924Board.org

[ted von Kampen]

I have a 1981 Porsche 924 NA with a lamda CIS system. This car sat for 15 yrs and I have spent 2-3 years restoring things. My present problem is lack of acceleration. To me it points to a problem in the fuel enrichment system connected through the Lambda system.

This car does start, run and is not too bad in town. However a WOT situation is such that it takes a long-long time to gain speed.

The coax input cable from the oxygen sensor to the under dash control unit is cut and part is missing. Happened long before I had car. From what I can read, the Lambda system uses the oxygen sensor output data to determine when and maybe how much to open the frequency valve to enrichen the fuel mixture. Since part of the sensor cable is gone, that does not work and of course neither does the frequency valve.

I am wondering a few things that maybe experienced members can answer.
If this oxygen sensor is disconnected and the frequency valve is therby not working, can this be the major source of poor to little acceleration?

Is there a way to bypass the oxygen control unit and manually test the frequency valve system to see if that valve can enrichen the fuel supply system? From what I can read about this system, the valve is electrically controlled and may be modulated by the control system. I notice on the throttle body system there are a few microswitches that switch when you reach WOT. could one of those be bypassed to run the freq valve as a test?

Long term I suspect the control unit may need to be changed. I don't know the condition of mine except it appears to be disconnected although in place.

Any comment or help or experience on this subject would be appreciated. I have gone through the pressure control system and setting, the WUR rebuild, the cold start system, etc so that functions.

Also, if there is any place to read discussions on this subject, please advise me.
Thanks for any help.
Ted

[Glemon]

I am the last guy to give any expert advice on fuel injection and electronic engine controls, but I am keying on the "appears to be disconnected" statement. Try reconnecting.

Greaetings from the other side of the state.
_________________
88 924S
68 TR250
02 IS300

[ted von Kampen]

Thanks Glemon... Yeah I am going to try that but I am suspect of the enrichment unit. That was why I was asking the rest of the information. I found a piece of Teflon insulated Coax so will try rewiring. The ox sensor is still in place and works. I have used that for setting mixtures. We shall see. The purpose of my post was to get a better understanding of the frequency valve and how it works. From what I read it basically changes the pressure in the fuel distributor to enrichen the mixture.

Tell me a little about your car. There doesn't seem to be many 924's in the state. I think there may be one in North Platte. I am in Scottsbluff. Send me a PM with your email address and I will send you some pictures.

I have been fighting the AC R134 freon leak. I pulled the car out of storage today and after 2 months it still works so I may have found and fixed the leak. Right now it is two cold to get a good test. Let me know if you know someone that wants one. I have too many cars and will sell this one once I get this fixed. ....Ted

[924RACR]

So the O2 sensor is only used to trim fueling at low throttle openings/idle. So that's not the source of your primary problem; it would mainly be noticed in a lack of fuel economy.

The main system, well yeah you probably ought to have it plugged in.

My first 924 had a bad WUR when I got it, and had similar results, extremely bad accel/power. You could indeed have this as an issue, CIS fuel pressure gauges will be needed to know for sure, yet at the same time without the Lambda controller working and in the loop, still hard to tell.

The microswitches on the throttle body are entirely relevant, ONCE the Lambda controller is plugged back in; IIRC there should be directions on how to check their adjustment, even I think in the Haynes manual.

When the Lambda system is functional, the frequency valve that adjusts control pressure/mixture should buzz during running...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[ted von Kampen]

Vaughn,
Thanks for the reply and info. I will be reattaching the Lamda system. During restoration I did remove, clean, calibrate and checked the WUR. It did need help. Also made a set of system pressure gauges and got that all setup. I hope the Lambda comes back to life. Appreciate your comments on the oxy sensor.
Ted

[CRF]

First, most 924 CIS systems will start and run without the frequency value connected. Second, the CIS lambda system is setup to normal operate at 50% duty cycle when the frequency value is connected. When it detects the engine is either rich or lean, the ECU will adjust the duty cycle to keep the engine at 14.7 air fuel. The default state of the ECU is to run at 50% duty cycle if the O2 sensor is disconnected. While 14.7 is OK for idle and part throttle, it too lean for full throttle, so Bosch engineers add a function to the ECU to run at a fixed duty cycle of 65% to make the fuel richer for better power, with a CO of around 13.5. We found that for the NA, the best power is found around 12.8, which requires a duty cycle of around 75%. The ECU detects full throttle via the full throttle switch. The full throttle switch is one of the two switches on the bottom of the throttle body with the white/black (ECU) and brown (ground) wires. The other switch down below is the idle switch (white wire). The other switch on the 81-82 that above the throttle body is the accelerator switch. A dead or bad ECU would also not generate a duty cycle.

One way to test the frequency cable is to test the voltage of the two leads. One side (red) should be 12 volts, the other lead goes to the ECU, which it grounds to generate the duty cycle. When connected and generating a 50% duty cycle, you should see about 6 volts at the frequency value.

Another way to test to see if the ECU is working is to connect to the test cable. This is the normal way of testing the ECU duty cycle in the 924. The test lead always generates a signal, even if the frequency value is disconnected (but only if the engine is running). The test cable goes to a round connector with three wires. On my car, the test connector is found on top of the driver wheel well. The three wires are, power (red), ground (brown) and ECU test (black). The ECU operates only when the engine is running.

I found two ways to connect to the test lead, a old points dwell meter, and a volt meters that measure frequency and duty cycles. I bought a cheap volt meter off Amazon. On my meter, it reads the duty cycle in reverse (smaller numbers for rich, bigger numbers for lean). The ECU runs at a fix frequency of around 60 Hz. Bench testing of ECU show that they can change the duty cycle from about 15% to 90%, but will always return to 50% when the O2 sensor is disconnected.

Carl

[Fasteddie313]

[ted von Kampen]

Thanks all for your tips. More work to do...Ted

[Fifty50Plus]

Carl did all the development work on my race car. He holds some patents on chips for the German cars. He knows his stuff!
_________________
1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

[Fasteddie313]

Wonder what Carl thinks of controlling a frequency valve with a megasquirt ecu running frankencis software..
.....

Sounds like he might even know a thing or two about cis-e including control thereof..
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80 Turbo - Slightly Modified