Fuel Pressure Woes

chucklehead · 11-01-05, 04:00 PM

My 85 has been stumbling over 2500 rpms and is slow to start when warm. All ignition components are updated in last 6months. Fuel pump replaced @ a year ago. Decided to start by changing fuel filter today and checked my fuel pressure today. Bad news = No pressure at Ignition ON and only 25 psi at idle. Could not reach 30 psi even upon revving to 3000 rpm.

Bad fuel Pump? Bad pressure regulator? Any HELP would be greatly appreciated. Also, O2 sensor and fuel pump relay have been replaced in the past month.
Thanks in advance.
Chucklehead

Robertwav1 · 11-02-05, 07:20 AM

Gee...well, I'm pulling out the injectors this weekend and sending them to TPIS. The warm start is an indicator that yours are leaking also. Might be worth checking out. I determined mine by starting the car warm within 10 seconds of shutting it down....it starts immeadiately. If it sits for 5 to 15 minutes it takes 3 to 4 seconds to start. Don't give the injectors a chance to flood the engine and see how it acts. They told me deposits form and have to have them cleaned ultrasonically...the techron thing is not enough. Good Luck!

MBDiagMan · 11-02-05, 08:46 AM

chucklehead,

You need to start by connecting your pressure guage and cranking the engine for at least a few seconds then turn it off. At that point, the pressure on the guage should hold for at least a few minutes, but usually for hours. If it does not then you have a leak somewhere like a fuel pump check valve, or the regulator. Possibly even an injector.

If the pressure held okay then do a fuel flow test. To do this disconnect the fuel filter outlet and use a hose to route the fuel into a container, crank the engine for fifteen seconds. At that point you should have a pint or more of fuel in the container. If not, remove the fuel filter and do the volume test again. If this time you get the pint you are looking for, replace the filter.
Actually the filter is probably the most common cause of what you describe.

If the fuel flow was adequate and everything else is okay, the problem is most likely the fuel pressure regulator.

Hope this helps,

chucklehead · 11-02-05, 08:55 AM

Quote:

Originally Posted by Robertwav1

Gee...well, I'm pulling out the injectors this weekend and sending them to TPIS. The warm start is an indicator that yours are leaking also. Might be worth checking out. I determined mine by starting the car warm within 10 seconds of shutting it down....it starts immeadiately. If it sits for 5 to 15 minutes it takes 3 to 4 seconds to start. Don't give the injectors a chance to flood the engine and see how it acts. They told me deposits form and have to have them cleaned ultrasonically...the techron thing is not enough. Good Luck!

Thanks, Robertway. I've definitely have something leakin' and the injectors are a likely culprit.
Chucklehead

chucklehead · 11-02-05, 09:04 AM

Thanks, Doc.
I've got around 20 psi on cranking and @25 at idle. When I shut it down, the pressure is gone in less than a minute.
The good news is that it idles and runs well at low rpms. Get it over 2500 rpms and she's stutterin'.
Chuckhead

Vettelt193 · 11-02-05, 09:16 AM

Quote:

Originally Posted by chucklehead

Thanks, Doc.
I've got around 20 psi on cranking and @25 at idle. When I shut it down, the pressure is gone in less than a minute.
The good news is that it idles and runs well at low rpms. Get it over 2500 rpms and she's stutterin'.
Chuckhead

if you disconnect the vac. line from the fuel pressure regulator, what does it read?

This sounds like a blockage in the line or bad fuel pump to me. Leaking injectors usually lead to hard starting, but they usually don't affect high RPM performance at all.

Because your pressure goes down so quickly, and is so low, I am betting it is a bad fuel pump

MBDiagMan · 11-02-05, 12:01 PM

How long ago has it been since the fuel filter was changed?

Again, since the pressure is leaking off, the problem is either the fuel pressure regulator, fuel pressure check valve or an injector. It's pretty easy to check and see if the regulator diaphragm looks okay.

Have a great day,
Doc

RWDsmoke · 11-02-05, 12:49 PM

I was having fuel problems on my 86. Finally found a bad ground was dropping voltage to the fuel pump under load. Showed 12v no load but under 7 volts the pump on. I ran a new ground wire from the engine to the tank and fuel pump. Fixed the fuel pressure problem and now my fuel gage is more accurate.

chucklehead · 11-02-05, 09:40 PM

Doc,
Changed the fuel filter yesterday. One of those days with one of the fittings on the fuel line not cooperating and twenty minutes of cool gas running down my arm.
Thanks for your help.
Chucklehead

chucklehead · 11-02-05, 09:45 PM

Thanks, RWD Smoke.
Good possibility. I'll trace the wiring and check the voltage. Gotta love these hot rods. One day it's running great, next day it's not!
Thanks,
Chucklehead

chucklehead · 11-03-05, 08:42 AM

Vettelt193,
Thanks for your help.
Chucklehead

chucklehead · 11-09-05, 10:17 PM

Quote:

Originally Posted by RWDsmoke

I was having fuel problems on my 86. Finally found a bad ground was dropping voltage to the fuel pump under load. Showed 12v no load but under 7 volts the pump on. I ran a new ground wire from the engine to the tank and fuel pump. Fixed the fuel pressure problem and now my fuel gage is more accurate.

Hey RWDsmoke,
I was getting ready to order a new fuel pump when I decided to do a little research before i pulled the pump. I found the pump ground wire soldered to the top of the sending unit covered with RUST. I figured, "What the HELL", I 'll cut the ground and reset it under one of the bolts with a nice clean contact. Took it for spin on the highway and it ran great through 4500 rpms. As this thing has been a PITA for the past month, I did not want to get too Cocky. Took her for another spin this morning and NO HINT OF A STUTTER!
Just stomped it and the ole L98 took off like a bat out of hell.
Thanks man. Looks like I got LUCKY with your help.
CHUCKLEHEAD

MBDiagMan · 11-10-05, 09:15 AM

Quote:

Originally Posted by chucklehead

Hey RWDsmoke,
I was getting ready to order a new fuel pump when I decided to do a little research before i pulled the pump. I found the pump ground wire soldered to the top of the sending unit covered with RUST. I figured, "What the HELL", I 'll cut the ground and reset it under one of the bolts with a nice clean contact. Took it for spin on the highway and it ran great through 4500 rpms. As this thing has been a PITA for the past month, I did not want to get too Cocky. Took her for another spin this morning and NO HINT OF A STUTTER!
Just stomped it and the ole L98 took off like a bat out of hell.
Thanks man. Looks like I got LUCKY with your help.
CHUCKLEHEAD

Great job!

I just hate seeing people start throwing parts at a problem without proper diagnosis. It is frustrating and expensive. You didn't do that, you diagnosed, inspected and found the problem.

Great work,

chucklehead · 11-10-05, 10:15 AM

Quote:

Originally Posted by MBDiagMan

Great job!

I just hate seeing people start throwing parts at a problem without proper diagnosis. It is frustrating and expensive. You didn't do that, you diagnosed, inspected and found the problem.

Great work,

Without the HELP from CAC members, there's no way that I would have figured out the solution to the problem.
Many Thanks,
Chucklehead