Before You Replace Your Fuel Pump...
Thread Starter
3rd Gear
Joined: Jun 2012
Posts: 3
Likes: 0
From: Los Angeles CA
Before You Replace Your Fuel Pump...
Was having a crank/no start situation today and thought I'd share a quick fix.
C4 had been sitting for a few weeks and wouldn't start.
Checked the codes and nothing that would affect the engine running.
Figured it was something mechanical so I checked a plug for spark and present.
Put my ear to the gas tank filler and the fuel pump spin up noise was VERY faint. No gas smell in the tailpipes either.
Figured the pump failed but didn't have a pressure meter to check. In lieu of using a meter I got a rag and released the valve to see the
amount of pressure in the rails. Gas just dribbled into the rag. So I turned the engine over to recharge the line to check it again and it started.
Seems the fuel rails/lines can develop a "vapor lock" even though it's a sealed system.
Never would have thought that have been the solution, but I thought I'd share this with everyone before buying a new pump.
C4 had been sitting for a few weeks and wouldn't start.
Checked the codes and nothing that would affect the engine running.
Figured it was something mechanical so I checked a plug for spark and present.
Put my ear to the gas tank filler and the fuel pump spin up noise was VERY faint. No gas smell in the tailpipes either.
Figured the pump failed but didn't have a pressure meter to check. In lieu of using a meter I got a rag and released the valve to see the
amount of pressure in the rails. Gas just dribbled into the rag. So I turned the engine over to recharge the line to check it again and it started.
Seems the fuel rails/lines can develop a "vapor lock" even though it's a sealed system.
Never would have thought that have been the solution, but I thought I'd share this with everyone before buying a new pump.
Last edited by Jackie Treehorn; Jun 7, 2012 at 01:40 PM.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
I've had to bleed off air in the rails before...
its just like old diesels that when run empty, you had to bleed the air out since they had no way of venting air out and until you did,. the fuel could not advance in the system.
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
The short cut is to prime key on, vent the air at the rail then crank again and it'll fire. Not unusual after being stored or parked for a while.
Its not so much a vapor lock as it is a liquid void, with no place for trapped air to go so the fuel cannot enter the rails..
its just like old diesels that when run empty, you had to bleed the air out since they had no way of venting air out and until you did,. the fuel could not advance in the system.
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
The short cut is to prime key on, vent the air at the rail then crank again and it'll fire. Not unusual after being stored or parked for a while.
Its not so much a vapor lock as it is a liquid void, with no place for trapped air to go so the fuel cannot enter the rails..
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 207
From: Orlando FL
I've always associated vapor lock with lots of heat. I have left my car lots of times in a garage many times for weeks while I was on a trip with no problem. Could you have a different reaction if the car was outside?
A few weeks is not a long time for a car in good shape to sit. Any number of things could affect starting. I'd sure like to know if there was injector pulses from the ECM. Purging the fuel line and having it start may or may not have been a coincidence. Sometimes things just wake up for no reason.
But I'll just leave it with Jackie's theory.
A few weeks is not a long time for a car in good shape to sit. Any number of things could affect starting. I'd sure like to know if there was injector pulses from the ECM. Purging the fuel line and having it start may or may not have been a coincidence. Sometimes things just wake up for no reason.
But I'll just leave it with Jackie's theory.
Thread Starter
3rd Gear
Joined: Jun 2012
Posts: 3
Likes: 0
From: Los Angeles CA
I think your assessment is spot on, but I've run out of gas before and didn't have to do this.
The tank was low when I parked it weeks back, but the reserve light wasn't on.
The car was leaning to the drivers side and may have sucked up some air.
Strange one, but it was definitely the fix.
Also checked the distributor module plug wire and it was full of white corrosion.
Brushed and lubed with dielectric grease and it fires up very sharp compared to before.
Anyway, hope this saves a few peeps some hair follicles.
Particularly on the road.
The tank was low when I parked it weeks back, but the reserve light wasn't on.
The car was leaning to the drivers side and may have sucked up some air.
Strange one, but it was definitely the fix.
Also checked the distributor module plug wire and it was full of white corrosion.
Brushed and lubed with dielectric grease and it fires up very sharp compared to before.
Anyway, hope this saves a few peeps some hair follicles.
Particularly on the road.
I've had to bleed off air in the rails before...
its just like old diesels that when run empty, you had to bleed the air out since they had no way of venting air out and until you did,. the fuel could not advance in the system.
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
The short cut is to prime key on, vent the air at the rail then crank again and it'll fire. Not unusual after being stored or parked for a while.
Its not so much a vapor lock as it is a liquid void, with no place for trapped air to go so the fuel cannot enter the rails..
its just like old diesels that when run empty, you had to bleed the air out since they had no way of venting air out and until you did,. the fuel could not advance in the system.
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
The short cut is to prime key on, vent the air at the rail then crank again and it'll fire. Not unusual after being stored or parked for a while.
Its not so much a vapor lock as it is a liquid void, with no place for trapped air to go so the fuel cannot enter the rails..
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
L98s use a circulating fuel system, so "vapor lock" can't happen. The fuel is pumped up to the fuel rails and then any excess fuel is bypassed by the fuel pressure regulator back to the tank. If you look at the connections on the fuel rails, you'll see that the fuel goes through the rails first before it gets to the fuel pressure regulator.
If you think the fuel rails somehow didn't get pressurized the first time you turned on the ignition (the ECM runs the fuel pump for 2 seconds to pressurize the fuel rails for starting and then shuts off the fuel pump so it won't run continuously if you turn on the ignition but don't start the engine), you can turn off the ignition, wait 10 seconds (for the ECM to go through its shutdown routine (reset IAC and do MAF burnoff)) and then turn the ignition back on.
If you think the fuel rails somehow didn't get pressurized the first time you turned on the ignition (the ECM runs the fuel pump for 2 seconds to pressurize the fuel rails for starting and then shuts off the fuel pump so it won't run continuously if you turn on the ignition but don't start the engine), you can turn off the ignition, wait 10 seconds (for the ECM to go through its shutdown routine (reset IAC and do MAF burnoff)) and then turn the ignition back on.
Burning Brakes
Joined: May 2011
Posts: 838
Likes: 2
From: Louisville Ky
[QUOTE=leesvet;1581006446]I've had to bleed off air in the rails before...
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
the way I read my 91 FSM, if the engine is rotating after the 2 secs prime, the dist is sending that reference signal to the ecm and the FP continues to have current, thru FP relay. does not have to wait for the oil pump switch. I think it also says that IF the FP relay is not in the circuit, the fuel rails were not primed for the 2 secs, and the op switch will have a longer crank time, but the oil pressure is only 4 lbs or so to engage the switch. I agree with Cliff that with the fuel return system it should not vapor lock.
In the C4...since the pump only runs 2 sec to prime, if the engines does NOT fire off then the dist cannot send the required reference signal to the ECM to get more pump run time....then its becomes totally up to the fuel pump oil pressure switch to supply power to the pump...which means more time cranking, and it WILL lite off eventually but you have to crank enough to build oil pressure.. After being parked for several weeks this can be a long long time cranking...
the way I read my 91 FSM, if the engine is rotating after the 2 secs prime, the dist is sending that reference signal to the ecm and the FP continues to have current, thru FP relay. does not have to wait for the oil pump switch. I think it also says that IF the FP relay is not in the circuit, the fuel rails were not primed for the 2 secs, and the op switch will have a longer crank time, but the oil pressure is only 4 lbs or so to engage the switch. I agree with Cliff that with the fuel return system it should not vapor lock.
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Michael S. Palmer
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Furthermore Vapor lock doesn't occur on a recirculating fuel system, such as those used on C4's.
Lastly, Vaprolock doesn't occur on 30+ PSI fuel pressure systems, as those found on C4's.
But even if somehow the fuel rails miraculously became "air locked" it is NOT at all like a diesel as leesvet suggested. Unlike a diesel, it is "self bleeding". Key on, system pumps fuel through the loop and "self bleeds". Any air trapped in the rail (LT1 only) gets "bled" out with the opening of injectors.
Summation of the first post? COINCIDENCE.
Last edited by Tom400CFI; Jun 11, 2012 at 08:53 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
This is 100% correct. The oil pressure switch is only there as a back up in case the fuel pump relay fails while driving.
Le Mans Master
Joined: Aug 2005
Posts: 7,392
Likes: 218
From: Merritt Island, FL
Coincidence. Not reality. "Vapor lock" which comes form the boiling of fuel doesn't happen on a cold engine.
Furthermore Vapor lock doesn't occur on a recirculating fuel system, such as those used on C4's.
Lastly, Vaprolock doesn't occur on 30+ PSI fuel pressure systems, as those found on C4's.
Lastly...even if somehow the fuel rails became "air locked" it is NOT at all like a diesel as leesvet suggested. Unlike a diesel, it is "self bleeding". Key on, system pumps fuel through the loop and "self bleeds". Any air trapped in the rail (LT1 only) gets "bled" out with the opening of injectors.
Summation of the first post? COINCIDENCE.
Furthermore Vapor lock doesn't occur on a recirculating fuel system, such as those used on C4's.
Lastly, Vaprolock doesn't occur on 30+ PSI fuel pressure systems, as those found on C4's.
Lastly...even if somehow the fuel rails became "air locked" it is NOT at all like a diesel as leesvet suggested. Unlike a diesel, it is "self bleeding". Key on, system pumps fuel through the loop and "self bleeds". Any air trapped in the rail (LT1 only) gets "bled" out with the opening of injectors.
Summation of the first post? COINCIDENCE.
