Out of nowhere I've started having an issue with my 1990 c1500 5.7 TBI. The truck will not idle or stay running unless I keep the gas pedal down 25% or more. If I keep the RPM's up with the throttle, it will stay running.
Ay suggestions on what could be the cause? Unfortunately I haven't had much time to troubleshoot the issue due to family obligations.
Check your fuel pressure.
Gearheadotaku (Forum Supporter) said:Check your fuel pressure.
This.
You're looking for 9-14 psi. Pretty annoying to check pressure on TBI trucks unless you've got the inline adapter for your gauge.
Your description sounds a lot like a fuel pump failing.
Also, coolant temp sensor will do this.
Any check engine light that we can jump with a paperclip? Do i need to come up in a couple weeks with my laptop and such and we do a full diagnostic run?
All of the above, but if it starts and idles ok in park i'd swap the MAP sensor with a known good one, and check the vacuum lines from the TBI for cracks or breakage.
Sounds like it may be a vacuum leak. Big enough that it won't idle but small enough that at higher air flow it's low enough impact that it can keep running.
ColoradoBob said:Sounds like it may be a vacuum leak. Big enough that it won't idle but small enough that at higher air flow it's low enough impact that it can keep running.
The gasket under the throttle body is a common failure point.
Grab a of carb clean and spray around to look for vac leaks. The motor will rev up if you hit one.
LS swap?
I recall that the tbi 5.7 was prone to vacuum leaks around the upper plenum. I always thought that a failing fuel pump would let you idle okay but would crap out under load. As mentioned above, too much fuel from a computer that's trying to dump fuel because it thinks it's -40 degrees can make it run terribly at idle but once you open it up it gets better. Those have an IAC, right? Is it possible that it's not operating? Can you use a wad of tape to crack the throttle butterfly open just a wee bit?
Throttle body engines use a map sensor, so any vacuum leak just makes them idle faster.
Make sure the egr valve isn't stuck open. Start it up, keep it running with the vacuum hose disconnected to the egr, and touch the base of it after 5 minutes. If you squeal from the heat, it's stuck open.
This talk of vacuum leaks isn't right. GM TBI is a speed density system, and a vacuum leak causes them to idle higher than usual.
It behaves the same no matter what gear it's in. Park, neutral, drive, reverse has the same issue.
I'll look through my fuel injection pressure test kit and see if it has the adapter for the TBI.
And I'll check the EGR as well.
If you want, i should be able to make it up there next weekend with my obd1 gm software.
Dusterbd13-michael said:Also, coolant temp sensor will do this.
Any check engine light that we can jump with a paperclip? Do i need to come up in a couple weeks with my laptop and such and we do a full diagnostic run?
I replaced the coolant temp sensor a few years ago when it had a no start issue. Not to say that it isn't bad again, I did replace it with a part from AdvanceZone.
I appreciate the offer to come help troubleshoot it. I've considered calling you to pick your brain about it. I just don't have much free time anymore since the little one is here
Dusterbd13-michael said:If you want, i should be able to make it up there next weekend with my obd1 gm software.
Thanks for the offer, unfortunately I have to work next weekend. Working swing shift, I have 2 weekends off then work the next 2 weekends. Just finished my 2 weekends off.
In reply to untchabl :
The offer stands for your next weekend off cycle just let me know. I'll bring the laptop and your HVAC module
EGR valve sticking can cause this, so can any vacuum leak,
In reply to wae :
there's no "upper plenum" on the TBI V6/V8 truck engines, just one large lump of aluminum/cast iron. You may be thinking of the 96-99 Vortec engines and they did have intake leaks. TBI vacuum leaks create the opposite proble, they idle fast.
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