CIS FD pressure plunger wear

[Walt Fricke]

My '82 SC motor, running fine last fall, started fine and ran fine moving around from trailer to garage and so on.

Right off on the race track it was clear something was very wrong. If I put the throttle down as one usually does, until WOT, over maybe 3 or 4,000 rpm it would go flat. If I babied the throttle, I could get to 6,000 rpm or so with normal song. That doesn't work when racing. Distributor was rebuild over the winter, and was advancing properly when checked with my dial back timing light.

System and control pressures were within normal limits, though at the high side of the band. Turning the adjustment screw 1/8 or more turn made zero difference. All plugs showed the same quite lean appearance - ceramic in center pretty white, barest hints of a little brown at the tip. The one injector I pulled sprayed better than I remembered from last year.

Because I had so much experience with the WUR last year, I opened it up, found no blockages, blew it out, put it back, no change. Aaargh.

Back home I just pulled the fuel distributor mostly apart. Found absolutely nothing which looked like it would interrupt the flow of fuel to the injectors, or mess things up at all. The main plunger moved like it should and looks perfect.

The one thing I did find was the piston at the far end of the fuel pressure regulator. It has what look like wear markings on it, although you can't feel anything with a finger nail. Pictures attached.





Photos best I can do.

Is this likely to cause the symptoms described? I can go to the chassis dyno, and check to see if system pressure, while 5 bar idling or reving in neutral, falls under load.

Is this, like the main plunger, irreplaceable because custom fit by Bosch at manufacture? Meaning the FD can't be rebuilt/fixed?

I've come up with an alternate possibility for the flat running - a failing ignition coil. It is the original Bosch CD coil. There was some crud around the steel/plastic joint - sort of like some dried grease. It pretty much wiped off with vigorous paper towel use. The tube the distributor wire plugs into had some dark discoloration around its base. And the distributor wire female connector down in the tube looks a bit corroded - dull, anyway - and often it seemed that ohmmeter probes weren't making contact. I recorded primary resistqances of 0.6 to 0.7 ohms (within limits I think. But the secondary was ~768 ohms, and varied. The Blaster coil I put in has 5.3K ohms secondary resistance.

Then there is the mystery of the wires.



The old coil wire had two resistor connectors, one on each end. Its resistance was ~1.163 K ohms. Readings varied some, perhaps because the end plugged into the coil seemed a bit corroded, or at least not a fairly bright dark brass color.

The new one I bought from Pelican had one resistor end, and one crimped plain end. It ohmed out at 1K ohms, nicely half of one having two of these. Resistor ends are wood screw style - they connect to the wire by a wood screw thread being screwed into the center of the wire.

From my junk box I found the third - both ends crimped. Its length suggests it came from my VW beetle race car back when, and unrelated to any CDI stuff. But for the Porsche wires, both from or for CDI systems, why one wire at twice the resistance of the other?

Here is the best I could do to show what the inside of the coil connector looked like:



I have a theory - the coil was on its last legs, extra resistance from the plugs didn't help, and while the spark was strong enough to start, idle, and accelerate while not under load, or under light load, when under the higher cylinder pressures of WOT loading didn't produce enough spark for consistent firing.

Of course, this is larded with wishful thinking. And I don't see how this would lead to lean plugs. Which is why I disassembled the FD, sort of hoping to find something I could clean crudded up. The most I found were the discolorations on the piston shown above, and some light deposits on the parts of the thin diaphragm where no fuel goes and it is just pressed between the two halves. This came off with paper toweling, and where stubborn with some carb cleaner added. Can't possibly have meant anything. Slits looked good, though I don't have a microscope.

All that looked like it might be a bit out of place were very little dents where the snouts of the differential pressure regulators sit on the diaphragm, but they were about the width of the snout which sticks out when you remove the spring and its associated parts, and were not on all six. With very homogenous spark plug readings, don't know how these could be a problem.

Could a microscopic enlargement of the cylinder clearance in the system pressure regulator cause too little fuel to flow under high load? The regulator sees the same pressure from the fuel pump all the time, doesn't it? Pump is a single speed, and is always seeing a 5 bar limit so it ought always to flow as much fuel, with the amount of return fuel being the variable? However, under load does the regulator assembly have to move back and forth to maintain that 5 bar, returning more or less fuel depending on the demand the slit and piston system creates?

The areas of the regulator piston don't look like they could be something which happened over the winter as the car sat. Or could they be oxidation related to the alcohol? Enough to affect something?

[jason2guy]

my system pressure piston has a similar mark on one side. i dont think thats causing a problem, main thing is that oring on the end. youve got a rebuilt distributor, thats good. i went with a M&W pro10 cdi box and an accel coil, and magnecor wires. its a great setup. get a new coil, they are not that expensive then you can rule that out. oh and yes ive read you can not replace that piston if it goes bad. you could always go to pickurpart get one and try it out

[T77911S]

did you check system pressure? if that is good the plunger is fine.

you have to have the ignition in good working order before you start on fuel porblems.
the resistances for the coil are good but the corrosion is not.
put on a good set of ignition wires.

lean could be caused by an air leak.
it could also have something to do with the frequency valve and electronics.
fuse 18 provides power along with the relay.
i believe there is a throttle switch that puts it in open loop mode at around 3k RPM. (?).

if the fuse is bad or the relay is bad it will run very vrey lean...if it would run at all.

the rubber O ring on the plunger is what seals or shuts off the fuel for the system pressure.
any wear on the plunger side should not really effect the pressure.
the return fuel from the WUR comes in the end of the plunger and out the hole in the side. then both that and sytem pressure retunr to the tank.
the other part that came out of the end of the plunger that looks like a golf tee with an O ring on it is actually a check valve for the WUR. when the fuel pump shuts off and pressure drops, the big plunger closes along with the golf tee. this shuts off the return from the WUR to help maintain a higher residual pressure.

[T77911S]

there should have been 1-3 washers at the base of the spring. those are what adjusts the system pressure. thicker washers reduce system pressure.
most people think that raising the system pressure means more fuel. that is actually backwords. it makes the engine run lean so then you have to lower the CP to make it run right.
its a balancing act betweeen the springs inside the fuel head and the system pressure pushing on them.

[Targalid]

If someone else had described those symptoms to me I would have said to change the fuel filter. Seems like classic plugged fuel filter to me: loses power under load, comes back when you let off the throttle. You probably already did that though.

[Walt Fricke]

Thanks, guys.

FUEL FILTER: I backflushed the filter and no particulates came out. And flushed it both ways with injector cleaner. And put a bunch of injector cleaner in it to get it to the FD and injectors. But it could still be varnished or something from sitting all winter. I suppose I could just replace it out of caution. I suppose another way to test the filter would be to measure the fuel volume for 30 seconds as Bosch suggests. I did that last year, but that was then. If the filter is clogged, the volume delivered should be too low, right? The spec is for a no restriction flow.

I am a bit puzzled by fuel flow, though. The fuel pump spins at a given RPM. The fuel goes to the filter, then the regulator, and then gets branched off to the control and system pressure sides of things, and the fuel return. When things are working right, the pump should deliver the same volume of fuel all the time, and whatever doesn't flow through the FD should be returned to the tank, along with what comes from the WUR?

If the filter is restrictive of flow, wouldn't that show up as a pressure drop by the time it gets to the regulator? Could a low flow still be enough to show 5 bar at low load, but falling at high load? I should be able to check this with the system pressure measurement system in place on the chassis dyno.

Some while ago I opened a failed fuel pump. I saw nothing wrong inside, so didn't learn much. I don't remember if these have an internal bypass, so that if there is too much restriction on the output, some of it bypasses back to the inlet so the pump doesn't get overworked. Holley carburetor pumps do just this, and it is also the approach taken on the 911 oil pressure pumps after 1976 or so.

SYSTEM PRESSURE is just fine - 5 bar same as it was last year. Ditto with CP - at the high end of the band, but within spec.

However, I thought that higher system pressure would, indeed, cause more flow. As long, perhaps, as a change was made in the WUR? A sort of hot rod trick was to make a restriction in the return line with some kind of clamp, which raised the system pressure? Not legal in my class, so I've never experimented, and I've not fiddled with the washers on the golf tee part.

Good to know that the funny looking markings, whether wear or not, on that part of the system pressure regulator are not likely to cause a problem. I've been trying to find a source of a Viton O ring of the dimensions needed for the end of that regulator for a while now, but haven't succeeded. I have been warned that that O ring can go bad, and create all sorts of hard to diagnose problems intermittently. Fortunately (or alas), it looks just fine.

If the multiple changes I have made end up with the motor running well, I will call it good, though I can at some point check by putting old parts (spark coil, wire to distributor, fuel filter) back in one at a time to see if one brings the symptoms back.

SPARK: I've got one of those "check your spark" things you plug in between the distributor and plug and can look at the spark jumping a gap. Nothing fancy. I wonder if that will show up a weak spark? It certainly would show up no spark.

FV: The frequency valve works fine despite the O2 sensor not being hooked up. You can feel it vibrating, and when its connection is removed at idle the idle goes to pot.

THROTTLE SWITCH: I can't see into the WOT (or 3k or whatnot) switch area very well, but am fabricating a connector so I can check to see when it closes or opens while manipulating the throttle. I can't see it going bad over the winter, but who knows.

AIR LEAKS: My understanding of air leaks is that they have a large effect at idle, but much less at high RPMs. Of course, a really large one, like I once had in the rubber boot connecting air measuring side on the right with the throttle body on the left, will affect everything (as it turns out, duct tape over a tear in this will let you run a long, long while before you get a new boot). But I don't think a failed auxilliay air slide valve - stuck in the open position - would have much effect of high RPM air/fuel ratios as long as this had been compensated for at idle by ways of decreasing other air flows (the idle adjuster) and increasing fuel flow (the 3mm set screw).

But I can blow some propane around the likely suspects and see if the idle increases. I checked all the other air components - AAV and the one which leans decellerations - last year, and all were working properly and the hoses were good.

[stormcrow]

You might want to check your catalatic comverter - Mine on my 78 had disintegrated and as a result I didn't have any power. Removed it looked down inside and the inlet side cell had crumbled into pieces blocking exhaust flow.

If all your pressures are correct now as they were last year when your P-car was running fine then I would check the Cat converter. You may be able to feel the exhaust coming out when you step on the throttle to see if you get good exhaust before you take it off.

But what's going on with your P-car sounds very similar to what my 78 was doing with the blocked cat.

[Walt Fricke]

Thanks - but I have headers and straight pipes - very loud. I suppose I could check to see if mice brought stuff up the pipes, but I'd have expected anything like that would have blown itself out - I checked my on--track data, and at least once nursed it up to 6,500 rpm - that's a lot of air moved.

[manbridge 74]

I seem to recall an old Bosch CIS manual stating there should be no dents in the FD partition. Not sure this could be the issue.... Got some pics?

[manbridge 74]

I seem to recall an old Bosch CIS manual stating there should be no dents in the FD partition. Not sure this could be the issue.... Got some pics?

From another thread.....

If you do that with a painted cast iron euro fuel head be very careful and only turn the 6 individual 3mm allen heads (that are under the 4mm allen head button screw covers) that adjust spring tension pushing down on the stiff stainless steel individual injector final fuel metering diaphram a tiny bit clockwise at a time or the metal diaphram will possibly be dented or dimpled and that'll damage it's ability to accurately and consistanly meter fuel against the small fuel orifices above it permanently.

[boyt911sc]

Originally Posted by T77911S

there should have been 1-3 washers at the base of the spring. those are what adjusts the system pressure. thicker washers reduce system pressure.
most people think that raising the system pressure means more fuel. that is actually backwords. it makes the engine run lean so then you have to lower the CP to make it run right.
its a balancing act betweeen the springs inside the fuel head and the system pressure pushing on them.

Ty,

I beg to disagree with your assumption about the thickness of the washer/s for setting the system fuel pressure. The thicker the amount of washer/s you place under the spring, the higher would be the primary (system) pressure. What made you think it is the other way around?

Tony

[Walt Fricke]

Sorry, I reassembled the FD without taking a picture of the dents/dimples. Small, way less than the diameter of the "plate" which the spring pushes down on the diaphragm.

Last year I adjusted the five screws whose covers I could remove to match the flow of the one I couldn't. I didn't think I screwed them all that far in, but maybe I did. Didn't hurt running last year, though, so why now?

Does anyone sell this diaphragm?

[boyt911sc]

Originally Posted by Walt Fricke

Sorry, I reassembled the FD without taking a picture of the dents/dimples. Small, way less than the diameter of the "plate" which the spring pushes down on the diaphragm.

Last year I adjusted the five screws whose covers I could remove to match the flow of the one I couldn't. I didn't think I screwed them all that far in, but maybe I did. Didn't hurt running last year, though, so why now?

Does anyone sell this diaphragm?

Walt,

The Bosch fuel distributor repair kit is available from American Precision Machinery located in Reno, NV. Not sure if they will sell just the diaphragm less o-rings. You could try and call them.

Tony

[stormcrow]

Walt

If you took the FD apart that is where your problem may lie. There are rebuilding kits you can buy along with the diaphragm. I have seen the diaphragm listed on e-bay for around $45 - $65 - I don't know if Pelican sells them. You can also buy the o-rings along with the springs to do a complete rebuild. Just do a search.

Taking the FD apart is a bear because there are small o-rings that fit around the opening to each of the injector ports. So you need to be careful should you decide to do this.

[boyt911sc]

Originally Posted by stormcrow

Walt

If you took the FD apart that is where your problem may lie. There are rebuilding kits you can buy along with the diaphragm. I have seen the diaphragm listed on e-bay for around $45 - $65 - I don't know if Pelican sells them. You can also buy the o-rings along with the springs to do a complete rebuild. Just do a search.

Taking the FD apart is a bear because there are small o-rings that fit around the opening to each of the injector ports. So you need to be careful should you decide to do this.

If you know what you are doing and understand how the system works, any DIY'ers could do this FD rebuild easily. Once you develop the skill and confidence to disassemble and rebuild a Bosch fuel distributor, there's nothing to it. It is the testing and calibration that takes a lot of time.

Tony

[T77911S]

Originally Posted by boyt911sc

Ty,

I beg to disagree with your assumption about the thickness of the washer/s for setting the system fuel pressure. The thicker the amount of washer/s you place under the spring, the higher would be the primary (system) pressure. What made you think it is the other way around?

Tony

you are right. just brain farted. thicker washer would make it close building pressure and reducing fuel return to the tank.
thanks.

[T77911S]

is this a full time track car?
was it running fine on the track before?

walt,
think of an outside water faucet that has a very very slow drip. now connect a hose to it and close the end of the hose. it may take a day or so but eventually that hose will build up to the water pressure coming into your house.
same thing with CIS.
low fuel demands dont show restrictions or fuel volume problems like at idle. hi RPM or extended hi RPM is where fuel supply is highest shows fuel flow problems.

you can do a fuel flow test. i think it is checked at the output or return from the FD to the tank. (it is on the 930). there is also a fuel flow spec to the WUR.

with the headers and no muffler you are already leaning out the top end due to low restrcition on the exhaust side. add an air leak and you really lean it out.

once it is running the way it was i would:
drop the CP a little
connect the vacuum retard. (this will help with a rich idle).
then i would richen the mixture as much as the idle can stand. blip the throttle and richen it until the RPM's drop then come back up. lean it out just enough so it does not do that.
( i have done tests with vacuum retard and idle. yes a less advanced idle timine will make it run hotter AT IDLE if you let it idle for a long time. BUT, it also leans out the idle. i tested this on my 77s and just checked it on my 930. it leaned it out 1 point. AFR was 11:1 at 950 and it leaned out to 12:1 with idle brought up to 950.)
colder plugs. i dont know what heat range you ar running but a track car really needs a colder plug. be careful here. with the rich idle that could foul a cold plug when starting or idleing.

[Walt Fricke]

It is a full time track car, and on the dyno last year was running at about 12:1 under load, which seemed fine. So the overwinter change is what is puzzling. Only change was rebuilding the distributor, and checking the advance with the dial back showed that didn't screw anything up. The distributor worked last year, but only with periodic flushing with brake cleaner and squirting white grease in, so the rebuild was mostly just cleaning - the bushings were fine.

The CIS gauges at the dyno should show up any fuel pump pressure issues (or blockages between pump and CIS).

I've got the vacuum lines attached. I don't think they do much good for track purposes, but I don't see harm either so easier to leave attached. That diaphragm isn't leaking.

I'd prefer to see CPs on the low end of the band, but getting there is not simple. At a certain temperature the bimetal arm disengages the top of the spring. At least that is what I understand. At that point, the CP is set by the basic spring strength and base location. So fussing with the bimetallic arm's base attachment won't do anything.

Instead, you'd have to move the coil springs' base locations down. It appears this can be done, as the assembly there, with diaphragms associated with vacuum or just ambient air pressure, can be screwed up or down. However, on my WUR there is no exposed Allen hex. I think I could cut a hole in the cover on the bottom, and so expose it and thus adjust?

But my thinking is that the 3mm screw adjustment can move the actual mixture (though not affecting CP in any way) to where I want it. And the mixture I had was rich enough and ran well enough last year that I am inclined to leave the WUR alone if I can fix my problem otherwise.

Thanks for the info on new FD diaphragms. I am aware from past experience of the six O rings inside. They seemed in fine shape (one reverted almost to its original round shape on removal), and I lubricated their outsides to make sliding things back together easier, which seemed to work.

As things are, no chance at all of the mixture fouling plugs.

The dyno should tell if the numerous changes have by accident resolved things or not.

[Walt Fricke]

Tony - looks like APM only sells via e-bay, and may be located in CA. It is a kit, but never hurts to have the other parts. I ordered one, and am going to put off reinstalling the FD until it comes and I replace the diaphragm just in case. R&Ring the whole assembly isn't that tricky, but awkward with the engine in the car. I had to loosen the right side rear engine mount to drop it a little.

You are right about this CIS stuff - the mechanical part isn't all that tricky to R&R. Knowing what it all does is an other matter. And there is lots one doesn't need to know - what is the diameter of the control pressure bleed hole in the diaphragm? As long as the parts makers know, all is well. APM has a nice PDF on how to R&R an aluminum bodied Mercedes FD, which at least shows how to do various things. Like how to remove the pieces which hold the O rings which go around the slits. At a minimum, removing these means you can get a closer look at the slots.

[T77911S]

if the engine was running fine before, i would not change anything then.


i would not jump into an FD rebuild. i would test other things first. if that is the route you are looking at i would test the fuel injector flow volume. make sure each injector puts out the same as the others. if that is OK then i would look at other things first.
do the fuel flow tests. there should be one for fuel volume out of the FD to the WUR and one out of the FD for the fuel pump.

i doubt tony will agree with me on this one but IF you do pull it apart, i would only pull the bottom off. leave the cylinder that the plunger slides in in the top. inspect the metal shim where the valves seat or close against it. look for little "dimples" if you have them, the shim is bad. (i dont know if the SC heads dimple the shim like the 930 ones do). if you get into the top of the HD, put a little grease on the O rings as you put it back together, this will help getting all those O rings back in. the other MAJOR problem is lining up the slits in the cylinder with the openings in the top of the FD.
the other thing is you have to "calibrate" the fuel delivery out of the FD. at the minimum you have to balance each injector.

i really doubt you have a FD problem.

i would also go back and look at the dist and advance.