MFI 911 Running Poorly When Warmed Up
 

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...

http://forums.pelicanparts.com/uploa...1639789287.jpg

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.

http://forums.pelicanparts.com/uploa...1639790455.jpg

Thanks! Which symptoms did you have?

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

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.

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.

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

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

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

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

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.

Could it be something super simple like fuel filter ?

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

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

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

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

http://forums.pelicanparts.com/uploa...1640299752.jpg

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....

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...

http://forums.pelicanparts.com/uploa...1640301356.jpg

And i have the drip line.

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

@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…

The gas cap test is a sign. If you haven't already, check to see if any of the vent lines are actually plugged. If the tank doesn't have at least one clear vent to atmosphere, it will run poorly once the fuel pump starts working against a vacuum. Sort out where the hoses need to connect later - I think that will be secondary to poor running condition.

The things you plan to do all sound like good ideas. Personally I'd want to find the smoking gun before throwing replacement parts at it.

You may also have a fuel pressure issue (multiple possible sources), which might be the next thing to look into once you get your vents sorted.

I’m replacing the evap lines and found something odd…

The large tee fitting closest to the tank, circled in this diagram

http://forums.pelicanparts.com/uploa...1641697229.jpg

Is blocked on the return (smaller diameter) side of the Y. It looks like a manufacturing defect as I can’t see a hole inside the fitting where the Y joins, just a tiny pin prick of light, so any thing that needs to drain through that side would be very slow to drain. The other two tees have the junction fully open.

Is this right? Or should I use file and open up that tee?

Small tee with junction clearly open…

http://forums.pelicanparts.com/uploa...1641757246.jpg

And large tee… tiny opening only, not really visible

http://forums.pelicanparts.com/uploa...1641757286.jpg

So I’ve been swapping all the old evap tubing and cracked tanks, no issues or big surprises, except the new vent chamber is slightly different at the top where the false to the charcoal canister attaches…

Old

http://forums.pelicanparts.com/uploa...1642454055.jpg

And new, which lacks that 5mm hose barb

http://forums.pelicanparts.com/uploa...1642454101.jpg

So I cut that off the old tank opened up the clamp, and pried out the barb and a rubber sleeve that seals it all up…

http://forums.pelicanparts.com/uploa...1642454256.jpg

Gave the opening on the new tank a little bevel to be able to press the barb and rubber sleeve into the new tank, and reclaimed it too.

http://forums.pelicanparts.com/uploa...1642454526.jpg
http://forums.pelicanparts.com/uploa...1642454526.jpg

And back in the car… I know Grady wants me to put good clamps on there!

http://forums.pelicanparts.com/uploa...1642454602.jpg

And some more pics of the evap lines and charcoal lines for completion…

http://forums.pelicanparts.com/uploa...1642454727.jpg
http://forums.pelicanparts.com/uploa...1642454727.jpg
http://forums.pelicanparts.com/uploa...1642454727.jpg

Done! Hopefully this is helpful for future readers.

And we’re all done. I used a plumbing tee to add a connection from the charcoal canister to the air box…

http://forums.pelicanparts.com/uploa...1643151695.jpg

So now my evap/vent system is completely overhauled, only kept the charcoal canister, the tees and most of the clamps. Made sure the air flow front to back was correct.

Now just need to go for a drive. Also excited to see how things smell after a fill up!

No more fuel smell after filling up! Still rough running, will continue to look into the suggestions of fuel press check valve and sock on the tank outlet.

You can see how fuel backs up into the vent system right after filling up.

http://forums.pelicanparts.com/uploa...1643319460.jpg

Did you use fuel resistant hose?

Yes, yes I did.

I replaced the pressure regulator valve on the fuel filter console but no significant improvement. Still misses at speed when warmed up. Couldn’t break 80 on the interstate tonight, started bucking and missing. I guess I’ll look at the tank screen and maybe add a fuel pressure gauge.

It strikes me that nothing you have done specifically addresses any differences in hot vs cold running. In other words, if any of these various hoses were indeed clogged or partially clogged, it would affect both hot and cold running. It's good that you have gotten these issues addressed, but they are clearly not the source of the problem.

Perhaps it is time to look somewhere else, like maybe ignition. Is the coil changing internal resistance as it heats up? The CDI box has already been suggested - maybe you should check it out. I noticed you said you are still running points - I would strongly suggest you replace them with one of the various electronic triggering devices, like the Pertronix. It's impossible to buy decent point anymore, made of proper, quality materials. I don't care what brand you are buying - there just isn't a big enough market anymore to entice the manufacturers to supply it with a quality product.

At the end of the day, you are still looking for something that changes as the car warms up, then "resets" when it is allowed to cool, even briefly. None of the plumbing/venting you have been working on fits that description. Start looking for something that does.

Would be interesting to see AFRs when running well, and when not. It would help you rule out a possible fuel mixture problem, vs. a possible ignition problem as Jeff suggests, and narrow the scope of your troubleshooting.

the connector on the airpox is just a drain tube. Run a hose from it to the underside of the car. Otherwise it will just make the back of your engine filthy.

Chris
73 E

I urgently recommend also to use a smoke detector tool (e.g. one of the Autool ones) to find false air leaks. The Porsche engines are heavily dependent on the mixture. Also use a CO sensor tool to find out, if the mixture is too lean or too rich. As you wrote - the whole stuff is more tahn 50years old - so every component is suspicious...

Thomas

update... it was the fuel pump. pressure would drop from 14 to 4 as things warmed up, and then it stalled. new pump, and a clean out of the sock/screen in the tank for good measure, and the car runs like a dream. 3rd pump in 10 years / 24,000 miles...

also as previously mentioned, i no longer have to smell gas after I fill up!

Are you running both air hoses to the pump?

I think you need to track down the reason why you are experiencing such a high failure rate on the fuel pump. These things typically go 100,000 miles with no issues whatsoever.

I might suggest a lack of use could be contributing. Condensation in the gas tank may be causing it to rust, which will clog the screen, but very small particles will pass through and possibly damage the pump. O-rings in the pump will dry out and leak, so it will not provide adequate pressure and volume. The little pucks on the pump impeller if allowed to dry out may rust or seize in place, causing a loss of pumping efficiency. Stuff like that.

You may want to take some measures to help "preserve" the fuel delivery system (and other systems as well) while the car sits undriven for extended periods. There are lots of products that are intended to do this. Maybe get the gas tank boiled out and coated internally with something that will seal it and prevent it rusting in the future. It sounds like you may just be getting a lot of crud in the system from sitting so much.

He was having problems with the electric fuel pump that delivers fuel to the MFI pump.

those first 2 pumps (after the original pump failed due to dried up seals after pulling the tank for a year of start and stops at fixing front pan rust) were both from Zimms, this third pump is apparently also from Zimms but is a different design for whatever that is worth...

I'll keep an eye on the tank and the pickup sock... tank was stripped 10(?) years ago, not sure what sort of sealant, if any, was applied after...

B

Well done, even without measuring the fuel pressure as mentioned.

Which fuel so you run in the car usually? With Ethanol? Afaik most fuels in US contain up to 10% ethanol, some much more. Probably this could be the reason for the leaks and damaged fuel pumps. You drive 2500mls avg per year. I recommend either fuel without any ethanol or to use a fuel stabilizer instead. This keeps the ethanol containg fuel being usable for at least 6 months without attracting too much water or prevent demixing fuel and ethanol which attacks the seals...

Thomas

I run ethanol free 93 probably 95+% of the time, at least the last 6 years since the car came back from CA and hasn't been on any long trips away from home and the convenient ethanol free pump.

Ok, so 93PON = 98RON here in europe.

BUT: here in europe/germany - every fuel from 95RON ("Super" in Germany) to 98RON/93PON ("Super Plus" in Germany) gas contains at least 5% and optional 10% ethanol!! No exceptions! Only those high performer 100+RON octane gases like "Aral ultimate 102" and "Shell v-power" are free of ethanol! These are superior blends and will be produced in only a few separated refineries. This is a common misunderstanding here. I guess there could be similar confusion in the US.

Don't get me wrong - but are you really really sure, that your favourite gas station sell the 93PON gas as a E0/0% ethanol gas???

Here's a list of US/CAN ethanol free gas stations:
https://www.pure-gas.org/

Fun fact: Aral and Shell are forced by law to denote on their pumps that those special sorts (100+RON) of fuel *could* still contain up to 5% ethanol, even they do not contain ethanol at all. They write these facts on their websites. (https://www.aral.de/de/global/retail/kraftstoffe-und-preise/unsere-kraftstoffe/faq/faq-aral-ultimate.html#accordion_ultimate-4)

Thomas