My 1993 c1500 and it's weird driving issue

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speedfreak78

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New guy here. I have this 1993 GMC C1500 305/4l60E and I think 3.08s..haven't actually checked the RPO code yet, but it's a dog. I've been reading this forum for years, finally had to sign up cause i got a head scratcher here i can't figure out and I can't find any post like it.

A little back story...

It was supposed to be my fathers truck, he bought it off the original owner about 15 years ago, who kept the truck cherry. It only had 100K on it. The guy had washed the truck, religiously, weekly... until a trash truck backed into it and took out the drivers fender, door, pillar/cab corner and half the bed. It sat for a couple months after that before my father bought it. I found a matching color, mint condition, door and fender we put on it. I pulled out the door pillar, welded in a new cab corner straightened the bed side out as best i could. Went through the whole truck and replaced anything that was suspect..exhaust, steering stuff, manifold gaskets, brakes and a bunch more I'm forgetting..I had a "friend" who was starting a body shop, and my father got sick, so we decided to help my "friend" out and let him finish the mud work and spray the whole truck in the original 2 tone blue/silver. He knew my father wasn't doing well and there was a time frame to get the truck done. Well..like happens alot.. 3 weeks in the body shop turned into months. At one point the truck was 75% sanded down to bare metal and left outside which pissed me right off. My father passed away during this time which both infuriated me with my "friend" and made dealing with him, legally, a real P'I'T'A. But eventually it got painted and I got it back and parted ways with my "friend"..then it sat for almost 10 years, being randomly started and driven around my property or worked on. I registered it last September and have been driving it since. Dealt with some basic issues on a truck that sat so long. Had electrical issues cause of a bad cell in the battery so it got a new battery. Cleaned up the grounds and some of the wiring connections that had corrosion. New Alternator. New fuel pump/filter. New plug/wires and verified timing.

Ok now the weird issue..or TL;DR...lol..

Occasionally, it drives like something is.... bound up? is the best i can describe it. Step on the gas and the engine sounds like it's trying to rev, but the truck has no acceleration. All sound, no fury. It never throws any codes. The weird thing is I can shut the truck off when it happens, immediately restart it and it's fine for anywhere from a few minutes to maybe 10 minutes, before it happens again. Then shut it off, restart..repeat. Seems like the hotter it gets, the quicker it happens again. I drove it to work most of the winter..about 20 min round trip and it only did it a couple times. Come spring time it started happening more frequently. When it's not happening the truck runs awesome. I'm guessing it's an electrical/ECU issue but without codes..I'm at a loss where to start. Since it runs really good sometimes, I don't wanna just start throwing parts at it. I've already gone over most of the wiring/grounds and had the tranny pan off to check the VB/wiring and it all seems ok? I just downloaded Tunerpro and ordered a cable so I can do live data streaming to watch what's going on when it happens. My W.A.G is restarting it put's it back into open loop where it runs fine until it goes back to closed loop and then a faulty sensor, or maybe ecu programming, is screwing things up.

Any ideas where i should start?
 
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Schurkey

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Verify fuel pressure when the engine runs poorly.

Connect "Tunerpro", look for codes, but more important, look at the sensor data and output data. Short- and long-term fuel trims, ignition advance, etc.

Will the coil fire a spark tester calibrated for HEI?

How old is the gasoline? How old are the usual "tune-up" parts--cap, rotor, PCV valve, etc? Any chance some bird, or insects, or whatever built a nest in the exhaust system (or the catalytic converter is plugged?)
 

speedfreak78

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Verify fuel pressure when the engine runs poorly.

Connect "Tunerpro", look for codes, but more important, look at the sensor data and output data. Short- and long-term fuel trims, ignition advance, etc.

Will the coil fire a spark tester calibrated for HEI?

How old is the gasoline? How old are the usual "tune-up" parts--cap, rotor, PCV valve, etc? Any chance some bird, or insects, or whatever built a nest in the exhaust system (or the catalytic converter is plugged?)
I have no way of checking FP at the moment. I've wondered if it's a fuel regulation issue. I thought about buying the parts I'd need to plumb in a gauge, but right now I'm just waiting for my cable before i go throwing any parts at it. Cable should be here tomorrow..fingers crossed. I had the fuel line off the TB once before, i don't remember why, but I do remember getting it back on wasn't fun, the threads just didn't want to line up and start.

The other day i visually checked the spray pattern of the injectors with a strobe. At idle the injectors seem to dribble a little, but once the throttle is opened they spray evenly.

Oh and I put a new FP relay in and a new IAC valve about two months ago. I was having low idle and stalling issues one day and when I went to check it out the old IAC valve it was so hot i burned myself. Ya know..the locking clip on the IAC connector broke and I never did go back and figure out a way to secure it, it seemed tight enough at the time. I know the truck dies when it gets unplugged at idle...i wonder how it effects things when it losses connection going down the road? I'll have to remeber to deal with that this weekend.

I have no way of testing spark when the issue is happening, at idle or free reving it in my driveway it's smooth as silk. I've also considered the Ign. module.

I've been driving it multiple times a week since September, just drove it today, probably put 5-7 tanks of gas through it? I just fueled up the other day. When I first started driving it back in Sept. I ran some fuel system cleaner through the first couple tanks just cause it sat so long.

Tune up was done around Nov.?, haven't changed the pcv valve but it checks out ok.

No nests, it blew those out back in September the first time i took it on the road...lol.

Don't think the cats plugged..I've done foot to the floor runs through the gears more than a few times and when the issue isn't there it scoots. Removing the cat is on my list of to do. When we did the exhaust we ran a aftermarket hi flow Y pipe i found online, OEM diesel 3500 mid and tail pipe, with a Thrush turbo muffler, but my father wanted to keep the cat so there were no issues passing inspection, even though it's old enough that it was exempt. I'm sure the smaller dia cat in/out pipe is the biggest restriction right now and i eventually want to fix that.

I did some more investigative work today. I wanted to test my open/closed loop theory, so i unplugged the O2 to force open loop and drove it all day and it seemed to be fine. You could smell it was running rich and performance was lacking, but it never did the issue I'm trying to diagnose. It also never threw a CEL either which i thought was odd because the CEL works when you initially start the truck.

So, it seems it getting bad signals in closed loop? I pulled the ECU out and opened it up last weekend to see if there were any obvious issues like corrosion or bad caps., but it looked really good. I wonder if the old battery that had an internal issue could've messed it up? I drove it like that for awhile chasing electrical gremlins before i figured out what was wrong.
 

PlayingWithTBI

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I had the fuel line off the TB once before, i don't remember why, but I do remember getting it back on wasn't fun, the threads just didn't want to line up and start.
If you unbolt the Throttle Body from the manifold, the fuel lines will thread on easier. Then bolt down the TB.

The other day i visually checked the spray pattern of the injectors with a strobe. At idle the injectors seem to dribble a little, but once the throttle is opened they spray evenly.
You may have a pinched or bad O-ring on the injectors. Need to fix it so they don't drip at all. You may be flooding it.

Ya know..the locking clip on the IAC connector broke and I never did go back and figure out a way to secure it, it seemed tight enough at the time.
That needs to be addressed

I've done foot to the floor runs through the gears more than a few times and when the issue isn't there it scoots.
If it doesn't bog down or have "lean pops" at WOT, odds are your fuel pump and pressure are good but, it's still a good idea to check pressure.

It also never threw a CEL either which i thought was odd because the CEL works when you initially start the truck.
OBDI systems may not throw the SES light but will store codes for ~30 - 40 key cycles.

I pulled the ECU out and opened it up last weekend to see if there were any obvious issues like corrosion or bad caps., but it looked really good. I wonder if the old battery that had an internal issue could've messed it up? I drove it like that for awhile chasing electrical gremlins before i figured out what was wrong.
Possibly but, I'd get the fuel and timing right before chasing after a "bad" ECM/PCM
 

speedfreak78

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Got Tunerpro hooked up and took a test drive. Injector BPW was in the red when the issue was happening and the O2 was showing lean. Figured it was a fuel issue and since i had a dribbly injector it was a good place to start. Pulled the TB and it was full of what looked like rust. Got a rebuild kit and a new fuel filter and purged the system till i had clean fuel at the TB.The truck is ..80% better? Still doing the same thing, just not nearly as bad. Might need an injector, the dribbly one got better, but still has a slight drip. I haven't done a data log since the work, hopefully I'll get it done in the next couple nights. Since I just put the fuel filter in less than a year ago and it looked like it was full of mud...I think I'm gonna drop the tank. Not looking forward to it.. when I did the fuel pump before, I had the bed off.
 

PlayingWithTBI

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the dribbly one got better, but still has a slight drip.
Send your injectors out for cleaning and flow tests. There are a bunch of shops that do this, here's one.
www.mrinjector.us/

Or buy flow matched ones. For a 5.0L yours s/b 5235279, YMMV

www.motormanfuelinjection.com/Rochester_TBI/fuel-injector-5235279.html
 
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