MFi running rich problem

adam911 · 05-19-2007, 03:05 AM

Just went out for a short trip in my 71 911 (2.2 mfi). It was the first trip of the day and only about 3 miles. When I was nearing home all of a sudden the car choked up and wouldn't rev and it started to backfire very loudly. Got out the car popped the lid, revvd it and there were plumes of black smoke coming out of the exhaust. It would idle, but as soon as you rev it goes flat and backfires. I'm was thinking it's something to do with the cold start solenoid. I checked the pipe from the heat exchanger to the pump thermostat and that's connected and appears to be working. I spoke to a local Porsche mechanic and he said that it was most probably the cold start mechanism in the pump thermostat housing siezing up. I took the cover off and behind it is a spring loaded rod with lots of washers on it. The washers at the top were thinner and made of a different type of metal. I'm assuming these buckle once warm thereby pushing the rod and closing off the cold start enrichment. Can anyone confirm if this is in fact the case?

Thanks in advance

Adam

adam911 · 05-19-2007, 09:13 AM

Ran the car and removed the thermostat cover and as I released the cover and took the tension off the spring the car stalls. If I push the rod with the washers on in I can start the car and it idles, release pressure on the rod and it dies. So, I'm assuming it's working. I'm guessing in colder weather some of the washers bend and pull the rod back thereby enriching the fuel system, not in, as I originally thought. Someone please correct me if I'm wrong.
I'm now begining to think it's ignition related, as the tacho is still going crazy. When the car is idling the tacho is bouncing up and down. The CDi unit is whistling as usual with the ignition on. I checked the points as I know that they can cause the tacho to behave irratically. I checked the points as I know that they can cause the tacho to behave irratically. They aren't pitted and the gap is correct.

rs dreamer · 09:19 AM

There's tons of info on the thermostat in posts here, just do a search on MFI. You are correct; the discs expand and push the rod. You can check that it is working by removing a little cover plate on top of the pump by the thermostat. There is a second hose underneath the left side of the engine that connects to the heat exchanger that is also needed to provide the hot air to the thermostat. You should check that that one is still properly connected. Gary

Here's a thread to check:

rough running MFI

adam911 · 05-19-2007, 09:18 AM

Gary, I thought that the washers would pull the rod to enrich, as when I take the cover off and the rod is released the car stalls, yet when I push the rod in (the same as in it's natural state with the cover on) the car idles?

As you say I'd better hunt down some posts.

rs dreamer · 09:31 AM

You are correct, I think. When the thermostat is cold, the rod is out and the mixture is enrichened. When the thermostat warms up, the rod moves into the pump and leans the pump.

Also, there is the cold start solenoid on top of the fuel filter console that you need to insure is off after the engine is started. It could be leaking fuel into the TB's and causing a rich mixture. This is only when the car is started cold. Gary

adam911 · 05-19-2007, 09:48 AM

I forgot to mention I had checked the cold start solenoid. I took the pipe off and it squirts once as the ignition is turned. I have another 911 that I'm going to start systematically swapping out. The other 911 has a permatune unit on it and the coil is larger, whereas the 911E I'm having trouble with has a bosch unit with the smaller coil. Does anyone know if the coils can be mixed between the two cdi's?

Grady Clay · 05-19-2007, 04:35 PM

Adam,
Welcome to the Forum

You will find a lot of info here, particularly with solving MFI issues.

The Pelican search function is your best friend.

I’ll encourage you to start by downloading the CMA document. It is the Factory MFI repair and adjustment manual. It isn’t complete as it assumes all new components and the P+A/Bosch MFI school attendance. It is the first (and most important) start for you.

Far more is available on Pelican.

I’ll encourage you to make your own (edited) manual about MFI. When you have a question that you don’t find with the search function, post your question and you will find many experts with answers. Just the process of building your own manual will instruct you about the function and subtleties of MFI.

Owning and maintaining a MFI 911 is a serious responsibility. Mostly it is to educate yourself and pass along the information with your very special car. Another is to be able to appropriately communicate with mechanics and other MFI enthusiasts.

Your initial posts indicate you have done some homework. There is more.

A “meat thermometer” in the outlet of the cold running thermostat on the MFI pump will help determine if it is getting the necessary heat to turn off the cold-running condition.

The recommendation de-actuating the cold-running enrichment mechanism in the MFI pump via the top access plate is very worthwhile. This tells you if the cold running enrichment works and turns off appropriately.

MFI was built by mortals and was designed to be serviced by mortals. It isn’t even Rocket Medicine. This is something you should become skilled at. We can help.

Best,
Grady

kwikt 911 · kwikt 911's Garage

First check the obvious like plug wires, how old are they, the same thing happened to me with my 72 911. Really before messing with MFI it could actually be a plug wire or your ignition points.

Kevin

adam911 · 05-20-2007, 03:07 AM

Thanks for the advice guys.

I am begining to think it's ignition related. I at first thought it was Mfi due to the plumes of smoke, but in reality that could also be caused by non ignition. Especially, as I can get the car to stall by actuating the thermostat. Plus the tacho is bouncing all over the place. I've read in a couple of threads this could be caused by the alternator misfunctioning, so I'm just about to check earths and voltages. Also, as I slowly try to rev the car it's goes to rev then dies as if the ignition were being switched on and of in quick succession. I'll post my findings later today.

Thanks guys.

PS where can I dwonload the CMA document, as I can find reference to it, but not the document itself?

domtoni · 05-20-2007, 05:12 AM

Adam,

Just to re-inforce Grady's comments, I had some problems with my car two years ago.

All I did was go back to the basics (a local Porsche mechanic did the work). We did:
- electrics
- valves
- a bunch of other non - starting related stuff
- set the MFI to the desired levels
- made sure the timing was right and the wires were good

The car starts like a dream, as it should !!!

Best
Dom

Jim Richards · 05-20-2007, 05:38 AM

Quote:

Originally posted by adam911
PS where can I dwonload the CMA document, as I can find reference to it, but not the document itself?

CMA

FA-18C · 05-20-2007, 05:59 AM

I would check your ignition before even thinking of messing with your MFI. I would check your points and plug gaps and timing first.

adam911 · 05-20-2007, 08:55 AM

OK, so I swapped over the CDi unit and the coil from my 72 car put it on the 71 car and it fires up and runs OK. I then fit the CDi and coil from the 71 car to the 72 car and it fires up and runs OK. Go figure, I guess I must have had a bad connection or something.
Thanks for the help guys.