Strange electrical issue

docstoy said:

Do you have a scanner to look at data ? Also as Road Trip stated service manual information is very helpful. I think if fuel pressure/ volume is within spec. and injector pulse and spark are good (tested wit hei spark tester) I would look at IAC . Has the minimum idle rate been messed with or adjusted ? There again specs. are in service manuals hope you've been looking at those. Not meaning to run you around cking stuff just not there to help with diagnostic thought process.

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I know that the plug for the min idle was removed so it might have been adjusted. I have tried adjusting min idle but every time I remove the IAC and (making sure it's in order of operations) short the A and B ports of the obd1, it idles around 2k.

Furthermore, I can see the pindle moving back and forth slightly when it surges

bogus said:

Furthermore, I can see the pindle moving back and forth slightly when it surges

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Is the engine completely stock?

You may see the pintle moving in and out because the ECU is trying to keep the engine from stalling. Or not.

Disconnect the IAC once the engine’s running, see what happens.

1998_K1500_Sub said:

Is the engine completely stock?

Disconnect the IAC once the engine’s running, see what happens.

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Okay it'll be a minute I'm at work

Don't just disconnect IAC, follow minimum idle rate procedure in service manual and use a tachometer not the one in your dash, or scan data. See if this helps problem, IAC may have been hunting for position or bottoming out. By trying to adjust idle rate IAC IS ALWAYS going to try and find its home position. This may be your problem, best of luck

docstoy said:

Don't just disconnect IAC, follow minimum idle rate procedure in service manual and use a tachometer not the one in your dash, or scan data. See if this helps problem, IAC may have been hunting for position or bottoming out. By trying to adjust idle rate IAC IS ALWAYS going to try and find its home position. This may be your problem, best of luck

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Would this lead to a stall like the one in my video?

An throttle opening (minimum idle rate) without proper adjustment per specifications which is what your ecm/pcm has been calibrated for and your IAC position which is also apart of the calibration of the ecm/pcm cannot operate correctly together, the answer to your question is YES it can or will stall and it will be hunting for its correct position (idle up and down) and it may or may not turn on a ck engine light because a lot of times it hasn't run or operated long enough for it to go closed loop to be able to recognize there is a fault. If run long enough IAC can be damaged because if minimum idle rate is adjusted wrong the IAC beating itself to death to close so that the calibrated r.p.m. the ecm is looking for cannot be obtained. Didn't watch your video just know that stalling is most often times caused by improper repairs being attempted, not being a sa. Just remember that all settings and adjustments are part of programming that your ecm has in it and when changes are made whether right or wrong ecm/truck has to go thru relearn procedure which is also described in service manual (fuel and emissions manual) which can be downloaded from sticky threads above titled 88-98 service manuals

docstoy said:

Don't just disconnect IAC, follow minimum idle rate procedure in service manual

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Just disconnect the IAC to see what happens... does the engine still stall, idle still lope, etc.

Then reconnect it.

Worry about the idle later. It shouldn't be hard to set, I can't figure what problem you were having when you tried to do it y'self.

Alternatively, expected range of IAC values at idle (IAC connected), engine warm, should be in the service manual. If it's outside that range on your scan tool, it's telling you something.

The simple fact that someone screwed with the TBI idle gap setting tells you something too. What's up with that? I asked you earlier "IS THE ENGINE STOCK?" Why do you not answer questions when the rest of us are willing to answer yours?

1998_K1500_Sub said:

Just disconnect the IAC to see what happens... does the engine still stall, idle still lope, etc.

Then reconnect it.

Worry about the idle later. It shouldn't be hard to set, I can't figure what problem you were having when you tried to do it y'self.

Alternatively, expected range of IAC values at idle (IAC connected), engine warm, should be in the service manual. If it's outside that range on your scan tool, it's telling you something.

The simple fact that someone screwed with the TBI idle gap setting tells you something too. What's up with that? I asked you earlier "IS THE ENGINE STOCK?" Why do you not answer questions when the rest of us are willing to answer yours?

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Because you ask a lot of questions lol. Yes the engine is stock. And I answer your questions the best I can. I'll unplug the IAC while running to see what happens and let you know

bogus said:

1994 k2500 5.7L 3/4 ton 6 lug
seems I can't post a video here so I'll leave a link to the subreddit where I posted this problem:

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I have cleaned or replaced every ground that I can think of.

I replaced the fuel pump when I got the truck in November 2023
New oil pump
oil pump pressure switch
EGR
EGR solenoid
vapor canister purge valve
MAP sensor
PCV valve
IAC
TPS
ICM
ignition coil
distributor cap
sparkplugs
plug wires
and all vacuum lines are okay.
replaced the inlet and outlet fuel lines to the TBI

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When it stalls, does the "check gauges" light come on?
Not the "Check Engine" light.
The "Check Gauges" light.
They are not the same.
When it stalls and you try to crank it again are you going all the way back to key off/ key removal before you try again?
When it stalls will it restart after it cools down for a while?
I suspect you have a dual intermittant heat related failure between the fuel pump relay and the oil pressure sensor.
Its a two path system where the truck starts off of the fuel pump relay and once you have oil pressure the fuel pump runs through the oil pressure sensor.
If you have an intermittant with the oil pressure that circuit reverts back to the fuel pump relay.
Its the on and off cycle that causes that little tiny coil mounted contact in the relay to weld itself together and then unstick and reweld itself.
Pull the fuel pump relay and look for slightly brown overheated plastic.
Go to NAPA, get two 30 amp Echlin fuel pump relays.
One for a spare.
If that new relay cures the problem you are halfway there.
Now you need to replace to oil pressure sensor too.

thinger2 said:

When it stalls, does the "check gauges" light come on?
Not the "Check Engine" light.
The "Check Gauges" light.
They are not the same.
When it stalls and you try to crank it again are you going all the way back to key off/ key removal before you try again?
When it stalls will it restart after it cools down for a while?
I suspect you have a dual intermittant heat related failure between the fuel pump relay and the oil pressure sensor.
Its a two path system where the truck starts off of the fuel pump relay and once you have oil pressure the fuel pump runs through the oil pressure sensor.
If you have an intermittant with the oil pressure that circuit reverts back to the fuel pump relay.
Its the on and off cycle that causes that little tiny coil mounted contact in the relay to weld itself together and then unstick and reweld itself.
Pull the fuel pump relay and look for slightly brown overheated plastic.
Go to NAPA, get two 30 amp Echlin fuel pump relays.
One for a spare.
If that new relay cures the problem you are halfway there.
Now you need to replace to oil pressure sensor too.

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Check gauges does come on with the battery light.
Its all the way back to key off, not removal, though.
It doesn't need to cool down I can just restart it immediately.
I have replaced both the oil pressure sensor and the relay, still stalls.