CIS Residual pressure -Timing

[Vereeken]

Following the spec book of CIS you need at least 1.2 bar of residual pressure after 20 minutes of shutting down the engine.

I still have 1.2 bar after 4 hours of sitting in a garage with 18°C or 64 F

Only after about 6 hours the residual pressure in the systems drops to zero.

Does this point to anything? Would a long held residual pressure contribute to a hard start problem? Would it show up as a problem in any way whatsoever?

[75 911s]

What year is your CIS? Cold start issues can be a symptom of CIS pressure outside tolerance.

[Vereeken]

1983 EURO with a 089 WUR.

CCP is 0.9 bar at 18°c without vac. 1.7 bar with vac at 400mg.

My question is also more theoretical. I did not yet master all of CIS specifics.

I want to understand a bit better....and now I am moving on to the nitty gritty....

[ossiblue]

If your other pressures are in spec, then the duration of residual pressure should not have any effect on starts. The residual pressure only influences warm starts, when the cold start valve is disabled. Without residual pressure in a warm engine, the injectors will not inject adequate (or any) fuel to start the engine on the initial cranking as the system will need to pressurize but cannot do so because the pump only runs during the start cycle. Having residual pressure means the system is pressurized and as soon as the pump starts, fuel will be injected and the engine will start and run. When the engine is cold and the residual pressure is zero, the cold start valve injects the fuel during the start cycle, the engine fires and continues to run because the pump has had time to build pressure and feed adequate fuel to the injectors.

I opened with a mention of the other pressures being in spec. This is important because long duration residual pressure may be due to a blocked return line. However, having such a situation would likely show up in other symptoms as well as out of spec system and cold control pressures.

[Vereeken]

Ok clear.

So how does the CSV get its fuel? I see it is branched off of the FD.

Building pressure from dead cold takes a couple of seconds. Why is the csv not affected by the lack of pressure during the first seconds of cranking?

I can imagine that the fuel drains back from the CSV during long periods of inactivity, so how does the csv gets fuel straight away?

I understand how the CSV gets voltage/signal but the fuel delivery of the csv is a mystery...

[ossiblue]

Some cars are plumbed a bit differently, but, in general, the CSV is fed directly from the pump, upstream from the WUR and separate from the pressure relief valve in the FD. The line to the CSV may be connected to the FD at the same location, but, internally, it is not dependent on the FD pressure relief valve. That means, as long as the pump check valve is working, pressure to the CSV will be maintained. As soon as the start cycle begins and the thermo switch is closed, the CSV will inject fuel.

[Vereeken]

Can a pump check valve cause a hard start?

Let me explain.

I assume that the check valve is ok because the residual pressure stays on, the CCp and Wcp and system pressure is within spec but....

What if the check valve is lazy and does not react quick enough during a cold start when cranking...

Is that possible? Would it cause an issue?

[Flat6pac]

The check valve or the accumulator both, if defective, release pressure.
On an 83, the check valve is attached to the pump, making it a low cost item to replace.
The accumulator is a diaphragm which if defective will dump fuel back into the return line to the tank.
Cold start injector will inject fuel to startup cold giving initial fire until fuel injection can catch up.
Bruce

[boyt911sc]

Originally Posted by Vereeken

1983 EURO with a 089 WUR.

CCP is 0.9 bar at 18°c without vac. 1.7 bar with vac at 400mg.

My question is also more theoretical. I did not yet master all of CIS specifics.

I want to understand a bit better....and now I am moving on to the nitty gritty....

Michel,

I just spent the last 3 days testing and calibrating a bunch of WUR-089's and the data are still fresh in my aging memory. Your cold control fuel pressure @ 18°C/64°F is too low and out of spec. At 0.09 bar (13 psi.) no vacuum applied and 1.7 bar (24.7) with vacuum applied has a differential of 11.7 psi. (?). What is your WCP (warm control pressure) with and without vacuum applied?

Pay attention to Larry's posts and he is giving you a very good guidance. I think you are getting confused between residual fuel pressure and control fuel pressures. They are not the same. The CSV relies on the primary delivery fuel by the FP. A defective FP check valve, fuel accumulator, or TTS would affect the operation or performance of a CSV.

Tony

[stlrj]

Michel,


Make sure the terminal on the red/black wire is grounded on the thermo time switch. That wire provides the ground for the cold start valve to operate. Without that ground, your cold start valve will never turn on.




Cheers,

Joe

[ossiblue]

To be honest, it sounds like you're beating around the bush a little. Do you have a hard start issue? If so, let us know exactly how it is occurring--warm or cold, after a long period of sitting, starts then abruptly stops, etc. Knowing all the details will help us help you. You've couched everything, so far, as theoretical, but from your persistent questioning, I suspect you have a real problem. If I'm correct, just let us know and things will be corrected readily. Lots of experienced minds here.

[Vereeken]

Originally Posted by boyt911sc

Michel,

I just spent the last 3 days testing and calibrating a bunch of WUR-089's and the data are still fresh in my aging memory. Your cold control fuel pressure @ 18°C/64°F is too low and out of spec. At 0.09 bar (13 psi.) no vacuum applied and 1.7 bar (24.7) with vacuum applied has a differential of 11.7 psi. (?). What is your WCP (warm control pressure) with and without vacuum applied?

Pay attention to Larry's posts and he is giving you a very good guidance. I think you are getting confused between residual fuel pressure and control fuel pressures. They are not the same. The CSV relies on the primary delivery fuel by the FP. A defective FP check valve, fuel accumulator, or TTS would affect the operation or performance of a CSV.

Tony

No you are right, just bad typing skills and poor PSI conversions.. who uses PSI anyway.

CCP with 400mmhg applied at 18° is 1.7 bar. I'd say that is spot on the graph.
WCP with the vac from the engine is 3.6 bar, so again bang in the middle of where it needs to be (3.4 to 3.8).

I am not confusing residual with control pressure. I am already passed that stage. It just struck me as odd why the spec book would call for at least 1.2 bar after 20 minutes while I see that and even higher numbers after several hours. So it is not that the system bleeds pressure at a high rate.

So my question was more out of intrest and to understand if the delayed bleeding of the residual pressure could have an effect on for example the CSV or anything else.

[Vereeken]

Originally Posted by ossiblue

To be honest, it sounds like you're beating around the bush a little. Do you have a hard start issue? If so, let us know exactly how it is occurring--warm or cold, after a long period of sitting, starts then abruptly stops, etc. Knowing all the details will help us help you. You've couched everything, so far, as theoretical, but from your persistent questioning, I suspect you have a real problem. If I'm correct, just let us know and things will be corrected readily. Lots of experienced minds here.

No it is winter over here....

But if you put it like that....

After prolonged sitting the car starts and then thinks twice before going to 1600/1800 rpm. I can only describe it as a car that thinks "Ok what am I supposed to do again" and then 5 seconds later the penny drops and finds 1600RPM.
So within the first seconds of startup you have 3 things that can contribute to the start-up sequence. The CSV, the thermal valve that holds off vac to the WUR and the initial CCP. Given that the CCP is in spec, the thermal valve opens within 10 seconds (depends a bit on ambient temp), I am only left with the CSV for which there is no spec, it works or it doesn't. SUre the TTV has a spec but I can not find anything wrong with it from the outside.

[ossiblue]

Originally Posted by Vereeken

No it is winter over here....

But if you put it like that....

After prolonged sitting the car starts and then thinks twice before going to 1600/1800 rpm. I can only describe it as a car that thinks "Ok what am I supposed to do again" and then 5 seconds later the penny drops and finds 1600RPM.
So within the first seconds of startup you have 3 things that can contribute to the start-up sequence. The CSV, the thermal valve that holds off vac to the WUR and the initial CCP. Given that the CCP is in spec, the thermal valve opens within 10 seconds (depends a bit on ambient temp), I am only left with the CSV for which there is no spec, it works or it doesn't. SUre the TTV has a spec but I can not find anything wrong with it from the outside.

Okay, gotcha.

I'm not sure what you mean by the "car thinks twice", but it sounds like the idle is hunting--moving up and down for a few seconds before finding a steady position. Is that what you mean? If so, that is consistent with too low of CCP setting. It is a symptom of an overly rich mixture. Whether or not that is your issue depends on the accuracy of your pressure readings as matched to the ambient temperature and the specific WUR. The CSV would not be an issue unless it is defective and is leaking.

[Vereeken]

LJ

No , not hunting. I know that typical behaviour.

I need to make a video of the RPM...

Basically you start and it does 1200 RPM (I just did it at 0 degrees), then some 5 seconds later (maybe 10) the car realizes , damn is it that cold? And goes to somewhere between 1600/1800. After that it is typical behaviour. It gradually drops the rpm as the AAR closes and FP rises....

This is a EURO car so no FV no 02. I see only two elements at play in the first 10 seconds, TTV and CSV/TTS.

Ofcourse all this can also be down to ignition but that will be another thread ;o)

Mind you all this is trivial as the car and engine are fine otherwise, but I simply want to understand if it points to anything other then being a 35 year old car.

[T77911S]

Originally Posted by boyt911sc

Michel,

I just spent the last 3 days testing and calibrating a bunch of WUR-089's and the data are still fresh in my aging memory. Your cold control fuel pressure @ 18°C/64°F is too low and out of spec. At 0.09 bar (13 psi.) no vacuum applied and 1.7 bar (24.7) with vacuum applied has a differential of 11.7 psi. (?). What is your WCP (warm control pressure) with and without vacuum applied?

Pay attention to Larry's posts and he is giving you a very good guidance. I think you are getting confused between residual fuel pressure and control fuel pressures. They are not the same. The CSV relies on the primary delivery fuel by the FP. A defective FP check valve, fuel accumulator, or TTS would affect the operation or performance of a CSV.

Tony

hey tony,
I know its not the sane WUR, but my 033 was new when I got my 77. the difference in pressure, hot or cold, with and with out vac was .8 bar.

[T77911S]

Originally Posted by Vereeken

LJ

No , not hunting. I know that typical behaviour.

I need to make a video of the RPM...

Basically you start and it does 1200 RPM (I just did it at 0 degrees), then some 5 seconds later (maybe 10) the car realizes , damn is it that cold? And goes to somewhere between 1600/1800. After that it is typical behaviour. It gradually drops the rpm as the AAR closes and FP rises....

This is a EURO car so no FV no 02. I see only two elements at play in the first 10 seconds, TTV and CSV/TTS.

Ofcourse all this can also be down to ignition but that will be another thread ;o)

Mind you all this is trivial as the car and engine are fine otherwise, but I simply want to understand if it points to anything other then being a 35 year old car.

I think it is probably fine, although my 77 never idled that hi when cold.
the time when it idles up could be the TTV opening and raising the CCP. if so I would leave it, but that's my opinion. the reason I say that is you are getting extra fuel for that cold start which is a good thing because that also prevents cold backfiring. just my opinion but I think that is why Porsche added the TTV.

( it probably idles up because at first it is too rich then leans out closer to what it needs for cold running).

[Vereeken]

yes my thought as well. A TTV is typically around the 5 to 10 second mark.

But it should not do that ofcourse because my WUR cold is what it should be so the fuel requirement is following spec.

That is the trouble with CIS. It is very mechanical and not overly accurate...

[boyt911sc]

Originally Posted by T77911S

hey tony,
I know its not the sane WUR, but my 033 was new when I got my 77. the difference in pressure, hot or cold, with and with out vac was .8 bar.

Ty,

I believe you but you have to demonstrate it now (?). When I was younger I could lift a 100 lbs. barbell. Today, I could barely lift a 40 lbs. bag of salt without straining my back. What matters most is what the device/s could achieve today not when it was brand new.

Tony

[boyt911sc]

Originally Posted by Vereeken

yes my thought as well. A TTV is typically around the 5 to 10 second mark.

But it should not do that ofcourse because my WUR cold is what it should be so the fuel requirement is following spec.

That is the trouble with CIS. It is very mechanical and not overly accurate...

Michel,

Search "TTV test results" and you will find an experiment I did with the TTV. A typical TTV would open after applying 12-volts to the heater between 20 ~ 30 sec. at room temperature. So a colder ambient temp. would be a factor to consider.

Tony