94 5.7 random stalling

[alicesage]

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hi y'all, my new to me 94 k1500 5.7l is having a few issues that i need to fix before she becomes my full-time work truck. the truck sat mostly unused for 4 years before i bought her a month ago. a mechanic friend did a full tuneup recently and i changed the oil on the way home after buying it. it stalled out on me at a light, had a tough time starting back up, and then we limped to the quick lube for an oil change. the oil was clean and at the appropriate level but had last been changed 3000 miles ago in 2021. it has a cracked exhaust manifold which i'm planning on replacing soon. it's been running mostly fine after that aside from a few annoying issues.

i'm currently struggling with my engine stalling out on me and dying when i come to a stop and take my foot off the gas pedal. it doesn't happen all the time, but it's happened enough times to make me paranoid. when this happens i shift into park, let it sit for a few seconds and then restart the truck. it's been struggling a little with cold starts and sluggish acceleration as well. i'm going to buy a scan tool this week, but in the meantime i used a paper clip to read the codes this afternoon. it displayed a 15 and 42 before I drove it for a short errand (without issue) and displayed the same codes when i got back. i haven't been able to find any vacuum leaks and the iac and pcv valve were replaced during the tuneup. there's no smoke coming from anywhere there shouldn't be smoke and the oil levels are fine.

i don't have a ton of experience working on cars aside from basic maintenance, but i work in construction and have some tools and a harbor freight a few miles away if needed. i rent so i don't have a garage but i do have a walmart parking lot and a parts store next door. my friend hasn't been able to diagnose the issue but suspects it could be the fuel pump or ignition control module. any advice for where to start?

 

[Schurkey]

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.

 

[sntrym]

Adding to Schurkey's post: New parts aren't always good. Make sure the parts that were installed are providing the correct data, especially the TPS and IAC if you're stalling at idle. You won't have misfire counts on a 94, but you can check the O2 sensor data, the TPS position voltage and %, and the IAC position.

I use an older Surface Pro with the ODB1 cable hooked up for live data and recording for tuning purposes with TunerPro in lieu of a scan tool.

 

[alicesage]

heard thanks fellas. i'm gonna try to get a scan tool this week but worst case scenario i'll get tunerpro. not sure how old the fuel filter is but i can go ahead and change it this week just to be safe. i'll replace the O2 sensor as well. i'll throw in the fuel system cleaner today. my original plan for diagnosis was just to run it until it dies on me but i depend on this truck to get to work.

are there any videos or written step by step guides showing me how to perform step 1? i'm assuming i can reuse the gas after i do this right? the tune up parts were all oem parts off the shelf from a local store. the distributor cap, spark plugs, ignition coil, iac valve, egr valve and pcv were replaced with oem parts a few weeks ago.

 

[Schurkey]

are there any videos or written step by step guides showing me how to perform step 1?

You can try Youtube, but I don't know of any step-by-step guide beyond what I've written.

i'm assuming i can reuse the gas after i do this right?

You're gonna push the pressure-relief button for two or three seconds. You'll lose less than one cup of fuel. Probably half-a-cup. I suppose you could pour it back into the tank IF YOUR DRAIN PAN IS CLEAN.

the tune up parts were all oem parts off the shelf from a local store. the distributor cap, spark plugs, ignition coil, iac valve, egr valve and pcv were replaced with oem parts a few weeks ago.

Yeah, that's about as good as it gets. Problem is, that's still likely to be Communist Chinese sourced. There's no promise that the stuff is good until YOU prove it's good. That's why the parts have a warranty.

 

[alicesage]

i'll make time to check fuel pressure today and i'll report back later this week when i get a scan tool!

 

[alicesage]

it was the fuel pump! it started wanting to die on me when turning and finally left me stranded one afternoon. the previous owners installed a really cheap ****** one. new fuel pump is in and she's running great now!