylon bits get sucked up off the bottom of the pan and voila...a trashed oil pump.

I'll start looking into the things you mention. I do know that the timing setup on the 91 is steel. Thanks again.
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1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-29-2018, 12:05 AM   #7
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Re: 5.7L Timing Gear/Chain Problem?

Quote:
Originally Posted by maxwedge View Post
The code may be because the sensor is picking up the engine noise. Someone has to check this noise onsite, from here it is almost impossible to accurately offer any help. Obd1 has limited info available, see if there are any other codes.
Ok...thinking about your comment again, I realize my first response wasn't very good. I didn't ask a pertinent question based on what you were saying.
So, since there is only an intermittent mechanical noise that I can hear, what else should I be looking for that's a "noise", in the sense you are talking about? The engine sounds pretty much normal. I do pay attention to the sounds my vehicle makes quite closely as it helps me avoid "getting stranded" problems. It doesn't seem to be missing, it has power except when the mechanical noise happens, i think I will have to look at the exhaust system immediately (per a comment below) because I do think I'm hearing something from that. I have thought it might be small leak in front of the cat somewhere.

I see a couple of posts below which have suggestions...so I'll be exploring those.

Thanks.
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Dave

1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-29-2018, 12:14 AM   #8
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Re: 5.7L Timing Gear/Chain Problem?

Quote:
Originally Posted by 777stickman View Post
If the timing chain jumped a tooth or two you would notice an immediate and constant change in power!!
I personally doubt that this is a timing chain issue.

When the KS hears a noise, such as pinging, it's job is to retard the timing. This is probably what you're experiencing when you feel the "slight hesitation".

Try running "premium" fuel in it for a couple of weeks, see what it does.

Other than that do a complete tune-up. Plugs, wires, cap, rotor, 02 sensors. Check for vacuum and exhaust leaks. Check that the dist is not worn out. Make sure "base timing" is set correctly.

If you're not sure how to do this, find someone who does.
Thanks much for this. After reading you comments I realized I wasn't thinking of engine "noise" properly...was focused only on the noise I heard. I'm going to check out a potential exhaust leak tomorrow. I can hear something that might indicate a slight leak ahead of the cat. I'll find a way to check on the timing (friend has a timing light). I'm not sure what you mean by "base timing" but will ask a friend who knows that kind of stuff. Distributor cap, rotor and coil was replaced two years ago, driven maybe 5,000 since then. I opened it up and it all looked good. I'll pull plugs and see what they look like. I'll give it a full tank or two of premium and see what happens.

Thanks again for giving me some things to consider.
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Dave

1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-29-2018, 12:29 AM   #9
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Re: 5.7L Timing Gear/Chain Problem?

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Originally Posted by Blue Bowtie View Post
There are numerous possibilities.... torque converter damage
Since I have a manual tranny, this is off the possibility list, correct?
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Dave

1991 GMC Sierra K1500, 226K miles, 5.7L 350, 5spd, Ext Cab, short box. Black.

1994 GMC Suburban C1500, 210K miles, 6.5L Turbo Diesel, various mods. Out of commission (Aug'18) till engine rebuild or replace.
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Old 08-29-2018, 07:35 AM   #10
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Re: 5.7L Timing Gear/Chain Problem?

Yes, you can take a torque converter off the list of suspects.
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