Home page
SPONSOR AD

Support our Vendors!  |  Advertise Here

Topic: Fuel Pressure Question

in Forum: C4 Fuel, Emission Control, and Exhaust Systems


Already a Member?
Login
Not yet a Member?
Register for Free!

Fuel Pressure Question (1/11)
 5/7/14 2:36pm
dvanloon
Standard Member
Send Private Message

Lehigh Acres, FL - USA

Vette(s):
1990 Targa Top 6 speed L98


Joined: 4/14/2014
Posts: 6

My 1990 L98 starts hard, particularly when the engine is cold, runs fine once started. I just connected a fuel pressure gauge and found the following. Zero residual fuel pressure on the fuel system. Turn key to "on" and I get about 8 psi for 2 seconds (indicating the pump turned on) but then it bleeds to zero immediately as the pump shuts off. I wait 10 seconds and repeat the procedure but no pressure is retained in the fuel system. Typically it takes 3 cranking cycles of 5 seconds before the engine will start and stay running. It maintains 38 psi while running. Is this a fuel pressure regulator problem or is it related to the diaphragm beneath the fuel pressure regulator?
Our Sponsors help support C4VR
Re: Fuel Pressure Question (2/11)
 5/7/14 7:54pm
396c4
Standard Member
Send Private Message

Joined: 12/7/2011
Posts: 219

Could be the pressure regulator or the fuel pump check valve. You should be getting a lot more than 8psi on the the key cycle and I wonder if the pump is going away. Unlikely injectors would leak that much, it would be flooded and not start at all. It should hold pressure for quite a long time. To isolate, you can pinch or block the return line to see if it's the regulator. If the pressure holds it's the pump check, if not suspect the pressure regulator.

At idle, failing pump might keep up but not be delivering properly at higher demand. I'd watch it under a road test too. If the vacuum to the regulator is working, the fuel pressure should be stable under varying vacuum loads and you should road test it to confirm. But, first test it with hand pump to see if fuel rail pressure changes with the hand pump vacuum.

When's the last time you changed the fuel pump filter? I've had one blocked so bad, I couldn't blow through it (and a new one  blows like it's a straw). Don't under estimate the importance of a blocked filter, I did once and it cost me a pump.

I assume it's stock because I switched to an adjustable Aeromotive style and it bleeds down within a few seconds too. Bothered me when I first put it in but when I called them, that's normal. These days I use their fuel pump controller so I pressurize via the override before cranking and have set the speed controller ramp so it the pressure regulator isn't working much below 3,000 (when it goes to full speed anyway).


______________
Re: Fuel Pressure Question (3/11)
 5/8/14 2:43pm
dvanloon
Standard Member
Send Private Message

Lehigh Acres, FL - USA

Vette(s):
1990 Targa Top 6 speed L98


Joined: 4/14/2014
Posts: 6

Thanks for the reply. I've only had this 1990 C4 for 2 months. Bought from a guy in another state - don't have his contact info. The car has only 50k miles on it, obviously wasn't driven much. In any event, I just noticed a small bolt on top of the fuel pressure regulator. It appears the original owner installed an adjustable pressure regulator. You mentioned that your adjustable regulator bled down rather quickly.
I tried adjusting the pressure and pinching off the return line at the tank. Pressure still bleeds to zero within a second but now I have 41 psi when running and about 28 psi max during the 2 second crank. Thank would be great if it didn't bleed to zero psi a second later.
I cannot locate a check valve in the fuel system. Fuel is definitely not bleeding into the engine through the injectors.
Re: Fuel Pressure Question (4/11)
 5/8/14 11:00pm
Adams' Apple
Standard Member
Moderator
Send Private Message

Duncanville, TX - USA

Vette(s):
1985 Coupe-L98/Auto,Bright Red/Carmine. 1974 Coupe-L48/4speed, Medium Red Metallic/Black Deluxe.


Joined: 3/18/2009
Posts: 2230

Sure sounds like the regulator has lost it's regulating capability...
If you pinched off the return line, and it still bled down that quickly, I'd have to suspect the regulator.


______________

Joel Adams  
My Link

 
   
(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Re: Fuel Pressure Question (5/11)
 5/9/14 2:37am
davep85c4
Former Member

Send Private Message

Joined: 4/4/2009
Posts: 254

Adams' Apple said:
If you pinched off the return line, and it still bled down that quickly, I'd have to suspect the regulator.

No. Not the regulator. With the return line pinched, you take the regulator out of play. If it still bleeds down with the return line pinched, then it's the check valve in the fuel pump.

I think the OP has a bad fuel pump.

Re: Fuel Pressure Question (6/11)
 5/10/14 5:27pm
dvanloon
Standard Member
Send Private Message

Lehigh Acres, FL - USA

Vette(s):
1990 Targa Top 6 speed L98


Joined: 4/14/2014
Posts: 6

I finally broke down and took the entire fuel pump/sending unit assembly out of the car. I found there is a rubber "insulator" that covers the bottom of the fuel pump and about half of the cylindrical portion of the fuel pump. It has a hole in the bottom where it connects to the fuel inlet filter. My rubber "insulator" was ripped into two parts - the round bottom piece was completely broken away from the cylindrical portion, exposing a portion of the pump.
I've ordered a new fuel pump and will advise if this is the "fix" after installation. This car was stored by the original owner for about 10 years. It appears it may have been stored without gasoline in the tank as there is rust on much of the sending unit assembly. Perhaps the rubber portion deteriorated being dry in storage. 
Re: Fuel Pressure Question (7/11)
 5/11/14 2:57pm
396c4
Standard Member
Send Private Message

Joined: 12/7/2011
Posts: 219

There you go.... The check is probably rusted too and so, not holding. Even more reason to change the filter ( if I were you). The good news is, the tanks (at least the later ones) have a rubber liner so they don't rust inside. I had trouble getting a good gasket seal too since the cover bends a little between the bolts. For quite awhile, there was a faint smell of gas around the filler door and it was a leak at the gasket.

Out of interest, you might look for my post on fuel level sensor, works neat but not a simple swap. Did fix the gasket leak though. I'm now rid of all the GM fuel related stuff except the rails and they're being modified for double feed. Hope yours works out.


______________
Re: Fuel Pressure Question (8/11)
 5/21/14 1:40pm
dvanloon
Standard Member
Send Private Message

Lehigh Acres, FL - USA

Vette(s):
1990 Targa Top 6 speed L98


Joined: 4/14/2014
Posts: 6

Thanks to all who contributed. My basic problem was hard starting, requiring about 3 cranks of 5 seconds each to start. Fuel pressure reached only mid 20 psi when turning key "on" and immediately fell to zero.
Final solution was replacing the fuel pump (and integral check valve). I also discovered the original owner had installed an adjustable fuel pressure regulator. I now get 40+psi fuel pressure when turning on the key. Pressure still bleeds down fairly quickly but this may be due to the adjustable pressure regulator. Bottom line is the hard start is fixed. With 40+ psi fuel pressure, start is instantaneous.
Re: Fuel Pressure Question (9/11)
 5/21/14 9:02pm
davep85c4
Former Member

Send Private Message

Joined: 4/4/2009
Posts: 254

dvanloon said: Pressure still bleeds down fairly quickly but this may be due to the adjustable pressure regulator.

I'm curious. Did you keep the "pulse damper", or eliminate it with a short piece of hose and two clamps? Many replacement pumps come with a short piece of suitable hose.  Hopefully you eliminated it, or it had been eliminated in a previous pump change out. These are notorious for leaking at their connections, and causing loss of pressure similar to "check valve" maladies.

Here's a pic of the pulsation damper.








|UPDATED|5/21/2014 6:02:39 PM (AZT)|/UPDATED|
Re: Fuel Pressure Question (10/11)
 6/4/14 2:28pm
dvanloon
Standard Member
Send Private Message

Lehigh Acres, FL - USA

Vette(s):
1990 Targa Top 6 speed L98


Joined: 4/14/2014
Posts: 6

Yes, I retained the pulsator. It's only a slip connection over the outgoing fuel line and my be the cause of the pressure bleed down. Regardless, the new fuel pump now provides 41 pounds of fuel pressure while craning and instantaneous starts. Thanks for the input, I'll remove it if the problem arises again. It appears the sole purpose of the pulsator is to dampen fuel pulse noise.
Re: Fuel Pressure Question (11/11)
 10/22/14 8:59pm
knnhon
Standard Member
Send Private Message

new poret richey, FL - USA

Vette(s):
1991 Coupe-L-98-auto, 1994 White Conv., l-98 6 spd.


Joined: 10/21/2014
Posts: 13

on my'94, it would hardly run, but would start ok. Chkd fuel press, and it was only 10 psi! Bought a 'top-of-the-line' Carter pump from 'Rock Auto'. In the directions, they SPECIFICALLY state to REMOVE the dampener! . Replacing the pump was easy! no gasket leaks, either. Runs like a champ, now!
Our Sponsors help support C4VR