Technical Tips > Post 84

I think we fixed the problem!

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Jethro:
Hi,

The other day me Vette just cut out on us when  we where driving... would run over but would not start.... hour later away she went, thought you tempremental %$#@#!%^& well went to start me vette (1988 C4 ) the other day.... no go! She cranked no worries but wont fire up. When I turned the ignition I didn't hear the fuel pump prime up and when I looked at fuel pressure gauge that is fitted under hood on fuel rails I had no pressure, even when I was cranking the motor over.  Well as the consensus with me mates was, looks like fuel pump, I decided to replace fuel pump and all looked good, turned ignition got instant fuel pressure 45 PSI Oh YEs! turned the the key and no worries fired up revved no worries So Chubbed up!. Off & On a couple of times no worries.... next day started straight off thought to myself, you beaut, I nailed this problem...... got in drove down the road 5 minutes into it, guess what? yeap no go go juice! pulled over on the side of the road dead as.........., tried starting her, no go popped the hood,  no pressure on fuel gauge that is fitted to fuel rails...... even cranked her and checked fuel pressure was non existent gave it another couple of minutes...... cranked no go...... another couple of minutes, turned the ignition on and  the fuel gauge popped up to 45 PSI on gauge! what the hell is going on?
Thanks
P.S anyone know where Fuel Relay is and what I'd be looking for?

Think we've found the issue! Well, fingers crossed anyways………

Well, eventually after replacing the fuel pump as previously stated then checking and isolating relays, wiring, ECU yadda, yadda, yadda  we were scratching our heads.... Found some interesting notes on the net about C4's and internal fuel wiring harness breaking down inside the tank....
“In-tank wiring is quickly becoming a major problem due to the age of the C4's. Many terminals are heavily corroded or rusting off. 
The generic in-tank wiring adapters supplied with other installation kits rely on the questionable factory wiring and adds another connection to the equation. This translates to more resistance / voltage loss and an extra potential failure point”.
http://www.lmspeed.com/index.asp?PageAction=VIEWPROD&ProdID=19
Came up with the theory, it's gotta be in the tank? We stripped the sender unit back out of tank and payed closure attention to the wiring harness from the top of fuel lid to pump. Looked sh#t and brittle and under close inspection in places it looked a bit melted? Anyway, decided to replace all the wiring harness from external “L” connector just outside the filler tank plate down to fuel pump and guess what? Vette is purring once again and even the miss / hesitation that I mentioned in overdrive. < hesitates under load especially in top gear and the hunting when hot> has lessened dramatically! I’m gonna drive her for a while and if this stops the intermittent fault eventually replacing the rest of the wiring on fuel system just as outline in “LMSpeed” foot note. Lets hope its fixed FINGERS CROSSED! Keep ya’s posted! P.S. four days later and 600klms still running fine !






;):ado

Cameron 77C3:
From the few I have worked on for some of the lads, there are two ways that the fuel pump is held on.

One is the start relay which is controlled by the computer.  It turns the fuel pump on for initial start up.  After a minute or so it drops out and the fuel pump is kept on by an oil pressure switch.  One of two located near the dissy.  If no oil pressure it shuts the fuel pump down stopping the motor from running.  By the same token if said oil pressure switch is faulty the motor will shut down.  But this is not in operation during start up.  The lock in relay does that.

The lock in relay, I have found them in a couple of locations depending on who did the conversion.  Some in the engine bay and some behind the dash.  If it's one of the ones behind the dash.....good luck.  LOL!!

One way to help pin point the problem is to temporarily by-pass the oil pressure switch that locks in the fuel pump.  You will know which one it is by disconnecting the wires of one of the oil pressure switches, and if you can get the motor running, see if the motor stops or the oil pressure gauge does not read.  This will tell you which one is which.

With the correct oil pressure switch by-passed you should be able to run the motor.  Mind you your fuel pump will not turn off when you turn off the motor so this is not to be left connected.

I'm suspecting a dickey fuel pump start relay or associated wiring connection.

This is only what I have found from experience and not to be taken as gospel.

Hope it helps.

PS there is a diagnostic chart that can help you find the problem.  This chart gives you a step by step, (flow chart), of how to pinpoint where the problem is.  You will need to purchase a full GM workshop manual, couple of hundred dollars, or find some one who has one who will copy the page and send it to you.

Cam

Cameron 77C3:
Ya gotta love Google. ;41

You may want to check this thread out on corvette forum:-  http://forums.corvetteforum.com/showthread.php?t=1940104

I grabbed some of the information off that site and have stored it on our server for safe keeping



Code 54 is fuel pump circuit. Most likely the relay. Its mounted on firewall next to wiper motor.






Click The Adobe logo below for a PDF printable version of the above circuit diagram and flow chart

Jethro:
Excellent,

Sounds the go I'll have to root around with it later in the week cause she's got the old Blood Pressure up right now! let you know what I find. Thanks Heaps for the other post and circuit you found on Google...
Regards, Jethro!

87Z51:
Switch is intended to keep pump running if relay drops out , not as oil pressure safety device.
At 5 lb setting engine would be dead before switch drops out.

Check connection on pump relay.Common problem for insulation to go bad as well as  bad or intermitant connections.

Jumper orange to tan / white at relay plug and pump should run.Easier to get to than oil switch behind dist.

Put a volt meter on the green / white to earth and see if it gets 12V when the key is turned on.(confirms prime signal from computer )

As a note if relay is not operating  , in a car with a decent battery, cranking should bring the oil pressure up enough to close oil pressure switch and operate pump.

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