MFI 911 Running Poorly When Warmed Up

[stownsen914]

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.

[brittbolen]

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

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



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?

[brittbolen]

Small tee with junction clearly open…



And large tee… tiny opening only, not really visible

[brittbolen]

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



And new, which lacks that 5mm hose barb



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…



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.




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



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





Done! Hopefully this is helpful for future readers.

[brittbolen]

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



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!

[brittbolen]

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.

[RSTarga]

Did you use fuel resistant hose?

[brittbolen]

Originally Posted by RSTarga

Did you use fuel resistant hose?

Yes, yes I did.

[brittbolen]

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.

[Jeff Higgins]

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.

[stownsen914]

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.

[cmcfaul]

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

[Schulisco]

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

[brittbolen]

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!

[porschedude996]

Are you running both air hoses to the pump?

[Jeff Higgins]

Originally Posted by brittbolen

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!

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.

Originally Posted by porschedude996

Are you running both air hoses to the pump?

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

[brittbolen]

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

[Schulisco]

Originally Posted by brittbolen

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...

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

[brittbolen]

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.

[Schulisco]

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