911sc idle surges when hot

DennisB · 05-02-2008, 02:54 PM

I was wondering if anyone has experienced a similar problem with their CIS? I have a 1982SC with the 02 sensor disconnected. The engine starts, idles and runs fine until it gets to its maximum running temperature. When it gets hot, I have an intermittent problem at idle. It becomes very rich to the point of a radical surging between a couple hundred RPM and about 1500 rpm. I have tried to lean out the mixture a little, but it doesn’t seem to solve this problem. The problem is intermittent, and it appears to be temperature related. It usually only does it when I am sitting in traffic. If I just get off the freeway where the engine has had a chance to cool down a little, the problem does not rear its ugly head until the engine gets to its maximum operating temperature again. Another thing that I noticed is that when I am in this “surging mode”, I can unplug the frequency valve relay momentarily and the engine will return to a normal idle for a minute or two, only to return to the violent surging. I am going to check my fuel pressure this weekend, and see if it changes when this phenomena appears. If anyone has any suggestions as to how to isolate this problem, or if you have experienced it, I would really appreciate your feedback.
Thanks!

Paulporsche · 05-03-2008, 04:46 AM

You should probably get your control pressures checked, especially your warm. Could be the WUR is slightly out of adjustment, giving you a too low wcp and a too rich mix when warm.

BTW what are you calling maximum operating temp?

DennisB · 05-03-2008, 07:04 AM

Thanks Paul,
I am going to check the pressure today. The thing is is that its an intermittent problem. And the engine runs very well otherwise. It seems more like something is failing when it gets too hot. Wouldn't I see a pressure problem with the WUR all of the time? By maximum temperature I mean the hottest the engine ever gets sitting in traffic on a hot day. I do not have a temperature gauge on the motor other then the dash gauge. It runs about 1/8 inch above the black mark (about 8 o'clock). Is it possible that one of the cold start mechanisms is kicking on, like a temperature switch failing? Anyway, I will put a gauge on it today, and see what my running pressure is.
Thanks,
Dennis

psalt · 05-03-2008, 09:04 AM

Hello Dennis,

The first thing I would do is hook up a dwell meter to the lambda test port and actually find out what the FV is doing when this occurs. Since you disconnected the O2 sensor, it should be around the 58 dwell default value. If you hooked up the O2 sensor you could also get an idea where the mixture is and how the system is working. If your car runs hot in traffic, the first thing to do is disconnect and plug the vacuum retard and reset the idle speed. You will be shocked at how much heat the retarded timing at idle creates.

Paul

Paulporsche · 05-03-2008, 11:43 AM

Ditto on the vacuum retard. I did this recently and my engine temps went down about 20 degrees F.

DennisB · 05-05-2008, 10:20 AM

Hi,
I put a pressure gauge and found the operating pressure in spec. I hooked my meter to the Lambda test port, without the O2 sensor connected. The dutycycle stayed at 50% until the problem occured. When it occured the duty cycle gradually increase to > 90%. If I disconnect the plug on the frequency valve and reconnected it. The duty cycle returns to 50% for a couple of minutes, then slowly goes back to > 90%. I am pretty sure that this points to the pulse generator in the Lambda control unit. I have not looked inside of it yet, but if it is not potted, I should be able to hook a scope up to it, and hit each component with a heat gun until the problem occurs.
Has anyone out there attempted to repair one of these? Are there schematics available? Any other ideas?
Thanks,
Dennis

psalt · 05-05-2008, 11:52 AM

Sounds like a bad ECU. The main temperature switch is located on the right side timing chain cover. Contrary to what the Bentley says, it is normally closed and opens at 15C, check with a meter that you do not have an intermittent short, causing the ECU to go back to cold open loop mode. You can leave it unplugged while hot and see if the symptom goes away. I would hook up the O2 sensor, check all the wiring and plugs, clean the grounds and try to get the system back to operational so you can test it.

DennisB · 05-05-2008, 12:02 PM

Hi Paul,
Thanks for getting back to me.
I checked that switch, and when I ground it, duty cycle changes to 64%. So that circuit appears to be working. I will do what you recommended and see if I can get it to work.
Thanks again!
ennis

DennisB · 06-03-2008, 09:17 AM

I finally solved the problem. It turned out to be the shielded O2 sensor cable. The shield was shorting to the signal wire. Apparently, when the engine got to the right temperature, something expanded just enough to short the two, sending my control unit a false signal that the engine was running to lean, increasing the duty cycle to 95%. This was very difficult to diagnose until I had to do a panic stop, then it started doing it all the time. So I guess I should thank the lady who pulled out in front of me when I was doing about 60 MPH

. Anyway, thank you for your help, guys. BTW, there is a wire on my Lambda control unit connector that is plugged into pin 11. The Bentley manual says it should be in pin 12. I believe it is the wire from the 15 Deg temperature switch. Does anyone know what the correct pin should be for this?
Thanks,
Dennis