I've bought a 1987 Z28, sometimes when I turn the key on I don't hear the fuel pump run and it won't start??? Then later I go back to it and turn the key on and I hear the fuel pump run and it starts right up, I'll go drive it for a hour and no signs if a bad fuel pump. I've switched the relays and that don't seem to matter. Anyone have any ideas?

 

Check connector C303. It tends to oxidize and cause just this: intermittent fuel pump issues.

Jack up the rear end and put it on jackstands. Get under, reach in above the pumpkin, there is a connector on the vertical wall, in the center. It has three wires. Unplug the connector and clean it well. Plug it back in. Turn the key and listen for the pump.

If that doesn't help, it could be other things, like a loose wire or such. In the worst case, it could be even the fuel pump, yes.

Lou

 

Thanks, I'll check that out. One thing that is odd, most the time it wont run is in the morning and after 9:30 it will run.

 

Check the oil pressure sending unit - it's either on the china wall just to the driver side of the distributor, or it's just above the oil filter. They are about $30 to replace.

The ECM only sends current (through the relay) to the pump to prime it. The OPSU sends the current to the pump for the actual running of the pump after it's primed. It could be that your pump is priming, but not running due to a bad OPSU. Maybe later in the day, warmer temps are allowing more fluid oil to get a better signal to the OPSU? (I'm stabbing in the dark here - but the pump won't run without a good OPSU).

 

What I come up with is the relay is clicking every time, then the pump never primes (and runs for 2 secs.). From what I can understand the ECM is doing it job and the relay is doing its job, so the pump should run for 2 sec to prime, (which it isn't doing) then the OPSU keeps it running. I've cleaned the three wires in the back above the pumpkin as advise by Lou, didn't seem to the be the problem. I had a friend test the Amps pulled by the pump(when it was working(not running) and was pulling more than 10Amps.(by running power from the battery,thru Amp meter to the gray wire that plugs in to pump relay) He says that's to many Amps and the pump needs replaced. Anyone not agree?

 

If the grey wire is pulling amps but the pump won't run, then it's safe to say that the pump is frozen and bad.

Also, I would like to clear up a misunderstanding: the OPSU is a BACKUP circuit to feed the fuel pump in case the ECM fails. The ECM turns on the fuel pump for two seconds if the engine isn't running but once it detects pulses from the distributor, it turns on the fuel pump again. Many people run without the OPSU, it is just a backup system. It was never meant to be a primary power feed for the pump.

Lou

 

Thanks,
The pump was working at the time it was pulling the Amps, what my friend tells me it should only pull like 4 Amp(I think that is what he said) and it's pulling alot more than that and a new fuel pump will take care of my problem.
I don't understand why at about 9:30 am it will go ahead and run the pump and I can drive the car all day?

 

The ECM actually supplies a ground to the relay. The relay is powered at all time.

The ECM also grounds the relay whenever it sees a rpm signal. The oil pressure sending unit is not required to run the car. It is only required in the event that the relay fails.

 

Toehead,
You saying the ECM needs replaced? Why would my fuel pump only work in the daytime? Thanks

 

No need to replace the ECM, it works fine.
Why your fuel pump works only during daytime? Could be that it doesn't "like" cold. Maybe once the sun is up and warms it up, it starts working.

Before you tear into the gas tank, though, I'd check the amps to the fuel pump when it's NOT working. That will positively confirm that it is bad.

The last thing to do is replace an expensive fuel pump (an involved job) if it's just a wiring problem.

Lou

 

Could also be that the relay has corroded connections.


If the car EVER works the ECm should be fine.

 

Lou,
Where I should check it is behind the C303 connector? Gray wire right? I can't get to the fuel pump, you know that. Scott
----------
Toehead,
I've swaped the relay with the one beside it (same part#) and I hear the relay clicking when I turn the key on, I just don't hear the fuel pump run until around 9:30 am. Thanks, Scott

 

The pump works and the car runs good and strong from 9:30 am till around 9:00 in the evening?

 

The time of day, I assume, is just coincidental with the temperatures. But that's just my guess.
Unless you can really say that if you try to start at 9:29 every day and it won't and then it starts once the clock hits 9:30. Then I'd worry about maybe some alarm with a built-in timer that the PO installed. Who knows?

You don't need to mess with the C303 right now. The easiest thing to check would be BEFORE 9:30, when the car won't start. Check the amps to the pump again, like you did before. If you see a lot of amps but the pump doesn't run, that'd be definite proof that it's bad.

Lou

 

Thanks, I'm going to leave it on jack stands and check it in the morning for current at the gray wire behind the C303 when the pump is not running. Will I only have current for 2 seconds after I turn the key to run? Or should I have current all the time at the Gray wire back behind the C303? Thanks, Scott

 

I think you meant to check for voltage on the C303? That would be present only for two seconds after you turn the key on. So it would be hard to check.

As I said before, it would be MUCH easier to do it the way you did it before, from the relay connector. Remove the relay first. No need to turn the key on. Use an ampmeter and put the red lead on the orange wire and the black probe on the tan/white wire. (If you have two orange wires on the connector, find the one that has 12V on it)

If you do want to mess with the C303 anyway, you can disconnect it and run 12V directly to the grey wire and ground to the black wire. If that doesn't turn on the pump, it's definitely bad.

Lou