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Help With '78 924 CIS Troubleshooting

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PostPosted: Mon Aug 27, 2001 10:29 am    Post subject: Reply with quote


We recently acquired our first Porsche -- a 78 924. This car had been sitting for about a year -- as the previous owner had problems getting it started --and (with the help of my trusty Haines manual)this is a project that I am working on with my son. We are enjoying working on the car, but still are on a learning curve and would appreciate any help from any more experienced 924 owners.

Overall the car's condition is not too bad and we are currently trying to get the CIS working right. The problem is that the car starts but stalls within about 45-60 seconds. It is obviously getting flooded -- strong gas smell and plugs are fouled. Here is where we are:

1. We checked the cold start thermo time switch and it cuts on and then cuts out in about 3 seconds (temperature is about 80F).

2. We pulled the cold start valve and it cuts the fuel flow when the time switch cuts out.

3. We pulled and tested the fuel injectors one by one and they do not appear to be leaking.

4. We pulled the auxiliary air regulator. In looking through the air passage at ambient temp (about 80 F) the valve is only open about a third of the way. We put it in the freezer for a while and it still did not open further. The heater resistance was about 50 ohms. With 12 volts applied to the heater connections the valve fully closes within 5 minutes.

5. We pulled the electrical connection from the Control Pressure Regulator and tried to measure heater resistance. This appears to be open.

So -- here are the questions--

1. Is the auxiliary air pressure regulator necessarily a culprit in this problem if it's valve opens but only about a third of the way? -- or this normal? Also the resistance reading was a little higher that Haynes spec (30 ohms) is this a big deal?

2. The Haynes manual does not specify the time it takes for the control pressure regulator valve to close. Is this within seconds after current is applied? Otherwise would this cause the engine to flood within 60 seconds?

In any event, we plan to replace the control pressure regulator. I have noticed that some parts suppliers are selling rebuilds of these which are about $50 cheaper than a new one (with a core return) -- is this a bad idea?

3. Finally it occurs that the combination of the auxiliary air regulator not being fully open for the first minutes of engine running AND the control pressure regulator being failed open could be working together to cause the problem. Is this valid? We are considering being conservative (read "thrifty") and just replacing the control pressure regulator but I am not sure if this is realistic.

Thanks for your help. If you have any other approaches to resolving this problem we would also appreciate hearing them.


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Joined: 16 Nov 2002
Posts: 1516
Location: Minneapolis, MN

PostPosted: Mon Aug 27, 2001 1:22 pm    Post subject: Reply with quote

First, good name. Anyone with that name is kewl.

This seems like one suggestion you might have tried- but have you checked the air/fuel mixture? If not, you could attempt to turn it down (remember where it was set first!!!) and see if that also helps...

but it sounds like you are getting even more fuel than what an air/fuel mixture could fix. Is the car mis-firing or running rough? One other thing you might want to check is ignition timing and any possible air leaks giving a false air reading...

Let us know if it works!

- Jon (The other kewl one)

Classic British Sports Car Restoration v6 + v8 Engine Conversion Swaps

Porsche 924 Wide Body LS1 Corvette 500 Horsepower Engine
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PostPosted: Tue Aug 28, 2001 4:25 am    Post subject: Reply with quote

a link to info on CIS
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PostPosted: Tue Aug 28, 2001 5:33 am    Post subject: Reply with quote

The aux. air regulator only should be open about 1/3 of the way. That is normal. All that this does is to bypass the throttle valve to increast the idle speed. To check for proper operation, it should be open when cold and when you connect 12 volts it should slowly open over a period of a few minutes. The warm up regulator is a common problem causing over rich running. I have never had trouble using a rebuilt. The injectors need to open at the correct pressure and not leak when closed. Also, the arm on the air flow meter needs to be set correctly. Otherwise you will have driveability problems. If it is adjusted too low you will have hard starting.
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PostPosted: Tue Aug 28, 2001 2:10 pm    Post subject: Reply with quote

Thanks for your replies. It will probably be a few days before I can get a replacement control pressure regulator. When we replace this we will double check for air leaks and try again.

I will continue to tweak on the system and appreciate all input.


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PostPosted: Tue Aug 28, 2001 2:28 pm    Post subject: Reply with quote

all porsche 924 owners go through this.

It's so common.

You should search for "WUR" and "Aux Air".

BTW if the car smokes and stinks like poop, you need to change the gas, don't worry it's not oil.
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