Auto mechanics: 98 GMC engine briefly "cuts out" and jolts the truck

[CountryBoy19]

5.7 L Vortec engine

Backstory: I've been experiencing similar symptoms for a few years now. It's taking me a while to track down the problems. Throw in the fact that this isn't my daily drive and it hasn't been too high on my priority list. That being said, it has been bumped up the list because of the severity.

I just replaced the distributor, cap, wires, & plugs and problem has "worsened". Fuel pump was a new, lifetime warranty Bosch 3 years ago.

Symptoms: prior to above work it would stutter and have a lack of power at low throttle positions (probably in about the 3-8% throttle position range by my estimate). As soon as I exceeded that "dead area" engine performance returned and all was well, however that led to me having to driving in a cyclic manner. I would have to accelerate then let off and decal, the range where the truck wanted to cruise was the range where the issue was occurring. I finally got some good advice from a local transmission shop of all places. He said it was the distributor gear worn out (he didn't even take it apart, he just knew it). Sure enough the distributor gear was pretty severely worn. I replaced the distributor and now the truck runs great except for one small (or larger) problem.

It still experiences "something funky" around that throttle position, but it only happens when letting off the throttle and it only happens once the truck is warmed up. Symptom: complete shutdown of engine, but only for a split second, not long enough that it actually dies, just long enough to make the drive train slam really hard.

No codes being thrown.

Things I have checked: MAF (checked by unplugging it to see if problem persists), all ignition components replaced except the coil.

Things I suspect but generally don't have a good way to narrow it down without buying/replacing them: coil?, throttle position sensor?, ignition advance sensor/mechanism?

The reason I think it "may" relate to ignition advance is because it only occurs when back off the throttle. IE, cresting a large hill as I back off the throttle it's like clock work, "BAM", engine cuts out briefly, slapping the drive-train then it resumes like nothing happened. I'm not sure how spark advance and such works but I believe it's mechanical in this truck, is it possible something is "hanging up"?

I have suspected TPS for a while but why would it only occurring when backing off the throttle?

 

[jlehman]

Probably not what is going on here, but I will throw it in the mix because I would have never guessed it with my situation. I was working on my 2014 jeep jk doing a major wiring project and then I had someone elese install power locks once I was done and I also had the tires rotated at a shop for time conveniance. Everything was good for a month or so and then all of a sudden I would get a power dip, my traction control lights would come on usually after taking foot of the gas pedal after accelerating. It got to be happening all the time and I was convinced that it was either something screwed up tying the power lock brain into the ignition wires or it was something screwed up by garage that did the tire rotation. To wrap up what could be an even longer story, when the power locks were installed the battery cable was tightend but not all the way, it loosened up over some time, and vehicle vibration / inirtia was causing the ever so slightest power flux on the battery cable causing my power drop.

 

[Woobie]

I'm assuming this is a 5.7L Vortec

Throttle Position Sensor is a cheap part, generally. You can check it though by using a fluke meter and putting the negative lead on your ground wire, and your positive lead on your signal voltage wire (the one that is not 12v). Key on engine off, have someone slowly press and let back out on the accelerator. Voltage should go from 0-5v evenly throughout the range. If it skips around, TPS is bad.

The spark advance is set by the computer based upon input from the Crank Position Sensor. It isn't as easy to check. It is located behind the harmonic balancer. I would just look to see if it's in good position, unless you have access to a snap-on modus, or similar tool. However, the engine should go in limp mode if it doesn't see the Crank sensor, so this is not likely the source of the issue.

Fuel delivery is based upon input from the Camshaft Position Sensor. If ithe computer doesn't see it, it will fail to demand fuel. The engine will not run when the Cam sensor is not sending signal, even if momentarily jarred loose.

These things are dependent on having good grounds, so make sure your ground wires on the individual sensors ohm to ground when the engine is off. Shake things around to check for something that could come loose and lose signal back to the ECM.

Trucks from that era were notorious for having fuel pump issues. They required about 55psi of fuel pressure at the schrader valve on top of the engine. Some wouldn't start if you had 52 psi, and the pumps do weaken over time. Some pumps will weaken after they run for a while, and once they cool off, regain pressure. To be honest, this problem doesn't sound fuel pump related, but it's something you can rule out.

 

[CountryBoy19]

I'm assuming this is a 5.7L Vortec

Throttle Position Sensor is a cheap part, generally. You can check it though by using a fluke meter and putting the negative lead on your ground wire, and your positive lead on your signal voltage wire (the one that is not 12v). Key on engine off, have someone slowly press and let back out on the accelerator. Voltage should go from 0-5v evenly throughout the range. If it skips around, TPS is bad.

The spark advance is set by the computer based upon input from the Crank Position Sensor. It isn't as easy to check. It is located behind the harmonic balancer. I would just look to see if it's in good position, unless you have access to a snap-on modus, or similar tool. However, the engine should go in limp mode if it doesn't see the Crank sensor, so this is not likely the source of the issue.

Fuel delivery is based upon input from the Camshaft Position Sensor. If ithe computer doesn't see it, it will fail to demand fuel. The engine will not run when the Cam sensor is not sending signal, even if momentarily jarred loose.

These things are dependent on having good grounds, so make sure your ground wires on the individual sensors ohm to ground when the engine is off. Shake things around to check for something that could come loose and lose signal back to the ECM.

Trucks from that era were notorious for having fuel pump issues. They required about 55psi of fuel pressure at the schrader valve on top of the engine. Some wouldn't start if you had 52 psi, and the pumps do weaken over time. Some pumps will weaken after they run for a while, and once they cool off, regain pressure. To be honest, this problem doesn't sound fuel pump related, but it's something you can rule out.

Several good points, including the fact that I forgot to include a few details in the OP (now updated). 5.7L vortec, fuel-pump was replaced ~3 years ago with a Bosch.

I've suspected possibly a crank/cam sensor but how the heck do you diagnose an intermittent problem like that? The truck has 325k miles on it, it's beat to heck, half rusted out, has a crack in the windshield, had to put a different tailgate on it (different color even) because the original rusted off... hate to put much money in it... and don't want to replace the truck because it only gets used 1-2 times a month...

I do intend to go over the ground straps/wires just to double-check but haven't gotten time yet. I did do a cursory check of the frame ground strap when I was under the truck a couple months ago.

I will follow your advice RE checking the TPS.

 

[churchmouse]

With that kind of mileage grounds could be an issue but I would think it would be more of a constant than specific problem.

 

[halfmileharry]

Several good points, including the fact that I forgot to include a few details in the OP (now updated). 5.7L vortec, fuel-pump was replaced ~3 years ago with a Bosch.

I've suspected possibly a crank/cam sensor but how the heck do you diagnose an intermittent problem like that? The truck has 325k miles on it, it's beat to heck, half rusted out, has a crack in the windshield, had to put a different tailgate on it (different color even) because the original rusted off... hate to put much money in it... and don't want to replace the truck because it only gets used 1-2 times a month...

I do intend to go over the ground straps/wires just to double-check but haven't gotten time yet. I did do a cursory check of the frame ground strap when I was under the truck a couple months ago.

I will follow your advice RE checking the TPS.

That sounds like a real truck. I'm tagging this thread as I have a buddy that replaced about everything on his. Same year and motor. It runs better BUT still has that intermittent sputter when Idling. It's worse when cold.

 

[rooster]

had this problem with a 94 chevy. Never did figure out what exactly caused it. I changed TPS,MAF and ton of things on it and it never corrected fully. The TPS sensor replacement helped but did not fully correct the problem. I took a job with a longer commute and sold it to get a gas saver before I ever figured it out. Prior to that I was just about ready to make it carbureted and be done with it. The older style of GM throttle bodies on those trucks are known for issues.

 

[Gary119]

I would check the ground on the left rear frame rail. Look up at where the bumper bracket bolts to the rail. Bolt will have a 13mm head. One of the main grounds for the truck.

 

[Tactically Fat]

Does it ever die on you when you're driving after you've been cruising at a certain speed and then slow to a stop?

 

[CountryBoy19]

Does it ever die on you when you're driving after you've been cruising at a certain speed and then slow to a stop?

Outside of 1 time that I can recall (more detail below) prior to putting the new distributor, plugs, & wires in, no... it runs like a champ. Honestly, it doesn't miss a single beat at idle or with your foot in it.

Details on isolate incident probably about 7 months ago, cool, foggy morning I started it to haul some firewood up to the house in preps for winter. I could tell something did seem right and it died at one point that morning. That whole day it seemed "off", but has never done that again; definitely hasn't even acted close to that since I did the ignition work.

 

[halfmileharry]

I have experienced some bad ECMs that cause weird problems and WILL NOT read on a scanner. NO codes read.
I have NOT worked in a GM dealership since 2001.

 

[churchmouse]

I have experienced some bad ECMs that cause weird problems and WILL NOT read on a scanner. NO codes read.
I have NOT worked in a GM dealership since 2001.

This same issue with the Dodge Ram van we just sold. Died when and where it wanted to. Fixed this and that from scans and seemed to get better. No codes. Then it just would not start.
R&R what they called the engine control module and off it went.

 

[halfmileharry]

Just talked to my buddy that has a similar truck. His EGR valve was showing 1% open at cold idle. He replaced with a junk yard egr and all is well now.

 

[Woobie]

Yeah, the ECM is sort of the final place you can look. The problem is, even with a $10,000 scanner, whatever the computer is doing to misinterpret data is also causing to lie to your scanner. So it's one of those things that you sometimes replace when nothing else will show.

Lile you say, cam sensor is about impossible to diagnose without a scanner, unless you can isolate the signal wire up by the ECM. Even then you're just kind of hoping to see some wierd or missing voltage.

Have you ever had a FRAM air filter on this?

 

[Woobie]

Just talked to my buddy that has a similar truck. His EGR valve was showing 1% open at cold idle. He replaced with a junk yard egr and all is well now.

I can see EGR effecting idle and power, but I wouldn't think it would just shut the engine off.

 

[halfmileharry]

The ECM is where the codes come from. They hardly read themselves as bad.

 

[Doug]

My son's car did something similar; turned out to be coils going bad.

 

[churchmouse]

I can see EGR effecting idle and power, but I wouldn't think it would just shut the engine off.

Bought the daughter a V-6 (3.4) GMC SUV. It would die on occasion. AS this gets discussed I remember being told to replace the EGR. Pulled one off a wreck at the yard a buddy runs. Never another issue "But" it would die and had to set for a while before it would restart.

 

[Woobie]

My son's car did something similar; turned out to be coils going bad.

That truck has one coil. OP, did you change the coil when you did the ignition work?

 

[Woobie]

Bought the daughter a V-6 (3.4) GMC SUV. It would die on occasion. AS this gets discussed I remember being told to replace the EGR. Pulled one off a wreck at the yard a buddy runs. Never another issue "But" it would die and had to set for a while before it would restart.

Well there you go, I stand corrected. Easily one of the worst contraptions ever put on a car. They have them on newer diesels now, and it's the first thing you do when you buy one: get the EGR delete kit.

Anyhoo, I'm still thinking this wouldn't be caused by the EGR.