Failing fuel pump causing low RPM idle? (1 Viewer)

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Joined
Jun 3, 2018
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Location
Salt Lake City, UT
Hello all,

My 1999 LC (150k miles) has been temporarily slowed by the dreaded boiling-gas-at-elevation problem and I am slowly figuring out how to get it back up to speed. First step was fixing some bad wiring that eventually blew my instrument cluster (replacement TBD when I find an acceptable used one). There was a 12v current running through my fuel tank and also fried my sender. That has been replaced and the old cluster remains in the dash (and it doesn't smoke, crackle, and pop anymore).

The truck runs like it always did, but has issues when idling. It turns over fine and then idles so low that I can barely hear it (tach is part of the cluster that isn't working). Eventually, it stalls. It isn't throwing any CEL codes (OBDII is part of of the cluster that IS working) and runs just fine once it starts going.

Any ideas? First thought goes to the fuel pump since that seems to be where most of my issues stem from, but it IS getting gas so I'm not sure. Dirty throttle body? ECU needs to recalibrate? Buying and replacing the fuel pump isn't a huge deal, but if it's still good then why do it, right? Any input is much appreciated!

Semi-related: does anyone have much experience replacing the instrument cluster for the 98-02 models? Can I simply just pull off the light dimmer panel and top of the steering wheel cover to get it out? Or should the entire steering wheel come off? Thanks!
 
Seems you now have handle on aftermarket wire issues at pump. But what else did PO play/mess with, keep working to get it back to stock.

Go through all vacuum lines including PCV and ones coming off power steering vane pump idler controller. I'll bet most vacuum lines bad if they've not been replaced.

Make sure you've a good air filter and that rodents didn't build nest in air box or air tube in fender well.

Sure clean the T-body and MAF sensor (use MAF spray). Disconnect battery to start the cleaning and end with connecting battery to good clean greased battery post.

Disconnect battery 20 minutes reset ECM. Always disconnect neg post of battery first and always disconnect battery when working on electrical or anything related to air bags i.e steering wheel.

I've not pulled the instrument panel, but seem the LC steering wheel will be in the way. Try search in mud D bulb replacement.

Consider PO had some issue that prompted him to jump fuel pump. If you've not figured out why yet, well it could be issue of idle your looking at.

Consider Fuel Injector off vehicle service and replace those seals at the bottom of each FI. Fuel Pressure Regulator on fuel rail will be suspect at this age. . Fuel damping device on fuel rail need inspecting also.

Check out each coil boots and spark plugs while doing a compression check.

Keep up the good work and have fun.
 
That's a great list of potential causes, thanks!

They found some damaged wiring in the harness between the instrument panel and the sender. They started at the cluster and starting working backwards trying to find where the short was coming from and ended up there. The mechanic actually thinks that the faulty wiring may have helped boil the gas - it was apparently sending an unfiltered 12v current through the fuel sender and into the instrument panel when the transistor in the cluster literally exploded. After they repaired the wire harness and swapped out the sender, they were getting good currents into the instrument panel. They were able to find an old cluster nearby for dirt cheap, but when they got it all plugged that cluster wasn't working correctly either (different instruments on the panel, don't think it is systemic).
 

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