Problems with MFI tuning!

[Baurice]

Hi from NZ!

I run a 1974 2.7 MFI Carrera. Some may remember from my previous posts that I've had the engine rebuilt in 2004.

Its been running very well since then and I'm very pleased with it - apart from the odd occasion when it has choked the spark plugs. I've put this down to the type of driving I do (low milage daily driver) and have changed the plugs to 6 to try and reduce this.

Now having got all the ten demandments correct I can get the MFi tuned absolutely spot on! Then after a short period (days) it goes completely out of tune (usually lean) and needs two whole turns on the rack to get back to running as per normal. It then needs more tuning back to where it was and we start again!

How frustrating!

What do you think?
Could the rack be sticking? I've taken off the rubber cap and pushed the rack back and forth. It doesn't seem to stick.
Do I need to send it for refurbishment?
If so where is the best place for this?

Cheers
Andrew
CHCH
NZ

[Baurice]

Ideas anyone?

[Early_S_Man]

Andrew,

For starters, you could remove the pump, clean the outside thoroughly, and remove the covers. Next, soak in a heated ultrasonic cleaner for a couple of days in B-12 Chemtool @ 120°F. I know the B-12 is hard to find in your part of the world [long way to Ft. Worth TX, I guess] ... so you could try a mix of 33% Zylene, 33% Toluene, & 33% Acetone, the principal ingredients in B-12. If a heated ultrasonic cleaner isn't available, you could just soak it in a covered metal pan, outdoors ... exercising the rack and turning over the cam drive a few turns every 8 - 12 hours.

Some people are plugging up the pump after filling the sump with Mobil 1, on the theory that combustion products don't need to be introduced to the internals of an MFI pump!

[Baurice]

Thanks for that Warren! What do you mean in your last paragraph? Is what you're saying that the pump isolated from the rest of the engine's oil supply once it is full of mobil 1?

Cheers
Andrew

[Grady Clay]

Andrew,

I’ll second Warren’s recommendation. What he is suggesting is something is sticking in the pump. A clue is your needing to use NGK BP6ES. That is usually way to hot, even in NZ winter. While it may be your use (as you say), it can also be from being way rich.

In Warren’s last paragraph, he is suggesting to isolate the MFI pump oil from the engine oil. This a good diagnostic tool. There is some controversy if this appropriate in the long term.

Any time there is a “mystery” issue with MFI, you absolutely need to check all the other CMA points (search CMA2). I can’t tell you the number of times I have seen the process go down repairing MFI components when the actual problem was internal to the ignition, fuel delivery, mechanical or something other. Don’t assume anything – test. Repeat regularly.

I have had situations where we went through the process many times before we found an intermittent problem.

Part of your solution is to do some regular long distance and sporting driving (I wish my docs would recommend this as a diagnostic tool). It is very difficult to diagnose the system if it is running poorly and never pushed for power. These are race engines that Porsche made suitable for the street.

Reporting test results and conditions will help us with diagnosis.

Note: If it has been very rich and never hot, you may have oil dilution with gasoline … err petrol. Very dangerous for your engine. Change the oil!!!

Best,
Grady

[frogger]

Andrew, is it the idle adjustment or the part load adjustment that you find yourself adjusting every few days?

[Baurice]

Thanks Guys
CMA is correct according to Jon Warring who I trust. I'm heading to the rack sticking as the whole system can be tuned perfectly. We have just replaced the microswitch for the rpm cutoff. We'll see if that helps.
Thanks for the advice on the oil Grady - I have replaced it as it was contaminated with petrol.
Frogger - its the partload adjustment. I tend to leave the idle alone

Cheers
Andrew

[frogger]

Here's a picture of the insides of the injection pump when the side cover is removed. I'm wondering if the toothed gear is worn, or somehow failing to stay engaged into the part load adjustment linkage.

[Baurice]

Interesting!

Thanks frogger