Fuel pressure
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Fuel pressure
85 L98
A little history,
my car starts on the first turn sometimes and then the next day or when hot, won't start at all unless you hit the gas. Just hooked up a guage and turned on the key. Car was cold. The fuel pressure went up to about 40 and then dropped like a rock even with the key still on. I thought that the pressure should hold there for awhile even after the key is off. What's this all about? Thanks!
A little history,
my car starts on the first turn sometimes and then the next day or when hot, won't start at all unless you hit the gas. Just hooked up a guage and turned on the key. Car was cold. The fuel pressure went up to about 40 and then dropped like a rock even with the key still on. I thought that the pressure should hold there for awhile even after the key is off. What's this all about? Thanks!
Team Owner
Joined: Jan 2003
Posts: 20,320
Likes: 248
From: Summerland B.C. Canada
Your syptoms sure do point to a leak in the fuel system and probably one or more injectors. The check valve in tank will also be suspect and add to the problem. Here is the info you need to diagnose http://members.shaw.ca/agent86/Fuel%...0Diagnosis.pdf
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Now I'm really confused. I pulled the gas tank book and crimped both hoses on the right as I was told one of them would be the return line. The guage still goes up to 40 and back to zero in a couple seconds no matter which line is crimped. How can this be. One of the lines I crimped would have to be the supply line. How could there ne pressure?
Pro
Joined: Jun 2003
Posts: 559
Likes: 6
From: OKC OK
You haven't mentioned black smoke while driving or any other symptoms of massively leaking injectors...
So...I'm thinking you're on the right track by looking to the check valve....but...
What kinda pressure do you have while the engine is running? Do you have a AFPR? Perhaps the diaphragm in the regulator has a hole or tear?
:
So...I'm thinking you're on the right track by looking to the check valve....but...
What kinda pressure do you have while the engine is running? Do you have a AFPR? Perhaps the diaphragm in the regulator has a hole or tear?
:
Le Mans Master
Joined: Feb 2002
Posts: 7,858
Likes: 2
Originally Posted by AGENT 86
Your syptoms sure do point to a leak in the fuel system and probably one or more injectors. The check valve in tank will also be suspect and add to the problem. Here is the info you need to diagnose http://members.shaw.ca/agent86/Fuel%...0Diagnosis.pdf
Burning Brakes
Joined: Nov 2002
Posts: 1,239
Likes: 1
From: Las Vegas NV
Originally Posted by Joslinm
I can't crimp after it when it gets to 40 because it drops so fast after peaking there.
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Drifting
Joined: Aug 2001
Posts: 1,550
Likes: 27
From: Minneapolis MN
The fuel pressure in my '86 also drops off right away with the key on, but none of the injectors are leaking. With the engine running though the pressure is just fine ... my check valve is bad.
Is there any harm in continuing to use this pump with a bad check valve?
Is there any harm in continuing to use this pump with a bad check valve?
Burning Brakes
Joined: Nov 2002
Posts: 1,239
Likes: 1
From: Las Vegas NV
Originally Posted by FlyboyZR1
The fuel pressure in my '86 also drops off right away with the key on, but none of the injectors are leaking. With the engine running though the pressure is just fine ... my check valve is bad.
Is there any harm in continuing to use this pump with a bad check valve?
Is there any harm in continuing to use this pump with a bad check valve?
Burning Brakes
Joined: Nov 2002
Posts: 1,239
Likes: 1
From: Las Vegas NV
And check the pulsator. The piece between the pump and the sending unit. I have many of these leak on GM cars. If you decide to replace the pulsator with rubber hose, make damn sure it is fuel injection hose ie. high pressure, not carb hose ie. low pressure or it will burst/split in a very short amount of time.
Last edited by crheinish; Apr 30, 2005 at 04:05 PM. Reason: got to work on my typing skills
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
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From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
here's the plug out of cyl #1 awhile back
I"ve always attributed it to a bad valve guide or seal. That's what I've been told. Could a leaking injector cause that?
I"ve always attributed it to a bad valve guide or seal. That's what I've been told. Could a leaking injector cause that?
Race Director
Joined: Jun 2004
Posts: 13,961
Likes: 707
From: WI
"If you decide to replace the pulsator with rubber hose"
I am going to be cleaning in there, the gas gauge isn't working to well. Would it be a good idea to replace the pulsator while I am there? I like just the hose idea. What is a pulsator anyways?
I am going to be cleaning in there, the gas gauge isn't working to well. Would it be a good idea to replace the pulsator while I am there? I like just the hose idea. What is a pulsator anyways?
Le Mans Master
Joined: Feb 2002
Posts: 7,858
Likes: 2
Originally Posted by Aardwolf
"If you decide to replace the pulsator with rubber hose"
I am going to be cleaning in there, the gas gauge isn't working to well. Would it be a good idea to replace the pulsator while I am there? I like just the hose idea. What is a pulsator anyways?
I am going to be cleaning in there, the gas gauge isn't working to well. Would it be a good idea to replace the pulsator while I am there? I like just the hose idea. What is a pulsator anyways?
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Originally Posted by AGENT 86
Your syptoms sure do point to a leak in the fuel system and probably one or more injectors. The check valve in tank will also be suspect and add to the problem. Here is the info you need to diagnose http://members.shaw.ca/agent86/Fuel%...0Diagnosis.pdf
Where is the check valve in the tank? How do I check?
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Originally Posted by MtlSphere
You haven't mentioned black smoke while driving or any other symptoms of massively leaking injectors...
So...I'm thinking you're on the right track by looking to the check valve....but...
What kinda pressure do you have while the engine is running? Do you have a AFPR? Perhaps the diaphragm in the regulator has a hole or tear?:
So...I'm thinking you're on the right track by looking to the check valve....but...
What kinda pressure do you have while the engine is running? Do you have a AFPR? Perhaps the diaphragm in the regulator has a hole or tear?
:I checked the pressure while running this morning. The pressure goes to 40 when the key is tuned on, then goes to 0 after a couple seconds. After car is started, the pressure holds at about 32psi
Racer
Joined: Oct 2004
Posts: 286
Likes: 1
From: Washington DC
The pulsation dampner minimizes the natural "pulsing" of fuel the fuel pump actually produces. This provides for a more consistent, reliable fuel flow and lengthens the life of the fuel system components.
I wouldn't remove it. If it ain't broke, don't fix it. If it is broke, get another one. Some people do replace it with hose...that's an option, but I'm not sure why you'd do that...It can reduce available fuel if it's leaking between the feed pipe and pulsator diaphragm or the pump and pulsator diaphragm. It becomes especially apparent below 1/2 tank.
I wouldn't remove it. If it ain't broke, don't fix it. If it is broke, get another one. Some people do replace it with hose...that's an option, but I'm not sure why you'd do that...It can reduce available fuel if it's leaking between the feed pipe and pulsator diaphragm or the pump and pulsator diaphragm. It becomes especially apparent below 1/2 tank.
Pro
Joined: Jun 2003
Posts: 559
Likes: 6
From: OKC OK
32 is low even for a 85. It should have a stock range between 34-40.
This sounds more and more like either a torn diaphragm or check valve in the tank.
When the engine is running can you crimp or remove the vacuum line from the FPR? Does the pressure go back up to 40? If so then the regulator would seem to be working properly.
Still no black smoke from the exhaust? Check valve seems the most likely source of the problem. Of all the Chevy's I've ever owned...fuel pump replacement in the C4 has got to be the easiest by far.
A note here...throughout the run of the L98 (from 85-91) GM raised the fuel pressure almost every year to a maximum of 47 psi. TPIS recommends 49-52 for the L98.
This sounds more and more like either a torn diaphragm or check valve in the tank.
When the engine is running can you crimp or remove the vacuum line from the FPR? Does the pressure go back up to 40? If so then the regulator would seem to be working properly.
Still no black smoke from the exhaust? Check valve seems the most likely source of the problem. Of all the Chevy's I've ever owned...fuel pump replacement in the C4 has got to be the easiest by far.
A note here...throughout the run of the L98 (from 85-91) GM raised the fuel pressure almost every year to a maximum of 47 psi. TPIS recommends 49-52 for the L98.