Unsticking a stuck MFI rack

[lopena]

Hello, gang:

This topic has been covered a number of times in the past, but I’ve learned a great deal about this in the past few months so I thought I’d pass on a few tricks.

The MFI rack, deep in the bowels of the MFI pump, is supposed to shift to and fro thereby changing the fuel mixture from rich to lean depending on the engine’s requirements. When the rack is all the way forward (towards front of car) the mixture will be full rich.

If your MFI rack is stuck and doesn’t move back and forth (mostly due to inactivity) your car will never run right. So, how do you know if you’re dealing with a stuck rack (or “control rod” in factory speak)? Here’s the process for diagnosing a stuck rack:

- With engine off, remove the air cleaner assembly.
- Locate the rubber plug on the front of the MFI pump (closer to the front of the car) and remove it.
- Get a M5 x 30 bolt and screw it into the end of the rack.
- Pull the rack towards the front of the car using the bolt and let go of it. If the rack is free it should snap back towards the rear of the car.
- If you weren’t able to pull the rack forward or if the rack didn’t snap rearward when you let go of the bolt you have a stuck rack! (believe it or not, the Workshop Manual says “replace the MFI pump if the rack is stuck”).

Now that you have confirmed a stuck rack how do you fix it? Actually, it’s pretty simple:

- Spray some WD-40 into the hole that the bolt is sticking out of…use the red aerosol straw to do this.
- “Wait a minute!” you’re saying…”shouldn’t you use carb cleaner?” No!! Carb cleaner isn’t a good lubricant and that’s what you need to lube the rack and get it moving. WD-40 is the right move…trust me on this.
- Using the bolt, start moving the rack back and forth…it may take several (30?) minutes of serious effort to get the rack moving.
- Continue spraying and manipulating the rack for as long as it takes to get the rack to move smoothly through its full range (about an inch or so).
- Eventually the rack will be free enough to move through its entire range and to snap back when you pull it forward and let go of the bolt. Don’t stop until you achieve snap-back!!

Now take the car out for a ‘brisk’ drive.

Good luck!
Alan
N.J.

[ejboyd5]

Another improper application of WD-40 based on a misunderstanding of its intended use and capabilities.

[lopena]

Except, it was the only thing that worked.

Alan
N.J.

[72targa]

Another improper application of WD-40 based on a misunderstanding of its intended use and capabilities.

And you recommend…

[2.5MFI]

This is a great forum with lots of helpful information given to guide Porsche owners with DIY repairs. I have to give thanks to people like Alan for providing information on their DIY repairs. Sometimes there are misunderstanding's so I would like to add a few items to this MFI topic.

- The control rack with the 5mm x 0.8 threaded hole on the end is the 2-piece control rack from a 69 year only. From 70 on it's a single piece rack with no 5mm threaded hole.
- The control rack is not what gets stuck/sticking. It's a plunger or two that gets stuck in it's cylinder which prevents the control rack from moving. Nothing you spray into the control rack will ever makes it's way up to the plungers.
If you have a 69 pump with the 5mm bolt hole, that has a sticky plunger you may be able to free-up the plunger by moving the rack back and forth as Alan has explained. If a plunger is frozen this will not work.
In the 20 years of working on MFI pumps, I have never had a sticky/frozen control rack on it's own. It's always been a plunger that is the cause.

[BOYER73S]

I believe Mark

[lopena]

Mark:

Thank you for the very useful and relevant information. Your MFI expertise is very much appreciated by all on this forum.

I think you’re right…when I manipulated the rack to and fro it probably freed up a sticking plunger. But I must say that after many minutes of trying I couldn’t move the rack at all until I doused it with WD-40. I also opened the side plate (the one with the oil return line) just a little and stuck the straw in to give it a couple of spritzes. Regardless of how it happened I managed to free up my rack.

So, let me ask you: short of removing the whole damn MFI pump, how does one free up a frozen or sticky plunger? I thought about opening up the high pressure fuel lines and spraying some carb cleaner down each hole but I was nervous that the lines might leak afterwards.

Your thoughts?

Many thanks!
Alan

[2.5MFI]

Well, First of all, why are the plungers getting stuck. 99% of the time this occurs because the engine has not run for a long time, like in storage for a year or more. The fuel left in the pump starts to evaporate and turns into a thin film or stain in the upper section of the plunger/cylinders. Doesn't take much because the clearance between the plunger and cylinder bore is about .00001 inch. ( I don't have the correct figure in front of me but will post it). To try and repair this with the pump on the engine requires the removal of all the fuel lines off the pump, remove pressure valve holders ( fuel line connector ) and all 6 pressure valves. The pressure valves are held in place with a teflon seal and when the holder is torqued down expand the seal and making this a little hard to remove without the threaded tool that screws down onto the pressure valve for removal. With all 6 pressure valves removed you can see the top of the plungers/cylinders. With the cog belt removed from the pump/cam you can rotate the pump pulley by hand and see the plungers that are stuck at TDC. Another problem occurs when rotating the pump, the cylinders might want to move up and out of their correct position which can cause the cylinder to rotate and misalign the fuel port holes.
If you get this far with no problems then you could try and spray some very light lubricant/cleaner onto the top of the stuck plunger. Then with a wooden dowel position dead center of plunger top, tap down with a small hammer ( sounds terrible ) while spraying lubricant. Still at the same time trying to keep the cylinders in place while rotating the pump pulley.
As you can see it's not a good idea to try this with the pump in place.

[lopena]

Many thanks, Mark.

So, it’s quite a process to get access to those plungers with the pump in place. Luckily I don’t need to do that right now…the car is now running fine.

You’re right about inactivity being the root cause of sticky plungers, but problems do occur within only a few short months. I often travel for a few months at a time and when I get home I invariably have a stuck rack. I blame today’s crappy gas laced with ethanol.

Would a fuel additive like Techron possibly help?

Thanks for all of your help and support!

Alan

[62S-R-S]

You’re right about inactivity being the root cause of sticky plungers, but problems do occur within only a few short months. I often travel for a few months at a time and when I get home I invariably have a stuck rack. I blame today’s crappy gas laced with ethanol...

Too bad there's no straight forward path to flow wd-40 at the top of the plungers, without removing the unit..