1972 911T 2.4L MFI Cold Start Fuel Enrichment System

[McOtto]

Car - 1972 Porsche 911T 2.4L

Problem - Not always, but very often during start-up I am experiencing fuel spilling into the exhaust system and leaking to the ground under the heat exchanger. The fuel leakage phenomenon looks very much like the flooding I previously experienced when I was having trouble with my ignition system (which has been corrected). By far, most of the leakage is on the cylinder 1, 2 and 3 side of the engine.

Engine Start-up and Performance - Cold starts involve spitting and sputtering. However, once the engine is running; idle is smooth (although a little high at just above 1,000 rpm), throttle response is great, so is engine performance at all RPM ranges. Warm starts are smooth and immediate.

Troubleshooting Efforts - All of the cold start fuel enrichment components test good. In addition to component testing, I also connected a temporary tube to the cold start solenoid outlet and captured approximately 50 ml of fuel (which seems right?). Multiple fuel line checks on the primary fuel system and the cold start fuel lines did not detect any leaks. When I disconnect the power supply to the cold start solenoid to disable it, fuel no longer leaks from the exhaust during start-ups. And, as depicted in the attached image, I think I have developed a reasonable layman’s understanding of the cold start system and how it should work.

Possible Explanation - I am scratching my head and hope someone on the forum can tell me what I am missing or may have an answer to this question: Is it conceivable that the asymmetrical plumbing design on the cold start fuel lines - coupled with low pressure, short duration fuel delivery - is sending fuel via the path of least resistance (blockage is another possibility, but the cold start fuel lines are new)?

In theory, this might point to the possibility that cold start fuel is not being evenly distributed, with disproportionally too much going to the shorter fuel lines feeding cylinders 1, 2 and 3 and because of the longer travel distance, too little to 4, 5 and 6 (possibly also explaining the cold start spits and sputters).

Thank you!

[Richy]

There is no way fuel is entering the exhaust and leaking to the ground . I believe the cold start fuel lines are leaking and the fuel is migrating to the heat exchanger . You need new fuel lines .

[cbpiii]

If it doesn't leak when you disconnect the solenoid then you have a leak downstream from there. Pull the little plastic injector nozzles/fuel lines off the stacks, as well as the line from the solenoid. On a bench with bucket pressurize the system to see where it is leaking.

I had a pin hole on the elbow of one of the cold start injector nozzles. It would spray fuel when the cold start was activated.

[McOtto]

Thanks gentlemen. Really appreciate your observations and guidance.

The previous owner replaced all of the fuel lines with new (which may have given me a false sense of confidence), and I have looked for leakage from the cold start fuel lines. Did not see anything. Given the clear line-of-sight location of the cold start fuel lines and stack nozzles, I would have thought it would be easy to see fuel dripping from the lines and accumulating on the engine cover immediately below. None of that going on. Topside is dry as a bone.

However, if my harebrained theory on surplus fuel carrying over to exhaust is impossible (although the tailpipe exhaust never smokes, but temporarily does after a leakage event), the problem being cold start fuel line related seems to be the only thing that makes sense. I will take another look.

I sincerely appreciate the help!

[dpetterson]

I’m curious, when you experienced the leaking, were you using the hand throttle to start the car and what was the temperature like?

[McOtto]

I’m curious, when you experienced the leaking, were you using the hand throttle to start the car and what was the temperature like?

Yes, for cold starts I typically follow Porsche’s hand throttle position guidance in the owner’s manual - full up. Recent garage temperature has been in the 75 - 85°F range.

[edmayo]

Maybe try pulling the plastic start nozzles from the stacks and manually energize the system to see if any are squirting noticeably more fuel. The squirt hole is very small. maybe someone put in the wrong nozzles on one side.

[McOtto]

Maybe try pulling the plastic start nozzles from the stacks and manually energize the system to see if any are squirting noticeably more fuel. The squirt hole is very small. maybe someone put in the wrong nozzles on one side.

Thank you, Ed! Good idea. I will try that.

[frederik]

Do you have hose clamps on the plastic injectors? I agree with Richy, it must be a fuel leak before fuel enters the cylinders.

[McOtto]

Do you have hose clamps on the plastic injectors? I agree with Richy, it must be a fuel leak before fuel enters the cylinders.

Yes, the hoses are clamped to the injectors.