Control Piston seized on Bosch CIS 911/930 Turbo

[ewave]

Help!

After letting my 1989 911/930 turbo sit for a few months, it wouldn't start. I identified the problem to be a seized control piston in the Bosch CIS fuel distributor.

I took the whole thing fuel distributor apart... Not sure if that was a good idea or not. But I still can't get the piston to move. I tried to grab onto it with a set of vice grips. I just wont move. It is pushed all the way in.

I've got it soaking in WD40 at the moment.

Does anyone have any suggestions, and has anyone else opened up there CIS like this, and been able to put it back together and have it still work? Is there a place to have these rebuilt, or do I need to buy a new one?

[Nostril Cheese]

soak in Kroil and use compressed air.

[safe]

I would be VERY careful using a wicegrip. These pistons are so sensitive that the slighest damage on the surface makes it useless, drop it on the floor and its toast!

[T77911S]

i have not taken one apart in a while, but how you have taken it apart has not done you much good, and i am not sure you want to take it apart the other way. if you had left the big nut on the bottom, the center tube that holds the plunger would have come out and i think you could have pushed the plunger down from the other end, but the draw back. there are 6 rubber orings that seal the center tube to the top housing. if you try to remove the center tube, put the bottom back on because i think the orientation of the tube is very important. the slits in the tube need to line up to the holes in the top. i would not remove the center tube if you can avoid it.
you may want to contact alan l, he has done a lot with his 930 FD and knows them pretty well.
you can grab that little nipple part on the plunger, careful with the flat part, that rides on the sensor plate.

soak it and keep working at it. you may put it back together, the control pressure could help push down as you pull it. carefull what you soak it in, you do not want to mess up those rubber orings.

good luck

[ewave]

Thanks for the advice!

I now have removed the piston AND carrier assemble. The control piston is still stuck, but now I can try to push it out from the bottom.

I carefully made a mark of the position of the carrier, so I THINK I will be able to line it up properly. When I was taking off the keyed locking washer, I THOUGHT that the carrier assembly was screwed in, and that the lock wash was there to adjust the depth-- Not the actual radial orientation. With the positioning mark I think I can get within 1 or 2 %. Do you think this will be enough?

Here's a picture of the control piston IN the carrier.

Wish me luck!

[T77911S]

ah man you have done it now. just kidding. i did the same thing on an old head, i had nothing to lose by taking it apart. when you put it back together, put a little oil on the orings, it will help them slide back in. as far as alignment, i think that notch in the top of the center part is used to line it up once it is back together. i suppose there may be a jig out there to do it.
i am sure you know, keep it very very clean.
since you are in it this far, maybe someone else may know if it is worth it to replace the orings.

[ewave]

Well I got the seized control piston out. No damage. It just had some varnish on it. I carefully took everythign apart, and then washed everything five times, until there was not a single spot of dust or grime on anything... Then in a clean room type environment, I carefully put the whole thing back together. I aligned the piston sleeve with the mark I made prior to dis assembly.

I put the whole system and car back together, and it started up first try. It runs perfectly!

Thanks for all the advice. This was the only thing/system in the car that I had not taken completely apart.

Here are a couple pictures. I'll post a few pics of the car later after I get the A/C working.

[JFairman]

Nice clear pics. Not many owners have taken the fuel head apart because who knows where to get all the new o rings and the diaphram that goes between the upper and lower chamber.

On top of that the 2 places that rebuild them usually won't take one in that has been taken apart by the owner of the car.

Congradulations on taking it apart, cleaning it and putting it back together with the origonal o rings and having it work.

I see 6 more little fuel filter screens to collect junk and inhibit flow on the inlets to the control piston cylinder... great.

[Paulporsche]

Good job!

[Dr J]

Congrats. A few years ago I did my engine drop. Did not put the engine back for almost a year. When I started it I had problems which were essentially the same thing, a stuck piston and had to get a rebuilt head. I'm glad you went through it.

My engine has been out for a rebuild now for 10 months. I'll keep this post handy!

[911st]

People talk about modifying the piston to increase fuel flow. I just do not see where that would have any advantage. There is plenty of room around it relitive to the size of the injector lines.

???

[JFairman]

Never heard of anyone modifying the piston with any kind of succes.

If you're thinking of the flowtech modified fuel head IA used to sell it's the 6 metering slits in the removeable cylinder the piston lives in that are CNC enlarged. I can see how that would make a huge difference, and it does.

They also shim the system pressure regulator to increase fuel pressure to approx 97psi and increase spring tension on the 6 differential valves to increase flow.

The standard IA modified head increases fuel 20%, but you can turn it down to less than that by lowering the spring tension on the differential valves while leaving system pressure the same.

[Oracle]

When my FD was rebuilt the screens were removed, they were clogged and one or two broken. The car never complained about those missing screens..