No Fuel Pressure
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
No Fuel Pressure
Got a problem with my '85 as its been hard starting and missing at idle. It doesnt miss all the time at idle but when it does the rpms drop to 580-600 on the verge of stalling.
Today I pulled the plugs, look good. I just did a tune up last fall so all the ignition stuff is good, checked all the wires too. I hooked up my fuel pressure gauge and got 0 psi when turning the key. The pump can clearly be heard running for a few seconds though. I start it up and the pressure goes up to about 5 psi and thats it. The pump is a new AC Delco unit installed last summer. Its got new injectors, O-rings, and in line filter. How is it still running on only 5 psi??? More importantly what could be wrong?
Today I pulled the plugs, look good. I just did a tune up last fall so all the ignition stuff is good, checked all the wires too. I hooked up my fuel pressure gauge and got 0 psi when turning the key. The pump can clearly be heard running for a few seconds though. I start it up and the pressure goes up to about 5 psi and thats it. The pump is a new AC Delco unit installed last summer. Its got new injectors, O-rings, and in line filter. How is it still running on only 5 psi??? More importantly what could be wrong?
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Though my FP gauge does depress the core in the Schraeder it sounds as though this may not be happening with your gauge?? Have you used this gauge successfully in the past??
You can diagnose fuel system leaks by clamping off one then the other fuel lines and monitoring the effect on the FP readings.
You can diagnose fuel system leaks by clamping off one then the other fuel lines and monitoring the effect on the FP readings.
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
Though my FP gauge does depress the core in the Schraeder it sounds as though this may not be happening with your gauge?? Have you used this gauge successfully in the past??
You can diagnose fuel system leaks by clamping off one then the other fuel lines and monitoring the effect on the FP readings.
You can diagnose fuel system leaks by clamping off one then the other fuel lines and monitoring the effect on the FP readings.
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
Ok I fiddled with the gauge some more and low and behold it works! I now have 15 psi when the key is turned to on and 25 psi while running, giving it gas raises the psi by a few. I'm talking to a friend who thinks its a bad fuel filter like its block it or something.
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
I replaced the fuel filter again, didnt do squat but it was worth a try. I pinched the fuel return line and turned the key on and it did not change the results, still 25 psi. It seems to drop fairly quick after that too. The FSM says its a possible bad fuel pump, but could it also be the fuel pressure regulator? I have a Holley AFPR that I could put on. I took it off because the diagphram broke and they sent me a new one but I never put it back on. When it failed I had fuel coming out of the vacuum port, but there is none coming out now. Also the FSM says that if pressure is above 37 psi ('85) then its the regulator, below 30 psi is a faulty pump, sock, or coupler.
Last edited by Demonic85; May 31, 2008 at 05:02 PM.
Instructor
Joined: Mar 2008
Posts: 111
Likes: 0
From: York PA
I know its not the same year but when I turned the key on my 95 the pressure went up to 35-38 then down to 0 when the pump stopped. It was the check valve in the pump. disconnect the supply line and see if you can blow into it. if you can it might be the pump. Or get someone to turn the key then before the pump stops close off the supply line with a rag and visegrips if the pressure holds then it might be the pump. My car ran good but it was very hard to start. 
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
I took the fuel pump out, nothing wrong with it or anything else in there. The sock still looks fairly new and nothing in it. The pressure holds after I shut the car off. It only started going down when I messed with the return line. My friend thinks the only thing it could be is a bad regulator. I dont want to tear it down if thats not it.
Like I said before my symptoms are hard start and low idle with a miss. Low idle as in 580-600.
Like I said before my symptoms are hard start and low idle with a miss. Low idle as in 580-600.
Race Director
Joined: Sep 2005
Posts: 10,878
Likes: 1
St. Jude Donor '07
Pull the vac line off the FPR. Smell it. Does it smell like fuel? If so, it's bad.
BTW: What are your pressure readings with the vac line pulled vs in place vs at WOT in 2nd gear?
If you decide to replace the regulator diaphram. replace the FPR with an AFPR while you're at it.
BTW: What are your pressure readings with the vac line pulled vs in place vs at WOT in 2nd gear?
If you decide to replace the regulator diaphram. replace the FPR with an AFPR while you're at it.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Team Owner
Joined: Feb 2006
Posts: 24,460
Likes: 7
From: sw Ohio
No fuel comes out of the vacuum port at any rpm, nor does it smell really bad of fuel. The pressure just stays at about 25-27 psi at idle and driving around. I didnt try WOT with the gauge attached though.
Anyway, I fixed the car's idle. I pulled the IAC and cleaned it up. There was carbon on it and the port. I reset it and also cleaned the TB. Fired it up and it has a smoother idle right around 710 rpms. Still has the occasional miss but its been there a while, probably cause its just old or something. I'll leave that for another day.
Anyway, I fixed the car's idle. I pulled the IAC and cleaned it up. There was carbon on it and the port. I reset it and also cleaned the TB. Fired it up and it has a smoother idle right around 710 rpms. Still has the occasional miss but its been there a while, probably cause its just old or something. I'll leave that for another day.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
If FP normally drops off but holds steady when the return line is clamped the FPR is faulty.
To test my fuel delivery system capability I disconnect the vac line from the FPR and block it, tape the FP gauge onto the windshield so that I can read it while driving and then watch the reading as I go from idle to WOT throughout a run through the 1-2 upshift.
When I had problems FP would drop from 48psi to under 40psi as RPM increased to the shift point; when the problem was fixed FP stayed rock steady throught the WOT burst.
BTW, since you had to fiddle with the FP gauge, try removing the valve core from the Schraeder valve. Here a tire valve remover tool works well.
To test my fuel delivery system capability I disconnect the vac line from the FPR and block it, tape the FP gauge onto the windshield so that I can read it while driving and then watch the reading as I go from idle to WOT throughout a run through the 1-2 upshift.
When I had problems FP would drop from 48psi to under 40psi as RPM increased to the shift point; when the problem was fixed FP stayed rock steady throught the WOT burst.
BTW, since you had to fiddle with the FP gauge, try removing the valve core from the Schraeder valve. Here a tire valve remover tool works well.
Last edited by 65Z01; May 31, 2008 at 10:20 PM.