CIS gurus, insight on values of leakage test

[stupidwhitecat]

Looking for some insight from CIS experts. I recently performed some pressure testing on my 74' and I'm trying to sort through the data to make sense of it. The one that really has me scratching my head is the difference between the values given for the leakage test between Porsche and Bosch. I have been going back and forth between the Porsche technical service manual and the Bosch K-Jet manual while testing. Almost all the normal values match up except for these, and there is quite a difference. For the leakage test at 10 and 20 minutes, Porsche gives normal values of 1.3 and 1.1 bar. While Bosch gives 1.7 and 1.5 bar. I'm not sure which to go by, or if there is any consensus on which is "correct." Based on the Porsche values my WUR passes, but based on the Bosch values it fails.

[PeteKz]

Stick with the Porsche values, but it doesn't really matter. The important test of residual pressure is how long it maintains RP. I say that should be at least 30 minutes, but longer is better.

How long does your system hold RP?

When you shut off the ignition, and thus the fuel pump, pressure is trapped in the system and held under pressure by the fuel accumulator (FA). Usually this starts around 1.5 bar, then slowly bleeds down. If the pressure bleeds off too quickly, you will have trouble with warm re-starts, in fact, difficult warm restarts are most often due to a bad FA (the internal diaphragm leaks). Other points that hold the RP are the SP valve on the fuel distributor, the FD, the WUR, and the check valve at the pump.

[stupidwhitecat]

I just did it per the manual at 10 in 20 minutes. I didn’t check any longer than that. I can recheck and follow up.

Two additional questions regarding the leakage test

1. What you mentioned does pose a question that I was wondering. How long should the system hold pressure? I.e. I left the gauges hooked up overnight and went out in the morning and the pressure was at zero. This is over 12 hours sitting. Should it still have some pressure in the system ?

2. And when doing the leakage test on the system pressure, am I correct to assume that you use the same values mentioned above?

[PeteKz]

Overnight, or after several hours, it will bleed down to zero.

As I said, I want to see it hold 1.1 bar for at least 30 minutes (more is better). I don't think that 10-20 minutes is long enough to get well past the warm restart difficulty zone.

Why are you testing your pressures? Are you having warm restart difficulty? If not, ignore the pressure readings and just drive the damn thing. "If it ain't broke, don't fix it!"

[stupidwhitecat]

Maybe opening a can of worms. Just trying to get it to go from running good to great. Had a slightly rough/irregular idle when warm. And feeling like it was slightly down on power when warm versus cold. Nothing too major. But now that I’ve checked these numbers system pressure is low. Cold control pressure is low. Warm control pressure passes. Leakage test passes per the Porsche value. Partial and full throttle tests, barely pass. I plan on running the test again just to see if it’s repeatable. But based on my first results, I assume that leakage test at 30 minutes will be acceptable. At 20 minutes it was 1.4 bar. So I can’t imagine in another 10 minutes it would leak down past 1.1.

[boyt911sc]

Originally Posted by stupidwhitecat

Maybe opening a can of worms. Just trying to get it to go from running good to great. Had a slightly rough/irregular idle when warm. And feeling like it was slightly down on power when warm versus cold. Nothing too major. But now that I’ve checked these numbers system pressure is low. Cold control pressure is low. Warm control pressure passes. Leakage test passes per the Porsche value. Partial and full throttle tests, barely pass. I plan on running the test again just to see if it’s repeatable. But based on my first results, I assume that leakage test at 30 minutes will be acceptable. At 20 minutes it was 1.4 bar. So I can’t imagine in another 10 minutes it would leak down past 1.1.

If you get 1.4 bar reading after 20 mins., your CIS does not have residual pressure loss problem. You need to know your CCP, WCP, & SP. Your control and system pressures could be the culprit to your problem. Test & verify.

Tony

[PeteKz]

If your SP is low, then the first step is to adjust that to the middle or higher end of the spec. Do you have instructions to do that? Then recheck the CCP and WCP.

[stupidwhitecat]

Yeah, the Bosch K-jet manual details it pretty well. Based on the manual I am going to start with 2 shims and recheck pressures. Current system pressure is 4.3. Per the manual 2 shims should raise it by about 0.3 bar. Before I do that though, I’m gonna check out the remaining components of the fuel system now that I’m this deep into it. I checked the fuel pump volume earlier this morning and that was right at 850 cc in 30 seconds. I also checked individual injector volumes and they we’re all within 1-3 cc of one another. Still going to check the cold start valve to make sure it is working properly/ not leaking.

[boyt911sc]

Originally Posted by stupidwhitecat

Yeah, the Bosch K-jet manual details it pretty well. Based on the manual I am going to start with 2 shims and recheck pressures. Current system pressure is 4.3. Per the manual 2 shims should raise it by about 0.3 bar. Before I do that though, I’m gonna check out the remaining components of the fuel system now that I’m this deep into it. I checked the fuel pump volume earlier this morning and that was right at 850 cc in 30 seconds. I also checked individual injector volumes and they we’re all within 1-3 cc of one another. Still going to check the cold start valve to make sure it is working properly/ not leaking.

The primary pressure adjustment is determined by the shim thickness used and not by the number of the shims. The stainless shims come in different thickness. As a guide, I use 0.010” to 0.016” shim thickness to increase 5 psi. SP.

Tony

[stupidwhitecat]

Got it. The Bosch manual only referenced 0.1 mm shims so that was the value I was basing it on. The manual states a 0.1 mm increase in shim thickness will increase system pressure by 0.15 bar.

Where do you find the shims nowadays? I did some Internet searching and the only thing I could pull up was SMC sells a fuel distributor rebuild kit that includes “shims “but it doesn’t mention how many or the thickness.

So I wanted to know where I was starting from before just adding shims, I wanted to see what was already in there. I pulled the screw plug on the distributor and found that there are already three shims in there. Two 0.4 mm and one 0.1 mm. For a total shim thickness of 0.9 mm. So with the 3 shims already I am still currently getting a low system pressure of 4.3 bar.

Is this too many? And would it be advisable to add additional shims on top of these? Again per the Bosch manual just to get into the range I would need to add at least another 0.2 mm.

[stupidwhitecat]

[boyt911sc]

Originally Posted by stupidwhitecat

Stu,

Could you post a side view picture of the PPV? I believe you have a FD-004 (fuel distributor). Please confirm the ID of your fuel distributor. Thanks.

Tony

[PeteKz]

If you need more shims, try Ace hardware or other well-stocked hardware store. They sell stainless washers in sizes that should fit in there. Measure the inner and outer diameters and find something that's "close enough." Measure the thickness and stack them up to get what you need.

[boyt911sc]

Originally Posted by PeteKz

If you need more shims, try Ace hardware or other well-stocked hardware store. They sell stainless washers in sizes that should fit in there. Measure the inner and outer diameters and find something that's "close enough." Measure the thickness and stack them up to get what you need.

Peter,

The PPV shims used in the early fuel distributors are solid disc shims. They are approximately 5.5 mm OD and come in different thickness. They are totally different from the shims used in SC fuel distributors.

Tony

[PeteKz]

Tony, thanks for the clarification.

[stupidwhitecat]

Originally Posted by boyt911sc

Stu,

Could you post a side view picture of the PPV? I believe you have a FD-004 (fuel distributor). Please confirm the ID of your fuel distributor. Thanks.

Tony

Correct, it is an 004

[stupidwhitecat]

Originally Posted by boyt911sc

Peter,

The PPV shims used in the early fuel distributors are solid disc shims. They are approximately 5.5 mm OD and come in different thickness. They are totally different from the shims used in SC fuel distributors.

Tony

Yes, quite different than anything I can find or have on hand. I was rummaging around in my parts bin and have some old parts from my triumph motorcycle carburetors that are pretty close to the diameter but they are washers and I didn’t want to just throw anything in there at this point.

After some more searching online, the closest I could find was McMaster‘s carr. They sell stainless steel washers in various thicknesses between 0.1 to 0.3 mm. is this something you think would be acceptable to use? I was thinking if I “sandwiched them” between the solid shims?

[PeteKz]

Given that the shims rest under a coil spring, I don't see how using washers instead of shims would make any difference. It's the total stackup that matters

[safe]

I would give the advice to stay the h*ll out of the fuel pressures if it runs good. Don't muck around unless you have or want bigger problems.
Replace the injectors if they are original, they are probably worn out.