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Fuel Issues

BigDummy89K5

BigDummy89K5

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Abbottsford, B.C.
Where do I start with this! Well I suppose, the very beginning. One day my '89 K5 just didn't start. Everything pointed toward the fuel pump, so I changed that and things worked well for a day. It wasn't the fuel pump because I am right back where I started. This is where it gets funny, or should I say confusing. If I hook power directly up to the fuel pump, it starts but this is not safe to go on like this everyday:doah: While the truck is running and wires get hooked up normally it keeps running fine. now if I shut the truck off and try it 3 seconds later, it starts back up.:confused: But if I wait a minute it won't start again. I have replaced the fuel pump relay and it is still the same. The ECM show no codes. So boys, please help the dummy get back on the road. Thank you.
 
Its prolly either your distributor module or magenetic puickup coil. If your ECM sees no/weak spark siginal it shuts off the fuel. Also check th wireing to the distributor and grounds.
 
x2 on the ignition module, just about any auto parts store can check the module for you. remove it and take it in.
 
Not to take this one off-track, but the oil pressure switch is SUPPOSED to keep this from happening, or so we think.

There was one other possibility in how that circuit operates, but it was never tested.
 
OK, Stan is a good friend of mine and we've been trying to figure this out over the phone. When it starts (jumped power to fuel pump) it runs normally. No codes, no indication there is any ignition problem or anything weird.

If the fuel pump is hooked to the correct tan/white wire it will not start. Using a test light he sees 12 volt at the tan/white for maybe 2 or 3 seconds when he turns the key on. If he jumps 12 volts hot to the fuel pump it will start and run normally. As an experiment he pulled the jumper off and switched to the tan/white while it was running and it stayed running then.

If he shuts the truck off and restarts within 5 seconds with the tan/white wire hooked up it will restart normally. Anything beyond 5 seconds and it will not restart with the tan/white hooked up.

My theory is the oil pressure switch is not providing the correct data to the ECM...the ECM sees a no run condition and kills power to fuel pump. During cranking I don't think the oil pressure switch is showing a high enough oil pressure reading to convince the ECM to continue with power to the fuel pump. It would seem to me that the oil pressure switch is part of a safety system. Blow a fuel line (or bad accident) and the motor will stall, and with no oil pressure the ECM cuts power to the fuel pump preventing an inferno. If the oil pressure switch is out of it's normal range, it would seem logical that the ECM would cut power.

Oh yeah, the fuel pump relay got replaced yesterday with no change in the problem.

Rene
 
Seems I'm not the only thinking along those lines Dorian, and it appears nobody ever definitively tested the OP switch as was mentioned at the end of that thread.

A few people have mentioned ignition module, and I can see how if it was fubar the ECM would not see spark and cut power to the fuel pump after the two second prime. Would the truck still run good with power jumped at the fuel pump though?

http://coloradok5.com/forums/showthread.php?t=168499

Rene
 
Ignition is handled through the module for cranking, ECM takes over after (IIRC) 400RPM.

It's possible the ECM isn't "seeing" the spark pulse if the module is shot, however, you can test that with a noid light on the injector connector. If the light doesn't blink while cranking, the ECM isn't grounding the lead, which means it PROBABLY isn't getting the signal from the module. Since it will run, I'd suspect the ECM is still good. Never played with TBI, but I assume a fuel spray could be viewed if the injectors are firing instead of a noid light.

Looking at the wiring diagram again, you could easily bypass the oil pressure switch by jumpering the two wires. Still shouldn't matter IF the fuel pump relay is working, and it's getting the signal to run from the ECM.

That 5 second time could simply be residual fuel pressure helping the starting process.
 
Well, I just talked to Stan and he is going to try and jumper the OP switch when he gets home from work.

The 5 second deal could be residual fuel pressure helping it start...however the question is why does it continue to have 12 volts hooked up to tan/white in that situation only, when other than that the tan/white gets two seconds of power and then nothing.

At any rate Stan will either eliminate the OP switch as a problem or confirm it this afternoon. I haven't seen exactly where the OP switch is, but Stan mentioned it looks like the distributor would have to come out to change it (if it is the problem) :doah:

Where exactly is the ignition module on a TBI set-up? I'm used to the old school HEI motors with the iggy module in the cap assembly...

Rene
 
Ignition module is still in the cap.

If its still got 12V on the tan/wht, it's possible there is +5 psi oil pressure for a few seconds, or the pressure switch is sticking.

Make sure he looks over the connectors (and just do a very good visual inspection) VERY well, I know the fuel pump relay wiring tends to come apart right at the connector.
 
Thanks guys for all of your responses. I was talking to Rene today about trying a jumper wire on the pressure switch. Not thinking that I would get any results, but lo and behold she started. So the pressure switch must be done or I have no oil pressure.:crazy: I will let you know how it turns out. And again, thanks.
 
Well, there goes the theory that the oil pressure switch doesn't really do anything Dorian... :p:

Rene
 
I know it does something, but it STILL could be that the fuel pump side of the wiring (not the oil pressure switch) could have problems.

That does bring up more questions though. I always let the pump prime before I roll the key to "start" so I don't have that additional cranking time with no fuel. It's possible the 2 second pump prime (handled by the ECM no less, and works in Stan's case, 12V for 2-3 seconds on the tan/wht wire) CAN cover up the 5PSI oil issue...

Not too sure how to test all of this, don't have time to look at the wiring manual. :(
 
Well, all that remains in my mind is to replace the oil pressure switch and see if acts normally again. :dunno:

Rene
 
Yeah, if it acts normally, who cares? :)

Even at least some of the GM manuals state that prolonged cranking is the result of the oil pressure switch being relied on instead of the relay. I just don't know how this can all be put together to make sense. Oil pressure switch is normally open, it takes 5 PSI for it to close, which is not instantaneous, how most EFI vehicles typically start.

I'd be real surprised on a "cold" engine if the fuel pump prime alone is enough to start the engine and get it up to 5PSI when the switch would turn the pump on. *confused*
 
Cranking speed should create enough oil pressure to close the switch though. My pet theory is this is GM's version of the inertia switch Ford used as a safety device. If the truck is in an accident and a fuel line is ruptured or broken you'd want the fuel pump to shut down as fast as possible. Fuel line breaks, motor dies, oil pressure is gone, power is cut to the fuel pump.

Even a tired sbc can generate more than 5 psi while cranking.

Rene
 
Cranking speed DOES provide enough pressure, that's why the switch closes at such a low oil pressure. But the prolonged cranking (if present) to generate 5psi is a telltale sign that the fuel pump relay (or relay circuit) isn't working right. At least according to GM.

As a for instance, I set my TPI to fire after only 4 distributor reference pulses, while stock was something like 8. At normal cranking speed, there is at most a split second where the engine is cranking but not running. Definitely no time to generate 5PSI. (good or bad :)) It is however, noticeable over "stock" cranking time.

All this takes to prove one way or another is to get a couple (or few) people with EFI (TBI in particular) to simply unplug the oil pressure switch while the engine is running and see if it keeps going. (or disconnect it, jump the terminals, start it, and then pull the jumper) Stan's truck doesn't count, it wasn't working right to start with. Volunteers? :)
 
Well, a little mini update. Stan starts the truck with the OP switch jumped...no problems. If he unplugs it while it's running it stays running. If he does not jump the switch it won't start. So, bad oil pressure switch which must have something to do with the starting circuit...but the ECM does not kill power to the fuel pump based on anything meaningful from it. Obviously the ECM sees ignition stuff going on and maintains power to the fuel pemp after it's started.

Anyways, Stan is going to change the oil pressure switch tomorrow i think.

Rene
 
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