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

[AlexJ]

Hi everybody
Recently been restoring an SC cabrio.
Regarding k-jetronic, all it was done was:
- rebuilt WUR and fuel distributor at a specialist
- new fuel pressure accumulator
- new cold start valve
- new fuel pump with new no return valve
- new fuel filter
- new injectors

Tests: control pressure (warm and cold) within the parameters, system pressure also ok.

The car starts roughly when hot after 20 minutes stoped. I have to hold the key at the start for about 4 seconds for the engine run. At first Fuel pressure seems to be lower than normal, but right after pressing the accelerator , everything sounds good. After that, the car runs perfectly.

And the worst thing is that the car didn’t have this symptoms before restoration...

[TracyC]

Originally Posted by AlexJ

Hi everybody
Recently been restoring an SC cabrio.
Regarding k-jetronic, all it was done was:
- rebuilt WUR and fuel distributor at a specialist
- new fuel pressure accumulator
- new cold start valve
- new fuel pump with new no return valve
- new fuel filter
- new injectors

Tests: control pressure (warm and cold) within the parameters, system pressure also ok.

The car starts roughly when hot after 20 minutes stoped. I have to hold the key at the start for about 4 seconds for the engine run. At first Fuel pressure seems to be lower than normal, but right after pressing the accelerator , everything sounds good. After that, the car runs perfectly.

And the worst thing is that the car didn’t have this symptoms before restoration...

Sent you a PM

[AlexJ]

Ok, there is already quorum(2 users with the same problem), to start a rescue!

[Walt Fricke]

How is the frequency valve behaving? All that stuff (except perhaps the O2 sensor itself) hooked up? Power to the relay under the passenger seat? Tried checking the pulse width modulation with a dwell meter (or, better, a scope) at the test port back in the left rear of the engine compartment?

[pmax]

What do mean by the CCP being spot on but the fuel pressure is low ?

[AlexJ]

Originally Posted by Walt Fricke

How is the frequency valve behaving? All that stuff (except perhaps the O2 sensor itself) hooked up? Power to the relay under the passenger seat? Tried checking the pulse width modulation with a dwell meter (or, better, a scope) at the test port back in the left rear of the engine compartment?

It is an euro SC.
It is more simple than that.

[AlexJ]

Originally Posted by pmax

What do mean by the CCP being spot on but the fuel pressure is low ?

Sorry I didn’t understand your question....

[pmax]

>Tests: control pressure (warm and cold) within the parameters, system pressure also ok.

Cold control, system pressures are fuel pressures.

> At first Fuel pressure seems to be lower than normal,

What fuel pressure are you talking about as low ?

[Walt Fricke]

Ah - I didn't notice the Madiera address.
Hot start issues are normally ascribed to a lack of residual pressure. Yes, you replaced the valve at/in the fuel pump, and the injectors, which are the likely sources of pressure bleeding off. You don't report the result of a timed residual pressure test. Did you do that, and it also was within spec?

It might be worth checking the operation of the thermo time sensor, which controls the CSV.
Hard to miswire that.

" At first Fuel pressure seems to be lower than normal, but right after pressing the accelerator , everything sounds good. After that, the car runs perfectly."

Are you saying that after the 4 seconds of cranking, and the engine starting, that it runs rough? A blip of the accelerator resolves that?

How are you measuring fuel pressure on a running engine? Easy to read control pressure. But if you leave the test valve closed for system pressure the engine won't run well or long, as the control pressure will become the system pressure, which will make things very lean. Though a momentary read seems to work. This assumes the standard CIS testing hook up.

[AlexJ]

Originally Posted by Walt Fricke

Ah - I didn't notice the Madiera address.
Hot start issues are normally ascribed to a lack of residual pressure.


Yes, you replaced the valve at/in the fuel pump, and the injectors, which are the likely sources of pressure bleeding off. You don't report the result of a timed residual pressure test. Did you do that, and it also was within spec?

It might be worth checking the operation of the thermo time sensor, which controls the CSV.
Hard to miswire that.

" At first Fuel pressure seems to be lower than normal, but right after pressing the accelerator , everything sounds good. After that, the car runs perfectly."

Are you saying that after the 4 seconds of cranking, and the engine starting, that it runs rough? A blip of the accelerator resolves that?

How are you measuring fuel pressure on a running engine? Easy to read control pressure. But if you leave the test valve closed for system pressure the engine won't run well or long, as the control pressure will become the system pressure, which will make things very lean. Though a momentary read seems to work. This assumes the standard CIS testing hook up.

Thanks for your messages
Yes, Maybe not very well explained, but my main issue is, after eliminating all the possible (in theory) causes, what can be causing the loss of residual pressure ?

Residualtests were: 1,9 bar residual right after switch of, 1,7 most of the time (up to 10 min) and I lost when it lowered to zero but for sure it was before the 20 min....

Yes, when I start it after de 20 min. It take a long time to start and when it starts, it behaves like the system pressure is low (for 5 seconds)... then it regains pressure after a blip or two, and the car drives well.

I am measuring the running pressure right before the fuel goes into de fuel distributor and the control pressure on the hose returning from the WUR to the fuel distributor. I have a very simple process only including a gauge and a T piece and a couple of adaptors. No test valve!

Hope now is everything clear.

Today I am starting with the clamping test... very little hope...

[AlexJ]

Originally Posted by pmax

>Tests: control pressure (warm and cold) within the parameters, system pressure also ok.

Cold control, system pressures are fuel pressures.

> At first Fuel pressure seems to be lower than normal,

What fuel pressure are you talking about as low ?

System pressure 4,9 bar
Control pressure hot: 3,1 bar
“Low fuel pressure behavior” when starting after the 20 min.

Residual pressure: right after switching of the car around 1,9. 2,0. 2,1
After 5 min, 1,7
After 20 min, zero. ( cannot specify when it goes to zero because I was not looking all the time)

[boyt911sc]

Originally Posted by AlexJ

System pressure 4,9 bar
Control pressure hot: 3,1 bar
“Low fuel pressure behavior” when starting after the 20 min.

Residual pressure: right after switching of the car around 1,9. 2,0. 2,1
After 5 min, 1,7
After 20 min, zero. ( cannot specify when it goes to zero because I was not looking all the time)

Alex,

Your WCP is out of spec and identify the culprit for your residual fuel pressure loss. The three (3) most likely suspects are:

• Defective fuel pump check valve.
• Defective fuel accumulator.
• Defective PPV (primary pressure valve) for the FD.

Keep us posted.

Tony

[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.

[AlexJ]

Many thanks for your help, both of you.

I will make new pressure readings tomorrow to have more accurate information.
By the way the PPV was not on my suspect list because the fuel distributor was the last thing to assemble, coming from a full rebuild at a Bosch specialist.

Thanks for pointing the mistake on reading system pressure.
I will proceed correctly.

Just to add some information: I have been asking for some help to the Person who rebuilt the WUR and fuel distributor.
He suggested to preform a “clamp test”. Camping the hose between the tank and fuel pump to verify if the check valve is doing its job. Second stage, camping on the return hose to the fuel tank. In both test, after 20 min. stop, the car reacted exactly the same, with the known symptoms.

I am starting to be suspicious if the electric system is having any contribution to this issue... I have been checking the fuel pump voltage and when I start the car, it is 11,3 V. After the car is running it goes up to 12,4 Volts. Is this correçt?

[AlexJ]

- Defective fuel pump check valve. Brand new!
- Defective fuel accumulator. Brand new!
- Defective PPV (primary pressure valve) for the FD. Brand new!

What a nightmare! This is taking me out of bed many times...

[boyt911sc]

Originally Posted by AlexJ

- Defective fuel pump check valve. Brand new!
- Defective fuel accumulator. Brand new!
- Defective PPV (primary pressure valve) for the FD. Brand new!

What a nightmare! This is taking me out of bed many times...

Alex,

Avoid using the “clamping method” to locate your mysterious fuel pressure loss unless you are using a clear plastic hose or tube. Use a hose with shut-off valve so you could verify and confirm that there is no fuel leaking. The test procedure is very simple.

Another thing you have overlooked was your fuel injectors including the CSV. Pull them out and have them pressure tested for drips or leaks. Lastly, are you certain that all the fuel lines are good and not leaking? This would be obvious and easy to detect but it happens.

You have seen and witnessed the gradual fuel loss in your tests but you did not know how to interpret the test result. Could you post a picture of your test showing where and how you connected the pressure gauge? Maybe there is a communication break down. Pictures would help. Thanks.

Tony

[AlexJ]

Originally Posted by boyt911sc

Alex,

Avoid using the “clamping method” to locate your mysterious fuel pressure loss unless you are using a clear plastic hose or tube. Use a hose with shut-off valve so you could verify and confirm that there is no fuel leaking. The test procedure is very simple.

Another thing you have overlooked was your fuel injectors including the CSV. Pull them out and have them pressure tested for drips or leaks. Lastly, are you certain that all the fuel lines are good and not leaking? This would be obvious and easy to detect but it happens.

You have seen and witnessed the gradual fuel loss in your tests but you did not know how to interpret the test result. Could you post a picture of your test showing where and how you connected the pressure gauge? Maybe there is a communication break down. Pictures would help. Thanks.

Tony

Tony,
Thanks for your message.
Injectors were new Twice because at first I suspected there was a system contamination and I thought one of the injectors could be stuck...
Cold start valve was new when I assembled the new WUR...

I will take some readings today and I will send you the data.
First I have to find a new reading gauge kit but with that safety valve you have mentioned.

[proporsche]

have you ever checked your sensor plate adjustment???That has to be done with the fuel system completely empty- and cold

Ivan

[AlexJ]

Originally Posted by proporsche

have you ever checked your sensor plate adjustment???That has to be done with the fuel system completely empty- and cold

Ivan

Hi Ivan,
That is a good one! But has been done...
By the way...
The fuel pump is activated by the micro switch at a certain point when the plate starts to move...
I am thinking if there is a way to actuate the fuel pump earlier than it is now... (???).
Is there a setup for this switch?

[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.

- connect wur plug, fuel pump running, car cold and not running, it starts at the up mentioned, 0,8 bar and starts raising, after around 1 minute, 1,2 bar, 2 min 1,9 Bar, 3 min. 2.2 bar.

- started the car. Control pressure goes right up to 2.2 bar and after 1 min running (car stoped), 2,4 bar and after 2 min. 2,9 bar.

- 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!

- switched of the car, (gauge in place), residual pressure, down immediately to 2.2. bar and slowly falling... after 15 min. 1.6 Bar, 20 min. 1.2

- after 20 min. started the car. Problem persists. Hard to start, taking a bit more than normal. For the first 2-3 minutes engine runs rough. If I run it high revs, pressing the gas, it starts becoming better. Note, at this point when I start the car again after the 20 min. period, the control pressure goes up to 4,7 bar and stays there!

- If I drive the car for a while, after 2-3 minutes driving normally the car becomes perfect! Once in a while, and I am not able to establish a pattern, the car doesn’t hold the idle. On a stop, the idle falls abruptly and the engine stops. If I keep on driving, the problem disappears... and it will show up again at some point...

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