Damaged FD Plunger

[brianhiggi]

A mechanic recently told me that the plunger in my Fuel Distributor was a little damaged. What symptoms would that result in?

My car is hard to start and has a surging idle when hot. Would it cause any of those problems? '78 SC

[boyt911sc]

Brian,

The FD plunger should slide up & down with very minimal resistance. You could test it by pushing it up and letting it slide down by itself. The tolerance for this plunger is very small and a slight deformation (scratch, burr, rust, etc.) would cause the plunger to bind. Do not use any sand paper to clean the plunger.

Tony

[E Sully]

The plunger is a very high precision piece that must move smoothly, yet prevent any fuel leakage to work properly. It controls the fuel flow volume to the engine. If it is damaged the only option is another fuel distributor. How did he determine this is a problem? Did he remove the fuel distributor and check the plunger, or is his diagnosis from how the engine is running?

[E Sully]

Here is a link to a post about rebuilding the fuel distributor if you want to know what is inside.
I don't see how in normal use the plunger could become damaged. Most of the time rust would develop if left to sit too long, but physical damage would only occur if the plunger was removed and mishandled.
CIS fuel distributer cleanout w/pics

[Vereeken]

Tony,

When you say the fuel plunger should slide up and down with minimal resistance. is this with the fuel pump running?

I have noticed that the plunger can remain seated in its housing when the engine is dead cold but once the fuel pressure comes on it drops and moves freely up and down.

Does this point to something or is this normal behaviour of Bosch FDs?

Thanks,

Michel

[E Sully]

I would say it is normal.

[boyt911sc]

Originally Posted by Vereeken

Tony,

When you say the fuel plunger should slide up and down with minimal resistance. is this with the fuel pump running?

I have noticed that the plunger can remain seated in its housing when the engine is dead cold but once the fuel pressure comes on it drops and moves freely up and down.

Does this point to something or is this normal behaviour of Bosch FDs?

Thanks,

Michel

Michel,

The FD plunger should slide freely up/down with no fuel in the system. With the FD filled with pressurized fuel, there will be a slight resistance to push it up but will drop down faster as you release it. In short, the plunger should be seating or resting on the air sensor plate lever all the time. It should not stay hanging or suspended away from the AFS lever because of the control pressure (downward force) acting on it. If it does, the plunger is slightly binding to the barrel.

Tony

[Vereeken]

Tony,

If it sometimes hangs in the FD (dead cold near freezing temps, no pressure in the system) but moves freely as you described on all other occasions would that cause any cold start issues.

Mine hangs in the 'air' after a long period of inactivity, but drops instantly the moment the FP is turned on. You feel it dropping on the AFS plate lever the moment the FP comes on.

Would this merit a FD clean? It is not that difficult from what I gather, but it seems time consuming...

Sorry too hijack this thread, but I feel it is also relevant to the original poster.


Michel

[E Sully]

While cleaning the FD may seem easy, it can end up in a leak between the 2 halves where the thin steel plate sits. Aside from o rings, parts are not available to rebuild them.
Your situation does not sound like a problem. At most I would say remove the fuel distributor and clean it without disassembling. Remove the piston carefully and see if there are signs of corrosion. If not then do not take it apart. Be very careful not to scratch the piston or cylinder.

[Vereeken]

Ed,

indeed.
That was my idea of a dissasembly as well. I do not intend to overhaul it completely only the plunger.

Michel

[boyt911sc]

Originally Posted by E Sully

While cleaning the FD may seem easy, it can end up in a leak between the 2 halves where the thin steel plate sits. Aside from o rings, parts are not available to rebuild them.
Your situation does not sound like a problem. At most I would say remove the fuel distributor and clean it without disassembling. Remove the piston carefully and see if there are signs of corrosion. If not then do not take it apart. Be very careful not to scratch the piston or cylinder.

Michel,

I completely agree with Ed's suggestion. Refrain from breaking apart a working FD. Remove the FD assembly and inspect the plunger. Do not use emery cloth or sand paper to clean the plunger's surface. The temperature drop could really affect the plunger's movement in cold weather due to contraction. As long as it does not bind when the FP is running, I'll leave it alone.

If it ain't broke, don't fix it!!!!! Are you having any sort of problem with the FD? BTW, are you aware that you have a different AAR in your car? I'm not suggesting you replace it.

Tony

[Vereeken]

Tony,

Sure. However. If it ain't broke, break it ;o)

I will leave it alone for now. I am trying to get this CIS thing properly understood.

The way I see it it it is a system that not many people are familiar with so I better get to grasp with it if I want to enjoy my car for as long as possible.

Yes, I know I have a different AAR. Somewhere on pelican there is an extensive post from myself on Cis troubleshooting on EURO SCs . In that process I have discovered 5 different types of AAR holes (from half moon shape to quarter moon shape to complete black holes).

Do I have a problem? Well yes and no. The original problem is long gone (bucking) but the FIRST cold start of the day is still not what it is supposed to be. The revs still are a bit too low in the morning. The weird thing howvere is that if you start from cold let it run for a minute or so and then come back again and start it again the problem is no longer there...

So I am guessing that either my ccp still is not good (although it falls in the 089 spec sheet from Jims site) or perhaps the FD needs time to wake up...

Perhaps I will experiment some more with lower/higher CCPs...see what that does.

[T77911S]

idont think the FD is your issue, at this point, at least until you check out some other things first.

first, i am curious. what did he say was wrong with the plunger?

cold start issues first need to be looked at more or less inthe following order:

air leaks. smoke test is best here or you can just go thru the system and replace evrything rubber related that has vacuum on it.

pressure.. system pressure, cold and warm control pressure.

tune up. plugs, wires, cap and rotor. along with timing and porperly working advance.

THEN mixture.

the cold start valve and related circuit needs to be looked at. usually if this is bad, it is extremely hard to start if it starts at all.


if the cold idle is low, along with the AAR, the mixture can do this too. if the mixture is off in either direction due to pressures or air leaks, the RPM's can be too low.


back to the plunger.
i would not worry if the plunger does not drop down with no FP. the tolerance in there is very tight. you could pull it out and clean it with carb cleaner and make sure there is no pitting or corrsion on it. other than that, leave the FD alone.
what you dont want is any "free play" between the sensor plate and the plunger.
with the FP on, lift the plate. if there is ANY movement in the sensor plate that does not have resistance, the plunger is not dropping all the way down.

[brianhiggi]

I actually don't remember the exact verbiage that the mechanic used to describe the plunger. He might have said that is was corroded or something along those lines. He did say he cleaned it up and put it back together (he had it apart) but wasn't completely confident it would function properly. The cold start problem started after this visit to the mechanic but it could be due to his adjusting of the air fuel mixture. He did a smoke test and found no air leaks.

Also How can I test the cold start valve?

[boyt911sc]

Michel,

I wish you well and succeed in your troubleshooting. After you had declared VICTORY about troubleshooting your Euro car in the past, I knew the euphoria would not last that long. I wanted to tell you time would prove you're wrong but I did not want to spoil the celebration. You are an experienced troubleshooter and so focused on your work that your forget to look around.

Now, you are back to square one. You should be able to start the car once maybe a second try if you have not driven the car for weeks. And drive the car out of the garage. You don't need a warm up period if your car is correctly maintained.

If you don't have any significant air leak in the system, all you need for cold start (including ignition & sparks) is an acceptable amount of low cold control pressure between 1.0 to 1.5 bar. If the cold control fuel turns out to be greater than 1.5 bar that works well for you, so be it. Just be happy and accept it that it makes the car starts on demand. If I were you, I'll check the WUR. I know you said it was perfect last time.

Tony

[Vereeken]

Oh no everything was fine. Until the WUR i assembled decided to rupture his membrame. (if you recall that was my other post on WUR control pressure that fell back to 2.5 bar after revving it). A rupture in the membrame. It did not hold its vacuum but it could build it up again, until the next vibration or whatever.

The issue I have now is far less troublesome. If I start from dead cold it fires right up but immediately settles to lets say 1.100 rpm where I would like to see 1.400 rpm for current temps. Muche nicer to drive when cold.

Here is the weird part after that initial start, you can turn the engine off. if you wait a couple of minutes and then start it up again it fires right up and setlles to 1400 then comes down as the pressure builds and the AAR closes.

THe wur I have now is a mix and match and sits currently at 1.0 bar at 7 degrees celcius.

too me it says an issue with ccp but I do not know if I should go up a bit or down. It seems up rather then down...Problem is I did not care to make this one adjustable.

[E Sully]

I think we should get back to the original post.
Brianhiggi,
How does the car drive aside from the cold start and idle issue? Does it run smoothly as you accelerate? What do the spark plugs look like?
You can apply 12V to the cold start valve and see if the solenoid clicks. Also use a meter on the electrical connector and see if it is getting voltage when you turn the key to start with the engine cold. Check out this thread on checking cold start problems.
Thermo Time Switch 1978 SC 3.0

[Vereeken]

Originally Posted by brianhiggi

My car is hard to start and has a surging idle when hot. Would it cause any of those problems? '78 SC

Right you are ED.

From the CIS documentation I have.:

Possible cause of a surging when hot:

Pivot bearing (of the air throttle plate) worn as a result the car looks for a steady idle but can not find it. this can potentially be tuned away with the mixture valve.

The pivot bearing is not the same as the FD plunger.

Another possibility: Decel valve broken diaphragm

Leaky CSV

Or simply too rich co-setting.

Vacuum leaks.

If you take this list from the bottom to the top (easy and cheap to difficult and expensive) it should put you on your way.

Do you have access to an AF ratio meter? Like an innovate LM1 or 2.

From first hand experience. My car ( a 83 SC) hunts at Idle with AFR at 12.8 it is steady as a rock from 13.2 upwards. And is economical (;o) from 13.8/14.0 and still delivers a smooth idle.

[T77911S]

2 topics at once! i have blended the 2 together.

[Vereeken]

Perhaps we need to list the components that can cause a difficult start from cold AND let the idle fluctuate when hot.

In my opinion this would normally rule out the AAR, AAV and Decel valve but would keep the WUR, CSV, TTS , air sensor plate as suspect.

Unsure about he FD plunger.