Vapor lock issues?

[91166 Carrera]

I have a '76 Carrera 3.0 Targa. The other day in very hot temperatures (90+), while driving in a PCA road rally, I got home and experienced a situation when I turned the car off. It sounded like gas was being pushed back into the fuel tank. It gurgled, sloshed or sounded like bubbling or backwash in the tank. The noise only lasted for about 10 seconds after the car was turned off. I turned the key to the start position again, let the fuel pump run for a few seconds, then turned it off, and the noise occurred again. Its not the typical buzzing or humming that the pump makes. Is it possible that a reverse fuel pressure is occurring and pushing the fuel past the check valve and back into the tank? The following morning, with the car cold, I did the same test and no noise occurred. I pulled the fuel filter and it was clean, in that no dirt came out from the filter when shaken. Anybody familiar with this situation and what's the fix? I've owned this car for 38 years and never experienced this situation. HELP!

Harry

[356911930]

I’m of sure I see a problem? What you probably heard was oil in the lines or tank. Just drive it and monitor the gauges.

Best of luck,

Rahl

[77Targa0912]

My Carrera 3.0 has developed vapor lock symptoms during this 100 degree heat wave. Cold start is fine. Once it’s heat soaked I usually have to crank it for 30 seconds and multiple key off and on to get it to stumble to life. Once it settles back in there is no more problems until the next stop and the problem repeats itself if it sits for an extended amount of time in the hot weather.

[91166 Carrera]

Its not an oil flow noise because it only occurred when I turned the fuel pump on and off. After turning the fuel pump off the noise persisted for about 10 seconds then stopped. In the meantime I changed the fuel filter. There is a fuel return line back to the tank, so if there are vapor locks in that line then as the fuel and vapor arrive at the tank a gurgling sound would be made. It just a theory, but I've owned this car of 38 years and never heard this noise before. Any thoughts on this? Thanks.

Harry

[Walt Fricke]

I can see how a failed fuel pump RPV might maybe cause this. One test would be to see if, after hearing this noise, you tried to restart the still very hot engine and so on. Hot start problems can be symptomatic of a failed RPV combined with high temperatures.

Your system turns the pump on when the key is in the run position, and I theorize that by running the pump a bit before restarting you may overcome the problems causing hot restart problems. Later 911s won't run the pump unless in the start position, or the engine is running.

[91166 Carrera]

Walt, forgive me but what is a RPV? Is that the check valve that attaches to the front of the fuel pump?
Thanks,
Harry

[boyt911sc]

Originally Posted by 77Targa0912

My Carrera 3.0 has developed vapor lock symptoms during this 100 degree heat wave. Cold start is fine. Once it’s heat soaked I usually have to crank it for 30 seconds and multiple key off and on to get it to stumble to life. Once it settles back in there is no more problems until the next stop and the problem repeats itself if it sits for an extended amount of time in the hot weather.

Do you have a carburetor or a fuel injection system? Thanks.

Tony

[Walt Fricke]

Harry - forgive me instead.
Because of the differences between the fuel pumps for the 2.7s and the 3.0s, I know more about these one way valves than I would like, but I have trouble remembering the abbreviation. Residual pressure valve? Something else? Anyway, you are thinking of the right part.

This is part of what you check when checking the specs on a CIS system - After the pump is shut off, the control pressure (system pressure?) should remain at or above X at Y minutes, and Z at Y+ something. I look these things up (I bet Tony has most memorized) when checking my SC race car's CIS rather than trying to remember. But as a result I know that there is a Turbo spec, which at least for the control pressures is different.

Did you convert this car to a Turbo? If so, did you add a second fuel pump? That about exhausts what I know of these turbos, other than their compression ratio (and hence pistons) is rather lower.

Or just change the CIS parts to the Turbo system? That is a bit puzzling.

[boyt911sc]

Walt,

I will be 80 years old in a few months and I’m having problem remembering where I left the tool I was using a minute ago. Sometime it was just sitting right in front of me and don’t see it. You should have a better memory than me because I’m probably 20 or more years older than you.

I have no idea what RPV stands for but there is FP CV (fuel pump check valve) that prevents the residual pressure from going down too fast.


Harry,

Vapor lock does not occur in CIS. Why? The typical system pressure in a CIS is 70 psi. and some FD have 100 psi. Gasoline is an incompressible fluid, vapor lock if any could not prevent the fuel in a CIS from flowing through the system.

Your hot start problem is not caused by “vapor lock” unless you have carburetor system that has a system fuel pressure less than 5 psi. Your hot start problem is caused by the low residual pressure or temperature related electrical problem. Not vapor lock.

Tony

[91166 Carrera]

Everyone- ok, I'm convinced, its not a vapor lock issue. This early Bosch CIS system has always been a problem, but less so since I had the fuel distributer rebuilt. In fact the entire fuel system has been rebuilt from front to rear a few years ago. Since this was a first time, isolated event, I'm just going to wait until it happens again and hopefully the system won't fail while I'm driving Guardmans Pass in Park City, Utah. In any event, thank you all for the help. In appreciation here's a photo of my Carrera. (According to Hagerty its 1 of 45 in the country).

Thanks again,
Harry