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
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...
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.
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.
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
From what you describe, I would bet on the fuel pump fuse mounted on the
firewall in the engine compartment.
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...
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.
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
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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