MFI Problem

Jimal92 · 09-27-2002, 06:01 PM

Hi

I got a 1972 911E with MFI. Had been of the road for a few years before i baught it in the middle of a 993 conversion. Anyway it was not funning very well at all no power spit and sputter when u got on the gas. I took in to have it fixed. Shop determined MFI pump was bad. I got a refurbished pump from Einmalig Porsche. Pump was installed and car runs nice except for an idle problem. Now when u step on the gas and let off it does not idle down it takes a few seconds before returning to idle speed. I noticed this on my way home so i turned around and took it right back to the shop. Any ides?? Micro switch? Speed Switch?? Mechanic is baffled!!!! Thanks James

pwd72s · 09-27-2002, 06:37 PM

James, most of today's mechanics are puzzled by MFI. My suggestion is to try to find a John Walker counterpart where you live. Somebody who has been around a few years, and knows what he is doing...

Jimal92 · 09-27-2002, 07:12 PM

Car is in the best shop in the western New York area a certified Bosch Fuel Injection shop.

pwd72s · 09-27-2002, 08:15 PM

Bosch has made several types of injection over the years. MFI was the first. Dig what I mean? You need somebody who understands the first system...not someone who understands the latest one.

jeff m · 09-27-2002, 08:21 PM

To test your speed switch and micro switch.
raise thje revs of your engine using rack, then push the micro switch (and release) the revs will drop then return to the previous level.
You will get many good results from searching "speed switch"
Jeff
011T

Early_S_Man · 09-27-2002, 09:02 PM

Sticking or slow-returning idle isn't electerical ... it is a linkage binding problem, or possibly adjusted too tight.

First -- try cleaning & lubricating all of the ball & socket joints at #6, #11, & #20 in the diagram below.

Check bushings #15 & #28 (bellcrank on driver's side of transaxle) for wear and play.

There are also bushings and ball & socket at the bellcrank on the left throttle bady at the location circled in red on the second diagram below.

304065 · 09-27-2002, 10:01 PM

Warren (as ever) is right on: what you have is most likely a sticking throttle linkage, worst case a sticky rack. The throttle linkage begins behind the gas pedal, which is where you should begin. Pull the pedal forward: you should hear a "pop" as the throttle linkage ball comes out of the socket on the back of the pedal. Try working the linkage back and forth, if you feel resistance, remove the wooden board that covers the pedal cluster, and lubricate the the bushings.

Jack up the car and support it with multiple jack stands (can't emphasize this safety step enough) and look at where the throttle cable exits the "tunnel" near the rear axle: have a helper work the throttle linkage and see if it's binding down there. The motion of the cable goes through a bellcrank into a solid link that connects to the MFI stacks, which should run straight back from the bellcrank, and have a slight bend in it. Again, if you can feel/hear it binding, lubricate and try again.

Because you have a new pump installed, I wouldn't think that the "rack" in the pump is sticking: more likely the linkage is binding up on you and causing the problem.

If you haven't already, download ALL the information about the MFI system from the Pelican Board: print it out, and read it over. Then read it again. (It took me about 10 runs through to begin to understand, which is perfectly normal). Overhauling the MFI pump is the LAST resort- 99% of the time the problem is somewhere else, and by understanding the system, you will not only be able to school your mechanic, but will be able to diagnose most issues yourself.

As a last resort, my mechanic, Cheech Fernandez, in Elmsford, NY, went to Bosch school on MFI in the late '70's and is one of few professionals who really understand the system. You might call him to ask his opinion if you are stuck.

Good luck! May you be hearing the howl of those stacks soon!

JFC

Swank911 · Swank911's Garage

I had a similiar problem with my MFI 2.4. Turned out the Ball at the end of the throttle pedal linkage was gummed up, not allowing the pedal to return all the way.

If the car is running smoothly at load and is idleing fine when it steps down, check the linkage.

Swank911 · Swank911's Garage

Also, a quick check would be to see if you can manually pull the throttle pedal back when it is at high idle.