Unique CIS/ K-jetronic problem, I think...

[boyt911sc]

Originally Posted by AlexJ

Back to this topic... finally found some time to do the measurements on the system, following your recommendations:

- gauge connected between WUR and FD, tap (valve) assembled, gauge at the segment on the side of FD...

- car not running, cold engine, wur plug disconnected, tap closed, pump running - pressure system 4.5 bar

- open tap (valve), wur plug disconnected, engine cold and not running, fuel pump running, cold control pressure, 0,8 bar. Detail: right after this test, when I disconnected the pump, cold control pressure goes up to 1,6 bar.

(edited)......

- went for a ride. Long enough to reach the working temperature. Back to the workshop and new readings with the car still running. Hot control pressure (gauge on place, tap opened, car running, working temperature), 4,6 bar!

(edited)........

Hope some of you can take any conclusions from this data. I’ll appreciate!

Alex,

Your WUR is out of spec. Look at your CCP (cold control pressure) it was only 0.8 bar (11.6 psi.) and the WCP (warm control pressure) was 4.6 bar (67 psi.) and too high. The WCP was equal to your SP (system pressure).

I thought you had a newly rebuilt WUR? The control pressures were out of spec. and causing your erratic running condition. BTW, what is the Bosch ID # of your WUR ? It should read something like 0-438-140-xyz. The last 3 digits would identify your specific WUR. Keep us posted.

Tony

[AlexJ]

Originally Posted by boyt911sc

Alex,

Your WUR is out of spec. Look at your CCP (cold control pressure) it was only 0.8 bar (11.6 psi.) and the WCP (warm control pressure) was 4.6 bar (67 psi.) and too high. The WCP was equal to your SP (system pressure).

I thought you had a newly rebuilt WUR? The control pressures were out of spec. and causing your erratic running condition. BTW, what is the Bosch ID # of your WUR ? It should read something like 0-438-140-xyz. The last 3 digits would identify your specific WUR. Keep us posted.

Tony

Tony, thanks for your comments.
Yes my WUR was rebuilt on a specialist. # xxx.089 ROW

That leaves me with a question....
If I set the cold control pressure, in this case, if I increase, wouldn’t the warm control pressure also increase?

[AlexJ]

Can this also be related to the WUR voltage?

[boyt911sc]

Originally Posted by AlexJ

Tony, thanks for your comments.
Yes my WUR was rebuilt on a specialist. # xxx.089 ROW

That leaves me with a question....
If I set the cold control pressure, in this case, if I increase, wouldn’t the warm control pressure also increase?

Alex,

Why don’t you have the WUR recalibrated by the rebuilder? Tinkering the WUR yourself might cost you to forfeit the rebuilder’s warranty. The control pressure adjustment for the WCP affects both the WCP & CCP. But the adjustment for the CCP does not affect the WCP.

So you make the WCP adjustment first and foremost. Followed by the CCP last. Just make sure you have a good and reliable pressure gauge kit. Keep us posted.

Tony

[AlexJ]

Originally Posted by boyt911sc

Alex,

Why don’t you have the WUR recalibrated by the rebuilder? Tinkering the WUR yourself might cost you to forfeit the rebuilder’s warranty. The control pressure adjustment for the WCP affects both the WCP & CCP. But the adjustment for the CCP does not affect the WCP.

So you make the WCP adjustment first and foremost. Followed by the CCP last. Just make sure you have a good and reliable pressure gauge kit. Keep us posted.

Tony

Yes, that is what I wanted do to... but in my location, to have the wur rebuilt it will take more than 8 weeks. As the owner of the car is almost killing me, I am trying to figure out a solution locally...

[AlexJ]

Something very strange happened today, after Tony’s suggestions..
As indicated, I decided to work on the WCP.
The gauge was still attached to the system after the readings from yesterday.
I started the car and went out for a ride.
After it reached the running temperature, headed back to the workshop.
When I arrived, the gauge for WCP was indicating 3.0 bar... everything was left as it was from yesterday,..

Why in two different days, two different readings?

[SkiVT]

Faulty gauge? Have you checked the screen in the wur connection. Almost seems like you have blockages that clear and then maybe clog/restrict again. Inconsistent behaviors always seem the most difficult to troubleshoot.

[boyt911sc]

Originally Posted by SkiVT

Faulty gauge? Have you checked the screen in the wur connection. Almost seems like you have blockages that clear and then maybe clog/restrict again. Inconsistent behaviors always seem the most difficult to troubleshoot.

Once in a while, I test my gauges for relative pressure readings. I run them in series and record their individual pressure readings. Since I don’t have a calibration standard for the pressure gauge, I use the individual readings to compare one gauge to another. So far, the S&G tool gauges have been very consistent and reliable. The oldest gauge is more than 25 years old and the most recent acquisition is a year old. The variance from the average is less than 1 psi. and considered them OK.

It is mandatory for fuel injection system troubleshooting work to have a good and reliable pressure gauge kit. Otherwise, you will have inconsistencies in your test results.



Tony

[AlexJ]

It is a new set (gauge, valves and fittings), I bought specifically for this issue, when I understood I was not measuring the correct way...

[AlexJ]

Anyway, considering that now I am reading the correct values (0,8 bar) CCP and (3,0 bar), WCP, I need to increase this values.
Any suggestions about what can I do locally to increase these numbers?

[SkiVT]

Some "resources" I have read but never had to try:

https://www.impactbumpers.com/forum/index.php?/topic/33747-911sc-bucking-at-1500-revs/page/4/

https://cis-jetronic.com/index.php?rt=product/category&path=65_66

Erratic WCP

[AlexJ]

I will have a look!
Many thanks!

[boyt911sc]

Originally Posted by Walt Fricke

Alex - Since you replaced the fuel pump/check valve, and the fuel accumulator, it makes sense to put these lower on the suspect list, although new parts have been known to be defective (which really makes diagnosis hard).

So I'd look at the PPV Tony mentions.

By the way - fuel flows from the FD to the WUR, and from the WUR back to the fuel tank. The WUR bleeds off pressure, lowering system pressure to control pressure.

The PPV sets the system pressure, which is lower than the pressure generated by the fuel pump. The PPV. The PPV also diverts excess pressure back to the fuel tank. The PPV has at least one (tiny) O ring, and if it goes bad it can cause problems. And if it is bad, it would act very much like a bad anti-return valve on the fuel pump.

If you are measuring fuel pressure before the fuel gets to the FD, you are measuring pump pressure, not system pressure. Most of us haven't measured pump pressure, as the Bosch testing system (and others like it, including home made, since it isn't complicated) measures system pressure by blocking the line to the WUR after where the gauge is Teed in. I bet you can figure out how to do that. You don't need to (and usually don't) have the car running to check system pressure, as it doesn't depend on what the WUR is doing. You just need to run the fuel pump.

Walt,

I was going over Alex’s posts and noticed your post above. There are several ways to measure the system pressure in CIS:

• The pressure gauge installed between FD & WUR with the valve fully closed.
• The pressure gauge installed before the FD with the valve fully opened.

The conventional way to measure the system pressure is between the FD & WUR where you measure the control pressures (CCP & WCP including the residual pressure). However, the fuel pressure entering the fuel distributor is not the fuel pump pressure but rather the system pressure caused by the PPV (primary pressure valve).

Below is the picture of my CIS bench tester measuring both the control and system pressure during calibration. The gauge at the left (control pressure) and gauge at the right (system pressure):



Control pressure reading (enlarged picture):


System pressure reading (enlarged picture):


Next time you have an opportunity to do some fuel pressure tests, install the pressure before the fuel distributor and you will get the “system pressure”.

Tony

[Walt Fricke]

Tony - I am sure you are way ahead of me on hydrodynamics (as with much else). If a pump can produce X PSI, but where there is work to be done by the pressure there is a recirculating reducing valve, does this mean that the pressure measured at the outlet of the pump will be the same as just after the reducing valve. I tend to get confused using Ohm's law hydraulic analogies backward to try to understand hydraulics. Would the readings be different if the pressure reducing system were of the non-recirculating type?

For testing I can see how measuring system pressure before the line to the WUR is convenient - on a running engine you won't shoot control pressure super high when you close the valve, and you don't need a valve for measuring there.

[AlexJ]

Keep on testing and finally manage to set the pressures to spec.
Set warm pressure first and today before start, set the cold pressure slightly punching the pin.
The values were accomplished. Went for a ride up to working temperature.
Checked warm pressure when arrived : 3 bar, correct for this wur.

Left it for half an hour, but still hard to start...
when finally started, the warm control pressure was 3,8 bar!

Is it supposed to change that much ?? in both situation the car was warm...

??????

[boyt911sc]

Alex,

When you did the fuel pressures testing, were you running the motor or just the fuel pump? I am not sure about your test procedures and if you could confirm which method you used, I might be able to explain some of the discrepancies you have observed. Keep us posted. Thanks.

Tony

[Walt Fricke]

Assuming measurements were made correctly, a warm starting control pressure higher than the last warm running control pressure makes no sense.

The cold control pressure is set by the warm up part of the WUR - the bimetal spring when cold presses down on the springs which press up against the diaphragm (which itself governs how much pressure is released). So the upward pressure is less, the control pressure lower, and the mixture richer. At a certain temperature, the bimetal part is high enough that it doesn't touch anything - it no longer has an effect.

So physically it is not possible (at least without something really weird in how the WUR was assembled) to have the control pressure rise after you turn the engine off. When you turn a warm engine off, you may see a bit of an increase in system pressure. Tony explained this as the effect of the pressure accumulator, and it is short term as residual pressure gradually bleeds off as the engine cools. But system pressure isn't control pressure. Physically, the WUR is not complicated, and it is easily reassembled.

3.8 bar would make it hard to start.

You have gotten suggestions that making sure the filter inside the WUR large metal cylinder is not clogged (which is one source of high control pressures) would be useful. In your case, though, the symptoms reported don't support the clogging hypothesis, as it should result in high cold CPs.

Restrictions in the return line system from the WUR to the fuel tank would raise CPs, but why would this be thermally related? Some have blown air through the return system to see if there are restrictions. I don't recall those leading to isolating an issue, though.

Located where you are, I can see that some diagnostic tricks are difficult. For instance, borrowing a WUR and a fuel distributor (the main controllers of these things) to swap in one by one, could help, but perhaps there are few of this model 911s on the island? The WUR swap is easy, the FD not so easy at all although possible.

Where does control pressure come from? The FD has an area which is at system pressure, fed directly from the pressure regulating intake stuff connected to the fuel line and pump. A tiny hole in the stainless steel diaphragm which separates the two halves of the FD allow system pressure (but very little volume) to get through to the control pressure part of the upper FD, which in turn is connected to the WUR. This hole could, in theory, get blocked, or alternatively get eroded and larger. Not clear how either would cause your symptoms.

The piston which air flow moves up (more fuel to the injectors) or down (less) has no rings or seals - it depends on close tolerances. The fuel the piston sends to the injectors is at system pressure. If it were to be worn, more pressure could leak up to the control system? These can stick from deposits, which can be cleaned off. They can't be replaced. People (Tony?) with experience with lots of FDs might know how often this piston/cylinder system gets worn or scratched - as well as whether this might cause your perplexing symptoms.

Did you, by chance, measure the residual pressure and control pressure just before you did the warm start?

['78 SC]

Originally Posted by Walt Fricke

Assuming measurements were made correctly, a warm starting control pressure higher than the last warm running control pressure makes no sense.
...

Where does control pressure come from?
...

Walt, I believe later CIS designs route the fuel exiting from the WUR through the pressure regulator on the FD. This "push-up valve" closes when there is no fuel flow, blocking the WUR return (part 4). With this closed, the pressure throughout the system would rise to whatever pressure the fuel accumulator could provide. I see this routinely when shutting off my 1983 with CIS Lambda.

I suspect this is to help maintain pressure after engine shutoff to improve warm starts.

Sorry for the interruption.

[Walt Fricke]

No interruption at all. He has an '82 Euro SC. I don't know if those, which don't have the Lambda stuff, have the push up pressure relief valve, or the previous one.

This schematic shows the push valve system.



I agree that the push valve would retain pressure in the whole system - the control pressure wouldn't flow past the O ring on the push valve, and the O ring on the other end of that valve would block off the return line to the fuel tank. Should help with hot starts, not make them harder.

[AlexJ]

Originally Posted by boyt911sc

Walt,

I was going over Alex’s posts and noticed your post above. There are several ways to measure the system pressure in CIS:

• The pressure gauge installed between FD & WUR with the valve fully closed.
• The pressure gauge installed before the FD with the valve fully opened.

The conventional way to measure the system pressure is between the FD & WUR where you measure the control pressures (CCP & WCP including the residual pressure). However, the fuel pressure entering the fuel distributor is not the fuel pump pressure but rather the system pressure caused by the PPV (primary pressure valve).

Below is the picture of my CIS bench tester measuring both the control and system pressure during calibration. The gauge at the left (control pressure) and gauge at the right (system pressure):



Control pressure reading (enlarged picture):


System pressure reading (enlarged picture):


Next time you have an opportunity to do some fuel pressure tests, install the pressure before the fuel distributor and you will get the “system pressure”.

Tony

When I see these test benches, I always wonder how do you reproduce the “hot running temperature” to set warm control pressure... (???).