Low Fuel Pressure
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From: Pearland Tx
Low Fuel Pressure
I recently purchased an 86 L98 with a 383 with a mild cam and Arizona Speed and Marine 52mm throttle body and after 100 or so miles the idle became very rough or would not idle at all and it runs poor. I checked the fuel pressure and it is at 30 pounds and as soon as the engine is turned off it drops to 0.
The previous owner said he rarely drove it for the last two years which may mean it has been sitting up for some time. It was very low on gas and I have filled it up and replaced the fuel filter and the fuel pump. After this it was still low on fuel pressure. I just installed rebuilt injectors from FCI and while I had it apart I changed the diaphram in the fuel pressure regulator which is adjustable, but adjusting it makes no difference in fuel pressure. The fuel pressure is still at 30 and still goes to 0 when the engine is stopped. Any ideas on what to check next?
Also tried to pull codes from the ECM but the service engine light does not come on then or when the key is first turmed on so there may be a bulb burnt out in the display. The only thing that lights up on the display is the security light when the doors are open. I see a lot of info on repairing the dash cluster but have not found anything about the display area for the service engine light module. Thanks for any help.
The previous owner said he rarely drove it for the last two years which may mean it has been sitting up for some time. It was very low on gas and I have filled it up and replaced the fuel filter and the fuel pump. After this it was still low on fuel pressure. I just installed rebuilt injectors from FCI and while I had it apart I changed the diaphram in the fuel pressure regulator which is adjustable, but adjusting it makes no difference in fuel pressure. The fuel pressure is still at 30 and still goes to 0 when the engine is stopped. Any ideas on what to check next?
Also tried to pull codes from the ECM but the service engine light does not come on then or when the key is first turmed on so there may be a bulb burnt out in the display. The only thing that lights up on the display is the security light when the doors are open. I see a lot of info on repairing the dash cluster but have not found anything about the display area for the service engine light module. Thanks for any help.
Safety Car
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I recently purchased an 86 L98 with a 383 with a mild cam and Arizona Speed and Marine 52mm throttle body and after 100 or so miles the idle became very rough or would not idle at all and it runs poor. I checked the fuel pressure and it is at 30 pounds and as soon as the engine is turned off it drops to 0.
The previous owner said he rarely drove it for the last two years which may mean it has been sitting up for some time. It was very low on gas and I have filled it up and replaced the fuel filter and the fuel pump. After this it was still low on fuel pressure. I just installed rebuilt injectors from FCI and while I had it apart I changed the diaphram in the fuel pressure regulator which is adjustable, but adjusting it makes no difference in fuel pressure. The fuel pressure is still at 30 and still goes to 0 when the engine is stopped. Any ideas on what to check next?
Also tried to pull codes from the ECM but the service engine light does not come on then or when the key is first turmed on so there may be a bulb burnt out in the display. The only thing that lights up on the display is the security light when the doors are open. I see a lot of info on repairing the dash cluster but have not found anything about the display area for the service engine light module. Thanks for any help.
The previous owner said he rarely drove it for the last two years which may mean it has been sitting up for some time. It was very low on gas and I have filled it up and replaced the fuel filter and the fuel pump. After this it was still low on fuel pressure. I just installed rebuilt injectors from FCI and while I had it apart I changed the diaphram in the fuel pressure regulator which is adjustable, but adjusting it makes no difference in fuel pressure. The fuel pressure is still at 30 and still goes to 0 when the engine is stopped. Any ideas on what to check next?
Also tried to pull codes from the ECM but the service engine light does not come on then or when the key is first turmed on so there may be a bulb burnt out in the display. The only thing that lights up on the display is the security light when the doors are open. I see a lot of info on repairing the dash cluster but have not found anything about the display area for the service engine light module. Thanks for any help.
1st, I'd go back to the FPR and pull it apart again and look closely at the seat and plunger. If that has signs of wear on one edge and not symetrical, its leaking fuel directly into the return line and the pressure is lost before it even gets to the rails. They CAN leak internally. The way to test is to pinch off the return and see if pressure spikes. Warning: pressure spikes CAN cause things to let loose...but it proves the point.
IF pinching the return makes more pressure, replace FPR.
Was a new inline filter installed after the pump and sock?
Was there much trash in the tank when you did the pump?
Only getting 30 psi old and new pump tells me the reg is dumping the fuel. The diaphram might be ok, but the seats sealing surface sure isn't.
As far as the SES lite....old trick with a troubled car is to place burnt out bulbs in the SES socket....and sell it. People do that to get past inspections as well. usually does not work but people try anyway. Sometimes they just get tired of looking at the SES lite or the passenger questions so they remove the bulb.
So it might be ON but you;ll never know. Works on some inspectors too. If there is one lite working then the whole display should work. its not been d/c, just disabled. That center info display is nowhere near as fragile as the digital cluster so they just don;t act up as much. Most times its only a bad bulb....how that bulb got bad is the only real question....normal wear or did it have help? Seems convenient (coincidence) that a SES lite would be out when a car has a problem so severe that it can;t idle....
The car should run at 30 lbs IF that is actually getting to the injectors. If the pressure is being dumped off then the inj are getting the residual pressure and very very little volume. Your pressure should be as follows:
key on 40+ lbs
run 40 to 48 lbs
eng rev drop of 5 lbs for no more than 1 second then recover to where the pressure was.
eng stop, a drop of 1-2 lbs and hold for an hour or more. A drop of 5 lbs/hr there after until 6 to 10 hrs have passed and its down to 20+.
A good system will have 20 the next morning.
basically once the system is charged with pretension, unless an injector opens there is no place for the fuel to go. It should hold that tension for hours. The pump has a check valve to prevent fast back flow thru the pump, but that is how and where the fuel should bleed down after hours pass. A very slow bleed down. This is so leaking inj can be identified by a rapid bleed down which would leave wet gas in a cyl.
If you left your 'pulsator' inline on the pump pressure output line, that could be leaking as well.
many folks just replace the thing with some submersable FI hose and forget about it. I have seen pressure gauges that were rock solid on cars without the pulsator...so its value is questionable. The tiny bit of flex in the 3" of fuel hose that replaces the pulsator probably absorbs the minimal pressure spikes in the system..
Bottom line, there just are'nt that many places for fuel pressure to go.
Its either going IN the engine thru a bad reg diaphram (test by checking the vac line for gas)....or thru the inj but you'd know that by the black cloud and smell.
or its not going to the injectors, which means it has to be going back to the tank. Only one way to do that....thru the return.
Or it was never there to start with...filter/pump/strainer obstruction/plugged.
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Joined: Aug 2007
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From: WV
On my adjustable FPR there was a small nickel sized disk that had to be put on top of the diaphram before the top part could be put on the FPR.. If you regulator is suppose to have this disk, and doesn't , it won't allow you to adjust your pressure up to 40-43 pounds pressure.. WW
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From: Pearland Tx
I did change the fuel filter when I replaced the fuel pump and it did not appear to have any trash in the tank, the sock looked to be in pretty good shape but I replaced it with a new one. I removed the 'pulsator' and replaced with a piece of fuel injection hose. It all seems to lead to the fuel pressure regulator even though I replaced the diaphram. I will check the fuel pressure tonight with the return line pinched off and see what happens. The car will start and run but the idle is ragged and the power is not what it first was.
Do you know how to get to the bulbs for the SES light, I assume there is more than one in that display since the security light works but nothing else. The car was inspected a couple of months ago, but in Texas on 25 year old cars, all you get is a walk around inspection. Not hooked up to the machines or anything.
Thanks for taking the time for such an in depth reply, You have given me enough info to trouble shoot and hopefully find the problem.
Calvin
Do you know how to get to the bulbs for the SES light, I assume there is more than one in that display since the security light works but nothing else. The car was inspected a couple of months ago, but in Texas on 25 year old cars, all you get is a walk around inspection. Not hooked up to the machines or anything.
Thanks for taking the time for such an in depth reply, You have given me enough info to trouble shoot and hopefully find the problem.
Calvin
1st, I'd go back to the FPR and pull it apart again and look closely at the seat and plunger. If that has signs of wear on one edge and not symetrical, its leaking fuel directly into the return line and the pressure is lost before it even gets to the rails. They CAN leak internally. The way to test is to pinch off the return and see if pressure spikes. Warning: pressure spikes CAN cause things to let loose...but it proves the point.
IF pinching the return makes more pressure, replace FPR.
Was a new inline filter installed after the pump and sock?
Was there much trash in the tank when you did the pump?
Only getting 30 psi old and new pump tells me the reg is dumping the fuel. The diaphram might be ok, but the seats sealing surface sure isn't.
As far as the SES lite....old trick with a troubled car is to place burnt out bulbs in the SES socket....and sell it. People do that to get past inspections as well. usually does not work but people try anyway. Sometimes they just get tired of looking at the SES lite or the passenger questions so they remove the bulb.
So it might be ON but you;ll never know. Works on some inspectors too. If there is one lite working then the whole display should work. its not been d/c, just disabled. That center info display is nowhere near as fragile as the digital cluster so they just don;t act up as much. Most times its only a bad bulb....how that bulb got bad is the only real question....normal wear or did it have help? Seems convenient (coincidence) that a SES lite would be out when a car has a problem so severe that it can;t idle....
The car should run at 30 lbs IF that is actually getting to the injectors. If the pressure is being dumped off then the inj are getting the residual pressure and very very little volume. Your pressure should be as follows:
key on 40+ lbs
run 40 to 48 lbs
eng rev drop of 5 lbs for no more than 1 second then recover to where the pressure was.
eng stop, a drop of 1-2 lbs and hold for an hour or more. A drop of 5 lbs/hr there after until 6 to 10 hrs have passed and its down to 20+.
A good system will have 20 the next morning.
basically once the system is charged with pretension, unless an injector opens there is no place for the fuel to go. It should hold that tension for hours. The pump has a check valve to prevent fast back flow thru the pump, but that is how and where the fuel should bleed down after hours pass. A very slow bleed down. This is so leaking inj can be identified by a rapid bleed down which would leave wet gas in a cyl.
If you left your 'pulsator' inline on the pump pressure output line, that could be leaking as well.
many folks just replace the thing with some submersable FI hose and forget about it. I have seen pressure gauges that were rock solid on cars without the pulsator...so its value is questionable. The tiny bit of flex in the 3" of fuel hose that replaces the pulsator probably absorbs the minimal pressure spikes in the system..
Bottom line, there just are'nt that many places for fuel pressure to go.
Its either going IN the engine thru a bad reg diaphram (test by checking the vac line for gas)....or thru the inj but you'd know that by the black cloud and smell.
or its not going to the injectors, which means it has to be going back to the tank. Only one way to do that....thru the return.
Or it was never there to start with...filter/pump/strainer obstruction/plugged.
IF pinching the return makes more pressure, replace FPR.
Was a new inline filter installed after the pump and sock?
Was there much trash in the tank when you did the pump?
Only getting 30 psi old and new pump tells me the reg is dumping the fuel. The diaphram might be ok, but the seats sealing surface sure isn't.
As far as the SES lite....old trick with a troubled car is to place burnt out bulbs in the SES socket....and sell it. People do that to get past inspections as well. usually does not work but people try anyway. Sometimes they just get tired of looking at the SES lite or the passenger questions so they remove the bulb.
So it might be ON but you;ll never know. Works on some inspectors too. If there is one lite working then the whole display should work. its not been d/c, just disabled. That center info display is nowhere near as fragile as the digital cluster so they just don;t act up as much. Most times its only a bad bulb....how that bulb got bad is the only real question....normal wear or did it have help? Seems convenient (coincidence) that a SES lite would be out when a car has a problem so severe that it can;t idle....
The car should run at 30 lbs IF that is actually getting to the injectors. If the pressure is being dumped off then the inj are getting the residual pressure and very very little volume. Your pressure should be as follows:
key on 40+ lbs
run 40 to 48 lbs
eng rev drop of 5 lbs for no more than 1 second then recover to where the pressure was.
eng stop, a drop of 1-2 lbs and hold for an hour or more. A drop of 5 lbs/hr there after until 6 to 10 hrs have passed and its down to 20+.
A good system will have 20 the next morning.
basically once the system is charged with pretension, unless an injector opens there is no place for the fuel to go. It should hold that tension for hours. The pump has a check valve to prevent fast back flow thru the pump, but that is how and where the fuel should bleed down after hours pass. A very slow bleed down. This is so leaking inj can be identified by a rapid bleed down which would leave wet gas in a cyl.
If you left your 'pulsator' inline on the pump pressure output line, that could be leaking as well.
many folks just replace the thing with some submersable FI hose and forget about it. I have seen pressure gauges that were rock solid on cars without the pulsator...so its value is questionable. The tiny bit of flex in the 3" of fuel hose that replaces the pulsator probably absorbs the minimal pressure spikes in the system..
Bottom line, there just are'nt that many places for fuel pressure to go.
Its either going IN the engine thru a bad reg diaphram (test by checking the vac line for gas)....or thru the inj but you'd know that by the black cloud and smell.
or its not going to the injectors, which means it has to be going back to the tank. Only one way to do that....thru the return.
Or it was never there to start with...filter/pump/strainer obstruction/plugged.
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Joined: May 2011
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From: Pearland Tx
Yes the disc was there. It must be a problem with the regulator other than the diaphram.
Calvin
Calvin
On my adjustable FPR there was a small nickel sized disk that had to be put on top of the diaphram before the top part could be put on the FPR.. If you regulator is suppose to have this disk, and doesn't , it won't allow you to adjust your pressure up to 40-43 pounds pressure.. WW
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Joined: May 2011
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From: Pearland Tx
Tonight I pinched off the fuel return line and did another fuel pressure test. The results were the same, pressure was 30 lb and immediate drop to 0 when the engine was cut off. I can find no fuel leaks or smell any gas in the engine compartment. The vacuum hose to the fuel regulator is dry. Tomorrow I will measure the fuel pressure at the tank and see what I find. Maybe the pump has a problem.
