MFI fuel pump? But it runs.

[kwikt 911]

Ok, sometimes my 2.4 mfi has a hard time starting when hot. Today it took around 10 minutes to get her fired up after bringing it up to running temp and shutting it off. Finally I got it started and I drove it home. The car idled and ran great as usual. Checked the fuel pressure while the car was running and it read 0 psi. I crimped the inlet line and the fuel pressure rose like it is suppose to but still reading 0 when uncrimped. I can hear the fuel pump buzzing so It does work. Now when it cools down to cold condition there is NO starting problem. Can it be a clogged line? Where?

Kevin

[Grady Clay]

Kevin,

As I understand, it is hard starting warm.
It starts OK cold after sitting.
Are you sure you crimped the ‘supply’ hose and not the ‘return’ hose when measuring the fuel pressure?
Please show us where you measured the pressure and what hose you crimped.

(I hope you have new hoses. Crimping is a precursor to a fuel leak and fire.)

High on the suspect list is the filter screen in the fuel tank outlet fitting. It may be partially restricted causing the low fuel pressure. Usually that also causes the engine to die under load, only to restart after sitting for a few minutes.

It may be the electric pump is ‘on it’s last legs’ and temperature sensitive but still pumping enough to run the car.

It is possible that for some reason the fuel is not circulating. When running, cool fuel is used in the engine. When stopped for a bit, the fuel boils in the MFI pump. When attempting to restart warm, there are only vapors in the fuel supply side of the pump and nothing gets to the nozzles. When let sit to cold, the vapors condense back to liquid and all is well to start cold (and the cold-start fuel circuit operates.)

What is the fuel level?
How ‘warm’ is it there?
If you restart immediately, does it start properly?
How long does it have to sit to not restart easily?

First post your test.

Best,
Grady

[kwikt 911]

Grady, I'm in the process of draining the half full tank. It has been 90-100* here in Florida and it starts hard when running at 180*to 200* engine temp. I checked the pressure at the return line(closest to the rear of the car at the console) coming from MFI pump back to the console. And it only starts hard when the HOT car sits for 10 minutes or longer. But sometimes it fires right up even when HOT. I also disassembled the fuel console and noticed the check valve was stuck open. Would this cause the 0 psi but still running condition?

[Grady Clay]

Kevin,

When you go to start the car hot, do you turn on the electric pump for a few seconds (like 5) before cranking?

You are measuring the pressure at the fitting closest to the rear of the car. Good.

Were you able to remedy the stuck-open pressure regulating fitting (center on the fuel filter console)?
How did you get it un-stuck?

What hose did you pinch off?


Progress.

Best,
Grady

PS: Make sure you don’t have ANY fuel leaks or remaining spill fumes.
We don’t want a ‘FIRE’ post.
G.

[kwikt 911]

Grady, yes I do turn the key on for a few seconds then start. As far as the check valve goes I used some penetrating oil then cleaned it with some contact cleaner numerous times until it finally seemed to seal tight once again.. Looking inside this it appears that the check valve does not use a steel ball but looks like some sort of fiber gate with a spring. Strange looking but Now it works!! If I remember correctly I pinched the line that connects to this check valve. When doing this the pressure would rise very quickly(within seconds) until I released the kink. I did not let the pressure rise above 25 psi fearing the low pressure rubber lines would split I also figure a new fuel filter will be installed when slapping it back together.

[kwikt 911]

Update Took it for a spin and it runs great and even seems to idle alot smoother. After bringing the engine temp up to 200* on this hot day and letting it sit for 15 minutes. It fired right up with a steady 14psi at the return line. So far so good. But we will see this Sunday during the start and stop of our local autox event.

[regency]

My vote says it has something to do with the cold start solenoid. Was having the exact same problem with my 911. I installed a small push button start bypass switch. My car would start in the garage using my normal MFI starting procedure. Then I'd stop at the store, come back 45 min later and no start?? Now, if it won't start, I give it one push ( can hear it spritz ) and it starts right up. I guess I could have replaced the entire cold start solenoid but this was quicker. Will eventually, someday.

Steve

73 911 T MFI Coupe, Aubergine

[Grady Clay]

OK,
my ’after-the-fact’ diagnosis is:

There was no fuel pressure at the MFI pump inlet due to the pressure regulating fitting stuck open.

When driving, the circulating fuel (even at zero pressure) was cool enough to not vaporize at the MFI pump piston inlet.
Your 911 ran normally.

When parked hot, the hot oil in the lower part of the MFI pump heated the fuel in the upper part to boiling or close.

When you tried to restart hot after 15 minutes or so, each ‘intake’ stroke of the MFI pump lowered the pressure at the inlet to below the boiling point of the already hot fuel causing it to flash into vapor.

The MFI pump won’t pump vapor so no fuel was pushed out the nozzles.
The engine would not restart hot.

After a long time trying, the cool fuel circulation (even at no pressure) cooled the upper fuel chamber in the MFI pump suficently, preventing the fuel from vaporizing.
The engine starts.

When hot, the cold start circuit does not function so it was no help.

When cold, the fuel had condensed to liquid and the cold start circuit functioned as normal so the engine started normally.

2¢


Now that everything is working properly, it is time for some exercise.
Win that autocross and add a few hundred km of ‘spirited’ driving.

Now is the time for a through ‘C&M’ part of CMA.
With everything running properly, you can set a baseline of measurements.
Before the snow flies.

Best,
Grady

[regency]

Grady, That's exactly what I wanted to say but couldn't.

Steve

73 911T MFI Coupe, Aubergine

[kwikt 911]

Ahhhhh but my cold start solenoid power wire has been dissconnected for years. Once upon a time a long long time ago the car flooded so I dissconnected the wire at the top of the solenoid and it finally started. Sometimes I DO connect it on very cold days here in Florida for instant starts but other than that it stays dissconnected. Maybe I'll hook up a switch to activate this one of these days.

Thanks Grady for the input.

PS. Took off the streets and mounted the Hoosiers and loaded the car on the trailer. Can't wait for Sunday

[kwikt 911]

Great day autoxing Placed 1st in class, FTD and 1st in pax (I believe). The car ran great as usual. Fuel pressure stayed at a constant 15 psi now. No HOT start problems so far. I truly believe the problem was the check valve.

Kevin