71E runs great cold….terrible when warm

[mhurwitz]

I’ve owned my 1971 911E since 1978. Car has 143,000 original miles. She ran great, but got tired of all the oil leaks. Fixing the leaks turned into a complete rebuild, a few months ago. Also decided to have the throttle bodies and MFI pump rebuilt. All tune up components have been replaced including plug wires.
My problem: car has not run good ever since all this was done.
Car runs great when cold, but as it warms up, it runs progressively worse.
Problems when car gets to running temp, (210),
1) Idle won’t go under 2200 for about 30 seconds after stopping at a light
2) rpm’s won’t go above 4,200 on acceleration
3) Car backfires when accelerating on take off

Timing and dwell have been checked

Fuel pressure from fuel pump to fuel filter canister is 2.5 bar/36 psi
Fuel pressure at bottom of fuel filter canister that goes to MFI pump is 1.3 bar/20 psi
Fuel pressure from canister to return to fuel tank is 2.3 bar/34 psi

I’m not sure what else to do or check….my mechanic doesn’t know what else to do.

Signed,
Frustrated (aka Mark)

[tobluforu]

Do you have the mfi pump hot air pipe connected?

[mhurwitz]

Originally Posted by tobluforu

Do you have the mfi pump hot air pipe connected?

Tobluforu, thank you for the reply. I went out and double checked. Yes the pipe is connected and it is blowing hot air out. I actually bought a new hose and it was actually 2 hoses….one inside the other. Is there a way to check the thermostat?

[tobluforu]

Have you gone through CMA, based on your post I'm guessing, yes?

[BK911]

Definitely go through the CMA manual.
Also don't see anything about synching the intakes, or CO adjustments?
To start, synch all the intakes.
Then a real quick test to see if idle CO mixture is good:
-Disconnect linkage to MFI pump
-Manually open throttle valves
Idle should slightly increase then decrease as the throttle valves open.
If this happens, it means the mixture is fine and set slightly rich.

[mhurwitz]

Originally Posted by BK911

Definitely go through the CMA manual.
Also don't see anything about synching the intakes, or CO adjustments?
To start, synch all the intakes.
Then a real quick test to see if idle CO mixture is good:
-Disconnect linkage to MFI pump
-Manually open throttle valves
Idle should slightly increase then decrease as the throttle valves open.
If this happens, it means the mixture is fine and set slightly rich.

BK911, I will run these tests today and report back…..thank you

[mhurwitz]

Originally Posted by mhurwitz

BK911, I will run these tests today and report back…..thank you

Intakes are all synced. Car idles at about 950.
Took the linkage off MFI pump. Idle lowered. Manually opened the throttle slowly and car died instantly

[mhurwitz]

I tested fuel output from fuel pump. Result was over 2.0 liters in a minute.
Does anyone know what the fuel output should be from the bottom of the fuel filter canister. (The fuel supply to the MFI pump after the fuel has gone through the filter canister) ?

[RDM]

MFI is a complex system. Start here and read everything:
Ultimate MFI resources thread

CMA will take you some time. I'm doing that process for my '71 Mercedes now.

[BK911]

Originally Posted by mhurwitz

Intakes are all synced. Car idles at about 950.
Took the linkage off MFI pump. Idle lowered. Manually opened the throttle slowly and car died instantly

Sounds like linkages are not adjusted properly.
Also sounds like CO not set properly.
Go through manuals posted above.

[mhurwitz]

Originally Posted by BK911

Sounds like linkages are not adjusted properly.
Also sounds like CO not set properly.
Go through manuals posted above.

Throttle bodies and linkages were done by Supertec in Fallbrook. I trust that he did them properly. They have such a good reputation. I guess I could start by checking the connecting rod between the pump regulator lever and cross shaft (114+-.2) mm

[mhurwitz]

Originally Posted by mhurwitz

Throttle bodies and linkages were done by Supertec in Fallbrook. I trust that he did them properly. They have such a good reputation. I guess I could start by checking the connecting rod between the pump regulator lever and cross shaft (114+-.2) mm

Connecting rod is spot on

[lateapex911]

It's interesting that the problems are only happening...and getting worse....as it warms.
AND you have the pipe hooked up correctly to feed the MFI system the heated air.

TO me (I admit I haven't been in the MFI game for too long) that suggests the Pump isn't reacting to the temp change.

You say Supertec did the work, did they take the pump off and have it serviced (By who, Gus at Pacific Injection?).

If so, the proper warm up behavior is on them, and I'd assume they tested it before returning the car?

Is this a problem that has appeared over time? Or was it like this when Supertech returned the car?

People have days when things don't go quite right, so I'd see what the people who did the setting up the pump and install say.

Seems to me that the pump isn't adjusting to higher temps.

If your guy is at wits end, and the pump rebuilder can't help, I see you are in Newport Beach, and there are like so many German car shops in Costa Mesa that you could stand on 16th street and throw a stone and hit a shop with each throw LOL.

Used to be that MFI was black magic, but thanks to the internet, that's not the case.

(BTW, I used to live in NB, wouldn't take much to bring me back ...soooo much Porsche support, great weather, bla bla bla!)

[mhurwitz]

Originally Posted by lateapex911

It's interesting that the problems are only happening...and getting worse....as it warms.
AND you have the pipe hooked up correctly to feed the MFI system the heated air.

TO me (I admit I haven't been in the MFI game for too long) that suggests the Pump isn't reacting to the temp change.

You say Supertec did the work, did they take the pump off and have it serviced (By who, Gus at Pacific Injection?).

If so, the proper warm up behavior is on them, and I'd assume they tested it before returning the car?

Is this a problem that has appeared over time? Or was it like this when Supertech returned the car?

People have days when things don't go quite right, so I'd see what the people who did the setting up the pump and install say.

Seems to me that the pump isn't adjusting to higher temps.

If your guy is at wits end, and the pump rebuilder can't help, I see you are in Newport Beach, and there are like so many German car shops in Costa Mesa that you could stand on 16th street and throw a stone and hit a shop with each throw LOL.

Used to be that MFI was black magic, but thanks to the internet, that's not the case.

(BTW, I used to live in NB, wouldn't take much to bring me back ...soooo much Porsche support, great weather, bla bla bla!)

Just to clarify, SuperTec did the throttle bodies only. I had the pump rebuilt by a guy that has done many MFI pumps for multiple shops in the area. I confirmed that he checks the pump thermostat on the bench. He has all the proper Bosch bench testing equipment.

[mhurwitz]

Any chance it could be the CDI box? Or would it not work when car is cold or hot?

[Corsair26]

Hi, Dont want to send you on a wild goose chase but it hasn't been mentioned that I can see so
- have you changed the overflow / regulator valve on the fuel filter console?
It looks like a hose connector but its not. Its on the top of the fuel filter and is the connector facing the engine or right side of the car. From here fuel returns to the main fuel pump.
I have the same problem exactly. 71 911E Rebuilt pump. New throttle bodies, CMA all done, all looks good but its not and this is my next stop.
Waiting for it to arrive now.

901 110 915 01 they're $175 so definitely not just a connector.

https://www.pelicanparts.com/More_Info/90111091501.htm?pn=901-110-915-01-OEM&SVSVSI=4237&DID=167201

I'll keep an eye on this post. If I find definitive answer I'll let you know. Hopefully one of us will.

[stownsen914]

Are you sure you got a correct fuel pressure measurement? 30+ psi is way high for MFI. It is supposed to be 15 psi measured at the fuel filter console (pre return line). Low pressure can cause MFI to run poorly, not sure what double the pressure does.

You can check the function of the warmup regulator on the pump. It's a bellows that visibly moves when heated. You can gently heat it with a heat gun (I wouldn't trust an open flame due to fuel proximity) to make sure it's doing its thing correctly.

One random thing to check is that the cold start spray valve isn't dumping fuel into the stacks inappropriately, that'll make it run poorly for sure. Easy enough to check. Once the engine is running, just disconnect the lead to the cold start solenoid that sits on top of the fuel filter stand and see if the behavior changes.

[Corsair26]

Maybe the pressure regulator / overflow valve on the fuel filter console. The hose connector (its not just a connector) facing the engine or right of car where the return line goes back to the fuel pump. It should hold pressure at 1ATM -+.2 at the MFI downstream from the filter. Ive got another thread going about this very problem.

[mhurwitz]

Originally Posted by Corsair26

Maybe the pressure regulator / overflow valve on the fuel filter console. The hose connector (its not just a connector) facing the engine or right of car where the return line goes back to the fuel pump. It should hold pressure at 1ATM -+.2 at the MFI downstream from the filter. Ive got another thread going about this very problem.

I took the connector off. The little plunger inside seems to be working and is clean.
The metal micro filter that goes into that port seem to be clean.
What dis the parts look like on the other thread?
Thanks,
Mark

[JackkTheMackk]

Wanted to chime in here. The 70E i picked up recently has the same exact issue going on as you. Symptoms are exactly the same when hot but my thermostat is not connected. The original style heat exchangers on my car do not have the MFI hook up so they’re obviously not original to the car. I haven’t had time to chase down the culprit and fix it yet but so far i have done all the following work

- New 123ignition dizzy/coil/wires/plugs hooked up to original CDI box
- New fuel filter, over flow valve, cleaned the tank, and new strainer in tank. Test all fuel pressure and they are correct hot and cold.

I have a set of NOS original SSI’s that are going to go on the car soon and I will be hooking up the thermostat then, which hopefully cures the issue.

You are 100% sure your hose connected to the mfi pump is blowing hot air? I would bench test your thermostat next. Can you hook up a wideband to the heat exchangers to read the AFR? That will help trouble shoot as well.