90 5.7l won’t stay idling

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Logan R

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Hi all,
I have recently acquired a 90 c1500 with the 5.7 in it. It had been sitting in a partially open garage for 10-15 years with no one touching it. Once I got it home I have replaced the ICM, full tune up, IAC, rebuilt the throttle body, 2 rebuilt or remanned injectors, siphoned out old fuel and put new in, had a new fuel filter but currently not using one to try and run it. Will be replacing computer today as I have not another clue what could be wrong. The truck will not idle and if it “runs” it only does so for a few seconds and at maybe 200 rpm at best. I have also dumped fuel down the throttle body while trying to start it to only get the same thing or a backfire/fire. Sounds like us is missing a few cylinders. I kept thinking it was timing as I replaced the pickup coil but so far that hasn’t seemed to be the issue. It also has no throttle response. Before I messed with setting the distributor right I had it set somehow to where it would start up, idle like a top for a max of 2 seconds, and then die. But would stay running with pretty much any throttle. Any help is much appreciated thanks!
 

95burban

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Have you pulled the spark plugs and checked them? Do you have compression? Don’t really care about numbers, you can physically feel a hole that’s down. Do you know why it sat? Or was it a “it was running great when papa parked it” deal.

Check fuel pressure? The rubber hose in the tank could bad or pump is really leak and sucked up rust because the sock is trashed.

There is also the timing wire to unplug when you time it but it doesn’t sound like it will even run long enough for that.
 

Logan R

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Yea I have checked spark as I tried to use my test light and gota pretty decent shock, also gettung enough spark on every wire to run a timing light. The truck ran and everything before it was parked. I feel like fuel pressure isn’t an issue as I can hear the pump running and it puts out a good amount of fuel. I plan on replacing the fuel pump and such relatively soon. I at least felt decent compression when checking TDC of cylinder one yesterday. I don’t no have a compression tester. Hoped soaking the cylinders in fogging oil would expand the rings enough. Also inspected plugs multiple times. Not wet at all. Thanks
 

Schurkey

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DON'T run the engine without a fuel filter.

Stop replacing parts and start doing actual diagnosis.

Many parts-stores have loaner-tool programs where you can get a compression tester, fuel pressure tester, and other items.

Connect a scan tool, verify EVERY sensor and computer output. (You won't be able to verify the O2 sensor until the engine runs long enough to heat it up.)

Oil does not "expand" rings.

Any chance the exhaust is plugged? Catalyst plugged, or some kind of nest in the tailpipe?
 

Logan R

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Well it would be great if the truck would run long enough to even think about a diagnostic tool. The exhaust doesn’t seem to be plugged as it blew a good amount of stuff out when it would run on throttle. And if it is there are a few rust holes in the exhaust to where I feel like it can run. I have even got maybe half a cup of water out the exhaust at one point, but that has quit I feel like it blew the condesnsation out.
 

95burban

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Put fuel in a spray bottle and try to keep it running.
 

Logan R

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Yea I did, turns out it was timing, it was off probly about 270 degrees when I first started, so I had to reset it multiple times to get it to run good, set it right the first time ran good for a little bit then went way outta wack again. Second time set it back ran like a dream. If there is any further questions I’m no expert but I can try and help. And if you need me to talk you through it I can too, as that may be easier for you. Just keep me posted!
 
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