Fuel Pressure Drops Immediately 86
Thread Starter
Racer
Joined: Sep 2008
Posts: 403
Likes: 7
From: Space Coast Florida
Fuel Pressure Drops Immediately 86
Hello C4 Forums,
Like the title says, I've been having rough cold starts, takes 20-40 seconds to turn over then runs fine.
Checked the relay and it's working. Hear the fuel pump engage when key is turned to ON, and with fuel pressure gauge connected to shrader valve I get the correct fuel pressure of around 43 psi before it stops to 0 shortly after the fuel pump is done priming.
I hear no signs of leaks and smell no gas near the engine bay (which is good) but I did just rebuild the fuel rail and do new injectors because I had a gas smell. What or where else should I look for leaks? Or how?
Like the title says, I've been having rough cold starts, takes 20-40 seconds to turn over then runs fine.
Checked the relay and it's working. Hear the fuel pump engage when key is turned to ON, and with fuel pressure gauge connected to shrader valve I get the correct fuel pressure of around 43 psi before it stops to 0 shortly after the fuel pump is done priming.
I hear no signs of leaks and smell no gas near the engine bay (which is good) but I did just rebuild the fuel rail and do new injectors because I had a gas smell. What or where else should I look for leaks? Or how?
Thread Starter
Racer
Joined: Sep 2008
Posts: 403
Likes: 7
From: Space Coast Florida
Racer
Joined: Apr 2016
Posts: 336
Likes: 93
From: San Antonio TX
Mine (89) had similar symptoms recently, turned out to be bad hose connecting the fuel pump to fuel line (replacing the pulsator) inside the tank. If you have a FSM, go through the diagnostic chart (involves clamping of supply and return lines and observing fuel pressure). That will steer you in the right direction.
Racer
Joined: Aug 2021
Posts: 409
Likes: 82
From: NW Arkansas
I agree with NavAir, if it is bleeding off to 0 in a very short amount of time it is likely something with the pump. There's a check valve inside the pump, or it could be a bad connection.
Turn the key to on to pressurize the fuel system. As soon as you hear the fuel pump stop priming, clamp the feed line at the tank. Then check your gauge. If the pressure is holding, the problem has something to do with the pump. This is easier to do with another person, as you have to be kind of quick to clamp the line right after the pump shuts off.
If the symptoms persist, you can try clamping the return line (this time before you prime the system), which will test for a pressure regulator problem.
Turn the key to on to pressurize the fuel system. As soon as you hear the fuel pump stop priming, clamp the feed line at the tank. Then check your gauge. If the pressure is holding, the problem has something to do with the pump. This is easier to do with another person, as you have to be kind of quick to clamp the line right after the pump shuts off.
If the symptoms persist, you can try clamping the return line (this time before you prime the system), which will test for a pressure regulator problem.
Thread Starter
Racer
Joined: Sep 2008
Posts: 403
Likes: 7
From: Space Coast Florida
Dumb question, I installed a braided sending line from JEGS that uses AN Fittings and an adapter. I know there was not an oring on that adapter because of the flared end and JEGS told me that's fine, but could this be the issue? Adapters on filter and fuel rail sides.
I lose pressure almost immediately and this was not like this before replacing the line.
I lose pressure almost immediately and this was not like this before replacing the line.
Racer
Joined: Sep 2021
Posts: 360
Likes: 59
I've thought of a method that should tell you if it is the pump or the regulator. you could blow air with a bycicle pump through the pressure line and see after the line pressurises, how quickly does it go to 0. I assume the pump has an anti drain back valve and it is likely it is leaking imo, if you said the regulator seems fine. Other than that, a leak anywhere else, like the injector or the line, you would smell the gasoline or would flood the cylinder, so it is less likely.
Thread Starter
Racer
Joined: Sep 2008
Posts: 403
Likes: 7
From: Space Coast Florida
I've thought of a method that should tell you if it is the pump or the regulator. you could blow air with a bycicle pump through the pressure line and see after the line pressurises, how quickly does it go to 0. I assume the pump has an anti drain back valve and it is likely it is leaking imo, if you said the regulator seems fine. Other than that, a leak anywhere else, like the injector or the line, you would smell the gasoline or would flood the cylinder, so it is less likely.
I think I found it... Diaphragm was pulled/broken by one screw. I think it wasn't smelly because it was on the underside of the plenum and hard to hear/see any gas signs. It's either that or a cross member o ring.
Ordered the Holley adjustable because this bolt tightening non sense is horrid for a fuel adjustment under the plenum. Fingers crossed, it should help greatly! Will keep updating until solved...
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Racer
Joined: Sep 2021
Posts: 360
Likes: 59
I’ m glad that helped. 
About the Holley regulator, you need to set it up to behave like the stock one especially at high load where Power Enrichment is based on the 3d map and other parameters otherwise it will alter the air fuel ratio. for example if the pressure at high MAP is lower it could run too lean, if it is higher then stock it would run richer. for example if the original psi is 35 psi at high load and you set it at 45psi the fluel flow will increase by sqrt(45/35) and AFR will go from 11.8 :1 to 10.4:1 which is a but too rich
About the Holley regulator, you need to set it up to behave like the stock one especially at high load where Power Enrichment is based on the 3d map and other parameters otherwise it will alter the air fuel ratio. for example if the pressure at high MAP is lower it could run too lean, if it is higher then stock it would run richer. for example if the original psi is 35 psi at high load and you set it at 45psi the fluel flow will increase by sqrt(45/35) and AFR will go from 11.8 :1 to 10.4:1 which is a but too rich
Thread Starter
Racer
Joined: Sep 2008
Posts: 403
Likes: 7
From: Space Coast Florida
Any suggestions then for setting the variable regulator? I had the South Bay version previously and I just sent to what I saw the recommended pressure was, believe 42.5psi for the Boshe D3, 22lb injectors.
Racer
Joined: Sep 2021
Posts: 360
Likes: 59
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
Set the pressure at 43 PSI with the vacuum line disconnected and the car idling. AT WOT there is no vacuum so the pressure will be set at 43 PSI and then when you close the throttle for cruising and idle the vacuum will reduce the pressure to keep the fuel pressure across the injectors the same with the higher intake manifold vacuum (Pressure differential) . That is why the regulator is vacuum referenced.
Racer
Joined: Sep 2021
Posts: 360
Likes: 59
Sorry, I got it backwards; thanks bjankuski for the clarification
So it is high pressure (43) with no vacuum aplied; that makes sense, to have higher pressure at WOT.
This is the important pressure to have it accurately set, with the vacuum disconnected. Then at idle with the vacuum line connected, it should go 30ish, small deviations from what the computer is calibrated at can be handled by INT/BLM.
21boy - did you fix it?
So it is high pressure (43) with no vacuum aplied; that makes sense, to have higher pressure at WOT.
This is the important pressure to have it accurately set, with the vacuum disconnected. Then at idle with the vacuum line connected, it should go 30ish, small deviations from what the computer is calibrated at can be handled by INT/BLM.
21boy - did you fix it?