Update on CIS Testing!!

OK, here's what we've been waiting for (I guess I should speak for myself). 8)

I tested the system both cold and warm, with the original and new (correct) vacuum hose routings, and here are the results:

Initial cold test (with electrical connections disconnected):

1. I connected the CIS gauge, ran the fuel pump, purged the lines of air, and obtained the following cold values: (a) the system pressure is around 65PSI and (b) the control pressure is around 28PSI.

Next, I started the car with the following results when warm:

With the new (per the diagram) hose routing:

1. With the new hose routing, getting the car to run was very difficult. I finally was able to start the car and get to the back so I could keep it running. The control pressure was 50PSI with the car warmed up.
2. I removed the vacuum line from the top of the WUR, and the pressure DROPPED to 45PSI, and the engine ran slightly better.
3. With the new vacuum routing, the engine did not run well.

With the original (incorrect) hose routing:

1. The engine ran better, and I obtained a value of 35PSI with the engine warm.
2. I removed the vacuum line from the top of the WUR, and the value INCREASED to 45PSI, causing the engine to run worse.

I also checked the operation of the Thermovalve, which operated properly. Also, the system held pressure, therefore the accumulator seems to be functioning properly.

Now, based on some values that I obtained, I have some comments and questions:

1. The system pressure is said to run at 80PSI, yet mine is running at 65PSI. Could this be an issue?
2. The cold system pressure is said to run at 32PSI-35PSI, yet mine measured at 28PSI.
3. The warm system pressure is said to run at around 50PSI, which mine does exactly with the new (as per the diagram) hose routing, yet the engine runs poorly. Could this be a simple mixture adjustment?
4. The warm system pressure runs what seems to be low (35PSI) with the original (incorrect) hose routing, yet the engine runs better. Could it be that the mixuture was "adjusted" to compensate for an improper WUR value due to the incorrect hose routing?

I'm going to do some work to the boat (it's getting very angry at me for spending so much time on the Porsche), but I'll check back to see what the consensus is on my results before I move forward. My initial thoughts are that the mixture is out of adjustment, so what do you think?

Thanks again everyone!!

Michael

Michael,

Your system and control pressures with the hoses connected correctly are pretty close to what you should have. Close enough that the engine should run OK if you adjust your mixture. If your car had been running OK with a control pressure of 35 psi, it will run very lean with a control pressure of 50 psi.

There is a mixture adjustment procedure that I've seen posted that you can do without an exhaust gas analyzer that should get your mixture close to correct. Try a search in the archives.

Questions I have. With the hoses connected correctly, and the engine warm, you said that the control pressure drops from 50 to 45 psi when you remove the top vacuum hose. What happens when you remove the bottom vacuum hose (the one from the thermovalve)? What happens when you remove both vacuum hoses?

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Bill Krause
'79 911SC Euro

OK, I adjusted the mixture as best I could, and seem to have a nice, solid idle at this point (with the new hose routing).

When I remove the top vacuum line (from the top of the WUR to the throttle body), nothing changes; pressure remains at 50PSI, and no change in engine idle.

When I remove just the bottom vacuum line (from the bottom of the WUR to the Thermovalve), pressure drops to 45PSI, but there is little or no change in engine performance.

With both vacuum lines disconnected from the WUR, I experience the same result as disconnecting only the bottom line.

There is a noticeable vacuum present on the line coming from the Thermovalve, and no vacuum present on the line from the throttle body to the WUR.

Based on my research and current understanding of the system, the vacuum system "operates against itself," so I would assume the throttle body connection will present a vacuum under hard acceleration, "overpowering" the vacuum from the Thermovalve, and causing the WUR to lower its pressure to provide a richer mixture.

I'll now wait until tomorrow morning, when the car has sat overnight, to re-test the cold control pressure, and see if these new adjustments/changes have an effect on the cold starting behavior.

Michael

Michael,

Your car is now working the way mine does give or take a few psi of control pressure and system pressure. There is a method for adjusting system pressure that involves adding or removing shims within the fuel distributor. I haven't done this, but it looks straight forward. I'll send you the procedure. I'll also send you the control presure graph for a 78-79 US 911SC.

Here's my understanding of how the two vacuum lines work.

The two vacuum lines are connected on either side of the throttle plate. The lower connection to the WUR comes from the intake side of the throttle plate. There is a vacuum on this line at idle. The other line comes from the other side of the throttle plate. At idle, there is very little vacuum on this line. This vacuum differential cause the WUR control pressure to go up (it leans the mixture at idle). When the throttle is wide open, the vacuum differential is small and the WUR control pressure decreases (this is full throttle enrichment). The thermovalve is used at cold start to shut off the vacuum on the lower line which causes the WUR to decrease control pressure and richen the mixture.

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Bill Krause
'79 911SC Euro

Thanks for the clarification. The system seems to work somewhat like I thought. I'd greatly appreciate it if you'd send me the control pressure data.

I must say that I'm truly happy I went this route, as opposed to sending the car off to the mechanic again. Because of the information provided on this BBS, I've gained a comprehensive understanding of how the CIS system works, and learned how to do quite a bit of troubleshooting. Being that this is my first Porsche, it really helps digging into these issues to understand a new technology.

I'll let you know how the cold start process goes tomorrow morning. I'm pretty anxious to see if this corrected the problem!!!

I have a little valve clattering, so that's probably my next home project!!

Bill, I owe you a debt of gratitude for working so closely with me on this. Without your guidance, I'd still be in a fog. Please send me your address over e-mail so I can at least send you a small token of my appreciation.

Michael

you don't say what problem you're trying to solve. cold control pressure usually is around 1.0 to 1.4 bar. a bar being 14.7 lbs. or one atmosphere. if it's too high during cold running the engine spits and pops, and is hard to drive for a few minutes. if it's too low, it runs too rich, idle surges up and down, and it smells rich, the warm control press. is 3.4 to 3.8 bar. the system press. is 4.8 to 5.2 bar. adding or subtracting shims in the fuel distributor check valve raises or lowers system press, which affects control press. accordingly.

[This message has been edited by john walker's workshop (edited 03-27-2001).]