88 930 idle problem

larrynp · 07-21-2005, 03:40 PM

I need help with an unusual problem I am having. When the car is stone cold it starts perfectly and idles for the first minute or so at 1200 rpm. After that it drops to 700 rpm. Driving around the rpm will rise as the temperature of the oil increases. If you continue driving until normal temp is reached it will idle at about 950rpm. If you stop before it reaches temp, say 170 or so, and park the car then return 10-15 min. later it starts right up and idles normally. If you reach normal temp and park the car and return in an hour or so and the engine is still pretty warm, it idles ok. As an aside, I have noticed that on cold start when it idles up to 1200 it can stumble slightly I.e. it seems like whatever is increasing the idle for cold start (WUR?) turns on then momentarily turns off, then on again. When that happened I touched the pedal and the idle stabilized.

When the car has been sitting for 1.5-2 hours and the oil temp is in the range of 100-170 is where the problem is. This is interesting in that at that temp range after cold starting and warming up the problem does not occur (as above). Anyway the car starts right up but the idle struggles to be at 500 rpm and will stall without constant throttle. If you keep on the pedal you can drive off and while it does not run great you can go. As the temp increases the idle slowly stabilizes and will, as it approaches normal (210) temp, rise to 950 rpm and idle smoothly. The car runs well then too. All this leads me to think this might be a heat soak issue. I have replaced the aux air reg. but it has not helped. Are there other CIS gizmos that have bi-metallic parts that could be affected? Its not a starting issue, its a transient idle issue. The best analogy to this is that it feels like an old manual choke car having the car run lean on start and needing to keep the choke on a bit or the car will stall. Could the fuel accumulator be on the fritz and with heat soak and not supplying enough fuel in these conditions? Maybe after running the car the engine fan is cooling whatever is being affected and that is why the problem goes away??
There are no vac leaks, by the way, and the by-pass valve is shimmed. I've turned the idle up too.

Anybody have any ideas?
Thanks!

gsxrken · gsxrken's Garage

Do you have the CIS fuel pressure tester? I'd be curious about your pressures during the problem. Your warm up regulator may be in need of replacing, but it's too expensive to just start changing on only a hunch and no data points.

larrynp · 07-21-2005, 05:32 PM

I don't have a tester. Are they easy to hook up? I could get one and do it if it is simple-if not I could ask my local Porsche Guru to try it.

john walker's workshop · 07-27-2005, 09:31 AM

something i've been doing lately is to disconnect and ground the wire to the 15° switch on the face of the right chain housing cover. the switch, thru the oxy sensor computer, changes the speed of the frequency valve, making the engine run richer until the switch reaches 15°C. i think the switches can be somewhat off and on. then i set the CO at around 3.5% to 4% at hot idle, (with the airpump hose clamped off) and it seems to fix that low rpm thing you describe. just did it on a 88 930 in fact. 80-83 SCs have the same problem.

Lorenfb · 09:47 AM

"then i set the CO at around 3.5% to 4% at hot idle, (with the airpump hose clamped off) and it seems to fix that low rpm thing you describe. just did it on a 88 930 in fact. 80-83 SCs have the same problem."
- johnwalker's workshop -

Way too rich!

1. Replace the oil temp switch if it's bad & don't leave disconnected.
2. Check for intake air leaks.
3. Check the WUR pressures cold & hot as mentioned.
4. Make sure that the AAR valve has 12 volts when the engine runs.
5. Set the warm CO to 1.5% with the O2 sensor disconnected.

Note: With the oil temp switch disconnected, the Lambda system does not
go closed loop.

john walker's workshop · 07-27-2005, 11:12 AM

loren objects to most everyone's suggestions whether they work or not. and it's not too rich loren. i forgot to mention to leave the oxy sensor disconnected.

Lorenfb · 07-27-2005, 07:08 PM

Operating at 3 to 4% forces the Lambda system to not function at its' ideal point.
Also, operating at 3 to 4% is not a good idea, other than race cars, with engines
that have a CAT. Furthermore, Porsche designed engines to operate properly
without having to offset the mixture to correct a problem.

john walker's workshop · 08:29 PM

disconnecting the oxy sensor forces the system to quit running so lean, gets rid of the cruising surge they all have, and gets you a better response accelerating off of idle. an idle set at 1.5% is kind of rough and uneven. they smooth out nicely at 3.5%. the temp also drops in stop and go traffic. remember, this is just an idle adjustment, it doesn't effect the whole rpm range, so the cats survive.

larrynp · 07-30-2005, 09:30 AM

Hey guys. My car has headers and exhaust (B&B) so there is no cat or o2 sensor. Does this change your thoughts on the low idle problem?
Thanks!

john walker's workshop · 08-02-2005, 02:41 PM

it probably still has the rest of the oxy sensor setup, which it needs to run well. you can remove the sensor from the system, but the rest has to be there, unless you backdated to a euro or 78-79 non sensor system completely.

larrynp · 08-02-2005, 06:21 PM

Thanks for the advice. Yes the o2 system is still there. By the way I shared your responce with my friend the Porsche mechanic and he knew exactly what you were talking about and that it sounded like a good idea. He is going to incorporate it into the diagnostics that he will be performing. I will let you know how things work out, probably in September.