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Discussion Starter #1
Hey guys, I have a cold start issue with my '78. I am losing fuel pressure after the car sits. When I turn the ignition to the "on" position, my fuel pump continues to run rather than building pressure and shutting off. I can hear to the fuel flowing past the regulator and back into the return line so I am guessing that the check valve is stuck. If so, where exactly is it located? If not, what else could cause this? I checked to make sure it wasn't bleeding past an injector. They all are new and are staying closed. Same goes for the cold start injector. Any ideas?

Thanks,

Jeff
 

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The pump turning on/off is not a function of fuel pressure. On a '78 there is a timer to run the pump for a few seconds after the key is turned to "ON", the pump also runs in the "START" position, and continues to run after the engine generates oil pressure. This is a common problem that the oil pressure switch has failed, strangly enough they always fail into the "always on" state which seems backwards for part of a saftey system.

Here's some more discussion;
http://zcar.com/forums/read.php?f=1&i=229802&t=229536

ConorP
 

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The check valve is located on the fuel pump outlet.
Your fuel pump should not come on with the key in the on position, only in the start position. My car just went through this a few days ago.
Remove the spade connector from the starter. Turn the key to the on pos. Does the fuel pump run (it should'nt). Now turn key to start pos. Fuel pump should run as long as the key is in the start pos.
Remove the relay box located on the passenger fender well. On the top row (inside to outside) is the fuel pump control relay, fuel pump relay, efi relay.
The f.p. control relay will have a six pin connector with only five wires going to it. Test that relay, per the FSM (factory service manual, if you have one... if not, better get one!). The test info is in the engine fuel section. You will need a digital multimeter and a few alligator clip test leads. Continuinty tests need to be performed on the relay pins with out voltage going to pin 95 (+) and 94 (-). Follow the FSM tests... but on pin 95 to 96, the meter will not beep but will show resistance, which is good. Then you apply 12volts dc to pins 94 and 95 and perform the same test, then ground pin 96 and test again. Look at the flow chart in the fsm, it will explain everything.
The oil pressure switch and alt send a signal to the f.p. control relay. It is a safety circuit so when the car stalls, accident, ect the relay will sense no oil pressure, or alt output and shut off the fuel pump.
Check all grounds too along with the oil pressure switch connections.
 

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I agree with the others. The continuous operation of the fuel pump is not a check valve problem, as operation of the pump is not determined by fuel pressure, but by a timer. Unless you have a fuel pressure gauge that has been added to your supply line after the filter and have verified that the pressure drops after the pump quits running, then it is unlikely that you can accurately diagnose a problem with the check valve. I did recently repair my check valve, however, due to a drop in fuel pressure on my '81 ZXT after the pump finished operation. You can find some information I posted about this procedure a week or so ago by a search utilizing my name as "author". I do believe that you have other problems, though.
 

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Discussion Starter #5
Thanks for the help guys. As soon as I get some time to work on it, I'll follow your advise. BTW, yes, I do have the FSM :) I looked in there first, but it didn't really explain when the pump should run and how it gets its signal. The reason I thought it was a pressure drop problem is that the engine takes a bit of work to get running after it sits. I just got the car running again after a few years of it being a garage queen. It runs good, but I have to give it wide open throttle to start. I figured that either the cold start injector is bad (I replaced all the regular injectors), or pressure is bleeding off when not running. Then, yesterday for the first time the pump continued to run in the on position.

Soon to be on the road...

Jeff
 

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There is NO timer on a 78. Other than the fact that it does not have an afm switch (replaced by the alternator and oil pressure) it operates much like the 75-77 models. I believe the confusion may be coming from the later models (and turbos) where the pump control modulator is involved (82-83 and possibly 81 models). If your 75-78 fuel pump comes on with the key 'on' and not cranking then there is a defect. I would be looking for a stuck relay or oil pressure switch. If you are having charging problems, a bad alternator might also be involved, as would if it had recently been replaced with a 'reman' alternator. Sorry I can't be more specific. Just moved (again) and manuals all packed away.
 

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Discussion Starter #7
Getting Closer (please continue help)

BINGO! JohnH240, I had JUST replaced the alternator with a reman unit. Okay, now let me go back over all the advise I have gotten here and fill the crowd in on the details again. My engine had not been run in a few years until just a week or so ago when I put it back together and fired it up. It DID NOT have this fuel pump running on problem in the "on" position, but it was hard to start. I had to start it with an open throttle which is not normal for a F.I. car. Once running, it was fine. I do have good oil pressure on the gauge, but it was NOT CHARGING. The charge light was bright and voltage was at 12V. It has been years since this engine had run, so I went to my parts stash in the basement and grabbed another alternator. It said "REMAN - INTERNAL REGULATOR" right on it, so I figured I'd try it. I installed it and turned the key to "on" Now this is when the pump kept running and I could hear the fuel flowing near the regulator and back into the return line. I fired it up which again took full throttle, and it ran well, but still failed to charge. The charge light was very dim and the voltage was right around 12V. When I would rev the engine it would sometimes begin to charge for a second or two. The voltage would rise and the light would go out.

Now, what some of you have said is that the pump runs based on the oil pressure switch, but I have good oil pressure on the gauge. The sender appears to be in good working order. JohnH240 then said the '78 does not use a timer and others said it does. It sounds like part of this problem is my alternator. Maybe the problem is in the wiring and my original alternator is actually fine? Does any of this narrow the possibilities down guys? I know now that I'm chasing multiple problems. One is the hard starting, and the other is the charging problem. While one or both of these is contributing to the pump run-on issue.

Again, thanks for all the help!

Jeff
 

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Re: Getting Closer (please continue help)

Take both alts to a parts store and have them checked. Also check the condition of the little plug that goes into the alt. Give the f.p. control relay a small tap, could be stuck. I think that was the problem I had this week. I believe that if the connector to the oil pressure sending unit is not hooked up the fuel pump will run in the on position... may want to test that and see it if happens. If so, check that connection, may not be making a good contact.
 

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Re: Getting Closer (please continue help)

As for the oil pressure sending unit, the gage reading is pretty much irrelevent to the switch part for the fuel pump. Inside the sender there are two functions. One for the gage (a variable resistor) and one a switch to ground. The switch should be closed when there is no oil pressure and opens when pressure reaches a certain level. Sooooo....just sitting there with engine off, you should be able to read with a multimeter, continuity to engine/chassis ground through one of the tabs on the sending unit. This is why you can disconnect the sender and make the pump run with the key 'on'. However on all that I have seen, you either have to disconnect battery to kick the pump off, or reconnect the sender and cycle the key on and off to kill the pump. It's sort of a whacky system with the relays, sender switch and routing through the alternator (provides another ground that disappears as alternator spins up) but when correct it works well for safety. Actually since they did (on later models) design the system to pressurize for a for a few seconds prior to cranking, I think they found that the f/i lines lost too much pressure during extended sitting periods, and may have contributed to starting problems, hot and cold. I know my Jeep pump runs like that for 3 or 4 seconds, before every start, to pump up the lines. I believe there is a way to install a spring loaded switch to actuate manually for a short time to 'simulate' this condition, just haven't looked into it recently. Have 1 and possibly 2, 78 project cars coming this winter so may look into doing that.
 

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Discussion Starter #11
Thanks guys!

Thanks for all the help. As soon as this holiday weeekend is over, I'll test everything out per your recommendations.

Jeff
 
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