[SOLVED] Stuttering, Jerking, and Misfiring for a 1997 GMC Sierra C100 5.7

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Hey everyone, I have an issue with my 1997 GMC Sierra C1500 with a 5.7 motor that's paired up with a rebuilt 4L60E. I've recently got the truck up and running from a bad distributor rotor and a distributor cap. While trying to find that issue, I've replaced the entire spider injector assembly with the electronic MFI design. I've replaced the timing chain and the cam and crank sprockets. And I also replaced the ignition coil module and the ignition coil a few months prior because the module quit on me. I know I've installed everything correctly, and I've placed all of the gaskets from all of those jobs. I don't think I have any obvious vacuum leaks and that's because I plugged off the throttle body plate, and then I vaped into the brake booster hose, and I didn't see any vape smoke come up, it held pressure.

The issue I'm having with the truck, is that the engine like to stutter and stall randomly. It's not completely random, it seems when it's under a load. For example, any time I turn the steering wheel all the way to the right, and push down on the brakes, the RPMs will drop real low, then start surging like it's wanting to stall out. It'll do this if I turn the wheel all the way to the left, but it seems to do it more often if I do it to the right. It'll also have a little stutter, like a sudden deep dip in power for a split second when I make a right turn or a left turn while going normal speeds, for example, like turn left at a stop light or something. And what's happened recently too, is that the engine will stutter while going straight. I'll be going around 45MPH and then I'll get a little jerk back multiple times in the span of a couple of seconds, the RPM will drop spike downwards and then jump right back to normal like nothing happened. Just the other day, I hit a bump in the road, and I suddenly lost throttle response, a couple seconds later, I shift it into neutral, and then rev it. It revs fine, so I put it back in gear and then I give it gas. It works fine, so then I try to floor to see what happens. The engine works up to like maybe 5000 RPM, and then it doesn't go past that. So I put it in park, check the codes, and it gives a transmission slipping code. Fast forward, I check the transmission fluid, and it's full. It smells a little burnt, but that's fine. Right after I clear the check engine codes, I hope back in, put it in gear, play the gears, and it shifts great. I've never had an issue the shifting before, I still don't to this day, and I even floored right after, and it shifted through the gears fine. So I'm thinking that sudden loss in power confused the computer and it thought it was slipping or something. That's so far all of the symptoms I've been dealing with.

The codes I'm getting for the truck are P0300 - which is a random misfire, P0103 - which is MASS AIR FLOW SENSOR CIRCUIT HIGH FREQUENCY, and that one is interesting because I've also seen the low frequency code pop up while also having the high code present as well. I've also had a fuel system trim lean code and a catalytic converting not efficient code, but those codes have been present ever since I bought the truck a year ago. I've also had a random misfire, but this is taking misfiring to a whole never level.
What I've done so far, is I've hooked up probes to the MAF sensor and tested it. I know it's better to have an oscilloscope or measure in Hz, but unfortunately, I do not have access to a multimeter with HZ on it, or an oscilloscope. With the key on, I proved the connector, and I've getting 12V for the power, a good ground, and the signal is 5V even. When I plug it back up and test the signal with the engine warmed and idling, the signal voltage sits at 4.45V. When I slowly rev it up, the voltage drops slowly which no sporadic variations. When I open throttle wide open, it drops fast. I don't see any problems with the reading. However, as far as I've seen, the voltage is supposed to go up, not down. That's the only issue I see with my test. I'm not sure if the trunk is different, but other GM cars around the 90s all go up, so I don't know. I've attached links to both of the two videos showing my test. Please excuse my squeaky belt. It's old and it squeaks a lot.

I'd really like to get this issue fixed. It's not hindering my daily driving, but I need some help before I load up the parts cannon and but a new MAF sensor. Any ideas anyone? What else should I test?

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Schurkey

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You need to connect a real scan tool, which will tell you MAF frequency directly. Trying to diagnose sensor operation with a multimeter is time-consuming and trouble-prone.

Code P0300 always scares me. It's real common for cheap tools to LIE about P0300. The "real" code might be cylinder-specific. P0305, for example. But a cheap diagnostic tool can't display P0305, cylinder misfire on #5; so it displays P0300 (random misfire) instead. Screws-up the diagnostics because it should be leading you directly to #5 but instead makes you think it's multiple cylinders in trouble, without telling you which ones.

A real scan tool would enable you to view all the sensor input data, including crankshaft/camshaft sensor synchronization; along with output data like EGR operation, IAC operation, spark advance, etc. I'd be especially interested in whether the IAC can keep up with load changes at idle.

Do you have a problem with your power steering? Excess pressure presents too-heavy a load for the engine. Maybe a pressure relief valve is sticking, or the variable-pressure power steering system on some trucks has failed.
 
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You need to connect a real scan tool, which will tell you MAF frequency directly. Trying to diagnose sensor operation with a multimeter is time-consuming and trouble-prone.

Code P0300 always scares me. It's real common for cheap tools to LIE about P0300. The "real" code might be cylinder-specific. P0305, for example. But a cheap diagnostic tool can't display P0305, cylinder misfire on #5; so it displays P0300 (random misfire) instead. Screws-up the diagnostics because it should be leading you directly to #5 but instead makes you think it's multiple cylinders in trouble, without telling you which ones.

A real scan tool would enable you to view all the sensor input data, including crankshaft/camshaft sensor synchronization; along with output data like EGR operation, IAC operation, spark advance, etc. I'd be especially interested in whether the IAC can keep up with load changes at idle.

Do you have a problem with your power steering? Excess pressure presents too-heavy a load for the engine. Maybe a pressure relief valve is sticking, or the variable-pressure power steering system on some trucks has failed.
That's what I've heard too, is to check with one of those fancy diagnostic machines, but unfortunately, I'll have to take the truck to a mechanic for that because I don't have one. I might take pretty soon if I can't figure something, so I'll update y'all on the various data. It's leaked before, so I put ATF in there, because I heard that it will help make the seals swell up, which to be fair, it doesn't leak anymore. Could that cause my issue? I've NEVER had any issues with it, and it's been in that reservoir for months. Do you have any suggestions before buckling down and taking to a shop?
I have a 97 Silverado 5.7. I forget how I figured the problem out but on mine it was the Fuel Pressure Regulator. It is on passenger side of the Intake system. It was random and did seem at certain rpm's.
When I replaced the spider injector assembly, a new fuel pressure regulator came with it.
 

Schurkey

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It's leaked before, so I put ATF in there, because I heard that it will help make the seals swell up, which to be fair, it doesn't leak anymore. Could that cause my issue?
Thousands of years ago--1960s, some of the 1970s--ATF was actually recommended for power steering systems.

Since then, ATF has become too specialized. It has ingredients that aren't welcome in PS systems. Thus parts stores selling quarts and gallons of "PS Fluid".

Whether or not this is causing problems with the PS pressure relief valve, you'd want to flush the PS fluid that's been contaminated with ATF in favor of fresh, genuine PS fluid.

Good time to check the PS fittings, and the pressure and return hoses for leaks, Remember that PS return hose is also "special"; do not use fuel hose. Ideally, you wouldn't use those crappy worm-drive slotted clamps that everyone else on Earth seems to prefer. "Fuel-injection" clamps are much preferred if you can get them to suit the hose diameter. Even OEM spring clamps work better than the worm-drive disasters.

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Alright everyone, I have an update. So I caved in and bought a new MAF sensor, and the truck runs great now. The check engine light is off, and all of the codes are gone. The only I'm getting now, is that bank 2 fuel trim is rich, and it's pending, which means it's not too serious, from what I understand. The truck has stopped stuttering, and it feels great to drive again. I haven't flushed out the power steering pump yet, but I will do that in the next couple of days, but as of right now, I am very happy with how it runs.

I really appreciate everyone's help! You guys are awesome!
 
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