Engine hesitates and backfires after driving an hour-

[G69]

My new MFI system seems to be working really well.

I had Marty at H&R rebuild the pump and I installed 6 brand new injectors.

The fuel console is cleaned-up and it has a new filter. The primer cold start solenoid is removed and plugged. Its wiring at the engine is new but it's pigtail for the solenoid is taped off at the filter console.

In the colder weather I had no problems. Now that we are seeing 90 degrees and more my 1969 911S will begin to hesitate after running an hour.

Since the car idles perfectly and there is no issues seen on the Tachometer I'm betting it is fuel related.

The fuel pump is very old and I wanted to see if there were any easy ways to troubleshoot this issue-

Since the car has to be at oil temp 100C and that temperature is only reached after 20 or more miles it is difficult to duplicate the conditions in my garage.

I checked the pressure by-pass banjo fitting with a pressure gauge when I first installed the MFI and it was stuck but I was able to free it up. It could be bad again. The car runs great for the first 40-50 minutes...
Is the phenolic bypass element something that would be influenced by under hood temps after running an hour?

Or would the fuel pump itself be the likely offender?
It is the same pump that was on the car when I got it and was running Webers with a pressure regulator in 1999...

What would be a good sequence to root out this intermittent problem?

Thanks,

Gene

[RSTarga]

Could be vapor lock or a dirty sock at the fuel tank.

[62S-R-S]

It doesn't really fix the problem, but a pair of these in the cockpit would be neat :

[frederik]

I had a similar problem, only occurred after at least 30 minutes. It turned out that the MFI thermostat (receiving hot air from the heat exchanger) didn't fully retract and kept enriching the fuel mixture a little bit even when the engine was fully warmed up. In addition to that, my MFI was set up too rich in general. Worth checking on your car.

[G69]

Thanks- I'm thinking a dirty supply from the tank would always be a problem not just when fully warmed up.

I haven't eliminated the fuel pump getting hot and faltering. Also a long shot since the car runs well until the problem arises.

But I spoke to the person who rebuilt the MFI pump and he is sending me another thermostat to see if that is the issue.
My situation isn't too rich- it is too lean...

If a new thermostat doesn't work he will send me another MFI pump to swap out.

That's customer service!

[edmayo]

We would start by draining tank and removing fuel sock to verify it is clean enough to see the internal tubes. Diagnosis is a process of elimination. Also if that check valve stuck once it will stick again. The phenolic has swollen.

[G69]

Thanks Ed, I ordered the valve.

I couldn't get the symptoms to appear today-

It still limits the range I'm willing to bet it won't happen again.

[Haasman]

G69- Similar temps today?

[frederik]

But I spoke to the person who rebuilt the MFI pump and he is sending me another thermostat to see if that is the issue.
My situation isn't too rich- it is too lean...

If a new thermostat doesn't work he will send me another MFI pump to swap out.

That's customer service!

If I understand the system correctly (being fairly new to this), the thermostat only enriches, and it should fully retract when warm. If you're running too lean this doesn't sound like it's the thermostat. Unless it's retracting at a too high temperature, so the system was dialed in with a partially active thermostat. Once it gets really hot, it might fully retract and then you end up too lean. In that case, with the new thermostat, you would end up too lean much earlier.

Let me know if there is a flaw in my reasoning as I'm happy to learn more.

[G69]

Rather than chase my tail, I'm holding off until I get the replacement check valve delivered next week. Without consistent fuel pressure controlled by a good check valve I have no baseline.

And I also believe Frederik is on the right track. I saw a comment on BAT about a 1972 911S where an owner said back-in-the-day his car ran its best (only) after a full 45 minute warm-up.
That is about the time it takes me to fully warm up to 95-100C on a HOT day and I experience the hesitation. On cooler days I don't see those temps- the car runs well.

I have been 'tuning' the MFI mixtures at 80C. I also thought the thermostat had reached its full lean setting stop by 80C but that may not be the case.
Having another bit of leaning range up around 100C would explain a lot of the anomalies I'm seeing. The car runs close to perfect at 85C and below. Once it hits 90-100C the hesitation starts. At that point higher engine bay and fuel temperatures could also be affecting the old phenolic check valve mechanism. If the valve binds open that sure would bleed off the fuel pressure I need at WOT. If it binds closed, MFI pump and fuel temps would rise.

100C isn't something I see every ride so tuning for a temperature seen only after a spirited drive of an hour on a HOT day may not be best for my car.
I also don't think 100C (212F) is good for the VR1 20/50 oil I'm running. I'd like to run at 80-90C at most.

Again- inconsistent mixture strengths may juggle my operating temps and conversely operating temps may be juggling my mixture strengths!

Marty sent me brand new thermostat so when I get the new check valve installed I'll insure I have correct fuel pressure and start again.

A bench test with a heat gun and infrared thermometer may show me what's up with the thermostat.

Thanks!