Stalling problem SOLVED!!

Ron Webb
06-14-2005, 12:32 PM
Like several other posts, I have been having a problem with my '92 pickup (3.1l V6) stalling then refusing to restart.

Last night, I figgered it out!!!

The Brain Box (ECU) is in control of the fuel pump. It'll shut it off (via the Fuel pump relay#1 in the fuse/relay box under the hood) anytime it decides to.

There is a BUG in the ECU software. It sometimes will shut the pump off inappropriately. GM knows about this. Their solution (instead of fixing the software) was to wire a second relay in parallel, driven by an oil pressure sensor. Now, any time you have oil pressure, the fuel pump should be running.

So the pump is driven by the ecu for 2 seconds after startup, then by this other relay after that (though most of the time both Fuel pump relays are energized)

If you lose this second oil pressure switch, or if oil pressure drops at idle to a low level that will not hold the oil pressure switch closed, you become dependant on the relay driven by the ECU (bug and all). It will sometimes shut you down.


The right way: Replace the relays, the Oil Pressure switch (not the main one), the oil pump, the oil filter, and the fuel filter. Also switching to a thicker oil will raise the oil pressure, and give the oil pressure switch a better chance of staying closed at idle.

The Army way: Boost the idle speed so that the oil pressure always stays up.

My way: Pull out both relays and throw them away. Then install a jumper in the #2 Fuel Pump relay socket so that it looks like the relay is always on. Now you will have fuel pressure any time the key is on.

This disables a "safety feature" in that the fuel pump will not shut off in a crash if the engine dies. I choose to live without this dubious (and recent) "feature". Let it strand YOU in a moving lane of traffic, and see how safe you think it is then!

There could be several possible combinations of problem (bad relays, bad oil pressure switch) that would have different symptoms. The jumper solution solves them all.

Hope somebody finds this useful!

07-16-2006, 08:42 AM
Hey, I like the idea! What did you do about the #1 relay? And what gauge wire did you use for your jumper?

Ron Webb
07-16-2006, 01:58 PM
My previous post was a year ago. I never posted "the rest of the story".

Everything I said in the last post stood up, but the mod is not necessary. Keep your relays, the occasional stall (once every month or 2) was there since new, and It'd start right back up.

The reason my fuel pump was kicking out, and the truck stalling and refusing to restart was that the ECU commanded it all right, but the ECU had a REASON to shut off the fuel pump!

Long and unlikely story - brace yerselves! Seems the ignition module (a standard GM HEI module) takes the output of the crank angle sensor, buffers it, and on startup uses it directly to fire the plugs, but it also has to go to the ECU because the ECU uses it to determine when to turn on the fuel pump. When the engine is running, an electronic relay pulls in, and stops the output of the buffer from going straight to the plugs, and instead uses an ECU output to drive the plugs, with all the fancy mapped delays.

Seems that for whatever reason, the buffer on the output of the crank angle sensor could drive the ECU alone, but when it was hot, it couldn't drive the ECU, and the plugs straight through. In that case, there was enough to drive the plugs, but not the ECU as well. Result: It'll run, but it won't start without a shot of starter fluid.

So I replaced the relays, removed the jumper, and replaced the ignition module (it is in the distributer). It ran fine for a year after that, then died of massive hemmorage (head gasket) with ONLY 220K miles!

So my little buddy Bruno, who happens to to be a 1992 Isuzu pickup, gets a transplant - a gen II 3.1L with 170 HP instead of the stock 140. Still in progress!

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