CIS Control Pressure Plunger & Cold Starts

[echrisconnor]

Please bear with me. I've been working with my CIS system for several months now tying to chase down a hot starting issue. Throughout this process I have verified my assorted pressures many times over and have verified things are working properly and not leaking. My problem was the WUR as allowing system pressure to bleed down too quickly after a hot run (yes, the WUR, after many tests and a replaced check valve and fuel distributor system pressure O ring), which I disassembled, cleaned re-installed. The car now starts flawlessly when hot after one or two hours. It also runs like it always has -- fast and strong.

So, the problem now is that after a hot run, the next morning when I turn the ignition on the injectors start spraying as soon as the fuel pump goes on (I have my pump jumpered to run as soon as the ignition is turned on). I can hear them singing away, and they will continue to do so for about a minute as system pressure builds and the control plunger drops down. If I lift up on the metering plate while this is happening, there is no resistance, the plunger has sucked itself up to the top overnight. if I keep my hand on the metering plate I can feel the plunger coming down slowly -- all the while the injectors are squirting away flooding the engine. This didn't happen before.

So, what I'm thinking is happening is that after a hot run and several hours going by, one side of the plunger is either getting negative pressure, or positive pressure, and the plunger gets sucked up.

As a remedy, if I just flick on the fuel pump for a few seconds (with a cold engine) and then come back an hour later, the problem is gone. The system is re-pressurized and ready to go. It starts pefectly then.

Help. I thought I was tuned in to so many of the CIS problems, but this is a new one.

[Paulporsche]

Did you ever verify that your fuel accumulator was working correctly?

When the engine is off, is the sensor plate in the right position?

Does the plate move easily through the extent of its travel? Plunger not sticking?

Have you confirmed correct functioning of the Cold Start Valve?

Are you sure you don't have a fuel vaporization problem? Or a leak? Are your fuel lines routed correctly?

I know you've been at this a while. I can't remember everything you have done.

[MotoSook]

Did you do something to the airplate-lever balance? Have you tried cleaning the fuel distributor? The pliunger should drop to the air plate lever...due to its own weight. If it is sticking in an elevated position, you are building pressure with the pump when you turn one the car (jumpered pump) slowly..because the flow is going to the injectors..as you described....until the plunger creeps back down...

W/o the fuel pump, the pumper should move up and down easily...

I might take the fuel dist. out and clean it...it's not difficult. Try searching for a thread I started when I clean my fuel distributor if you think you'll need it. It's pretty straight forward..

[echrisconnor]

Thanks to both of you for responding. I'm really stumped here.

In my assorted tests, I've confirmed the following:
1. No fuel leaks
2. CSV & thermotime work and do not leak
3. Injectors do not leak & spray evenly
4. New check valve & it doesn't leak
5. Fuel accumulator is working properly
6. Fuel distributor 0 ring is replaced & system pressure is in-spec
7. WUR no longer leaks & hot & cold pressures are in-spec
8. System properly holds pressure to factory specs.
9. Hot starts perfectly

Also, the fuel distributor control plunger moves freely! I did a pressure test a while back where I had the system pressurized (engine off) and I would lift the metering plate. It would push the plunger up and it would then come right down again and I could see the system pressure change. Also, it runs fine when it's going. No binding.

Something is forcing the plunger up. Either it's negative pressure on the WUR side of things now, or it's positive pressure on the fuel distributor side. Although, the two are open to one another just through a restriction valve.

I'm stumped. Please help.

[MotoSook]

I wonder if you are draining through the regulator-valve on the fuel distributor. When the plunger is up...is the is the airplate up too??


I've never had a 3.0C fuel dist. in my hands, but what o-ring are you talking about? My '76 fuel dist. did not have an o-ring that needed replacing...just the one that seals the airbox from atm.

[echrisconnor]

Souk,

The O ring in noted in the below diagram. It's in the system pressure regulator in the fuel distributor and keeps return fuel pressure from returning to the tank after the pump is shut down. It's one of the things to check in dealing with hot starting and pressure loss issues.

On the air metering plate. It sits level and the counterwieght is not allowing it to rise up and lift the plunger. Clearly, the plunger is lifting somehow else.



Note -- my car is not a lambda car, and does not have the frequency valve circuit, and the system pressure valve is a little different, but this is a good diagram for showing all the components at play in this area.

[MotoSook]

Ok....I was fixated on the plunger...

I would not out rule fuel reg. The new O-ring as a potential cause of your problem. The regulator seat is not related tot he o-ring.

Put your CIS pressure tester on the system. Leave it connected and run the car at idle, then shut off the engine and quickly turn the valve on the pressure tester so that there is no flow to the WUR. Watch the pressure.

If the fuel is draining and causing the pluger to rise, fuel can flow through the check valve at the pump (not to be out ruled even though you replaced it), the the regulator on the dist. or the WUR, right? By the above test, you'll elimnate the WUR. The residual pressure should keep the plunger down, but it seems you are loosing residual pressure.

I don't think any drainage through the injectors will cause the plunger to rise....

I think you are on the right track....

Eliminate the WUR as a potential...

Check all you fuel connections,..

I've fallen a victim of my own mind and neglected to check my won work..making assumptions...

[Tim Hancock]

Chris what did you do to the WUR to fix it or did you replace it? Be careful with running the injectors w/out the engine turning. When I tore my SC down for what turned out to be an overhaul, I found a bent rod due more than likely to the PO's non P-car mechanic screwing with the fuel system (It appeared to be cause by a fuel hydro-lock as the top of the piston and the valves were undisturbed).
The car still ran fine, but I am thankful for finding it before more serious damage ocurred.

Souk's advice to pull the plunger seems logical as a small pc a debris could cause it to hang up. I was told when doing this, to be extremely careful with the plunger as it is a close tolerance fit in the bore and needs to be scratch free.

[echrisconnor]

Whatever is happening is happening very, very slowly.

I've tested the check valve by hooking up my pressure gauge just after the fuel accumulator and pressurizing the system. No drop in pressure after over an hour, and when I bled the system down it held pressure around 20 psi even after letting a substantial amout of fuel bleed out (as the fuel accumulator released its volume).

I had done the WUR elimination test just the other day and had confirmed that the system held pressure for an extended period of time when the WUR was removed from the equation. This is what lead me to rebuild the WUR. I just got done opening up, cleaning, and generally reconditioning. I've now tested it following its rebuild and it now holds pressure much longer and the hot starting problem is completely eliminated. Hot and cold pressures have been re-set and confirmed.

So, I'm confident the plunger isn't sticking -- since it would never be fully retracted and moves freely when I have tested it. I will re-check the O ring, but even the old one was in great shape, and I don't see any noticeable loss in pressure over 30 minutes when the WUR is closed.

What it seems I need are two gauges. One which I can hook up on the pressure side, and another on the return side. Then, about 8 hours to sit and watch them (not likely to happen; my wife is due with our second child any day now).

Tim, I believe Souk was telling me to re-check the plunger in the pressure regulator -- not to remove the control plunger. Can you do that with the fuel distributor still in the car? Also, I have been very careful when the injectors have run for more than a few seconds. The time when it ran for a minute as they pressurized, I slowly turned the car over by hand first, then cranked.

Thanks, and keep any suggestions coming! Look at the above diagram though and tell me how one could get a pressure difference enough to lift the plunger? By the Bosch manual, the pressure differnece between the two sides is created by a restriction valve and the pressure drop is only .1 bar.

[Tim Hancock]

This is probably going to sound stupid and I admit I am grasping at straws, but since you tore the WUR apart (that is what you did right?), is there a chance you did something wrong or maybe hooked it up backwards (I am not sure if you even can but I can't remember for sure)? I really am having a hard time trying to see what would cause low pressure after the pump is off as the orifice should allow pressure to equalize above and below the piston.

[echrisconnor]

I don't want to suggest that I'm infallable, but I'm extremely confident that it went back together properly and was hooked up right. And, the hot start problem which I was experiencing has gone away completely. It starts perfectly after one to two hours after a good hot run. Whereas before, when hot, it would be fully depressureized and would require re-priming the injectors by manyally lifting the throttle plate.

If I do the thing where I just run the pump for a second after the car has cooled down and then come back an hour later and try to start it, the plunger has been pushed back down and the car starts perfectly. It's got something to do with the temperature and maybe fuel continuing to boil or change in volume after several hours. After its out of good hot start zone. I have not yet tried to do a start after about 4 hours of a hot run.

I'm confounded. No search on the forum that I can reproduce shows anything like this. The few cases that are related are all due to stuck/frozen plungers, and then the car's don't run well anyway.

[jstobo]

Here's a guess. Could one of the equalizing restrictors be clogged(#7 in above diagram.) My guess would be the one on the inlet pressure side. This way, when the system pressure maintains its 20 psi you have no pressure above the piston(control pressure bleeds off). It pushes it up. After starting as the control pressure builds it pushes the piston down. Once warmed up the control pressure is where its supposed to be and the car will run fine as the fuel flow is determined by the air plate and differential pressure chamber. Maybe a through cleaning of the fuel distributer is in order. Good luck.

[echrisconnor]

That was one of my first thoughts. However, if the one shown in the lower right on that diagram were clogged, no fuel would get to the WUR, and the system pressures would be way out of whack. If pressure didn't get to the second one (above the plunger) then I'd expect the car would run poorly, since the pressure wouldn't change during running and the mixture would be way off.

Maybe it's a partial obstruction which only really appears at low pressures... I dunno. Maybe a strong dose of Techtron is in order.

[echrisconnor]

After hours of searching old archives I've found one reference which sounds related:

___

Perhaps I was simply lucky and lucked into the rebuild. I did have to clean and reseal the diphram a couple of times because I used too much sealer. There is a small orifice that allows lower chamber fuel to pass to the upper half for fuel piston counter pressure. If this tiny hole is plugged the air plate will swing to the top and will not come down. Obviously not a good situation . I do not know if this is much help but I had good luck and learned a great deal in the experience.

___

Maybe all of my opening things up, cleaning and testing clogged this opening... If I did plug this opening, does anyone have any recommendations on how to best clean it? I can try techtron in the tank, but would anyone recommend a blast of carb cleaner through one opening or another to dislodge any crud?

[echrisconnor]

I cleaned things up on the fuel distributor last night.

I pulled the line to the WUR, and pulled out the system pressure regulator. Then I made a seal around the straw on my carb cleaner bottle and blew carb cleaner through both sides. I was able to easily blast cleaner from one side to another. Additionally, I was able to use the pressure from the carb cleaner to push the plunger back down. In doing so, I was able to cycle plunger several times and presumbably draw cleaner through all the relative orifaces. Left it full of cleaner last night, and will drop a bottle of Techtron in the tank tonight and go for a good hard drive.

Interestingly though, I talked to a friend of mine who also has a CIS car. He said that he tried lifting the air sensor plate on his car when it was cold and not running, and it moved freely without any resistance (as in the plunger on his car may be retracted as well). So... am I finding problems in things that are normal? Anybody else got a CIS car that's cold right now? Can you go and see if your airflow plate has zero resistance on it?

Let me know. If nothing else, hopefully this cleaning helps.

[MotoSook]

Just checking back in to this thread....

Interestingly though, I talked to a friend of mine who also has a CIS car. He said that he tried lifting the air sensor plate on his car when it was cold and not running, and it moved freely without any resistance (as in the plunger on his car may be retracted as well).

That's not unusual IMO. My airplate moves freely up and down and I can feel the plunger follow it.

Cleaning the fuel dist. is a step in the right direction...

When you out the plunger back in during installation of the fuel dist. make a mental note of how the plunger travels...it should drop out of the fuel dist. while installing unless you use a finger to hold it up while you lower the fuel dist. Hold the air plate up while you are installing...so the airplate meets the plunger, and the plunger doesn't drop down and out of the fuel dist.

[echrisconnor]

Thanks Souk! I actually did my cleaning with the FD in the car. I never removed the control plunger. I removed the system pressure regulator plunger and used that as a port for my spraying and the output line for to the WUR as the other side. It may have been my imagination, but after a few cycles of carb cleaner and sliding the plunger up and down, but it appeared to move more freely.

We'll see what happens.

So, in your experience, it's not uncommon for the control plunger to be elevated above the airflow plate on a cold car?

[MotoSook]

No Chris..the plunger should not move up when the car sits...at least I never noticed it. As a mater of fact, I seem to recall the plunger coming back down if I push the plate up and let it settle...I can check on my '76 tonight for you.

Removing the fuel distributor is not hard...I'd take it off and give it a good cleaning with the plunger removed. It would be difficult to get all the comtaminate out of the FD by cleaning through the regulator.

[Paulporsche]

Chris,
Some have used emery cloth to clean up their plungers. If you remove the FD, don't let the plunger fall out of the bottom.

[MotoSook]

I would not use any thing "abrasive" to clean the plunger...if it needed polishing, I would use something VERY gentle on the surface....

When I had my FD apart (2x) the plunger showed no signs of corrosion and the surface was as polished as could be expected. I just soaked everything with carb cleaner...blow carb cleaner through the flow paths. Store the plunger to avoid damaging its surface...any nicks or scratchs won't be good.