83 SC surging idle when hot...
 

I drive my 83 Cab 400 miles to/from (each way) on business trips 2-3 times a month during the summer. Last year I sorted various vacuum leaks, my WUR was out of spec so replaced that with a Tony-rebuilt one, etc. I have verified fuel pressures (don't have the data in front of me, I'm on one of those trips now) and have done multiple smoke tests as I fixed various leaks and there are not visible leaks now. I also do see/hear an RPM change when I remove the oil filler cap, telling me that I don't have any gross air leaks. Now to the problem...

The idle speed is stable just a tick below 1000RPM UNTIL I've been driving and the engine really warms up. This is most noticeable after driving at highway speeds for 30+ minutes. Then, when I come to a stop, the idle surges between 800-1200 RPM. If I just drive around town so the engine doesn't really get warmed up, I never see this problem. If I stop the car (say to run an errand) and then return 5-10 minutes later, idle is fine until the engine really gets warmed up again. It really seems like some CIS high temp feedback loop is not quite working properly but after going through all of the CIS test procedures, I haven't been able to locate it.

What else can I check/do to figure this out?

cheers,
Michael

I'm having this same issue right now. If I unplug the O2 sensor my idle is normal when hot, but when my O2 sensor is plugged in and the car is hot it will sit at ~1500 RPM. Try unplugging your O2 and see if your hot idle is normal.

Hi boys...if surging on hot engine it is good to disconnect the o2 sensor and also no reason to connect back until you got for a smog check.
Also the cause is usually too rich fuel mix

Ivan

While I'm okay with a band aid fix in the mean time, I would still like to find the issue and fix it. I already replaced my O2 sensor and O2 sensor relay, no change. OP I suggest you try the same. Is your OXS light on? Mine came on when all this stared for me.

No OXS light. I have tested the O2 sensor relay under the passenger seat. I do suspect I'm too rich too.

Did you set the CO to spec?

Not with a meter, I used the "field plunger test".

Michael ..i suppose you do not have catalist on your 911 right?
Ivan

Just perform the simple "2 finger test"....well, Ok, a finger and a thumb:

1) Prior to starting the engine, reach with one hand under the passengers seat and grab the O2 relay between a finger and your thumb,

2) While holding the relay in (1) above, start the engine. Did you feel a "click" of the relay when starting the engine? If so, it is working. If not, it isn't working.

I can tell you from experience, that if the engine (in my case a U.S. model '82SC) is tuned to spec with regard to the CO, and the Lamda system shuts down, it will go extremely lean to the point of the rear end of the car shaking like a paint shaker and you might have the power of a Subaru Justy hitting on 1 of the 3 cylinders!

Ivan, the car is stock US with a CAT and working lambda and O2 sensor.

Thanks SCadaddle. I'll see if I can contort to do that. The car pulls strong and runs great otherwise. It isn't until I've done prolonged driving and the engine is well heated that the issue kicks in.

I should also say that I have monitored the lambda duty cycle so I know the lambda system is working.

Michael .the reason i have asked is you have measure before cat the c/c mix...there should be a little nut cover to insert testing tube...
Ivan

I've had a similar situation with my 81 SC, and replacing the O2 sensor fixed it for me.

One of the best tweaks I did to my '83 Euro was put an AFR gauge in place of the clock. I can monitor my CO constantly and it not only tells me when I have it adjusted to my sweet spot (richer than normal) but how well the WUR and CIS are working when I put my foot in it.
I did this so I'd never run too lean as the 930-10 has a 9.8-1 comp and to not have to worry about a lean situation leading to a blown airbox.
It is an analog so matches the other gauges.

@proporsche - I'll have to look for that as I don't recall seeing this when I've had the engine out.

@aoncurly - not sure that replacing the O2 sensor makes sense as the lambda system is functioning correctly otherwise and I see the correct duty cycles. This would not be the case if the O2 sensor were bad.

@Reiver - I absolutely want to do this and have the wide band O2 sensor and gauge and a data logger. The issue, for me at least, is getting the bung welded in. I can't get the front bolts off the cat (the rear muffler bolts are new as of last May) to remove it to have the bung welded on.

944 owner here...so please excuse that I don't know if your SC even features an Idle Control Valve (ICV) - but if it does...you may want to unplug it to see if anything changes. My 944 demonstrated the exact symptoms you've described...which I fixed (after verifying symptoms as described) by rebuilding my ICV.

Michael...look for something like this .On picture is a aftermarket but the location for testing should be the same

Ivan

http://cdn4.pelicanparts.com/techart...small/Pic2.JPG

Thanks Ivan, as many times as I've crawled around under, over and around my engine I don't recall seeing that! I'll look again now that I know what and where to look.

Also Michael why do still have the Cat?If you would just replace it with strait pipe you will get couple of horses and nice sound.Do you have to have it in MA?Or just for the smog check?

Ivan

It is all stock, so yes it has a cat. Technically it is illegal to remove a cat here in the US and I'd rather not go down that path. Although, my car is exempt from emissions testing due to its coolness! (actually due to its age)

In any case, I still need to figure out how to get those 6 front bolts off. Major PIA.