MFI 911 Running Poorly When Warmed Up

brittbolen · 12-17-2021, 04:04 PM

Hello! Long time no talk!

So lately my 911E starts to run poorly after it warms up, eventually stalling.

The problem occurs after the car has come up to temp, and usually happens faster and more severely if it’s > 80F outside, or the oils been up over 200.

Symptoms start out as popping/backfiring after a heel&toe downshift and quick acceleration onto a different road… from there it escalates into missing/stumbling/hesitating at WOT at 6000, then 5000, eventually it just won’t go past 5000 rpm, and can be hard to even get up to 60 mph, from there the car won’t idle at 1200 and eventually it will just stall out if i’m at a light, or a stop sign, or just cruising above idle…

Besides that behavior when warmed up, run for 30 minutes or so, the car just feels a little rough, it hesitates, and seems to be running a bit richer than usual even when warm.

if i park and wait and cool off for as little as 10 minutes, it’ll fire up like nothing happened.

I'm looking for suggestions on what I might look at next... fuel pump?

Fuel pump? Seems to happen faster / more severely when at < 1/2 full... but not always, i was missing on 7/8 full last weekend...

Car details:

1970 911E, stock MFI motor, w/ points and everything. 114,000 miles, only engine work was rocker shaft broke once, and I’ve got carrera tensioners…
Zims fuel pump
Good oil pressure
Starts well when cold, or shortly after shut-off… wait 1 hour and it takes some cranking to get going.
Normally idles at 1200, was recently idling at 1800-2100 when warm, so I put a drop of oil on the felt pad under the rotor and that seemed to clear up the idle, but not the missing/hesitation. (this is not the first time I’ve fixed the idle problem in this way)

I recently checked timing, dwell and adjusted the valves (not that I think the valves have anything to do with it, just due) nothing of interest there that I could see.

Thanks for any suggestions on things to try next!

And if you've read this far... obligatory recent photo...

mar2mar · 12-17-2021, 04:26 PM

Had a similar problem with similar symptoms on my MFI 70S. It turnout to be a broken overflow valve (circled in red) in the fuel filter console.

brittbolen · 12-17-2021, 06:04 PM

Quote:

Originally Posted by mar2mar

Had a similar problem with similar symptoms on my MFI 70S. It turnout to be a broken overflow valve (circled in red) in the fuel filter console.

Thanks! Which symptoms did you have?

kltarga72 · kltarga72's Garage

Fuel issues will cause the symptoms you are experiencing. Check the fuel tank outlet filter, loose fuel line connections, old rubber hose lines will crack.
I had a new Bosch fuel pump on my 72-S that until I put my hand on the pump while the engine was running I could feel the pump was intermittent or cavitating. After replacing the fuel pump problems were solved

stownsen914 · 12-18-2021, 02:15 AM

If fuel supply is the issue, it will show up as decreased fuel pressure which can be measured. The mechanical MFI pump likes 15 psi and will become unhappy much below that and the engine will run poorly. (I had the exact same problem as mar2mar, resulting in severe engine stumbling. The valve he mentions is a check valve that keeps fuel pressure at 15 psi. When it fails or the spring becomes weak over time, fuel pressure drops. As mentioned above, a failing fuel pump will also cause low fuel pressure.)

Another possibility since your case seems to be warmup dependent is that the warmup circuit on your mechanical pump is stuck. If you are getting an enriched mixture all the time, the engine could run poorly when warm. Check the small hose that goes from the heat exchanger to the pump - it delivers warm air to the pump. If that has fallen off or ruptured, the pump will think the engine is cold all the time and stay rich.

Jeff Higgins · 12-18-2021, 04:40 PM

Quote:

Originally Posted by brittbolen

Hello! Long time no talk!

if i park and wait and cool off for as little as 10 minutes, it’ll fire up like nothing happened.

It's not going to cool off enough in ten minutes to make any difference. What it will do in ten minutes, however, is suck enough air past a plugged gas tank vent to relieve vacuum in the gas tank. It will then run normally for awhile once again, until it again creates a vacuum in the gas tank. Try taking the gas cap off and driving it.

RSTarga · 12-18-2021, 06:02 PM

Try cleaning the sock filter at the bottom outlet of your tank.
Classic problem and a simple fix

brittbolen · 06:28 PM

Stownsen914 my warm up hose was disconnected, probably when I rechecked a couple valves. Ok 1 check/suggestion done!

brittbolen · 12-18-2021, 06:27 PM

Jeff and RSTarga I’ll get on both of those checks, thanks

mhurwitz · mhurwitz's Garage

I have a 71 E that was doing the same thing, Ended up having to rebuild my throttle bodies. Runs great now.

Rosco_NZ · Rosco_NZ's Garage

I’m guessing you’ve already checked spark when at temp / stumbling and ruled out the old CDI box?? CDI failure usually engine temp dependent.

scotricker · 12-20-2021, 01:41 PM

Could it be something super simple like fuel filter ?

rfuerst911sc · 12-21-2021, 02:30 AM

Brittbolen check your PM's , been trying to get a hold of you . Thanks

Schulisco · Schulisco's Garage

Hi,

I have to admit that I'm not in the MFI and F-model field - but I also struggled heavily in the past with back firing, poor and below 40°F impossible cold start (ok, not here) and poor power delivery together with a heavy fuel consumption on my '81 SC.

What I figured out - the Porsche engines live and die with the mixture! Especially the fuel injected engines.

Either too lean or too rich produces several of your experienced symptoms as well! Guessing you're experiencing a high fuel consumption as well (for sure the carbs itself aren't that fuel efficient) of 11mpg or less, right?

If you haven't modified anything on the engine there must be sth wrong. Either air leaks or more likely misadjusted injection and mixture. Did you checked the fuel pressure? Probably it leans up while reving high due to dying fuel pump?

To verify the mixture I recommend to make tests drives with a lambda tool applied (e.g. innovate LM2) while driving with several loads at several temperatures and speeds on the road or even better on a dyno!
Best for performance of an aircooled engine is 0,85 lambda / 12,5 AFR on WOT, not more and not less! Also around 1 / 14,7 on idling and cruising with 30% throttle...

Regards, Thomas

brittbolen · 12-23-2021, 01:26 PM

ok, did another test drive, better now that the heater hose for the MFI thermostat is hooked up, but same problems eventually... I did some reading on the tank ventilation system, and neither hose in my engine bay was hooked up at all...

Looking at the diagrams here...

https://forums.pelicanparts.com/porsche-911-technical-forum/299053-tank-ventilation-mfi-diagrams.html

in that first diagram from Grady lines 5 and 6 were disconnected.... I can tell which is which because only one is long enough to be #5, but I can't tell where #6 "Purging line from activated charcoal container to engine air cleaner" hooks my air filter... so thats my next question/research...

B

brittbolen · 12-23-2021, 01:52 PM

yeah, my air filter assembly doesn't have a connection for the charcoal canister...

Based on another thread on earlyS registry i should have 2 connections on the back of my air cleaner... I don't recall changing my air cleaner assembly in the 30 years I've had the car...

any guesses on how this would have been hooked up? or have I just never had this connected? nah....

RSTarga · 12-23-2021, 02:12 PM

I may be wrong as I am doing this from memory.
One line goes from oil tank to engine thermostat housing
The back of the air filter should have a fitting with two different diameter fitting. It appears yours has been modified.
One connects to the breather fitting on the neck of the oil tank, the other comes from the charcoal filter.
Another thinner line comes from the charcoal filter and goes into the engine shroud fitting behind the alternator.
The thin fitting from the air filter goes into a drip tube at the rear of the engine.

brittbolen · 12-23-2021, 02:16 PM

Quote:

Originally Posted by RSTarga

I may be wrong as I am doing this from memory.
One line goes from oil tank to engine thermostat housing
The back of the air filter should have a fitting with two different diameter fitting. It appears yours has been modified.
One connects to the breather fitting on the neck of the oil tank, the other comes from the charcoal filter.
Another thinner line comes from the charcoal filter and goes into the engine shroud fitting behind the alternator.
The thin fitting from the air filter goes into a drip tube at the rear of the engine.

From what I can tell from pics and a thread from Grady, the 69 MFI filter housing didn't have the second connection on the back of the air filter...

https://www.early911sregistry.org/forums/showthread.php?75913-Correct-air-cleaners-for-various-MFI-years&p=572146&viewfull=1#post572146

Everything else you describe matches what I've seen as well...

Here is my disconnected hose for the charcoal tank to the engine shroud...

And i have the drip line.

So where would the charcoal canister connect on a 69 MFI car?

stownsen914 · 12-24-2021, 04:03 AM

@brittbolen - do you know if any of the vent hoses to your fuel tank are plugged? I like Jeff Higgins suggestion above to see if the poor running goes away if you remove the gas cap as a test.

Good you fixed one issue, sounds like you have a few things going on.

As for the hose connections, it will be good to sort those out. I'm suspecting though that the disconnected hoses you're investigating now wouldn't cause the poor running condition.

brittbolen · 12-24-2021, 07:06 AM

Quote:

Originally Posted by stownsen914

@brittbolen - do you know if any of the vent hoses to your fuel tank are plugged? I like Jeff Higgins suggestion above to see if the poor running goes away if you remove the gas cap as a test.

Good you fixed one issue, sounds like you have a few things going on.

As for the hose connections, it will be good to sort those out. I'm suspecting though that the disconnected hoses you're investigating now wouldn't cause the poor running condition.

I did a test without the cap it still ran poorly, but when I put it back on it ran even worse. So it did make some difference I think. But it wasn’t night it day.

The tank vent system is where I’m looking now. All the parts except for the connection to the air cleaner appear to be there. The clear lines are present, rock hard and connected. Both expansion tanks are present. I’ve always had gas smells for the first few miles after a fill up so something needs fixed.

But nothing is obviously blocked. I’m thinking of replacing all the tank vent lines and vapor capture bits, it’s all 52 years old now…

Just wish I knew how the charcoal tank connected to the air cleaner in 69.

I’m also going to either buy the parts to rig up a fuel pressure gauge or just replace the pressure release valve in the console as I think there has been issues there in years past…