Cold Control Pressure on WUR - hard stop? CIS Experts!

[Vereeken]

As we all know by now the ccp on a SC is crucial for getting good cold start and driveability from the get go.

I am adjusting my NOS WUR that came with an out of spec ccp.

My question is (thanks to Jims CIS primer for the graph) the following.

The Bosch graphs always show WUR pressure tables starting at 10° Celcius.
I have noticed a couple of forum members extending the graph to freezing temperatures suggesting that the ccp should be lower the lower the temp goes.

I am not sure that this extrapolation is accurate. It would be imaginable that the ccp should be 0 bar at a give negative temp. Which I do believe would prevent the car from starting.

So what I am thinking is that the ccp should be as low a the 10° indication indicates and should not drop beyond this point even if the temp gets lower.

Is this correct?

Relative to that, I have noticed that my SC starts better cold with a ccp at the high range of the table for the given temp. So in my case the car prefers 1.4 bar ccp over 1.0 bar ccp. Has anyone else noticed this? Or does this indicate another issue or worn fuel components...?

Thanks

Michel

[Bob Kontak]

Good question. It's not like Porsche ran out of graph space so they had to keep the graph small.

I am one of those who drew a graph to freezing and made an assumption. I would like to know the answer as well. From my layman's viewpoint I was thinking 10C/50F "absolutely" has to take less fuel that the car starting at 0C/32F (let alone -10F) but that was not addressing the operating limits of the hard parts. Maybe 1 bar cold control pressure is all that is needed even for the coldest mornings.

One of the CIS guys has to know.

My pressure ranges are in the middle so can't talk to your second question.

[Vereeken]

Bob, I was referring to your drawing but did not want to make it personal

I really think the 1 bar is the hard stop. Otherwise you would not make a graph that starts at 10° but then again who knows what a German engineer was thinking in the late seventies.

Ofcourse this graph was made in the pre-microsoft era and changing a graph where you made a mistake in the scale must have been a b$tch

Lets see if one of the GIS guru's comes up with an answer.

Michel

[Bob Kontak]

Michel, I have made more than a few incorrect assumptions here. Corrective feedback is welcome. Even deserved beatings, painful as they, tend to clarify focus. :-)

Regarding the graph, if you extend the X axis to the really cold ranges you would be less than zero bar ccp before long. That is not going to happen mechanically. I bet you are correct regarding hard stop.

[Targalid]

Could these questions not be answered with a fuel pressure gauge and a thermometer? Does the CCP change below 10C or not? That is the question. I guess the real question is what happens with the bimetallic spring inside the WUR below the temp on the graph. Is is already at the end of its travel? Another way to answer this question is to remove the WUR and place it where it can be viewed at different temps, in a temperature controlled environment like a refrigerator or freezer, and see if the parts move at temps below 10C.

[RarlyL8]

It has been my observation that the 10°C terminal end of the graph is the lowest setting for a couple of reasons. One is that the reduction in vapor pressure of gasoline reaches diminishing returns at that point. The other is that the CSI (cold start injector) is activated at ~45°F temporarily adding to the enrichment.

[boyt911sc]

Originally Posted by Vereeken

As we all know by now the ccp on a SC is crucial for getting good cold start and driveability from the get go.

I am adjusting my NOS WUR that came with an out of spec ccp.

My question is (thanks to Jims CIS primer for the graph) the following.

The Bosch graphs always show WUR pressure tables starting at 10° Celcius.
I have noticed a couple of forum members extending the graph to freezing temperatures suggesting that the ccp should be lower the lower the temp goes.

I am not sure that this extrapolation is accurate. It would be imaginable that the ccp should be 0 bar at a give negative temp. Which I do believe would prevent the car from starting.

So what I am thinking is that the ccp should be as low a the 10° indication indicates and should not drop beyond this point even if the temp gets lower.

Is this correct?

Relative to that, I have noticed that my SC starts better cold with a ccp at the high range of the table for the given temp. So in my case the car prefers 1.4 bar ccp over 1.0 bar ccp. Has anyone else noticed this? Or does this indicate another issue or worn fuel components...?

Thanks

Michel

Michel,

You are correct!!!!! I did some testing on assorted Bosch WUR's (0-438-140-XXX) and found out that the lowest CCP's range from 0.8 to 1.0 bar. The temperature could drop to 0°C/32°F or below and the CCP would still register in the 1 bar value. The main reason for this set-point is to maintain residual fuel pressure. Imagine if the CCP is allowed to go down to ZERO (0), this is like having a bad FP check valve, a leaking FA, etc. resulting to starting problem.

In my case, my '78 SC has CCP just above 1.1 bar and I could start the engine even when the temperature is well below freezing point. Look very carefully at the numbers of the fuel pressures from the chart. The median value has a range of plus or minus 10%. So what ever value makes your engine run great, maintain it and be happy with it. HTH.

Tony

[Bob Kontak]

Originally Posted by RarlyL8

One is that the reduction in vapor pressure of gasoline reaches diminishing returns at that point.

Just asking here. Does "at that point" mean 10C or when the CCP is say, less than one bar? A little confused as high Reid Vapor Pressure winter gasoline is more volatile than summer gas and it does a better job creating vapor at 20F than summer gas would. Am I missing the context?

[Vereeken]

Thanks guys, i guess it confirms my idea.

I can only add my personal experience.

0.8 bar at 4° celcius sees to it that the car refuses to start, and when it starts after cranking for 20 seconds it runs at low RPMS.

I am now a little bit too high (2.5 bar at 14°) and the car starts from the first second.

Next week we are supposed to be getting a cold spell with night temps aorund 0°.

I will let you know what that does to the car and the ccp.

From what I understand now 1bar is the very minimum for all temperatures below 10° celcius. I feel it is better to have 0.1 bar too much then too little but all of this is ofcourse based on the experience of 1 car.

I never imagined that this car was so sensitive to fuel pressure.

Michel

[Jim Williams]

Here is some information that might add some insight into the issue of WUR cold control pressures, and what the pressure should be.

First some background: By a twist of fate, I came into a 3.1 liter engine some years back. There is a thread or two on this forum about that. Because it was misrepresented to me at the outset, I thought I was buying a rebuilt stock 3.0 liter. It turns out that the 3.1 liter engine carries it's own special CIS. Since there are no guidelines in the factory manuals about this engine or it's CIS, I have had to do some experimentation. The engine came with a 089 WUR. I have to assume this is what it was delivered with, so when I rebuilt the engine, and installed it in my '73 RS-look, this WUR is what I used. I even had a respected rebuilder check the 089 and set it to factory specs.

When my rebuilt engine is adjusted to a reasonable CO reading when warm, and further checked by raising and lowering of the airflow sensor plate to verify that the mixture is in the ball-park at least at hot idle, the engine pulls like I'd expect it to.

But at cold start, although it started fairly quickly, it was obviously way too rich. It smelled bad, and hunted, and idled poorly, and required opening the throttle significantly to keep the engine running. (A note: This is a garaged car, and starts up at a temperature seldom ever less than about 60˚F.) I have a dash gauge that reads WUR fuel pressure, so I can see from start-up how the control pressure increases as the engine warms. While at startup, engine (and WUR) still cold, if I immediately did the simple airflow sensor raise/lower check, on pulling down on the airflow sensor plate, the engine speed immediately increased several hundred rpm, indicating the cold mixture was way too rich. By the way the AAR is a 206, stock for a Volvo.

The 089 WUR is spec'd at about 1.2 bar at 10˚C, with vacuum(lower with no vacuum). If the thermovac switch holds off engine vacuum for about 20 to 30 seconds at start-up, this is what the WUR produces. Even with a vacuum, this is still the spec. So the engine's actual mixture requirements at my "cold" start temperatures were way more lean than the rich mixture the 089 was providing. Even bypassing the thermovac switch didn't improve things.

In looking at the specs for an 045 WUR, I see that the cold control pressure is around 1.8 bar, and the warm control pressure is 3.4 Bar compared to the 089 warm pressure of 3.6 Bar - not a lot of difference.

Having just checked the cold and warm pressures of both the 089 and the 045 on the workbench with my test setup, and verified the pressures, I swapped out the 089 for the 045. Bingo - everything is now copacetic. Engine starts immediately, goes to around 1900 - 2000 rpm without touching the throttle, and gradually falls off, even as the WUR pressure increases as I note on my dash gauge. No over-rich smell. I expect there is still some minor tweaking to be done, but the lesson is clear, it matters not what the WUR warm-up specs are on the graph if they don't match the needs of the engine.

So the conclusion is, before just trying to make the WUR match a graph, it would be good to determine if this will meet the needs of the engine.

[Vereeken]

Jim,

That sounds a lot like my engine.

I am going to put it at 1.8 ccp no vacuum and see what happens.

I have already established that 0.8 is no good and that 1.0 (the lower threshold of the graph) is only marginal. 2+bar is good but it seems to almost eliminate the high rpm mode at cold start.

1.8 would actually be the top spec at 10° for a 089.

Finally I see some sense in all this CIS.

Michel

[Jim Williams]

Do the push/pull test on the airflow sensor plate with a just started cold engine. You can get an idea if the mixture is too rich, or too lean, for that matter. Then you'll know which way you might need to go with the fuel pressure. Do you have the pressure gauge setup for checking the System and WUR Control pressures?

[T77911S]

Originally Posted by Jim Williams

So the conclusion is, before just trying to make the WUR match a graph, it would be good to determine if this will meet the needs of the engine.

nice work from jim and tony.

i have always looked at it this was. if porsche (bosch) thought the specs were needed for below 10c, they would have made the chart show it, after all, look how cold it gets in germany.

the condition of the engine/CIS can also make what jim said more true.

but jim, if you have a 3.1, could that make a difference as far as your CCP? and what about the FD/AFM, is the WUR matched to them or does it make a difference?
bigger cams, higher compression make a difference?

to the OP, make sure the mixture is correct for warm running. if you are too rich/lean, that will also effect cold starting/running.
timing will effect it too. advanced timing at idle makes it richer, retarded (from spec) makes it leaner.

[Jim Williams]

Originally Posted by T77911S

nice work from jim and tony.
but jim, if you have a 3.1, could that make a difference as far as your CCP? and what about the FD/AFM, is the WUR matched to them or does it make a difference?
bigger cams, higher compression make a difference?

Here's the Bosch part number of the 3.1 FD:



As I mentioned, the 089 WUR and the 206 AAR were on the engine. The AFS is an 070, like the '78 - '79 3.0 liter. NO ONE seems to know any details about the FD except that it was modified from the stock 031, even the folks at Porsche who at the time were involved in the development of the 3.1 engine including Rolf Sprenger. There are no official records existing at Porsche on the 3.1 engine, just info in some private files. Again, there are no factory specs on the CIS components, not even a list of the part numbers.

For all anyone knows, the specs for the needs of the 3.1 liter WUR were figured out manually by trial and error and the closest WUR in the inventory at the time was chosen. Manually dial in a warm engine control pressure value that matches 95% of the stock WURs, set the optimum warm running mixture with the 3mm set screw adjuster, then wait for a cold engine the next day and manually set the cold control pressure to create a reliable cold start. Manually increase the control pressure vs time to get a value for the internal warm-up resistor. Then, based on these numbers, pick a WUR. Of course all the preceding is a total guess.

The AAR would be picked to provide adequate idle air, somewhat matched to the gradual leaning out of the mixture by the WUR, decreasing the added air bypass around the throttle until no longer needed (idle speed of 950 or so attained). Maybe the '77 2.7 liter AAR (206) matched up here.

The cams for the 3.1 were the same as used in the 3.0 liter. The compression was stated as 9.5:1 with 97 mm P/C (some notes imply 9.8:1, the ratio I measured). As far as I know, there are no more of these P/Cs in existence.

[Vereeken]

Guys,

I really need some extra input. This car is driving me crazy.

After a night I can not get it to start like it should.

The weird thing is that the car does its start-up routine perfectly after the first start.

The first start in the morning is as follows:

-Very long cranking to get the car going
-followed by a low idle
- if I leave it running for 1 minute the rpm creep up to 1.000rpm.

If I shut off after 1 minute and wait for a couple of minutes to restart the start-up is perfect (even weirder, teh second start is also good when all 12v sources to AAR, WUR and TTV are disconnected so basically my second start starts with the same pressure values and AAR opening!).

-immediate start.
-1450 rpm
- lowering down to 1000 rpm over the course of five minutes.

I can bring my Co and ccp and wcp into spec or start of with a higher ccp nothing seems to matter. It needs that first laboured start before it does its routine perfectly.

The CSV and TTS work fine. I have verified again with a test set-up with the CSV out of the car. It sprays a nice pattern of fuel from cold. When I disconnect the TTS the CSV does not spray. I am going to do one last test with the CSV to see if it does not leak under 4,9 bar pressure.

Any suggestions?

Michel

[Scott R]

You have a ROW 81' with no Lambda system correct?

[Vereeken]

Yes indeed, actually a 83 SC last of the line with no Lambda and with a 089 WUR with a vacuum connection for increased AFR at cold starts.

Michel

[Scott R]

Given that then, are you getting enough lift on the sensor plate while cranking? I've seen an SC that took a lot of cranking to develop the vacuum to lift the plate and start the pump and injectors.

It's something to check I suppose.

[Vereeken]

This is somthing that I have been contemplating. Only thing is how do I create more lift? And why would this go away on the second start? A fuel plunger in the FD that is lazy?

Michel

[Scott R]

This particular instance the piston was sticky and after cleaning we needed to adjust the plate height as well. After that the lift came up right away and the car started fine.