MFI 2.2 Running Lean after long drive

['71 911E Coupe]

Hey everyone,

The car is a 71 911E with stock 2.2l with MFI. I know all about CMA, just wanted to see what anyone thoughts were with the problem I encountered yesterday.

I was driving about 50 miles, a little over an hour, and the car was running great. I was on the highway cruising in 5th gear at about 65 MPH going up and down some hills and the engine started to lose power, I heard that "popping" sound out of the intake stacks, a few lean backfires, so I pulled off the road and the car stalled. Could not get the car started again, kept trying and if it would turn over it had a really lean misfire then would die out again.

I let the engine cool off (about 200 degrees) and felt the electric fuel pump which is located near the engine, in front of the rear wheel. It was pretty hot, so I believe that I wasn't getting enough fuel up to the MFI pump. After waiting a while the car started up and ran normal again. Went to drive the car home, started up like normal, but at about the same distance and time when I encountered the problem in the morning, on my way home I had the same issue about ten miles from home. Let it cool off again, then was able to get it home.

Is it possible the electric fuel pump is getting too hot and the brushes are not making contact so the current running through the pump is causing it to get hot? I never drove the car that many miles on a hot day before.

The pump was taken apart about 4 years ago and cleaned up by a good Porsche mechanic, Chris Radbill in West Chester, PA. However the pump is 43 years old, so this is nothing with the work he did. Also the car has about 90,000 miles on it.

Thanks!

[Flieger]

If the screen filter in the tank is clogged it will mean the pump starts sucking so hard the fuel boils (cavitates) and will not have sufficient mass flow to cool the pump.

['71 911E Coupe]

Originally Posted by Flieger

If the screen filter in the tank is clogged it will mean the pump starts sucking so hard the fuel boils (cavitates) and will not have sufficient mass flow to cool the pump.

Thanks, I will check the screen out first.

['71 911E Coupe]

I actually had that tank filter out several years ago. I remember I had two of them. Neither were in excellent condition but I put the cleaner looking one in the tank so now that I think about it, with all of the crap in the bottom of the tanks at gas stations, I would not be surprised if this is clogged.

If this is the case, is there a source to buy a new one of these?

BTW, I just replaced the main fuel filter which is before the MFI pump less than 500 miles ago (about 1 year ago)

Thanks!

[The cap'n]

I've always ordered the screen filter from the dealer. Be specific on the application as there are multiple types.

The Cap'n

['71 911E Coupe]

Hi All,

Finally got around to replacing the screen in the tank with a brand new one. Only $41 dollars and it came packaged real nice. I love buying Porsche parts.

I am also going to relocate the fuel pump from the rear near the heat exchangers to the front of the car.

My question is, I know many people have them mounted near the steering rack but I was wondering if I could mount it in the smugglers box like the RSR cars? Just for easy access when I open the hood.

Does anyone think the pump will have trouble pulling fuel "up" from the tank?

[zedsn]

Mine is mounted below the gas tank up front inside the removable cover.

[The cap'n]

I wouldn't put a pump designed as a "pusher" higher than the fuel it's likely to be pulling. MFI pumps are pretty much NLA, and hastening its demise might not be a good idea.

The Cap'n

[Flieger]

RSR's ran MFI so I would think smuggler's box is ok. Cavitation doesn't set in gradually, it's kind of an all or nothing deal where you are sucking so hard on the fuel that it boils, and I think this type of pump is more tolerant of the cavitation bubles than an impeller type.

[stownsen914]

Your fuel system may not be making enough fuel pressure, which will definitely cause the MFI system to not run well. It needs 15 psi. You may get away with 12-13, but no less. There are various reasons why pressure may be too low. Clogged filters are a common on as suggested already. Fuel pump could be going bad. Check the volume your fuel pump is putting out by disconnecting a fuel line and measuring how much fuel per minute or whatever the spec is.

There's a less obvious reason for the issue you're seeing, which I discovered the hard way. The fitting for the return line on the fuel filter console actually has a check valve in it. It's a simple, spring-loaded device that's supposed to only open when pressure reaches 15 psi, which is what the mechanical pump requires. When the spring gets old (as they all are now), it can get weak and not hold 15 psi anymore. The symptom of this is generally crappy running at all RPMs and loads, but more evident at heavier loads. When this happened to me, I discovered it by measuring the fuel pressure, and fixed it by putting an aftermarket fuel pressure regulator in the return line to act as a replacement for the check valve.

Scott

['71 911E Coupe]

Hi All,

Thanks for your replies. The fuel filter has less than 200 miles on it and I have replaced the check valve at the fuel filter console. Several years ago the car would continue to stall, turned out to be that valve.

I would not doubt if the pump is on its way out, I'm sure it is the original one and along with being located back near the heat exchangers all of those years it might not be performing as well as it should 100% of the time.

However, I am going to do a pressure and volume check as well a vacuum check on the intake side of the pump. Hopefully this will shed a little more light on how the pump is performing. Then I will decide on where to locate it. If the pump needs to be replaced I might try the Zim's replacement kit.

I am really starting to miss that MFI sound so I have to get in gear and work on the car this weekend. I'll keep everyone posted.