89 GMC 5 second run

Went out today fired up the truck it ran for 5 seconds and shuts off... It shuts the fuel off, thought oil pressure at first not letting fuel pump run,
but it has almost 60 psi at start up. Ran codes and got a 45 (rich O2 sensor)... it only runs on the 5 second timer... Anyone with the quick fix on this ??? I looked back through the posts I'm sure it's there somewhere but didn't see it yet...
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I guess I'll try the oil pressure switch... I know it has two circuits in it one for gauge and one for engine running.... I'll see what that does...
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It
O2
it
Don't think it will help and don't have enough info to try to help. Did you check for any stored codes?
Al
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Like I said in the initial post the only code is a 45
It's a 5.7 1989 GMC TBI ? it starts runs for about 3-5 seconds then shuts off... when it shuts off the fuel stops instantly. We used a squirt bottle and sprayed fuel in and it keeps running. Going for the op switch now try that and see if it fixes it...
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I can't remember for certain; but I think the oil pressure sender for the gauge is separate from the oil pressure switch for the fuel pump. I had one fail on my '88. There's also a fuel pump relay and a fuse... but I can't remember whether they are used during start-up or while driving. It seems to me that the oil pressure switch acts through the fuse and relay. The fuse is in a black "holder" mounted to the firewall... it pulls out from the bottom if I remember correctly. You can also activate the fuel pump "manually" by using a jumper wire in the ALDL plug under the dash... I drove mine that way for a week or two while my rebuilt ECU was on back-order. That way you can get the thing running so you have oil pressure to check the switch if you want to. I would check both the fuse and the relay. Sorry... it's been years since I had that truck. David

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OP switch not the answer, O2 sensor not the answer. Still starts and runs for the 3-5 seconds or so and then shuts off... Luckily we had a donor truck same year same engine that we scrapped a few months ago, but saved the harness, TBI, O2 all sensors and computer... (to put into an older car) so... we might throw the computer on it next what the hell... I'll look at my wiring schematic and see if I can see which ones to jump and see if that lets it run for any length of time. It is weird because we use the truck regularly... we parked it one night next morning came out and now this ... never had a problem before it has been the most dependable truck. It only has 215,000 miles on original engine, never had anything apart on the engine. Holds 40 psi. oil pressure in gear at idle, almost 60 psi. diving down the road even checked it with a manual gauge and it's right.
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The reason it starts is because the computer turns on the fuel pump itself, to build up pressure so it starts easily... so it's probably not the computer. From what you describe, I would bet on the fuel pump fuse mounted on the firewall in the engine compartment. David

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Double checked the fuse it is good, put a meter on and monitored it. That is the fuse that runs the pump by the computer. It has 12.6 v until pressure builds then it turns off. Start engine and it has voltage even after the engine shuts off, but it is not injecting. For what it's worth, we..... Swapped TBI's found no vacuum leaks or any other stuff. Started same thing, swapped computers no change... Right now throwing on the MAP and the whole cluster there... Didn't fix it.... This eliminates almost everything , but possibly the distributor isn't giving the ref. pulse, but the hold down bolt is froze tight into the aluminum manifold or that would have been changed already too..... Going out now to try to loosen up the bolt and get it moving...
Ready to throw on a carb.... I hate carbs and hate fuel injection worse when it doesn't work...
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P).S. not the distributor..... I might try retorqueing the lug nuts.... lol
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What about the relay for the fuel pump? Maybe the relay isn't switching... so the power isn't getting to the pump when the computer releases it? Wish I could remember more about how that all worked. David

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Yes replaced that too.... the list is so long now only one I can think of now is 5 gal of gas and a match... It's nice having a running truck to swap parts from, I know it's going to be something sooooo stupid that I just keep overlooking it. I listened to the pump it sounds like it runs before starting and then doesn't run again until it shuts off... We are hooking up the fuel pressure gauge right now to verify that it loses pressure completely before turning back on. I'm hoping it's not the pump, we just replaced that along with the sender unit, fuel lines etc... about a year ago...

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69SScamaro wrote:

How about jumping a 12 volt line direct to the pump and seeing if it runs then.
--
Steve W.
Near Cooperstown, New York
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My oil pressure switch on my 89 K5 connected or not, has no effect on whether it runs or not. Disconnected, it registers max pressure. Have you tried replacing the oxygen sensor ? Do you have any intake manifold leaks ? TBI base...etc. ?

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