ashame1

I am considering the purchase of a 1980 SC.
When cold, the engine will not idle.
When warm, the idle is about 1400 RPM's

Does anyone have any thoughts as to what might cause this?

Any idea on potential repair costs?


Thanks...

lagoon

I would check your fuel pressures first. The WUR could be not set right and/or your mixture is off. 81'SC

john d 81SC

Take a look at the last time the oxygen sensor has been replaced. I don't want to misquote Steve Weiner (Rennsportsystems.com and a regular contributor to this board), but one symptom of needing to replace the oxygen sensor is a car's inability to idle smoothly.

Bill Gregory

Other possibilities, the auxilliary air valve is not opening, which would impact idle when cold. It's also possible the air sensor plate needs adjusting. On the warm engine idle, check for air leaks. Use a canister of propane gas and with the gas turned on (BUT NOT LIT!!) move it around any junction where air could possibly enter the system. If you find an air leak, you'll hear the engine rev. CIS fuel injection is intolerant of unmetered air, so air leakage can mess things up.

Also, assuming the SC you're looking at has some sort of records, review them for when any CIS-related components were replaced. If there are limited-to-no records, then make sure you get a very thorough pre-purchase inspection. You've probably already read the 911SC FAQ at <a href="http://tech.rennlist.com," target="_blank">http://tech.rennlist.com,</a> which may help answer some questions.

Nate71c

I have a 78 SC with cold idle problems. The car is really hard to start from cold, requiring a lot of cranking before it begins to fire, and then another 2 or 3 cranks before it reluctantly begins running, oscillating between about 400 rpm and 1200 rpm for a few seconds each. After a couple of minutes of this, the idle settles down to a perfect 800rpm.

I live in Australia, where we have a choice of either lead-replacement fuel or 96 octane unleaded. When I bought the car last year it had been running on the lead-replacement "super" fuel, but on the advice of my local Porsche specialist I switched to the 96 octane unleaded. I reckon the reluctance to start had become worse since the switch.

The car has just had a full service and tune, and is no better.

Nate

ChrisB

Now that you guys are bringing up this topic, I'll tell you what I'm experiencing. Starts fine but I have to use a slight bit of throttle to keep the idle just above stalling for the first minute or so. After it warms up, the idle oscillates between 300rpm (sometimes it dies) and 1300rpm - about a 2 or three second frequency. When coming to a stop (in neutral), I have to use throttle to keep the rpms from dropping too quickly. If I just let off the gas from, say 3000rpm, the rpms drop too low and overshoots normal idle and it stalls. I've checked the aux. air regulator and the warm-up regulator (even tried a new one with no change noted).
Any ideas would be appreciated.

Thanks,
Chris

ashame1

[quote]Originally posted by ChrisB:
<strong>Now that you guys are bringing up this topic, I'll tell you what I'm experiencing. Starts fine but I have to use a slight bit of throttle to keep the idle just above stalling for the first minute or so. After it warms up, the idle oscillates between 300rpm (sometimes it dies) and 1300rpm - about a 2 or three second frequency. When coming to a stop (in neutral), I have to use throttle to keep the rpms from dropping too quickly. If I just let off the gas from, say 3000rpm, the rpms drop too low and overshoots normal idle and it stalls. I've checked the aux. air regulator and the warm-up regulator (even tried a new one with no change noted).
Any ideas would be appreciated.

Thanks,
Chris</strong><hr></blockquote>

Flyin Elvis

the Rat does this; cold starts fine but revs 800-1200 and after about five minutes she pegs a high-idle at about 14-1500... she then smooths out after another 10 minutes and is fine after that, runnign at 800..I like the 02 sensor as a first check; if it had a vacuum leak it would flux all the time, correct? the Rat has a buck-sixty on the clock and as a recent owner I'm betting the 02 hasn't been changed in many years, if at all. Fuel pressure next??

1983 911sc
1998 BMWr1200CA

ChrisB

Rat,

I think you're right about the vacuum leak making it flux all the time. My 3.0 doesn't have an O2 sensor so I'm kind of stuck at this point. My wife won't drive the car now because it's difficult to keep it from stalling as I slow down to a red light. It's improving my heel/toeing though!

Todd

I have exactly the same trouble with my 73.5. I'm also interested in hearing more about this.

Hladun1

Some points to consider (let’s assume it’s CIS related, not ignition, fuel pump, etc.);

1. If it won’t idle at 900 to 1000, try 1000 to 1200. This doesn’t solve any problems but it helps prevent stalling. (When you’re driving you’re at redline anyway so why worry about the idle speed, right?)
2. If you can’t adjust the idle down with the idle screw, then you’ve probably have a leak in the AAV, AAR or deceleration value. I suggest taking the AAV out (it causes the cold surging and startup backfiring). The AAR must close when it’s warm. The deceleration valve can have the output line disconnected and plugged (both sides of the disconnection point) to check if it’s a culprit.
3. Check for vacuum leaks with a vacuum gauge. (My 78SC draws 15 inches of vacuum at idle, but that’s with a well worn engine.)
4. The deceleration valve should make the rpm drop to 1500 or so when the throttle is released and then hold for a moment or two and then drop to idle. It’s adjustable. It’s this device that’s mainly supposed to stop the car from stalling when you take your foot off the throttle.
5. Check the CIS fuel rest pressure, if it doesn’t hold you’ll have starting problems.
6. Make sure you’re thermotime switch is connected with red/black to the switch side (not heater) or else your cold start injector won’t work and you’ll have starting problems.
7. The AAR is a "fast idle" device, you engine should idle fast when it’s warming up. (1500 to 2000 rpm for a few minutes). It’s adjustable.
8. You must set the CO level with a meter. If you don’t have a O2 sensor set the CIS at the rich (high CO) level. My CIS system is quite sensitive to adjustment so "playing" with it will soon have you in trouble.

--Moe--

I just found the culprit of some of your problems this morning. My 914-6conv with a 911T 2.4L CIS engine had some of the same symptoms.

Easy to start cold, but would not idle by itself. If hold the throttle to keep it from stalling it's idle would oscillate between 800-1400rpm. When coming to a stop I'd have to hold foot on throttle to keep engine from stalling. Once it was warmed up, all is normal, no stalling or oscillating idle.

The culprit was the Warm Up Regulator not getting power. I had cleaned the engine bay recently and must have shorted out the WUR circuit popping it's fuse. I replaced the fuse and whala! engine fires right up and idles smoothly.

Look at your WUR as the culprit, check it for power. If its getting power then look at having a shop adjust it or replace it if to far gone.

--Moe--
morgan_harwell@stanfordalumni.org

Todd

&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; Headed to garage... <img src="graemlins/wave.gif" border="0" alt="[byebye]" />