Diagnosis help

[P10crew]

I need some advice from the experts please.
1990 c1500 5.7 120,xxx miles.
On Saturday I was in a business and when I got back in the truck it went about 20' and died. I had pulled out of my parking spot and there was another car backing out of a spot so I had to stop as I got in the alley way. My truck died as I let off the gas to coast to a stop.
I cranked on it for maybe 5 seconds and it would not fire. I cycled the ignition switch again and it fired right up. Ran just fine I then pulled out on a side street and made it 2 blocks and as I let off the gas to coast a small hill the ole bugger died again. Same deal I cycled the key twice and it fired right up. I was at 1/4 tank of fuel so I went to the nearest station and filled up. And headed home. Now this morning on the way to work it did this same thing dieing as I coast to a stop. From that point on I kept some throttle in it as I came to stop lights and all was fine. Definitely fuel related and I will change the filter tonight but am a little confused of how to diagnose this other than driving in traffic. ? Do these 90 TBI have a fuel pressure tap somewhere under the hood? BTW what ever is going on has not set the SES light. Thanks in advance.

 

[Jethro_Bob]

On older cars with no codes my first step is unhook the battery then let it idle for three minutes on the first start before killing it. This has "cleared up" many odd issues for me. On occasion, it has tripped the check engine light shortly there after allowing me to better track down the issue.

Just my opinion, but I think cars need to be "rebooted" every once in awhile.

Edit: first step was going to be check for pending codes but op has a obd1 system.

 

[Donald]

The most likely thing is the in-tank fuel pump. I had a 1993 Suburban with 5.7L and replaced the fuel pump 2 or 3 times. While there is a fuel filter there is also a sock on the fuel intake in the tank.

If the fuel pump has not been replaced recently its likely you will snap some fuel line fittings and need a fuel sender assembly with pump. And the sock if not included. I would go with OEM or a top brand. I replaced the NAPA fuel pump with a year.

My tank had a lot of waxy goo covering the hole where the fuel sender assembly goes. You need to scrape it all off but it will be clean when ou do. I used wax from a new toilet bown ring to replace the waxy goo.

I dropped the tank a few inches, undid the electrical and fuel lines then removed the fuel sender assembly.

 

[Donald]

You can stick your ear near the fuel tank and have someone turn the key to ON and maybe hear the fuel pump. Also can check the fuel pump relay and fuse, but its most likely the fuel pump.

Never run out or get very low of gas with GM vehicles with TBI and in-tank fuel pump.

 

[Lou_Boyle]

Had similar symptoms with both plugged filters and failing fuel pumps.

 

[ragtoplvr]

Originally Posted by Jethro_Bob
On older cars with no codes my first step is unhook the battery then let it idle for three minutes on the first start before killing it. This has "cleared up" many odd issues for me. On occasion, it has tripped the check engine light shortly there after allowing me to better track down the issue.

Just my opinion, but I think cars need to be "rebooted" every once in awhile.

Edit: first step was going to be check for pending codes but op has a obd1 system.


This is bit unclear. Never unhook the battery on running engine.

Unhook battery engine off. wait 20 minutes. then connect battery and let idle for 3 minutes.

Rod

 

[mk378]

Does it always keep running if you stay on the gas pedal? Starving for gas affects mostly driving-- especially heavy throttle like pulling hills, not idling.

There is an idle air valve in the throttle body, if you take the air cleaner off there is a bypass hole in the corner where a stepper motor (round thing with 4 wires) is screwed into the side. That hole should be flowing air when idling.

There is no fuel pressure test port. Pressure testing requires replacing one of the fuel lines with a special one having a port teed in.

 

[JMJNet]

I agree check fuel pump and fuel filter.

Load test the battery?

 

[Kawiguy454]

My 02 f150 was similar problem it Was a bad fuel pump. Cycling the key would get it to work for a while until it quit completely.

 

[P10crew]

All so far. Why if the pump is failing would it s the bed at its least demand....., idle. I can hammer down on her and she runs strong as it ever did.
Is the ecu changing voltage to the pump as demand changes?
Thanks

 

[Jethro_Bob]

Originally Posted by ragtoplvr


This is bit unclear. Never unhook the battery on running engine.

Unhook battery engine off. wait 20 minutes. then connect battery and let idle for 3 minutes.

Rod



Thanks, you are correct, I quickly typed that on my lunch break, unfortunately it's to late to edit. I will do my best to write better posts in the future.

 

[P10crew]

Originally Posted by mk378
Does it always keep running if you stay on the gas pedal? Starving for gas affects mostly driving-- especially heavy throttle like pulling hills, not idling.

There is an idle air valve in the throttle body, if you take the air cleaner off there is a bypass hole in the corner where a stepper motor (round thing with 4 wires) is screwed into the side. That hole should be flowing air when idling.

There is no fuel pressure test port. Pressure testing requires replacing one of the fuel lines with a special one having a port teed in.

It has not died with me holding part throttle.
It will be a good test getting home tonight as I have 15 lights to possibly stop at. If iac is not allowing air flow at idle will it die out the motor??.

 

[P10crew]

Ok I'll start this reply like this MK 378 you da man!
Your post sparked enough interest that I pulled up a YT video on GM aic. The vid describes my exact symptoms. So on the way back into town I called my buddy at GM parts and picked one up. I changed it in the parking lot at work. Took longer to open the hood and get the air filter off than to change the part. As soon as I fired it up I could tell it made a big change because it's never had a nice high cold warm up idle since I've owned it. As it rolls to a stop light it has a nice reliable idle and really don't get down to full (lowest) idle until it gets completely stopped.
👊Thanks man!

 

[WyrTwister]

Congratulations on the success !

Just as preventative maintenance , consider replacing the fuel filter . And any other scheduled maintenance .

Best wishes ,