My Nightmare-1996 Sierra 5.0 stumbling and stalling

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Jason012

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OK, so my Sierra has been running horribly now for a while. It was dropping misfire codes, and I replaced the plugs, the throttle position sensor, the fuel filter, not in that order and not in relation to the codes only. I took it to a shop that was able to get it to run better, but what it does is stumble on accel and then when cruising at speed it will “hiccup”, I would call it like a “micro stall”. I’m guessing at this point there may be multiple issues, but that there is a fuel delivery issue. I thought previously however that a fuel pump would either be working or not be working, it would not taper off toward its demise but simply stop working.

Is this not true? Can the fuel pressure be tested without getting at the fuel pump which is above the tank/below the bed?

This is made especially worse right now because my 99 Tahoe is also in the shop because someone tried to steal my wheels, didnt finish the job, and the RR wheel fell off while I was driving. Bad GMT400 week.

Any thoughts? I’m replacing the coil since I may as well and it might improve things, and then why not also the ignition module since I will have the part out. Thing is, I thought this was a fuel delivery problem last year and the shop I took it to was like no, the fuel is fine (they may have pressure tested it then actually, I dont know for sure).
 

Eveready

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From all I have heard the odds are pretty good that your fuel pump may be the culprit. I would get a pressure test before throwing any more parts at it . "Stumbling" under load definitely sounds like a fuel delivery issue and the pump would be numero uno on my radar from your description. Be sure to post what it turns out to be.
 

honkon

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Does it stumble right when you get into the gas when accelerating from a stop? My Yukon was doing that, and also locking the torque converter up really harshly at highway speeds.
It was the ignition timing. Do you have any cam sensor codes? Not sure if its the same on the 5.0, but on the 5.7 you need a scan tool to set the timing.
Get yourself a scan tool that can monitor the fuel trims. If it's a fuel delivery issue they'll be above 0, probably nearly pegged out around 30% during the stumbling, hiccuping events. Fuel trims are the very first place you should look when having drivability issues.
Basically; scan tool, scan tool, scan tool.
 
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Jason012

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Thanks honkon, that makes a lot of sense and was what I started to think today. The auto parts store bench tested my ignition control module and said it was failing. I asked them to check after I bought a replacement since they are kind of expensive. At any rate, I replaced my ICM and the coil and it runs and idles so much better, no stalling, better shifts, smoother running, no bogging, but at 2 points during my 20 minute test run it did the highway "skip a step" stumble, although not nearly as severely as before. After the 20 minutes was nearly over, check engine light went on again but it was still running fine.

I haven't checked fuel pressure yet, but I park on an upward slope and the truck starts at first no problem, hot or cold so it hard to think its less than 50 lbs. What it would do is stall when it was hot, like at a red light, and would not start again for about 30 seconds, which I think was the fritzy ICM overheating and needing to cool down, allowing the restart, or it would stall under the first heavy load of the drive, like pulling away from the first stop.

I will look into the timing next. I'm very happy so far that at least it is this much improved.
 

Jason012

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So, I'm still kind of in the dark on how timing is set on these if there is a control module, doesn't that adjust the timing as an ongoing thing? Anyway, I replaced the distributor, rotor and plug wires. The check engine light is gone and everything I do improves the drive-ability but its just not perfect yet. I think the issue is more air in the air-fuel-spark trinity but I suppose there could be a compression issue/intermittently stuck valve. Pretty soon it will be going in to a shop for them to do the rest but I will post updates anyway, thanks,
 

honkon

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You have to set the base timing for the computer to be able to control it correctly. You do this the by using a scanner to monitor the cam position sensor (CMP, located in distributor) offset, holding the rpm at 1200, and turning the distributor until the reading is 0 degrees +/-2
 
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Supercharged111

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You have to set the base timing for the computer to be able to control it correctly. You do this the by using a scanner to monitor the cam position sensor (CMP, located in distributor) offset, holding the rpm at 1200, and turning the distributor until the reading is 0 degrees +/-2

That's not setting the timing. The PID you are setting is CMPRET. Getting it right makes the distributor happy. It has absolutely nothing to do with ignition timing.
 

honkon

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OP; it's unlikely that this is your problem, it could be any number of things. Reading the trouble codes would be the first step in tracking this down. But you say you're taking it to a shop anyway.

That's not setting the timing. The PID you are setting is CMPRET. Getting it right makes the distributor happy. It has absolutely nothing to do with ignition timing.
You're being semantic.
No, you cannot adjust the ignition timing and the cam position signal has no effect on drivability, but if the distributor isn't adjusted correctly it will cause crossfire. It is impossible to time the ignition if the distributor is cross firing. So it must be adjusted to allow the VCM to properly control the ignition timing.
Claiming that an incorrectly adjusted distributor has "nothing to do with ignition timing" without any evidence to back your claim is nothing but misleading. "Base timing" is incorrect, misleading terminology on my part though.
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Jason012

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Yes, I understand what you’re talking about now-sort of like having the right baseline from where automatic adjustments are made.

My codes say MAF and TPS. I went ahead and replaced my MAF, I’ve cleaned it before, but I replaced it. Again it helped some but not totally. It comes in phases, at its worst it lacks power, wont rev above 4K and sputters. I have already replaced my TPS but maybe something in the truck is “breaking” it. This just makes me insane. I’m starting to wonder if there is a problem in a wiring harness somewhere or a vacuum leak.
 

Jason012

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This issue was ultimately resolved by cleaning and securing the inputs to the computer on the driver side front wheel well under the hood. They had loosened and were filthy causing all kinds of problems. Truck runs awesome again. It is also for sale.
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