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Vapor lock, hot start?

cfisher

Proven Member
451
4
Dec 21, 2002
olathe, Kansas
I've been having problems with my car after I drive it. It starts just fine when cold but when I turn it off and turn it back on it struggles around 300rpms and sounds like its on 3 cylinders. This lasts about 10 seconds and then the rpms go to 800rpms and everything is fine. I have no stutter problems or anything while driving but it's just when I go to start it up after driving. Also, it seems to be getting worse and last night I went to go start it again after leaving a QT and it died on me. I've researched a little on hot starts/vapor lock but I'm thinking maybe I have a bad o ring on my walbro 255. Any help would be appreciated. Thanks guys.:thumb:
 

cfisher

Proven Member
451
4
Dec 21, 2002
olathe, Kansas
Okay, I think something is wrong with the fuel pump. It did not want to start this morning and all of my readings on my logger are going leaner. I had to richen up the SAFC by about 5% to get my fuel trims back in line. My gauge on my AFPR states everything is good pressure wise. Also, when I give it gas and let off real quick the car will almost die and then jump back up to 800rpms or so.:notgood:
 

FORMONTOYA

DSM Wiseman
2,260
57
Oct 7, 2004
Houston, Texas
If you are having problems with "hot starts", driving the car...shut it off to go into someplace for a couple minutes...then come back out and have problems, more than likely it is the fuel pressure solinoid. It is designed for just that...hot starts. At a certain temp it will activate and pull from the outside instead of from the intake mani under vacuum. It is the same as removing the vac line from the fpr. It raises the fuel pressure to avoid "vapor lock" in hot conditions.

Easy enough to check, try starting at the house shortly after a cruise. If it is giving you problems, shut off the car, remove the vac line from the fpr and see if that lets the car start as it should. The o-ring is a possibility, but I would think you would have drivability problems as well.


Jim
 

cfisher

Proven Member
451
4
Dec 21, 2002
olathe, Kansas
FORMONTOYA said:
If you are having problems with "hot starts", driving the car...shut it off to go into someplace for a couple minutes...then come back out and have problems, more than likely it is the fuel pressure solinoid. It is designed for just that...hot starts. At a certain temp it will activate and pull from the outside instead of from the intake mani under vacuum. It is the same as removing the vac line from the fpr. It raises the fuel pressure to avoid "vapor lock" in hot conditions.

Easy enough to check, try starting at the house shortly after a cruise. If it is giving you problems, shut off the car, remove the vac line from the fpr and see if that lets the car start as it should. The o-ring is a possibility, but I would think you would have drivability problems as well.


Jim

Thanks Jim. I don't even use the FPR solonoid though. I have an aeromotive afpr with -6an stainless lines and the vacuum line goes straight from my intake manifold to the afpr. The solonoid is plugged in but just hangs there. I didn't cap it off or anything, I just left it plugged into the harness. Should the solonoid be capped off or are you supposed to route the vacuum line to the solonoid/then afpr?
 

FORMONTOYA

DSM Wiseman
2,260
57
Oct 7, 2004
Houston, Texas
cfisher said:
Thanks Jim. I don't even use the FPR solonoid though. I have an aeromotive afpr with -6an stainless lines and the vacuum line goes straight from my intake manifold to the afpr. The solonoid is plugged in but just hangs there. I didn't cap it off or anything, I just left it plugged into the harness. Should the solonoid be capped off or are you supposed to route the vacuum line to the solonoid/then afpr?

In Houston it get pretty damn hot so I used the FPS on mine. Intake mani-FPS-FPR and have not had any issues. Although a lot of people do the same as you and remove the FPS from the mix with no problems.

But you can check if "vapor lock" is the issue by trying the above. Drive the car around to get it good and hot, come back to the house, shut it off and shortly thereafter try and start it. If you have the issue show up...shut the car off, remove the vac line from the FPR and see if the car starts correctly. If it does, then you know you should put the FPS back into the mix.

We are starting to use the AC again here, and Kansas gets pretty damn hot also, might be that simple.


Jim
 

MyBeatGSX

Proven Member
1,335
27
Jul 17, 2005
Southington, Connecticut
Mine has the exact same problem with the exact same fuel setup. You aren't alone. Let me know if plugging the FPS back in fixes the problem. But how does that effect fuel delivery at other times?
 

FORMONTOYA

DSM Wiseman
2,260
57
Oct 7, 2004
Houston, Texas
MyBeatGSX said:
Mine has the exact same problem with the exact same fuel setup. You aren't alone. Let me know if plugging the FPS back in fixes the problem. But how does that effect fuel delivery at other times?

The way the FPS operates is under normal condition it is "closed" allowing vacuum to get to the FPR during starting. Under hot conditions the FPS is "open" to atmosphere which allow the FPR to raise fuel pressure to the base setting as you would see with the vac hose off the FPR, which helps to control "vapor lock".

Under normal driving conditions the FPS is doing nothing allowing the FPR to see whatever is going on inside the intake mani or wherever you source the line from.

The FPS is really designed for "hot starts".


Jim
 

MyBeatGSX

Proven Member
1,335
27
Jul 17, 2005
Southington, Connecticut
Well then clearly that's our problem. Why would anyone eliminate it if its only there to help and not hurting/changing anything?:confused:

Which side do I need to hook to the FPR and which side goes to manifold vacuum?
 

FORMONTOYA

DSM Wiseman
2,260
57
Oct 7, 2004
Houston, Texas
MyBeatGSX said:
Well then clearly that's our problem. Why would anyone eliminate it if its only there to help and not hurting/changing anything?:confused:

Which side do I need to hook to the FPR and which side goes to manifold vacuum?

A lot of people will eliminate it with no problems (especially up north) just to clean up the engine bay and eliminate potential vac/boost leaks.

If you look here...
http://members.shaw.ca/dsm.1000q/Engineprimer/2G/solenoids.htm
...the picture on the bottom is the FPS, the black hose goes to the intake mani, the red hose goes to the FPR.


Jim
 

Daaan

Proven Member
229
0
Aug 15, 2007
Courtice, Ontario_Canada
cfisher, let me know if you find a solution to this problem. I have the exact same thing happen to my car. It starts fine when the temp is at zero, but when the temp is high and i try to start it, it bogs on 3 cyls and turns off about 5-6 times. I have found that if i leave the key on and run the a/c for 30 or so seconds, it will start. The fans kick on because of the a/c and clear out the "hot air". This is a pain in the ___. Ive replaced the ignition coil, fuel filter, spark plugs, battery, and many more, so those are ruled out.
 

DSM1G90

Proven Member
5,101
66
Apr 6, 2008
Nampa, Idaho
What I've done with other cars is put some copper water pipe thermal insulation (fibreglass preferably) around the main fuel line from filter to rail to insulate the line from the heat under the hood to help prevent vapor lock.
 

Daaan

Proven Member
229
0
Aug 15, 2007
Courtice, Ontario_Canada
Jim, i was wondering if you know where the FPS is located on my 1992 eagle talon. I have been looking in my haynes manual for it. The manual mentions nothing about the FPS (feul pressure solonoid) under the fuel and exhaust system section. The link you posted shows the solonoids on the top right side of the engine bay. My car has nothing in that spot probably because its a non turbo.
 

Khardy

Probationary Member
1
2
Mar 20, 2016
124 Campbell Cove, Mississippi
I'm afraid most of you guys are completely off base. I've had the same hot starting issue in my 2004 ECLIPSE for years, and was not able to figure it out until two days ago. The hot start/stammer problem started about eight years after I purchased my Eclipse brand new. It was also associated with a low rumbling sound coming out of the rear of the car which went away after the stammer resolved and RPM's stabilized. In 2011, I replaced the factory fuel pump (but not the fuel pump assembly), and the problem did not go away. While not worsening, the problem has been annoying to say the least, and a concern to buyers now that I am trying to sell my vehicle. Convinced that the problem had to somehow involve my fuel pump, two days ago I replaced my fuel pump again, only this time with a complete fuel pump assembly.....and the problem was resolved. Out of curiosity, I disassembled my old fuel pump assembly and found a leaking seal on the main fuel pump output stem.

So here's what was happening: The leak was small and thus didn't interfer with fuel pressure during normal driving, cold starting, or immediately after a hot stop. However within 10 minutes after a hot stop, hot fuel under pressure would leak backwards into the gas tank, lowering fuel pressure to nearly zero in the hot fuel rail. Hot fuel under low pressure near the hot engine would vaporize and vapor lock would develop almost all the way back to the tank. On a delayed hot start, the fuel pump would fire up normally, but could only pump mostly fuel vapors into the injector lines, leading to misfire and stalling until all the vapors were either pushed into the cylinders through the injectors, or bypassed back into the fuel tank through the fuel return line. Once the vapor lock was broken by fresh gas pumped into the injectors, the fuel pressure returned and the stammer would stop. Interestingly, as the vapors were pushed through the return line deep into the gas tank, a loud bubbling sound would be created......precisely the sound no one (but me) was able to diagnose all these years. If either an Eclipse fuel pump check valve, or an assembly output seal go bad, this same condition can occur. The cure is a new fuel pump AND assembly. AND, be careful to completely seal the new fuel pump into the tank or the CEL will soon flash, giving a P045 (small emission leak) code. I was also having this code flash for years after my first fuel pump was replaced, because my mechanic didn't tighten the replacement down completely. After tightening the old fuel pump, that "small emissions leak" code was easily erased for good....but the stammer didn't resolve until the new fuel pump assembly was installed. These two longstanding and perplexing problems are now clarified, and resolved. P.S. Mitsubishi and their "Geniuses" we're completely worthless and never did figure either out. And keep in mind that cheap, Chinese fuel pump assemblies are going to fail much sooner after replacement than OEM or higher quality replacements.
NO CHARGE FOLKS. :)
 
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