Fuel pressure not holding on a '84?

[Laredo]

Was over at tommyb's place on Sunday putting his '84 back together (swapped over to PE, 35# injectors, intake, etc). All is done, got new TPS set, runs great, normal fuel pressure, but...noticed fuel pressure drops to 0 instantly after engine is shut off, or after the key-on prime. I know on the 85-1/2 - 86 single in-tank pump it's supposed to hold pressure. Is this also the case for the 84-85 with the in-line external fuel pump on the frame rail? I was thinking the hose in his tank is ruptured/rotten and the only reason he has good pressure while engine is running is cuz the external pump is "sucking" fuel rather than having it force fed by the in-tank pump. Correct?

[Kendal Coker]

The high pressure pump on the frame rail will pull fuel through the in tank pusher pump if it is not blocked. There is a rubber hose connecting the tank fuel pump to the line that could be bad. The new gas has been known to mess up the rubber hoses. I would look for a loose connection at the pressure pump and the fuel rails.

[MikeFleming]

Two places for that to leak from on a dual pump system (assuming there's no other large fuel system leaks):

1) FPR. You can gently clamp the return hose and test again.

and

2) The check valve is in the outlet of the HP (chassis-mounted) pump (there is no check valve in the low-pressure, in-tank pump). The hose inside could still be bad though.

And it could be both of the above. Normal spec is to loose no more that 10 PSIG after 20 minute.

It's cheaper to replace both pumps with one in-tank HP pump. DO NOT fail to bypass the in-tank harness ground resistor.

[tommyb]

There do not appear to be any fuel system leaks.

We put on a brand new Adjustable Fuel Pressure Regulator with the job so I am doubting that it is the FPR.

I already have a spare correct new BOSCH external fuel pump in my parts supply so maybe the easiest thing to do is replace that first since that money is already spent. Is there anything wrong with driving the car with a bad check valve, I am assuming no. THANKS!

Tom

[MikeFleming]

Is there anything wrong with driving the car with a bad check valve, I am assuming no. Most common issue is hard starting when hot. Usually longer crank times to build up pressure.

[TheSVOTrust]

I would use the
KOKO
KOKO
KOKO ......then go to cranking the motor !

That way the pump primes the lines and injectors and your motor doesn't crank dry until you get pressure to the injectors.

Mark

[tommyb]

Thanks for more tips will replace the fuel pump when I get a chance and then move on to the next issue!! Whatever it might be.

Tom

[Meotchh]

Sounds like we'll be fixing some issues at Sturgis. That's what we got the garage for. We have a compressor available at the house & between everyone that is going we should have all the tools needed. Let us know what needs to be looked at/ finished so we can prepare ahead of time.

[MikeFleming]

I would use the
KOKO
KOKO
KOKO ......then go to cranking the motor !

That way the pump primes the lines and injectors and your motor doesn't crank dry until you get pressure to the injectors. Since it looses all pressure in a second, this won't do anything useful. As soon as a Key Off is made, it drops back to zero immediately.

[Patrick]

Somebody pack a Mityvac so that we can check the fuel pressure regulator.

[tommyb]

My car will not be at Sturgis as Doug(laredo is using my trailer to bring his) so nothing will be fixed next week on this front. Car starts right away cold but I haven't driven it to warm it up since the discovery to see what happens warm. I will be driving my 2006 Supercharged GT unfortunately.

[Meotchh]

Bummer:(

So you going to enter the autox to get beat by a group of drivers in nearly 30yo pinto powered mustangs?

[Laredo]

My car will not be at Sturgis as Doug(laredo is using my trailer to bring his) so nothing will be fixed next week on this front. .... I will be driving my 2006 Supercharged GT unfortunately.


Bummer:(

So you going to enter the autox to get beat by a group of drivers in nearly 30yo pinto powered mustangs?

The real story....he's not bringing the SVO cuz he says it's too "unreliable" I can't imagine what is he thinking?? ;)
SVOs are VERY reliable (cough, choke, gasp)

[tommyb]

It is the perception of unreliability since I haven't driven it for 2+ years maybe next year! Therefore starting this fixit thread. I know better than to enter the autocross my sticky tires will not make the trip, but the GT does have the Ford Racing Handling Pack upgrade still no SVO.

[MikeFleming]

"Fuel pressure? We don't need no stinkin' fuel pressure!!"

[brsvo]

My 84 does this long crank hot start thing and I was wondering what could cause that. Runs great otherwise and starts right off cold without a problem.

[MikeFleming]

^
Get a pressure gauge on it and check to see if that's your issue.

[brsvo]

The only pressure gauge I have is part of the 86 and I am having too much fun driving that car right now to take it off. Now I have a possible clue though! Thanks

[tommyb]

Fixed, Prize goes to Mike Fleming for the correct diagnosis! Check valve on the frame mounted fuel pump must have gone bad now have a new Bosch fuel pump and Wix fuel filter. Much thanks to Laredo for all the help!

[MikeFleming]

So I win One Internets Prize?? Kewl!!

Glad to hear it's running proper again.

[Meotchh]

Yes, now before you go, let me tell you what you've won...

One complete day's banning at no cost to you.

Tom also receives a banning, until his next post, for giving Mike an even bigger head than he already had.


So I win One Internets Prize?? Kewl!!

Glad to hear it's running proper again.

[MikeFleming]

When does my Free Complete, One-Day Banning start?? I wouldn't want to waste any of that!