82 SC Poping on cold aceleration

[Bill wjw]

I have spent some time searching and nothing really fits my problem. 82 SC . It fires right up and after a few seconds starts to hunt, After warming up a few minutes the idle goes steady to 1200-1400 rpm then settles to 950. I drive about 1/2 mile on flat road at 3000 rpm and go down 1/2mile long hill. At the stop sign at the bottom of the hill she backfires and pops a couple of times and then runs fine. Drives at 65-75 on the highway for 12 miles to work and usually no problem. Occasional stumble upon accelerating after a stop, more so when cold New, plugs, cap rotor.
I have a break in the muffler where it attached to the cat. I need a new muffler and cat. Choices are many, but I lean towards close to stock system.

I know I need to check the system pressures- I have done this many times and WUR could be the problem. I need a new timing light suggestions? simpler is better for me. Co adjustment to be done by others.

SC owners where would you go from here? A popping Porsche at stop signs is embarrassing.

Thanks,
Bill

[T77911S]

you have a lot to look at.
cold you are open loop (no O2 sensor) so the WUR is controlliing the mixture. (all right, the oxs control unit is controlling the frequency valve (FV) at a steady setting). so control pressures are the first thing i would look at. ignition timning and advance. make sure all that is working. air leaks and exhaust leaks. both can make it pop. air leaks will make it run lean making it pop.
warm adds in the O2 sensor. also the enrichment circuit that is actuated by a switch on the throttle body. and of course air leaks and fuel pressures.

you could also have dirty injectors. put in some techron with about a 1/2 tank of gas or less and drive it with the RPM's held hi. do this every other tank for a while, see if that helps.

looking back at your post, hunting after start up makes me think it might be a control pressure problem. IE less likely an air leak. but it never hurts to check intake bolts, injector O rings and vacuum hoses.

[TibetanT]

Bill:

When I had this problem it turned out to be timing issue and fuel mixture.
New spark plugs...did you see any signs of oily plugs when you changed them?
Did you get a chance to set the valves and timing?

If you need a stock muffler for your 911SC, I have a good one. PM for pictures and details. I am in Washington State.

Hope this helps.

[Peter Zimmermann]

The cold hunting is OK, but "a few minutes" is not acceptable. Time it, it might only be 30-45 seconds, which is OK.

At first op fill the tank with Chevron Premium.

CO% to be set by others? Be absolutely sure "others" know how to do it, i.e. engine at full temp, idle speed (900+/-50) correct, exhaust sample taken pre-cat, OXS disconnected. 0.4-0.8% is the spec.

Spark plugs; what make? what gap?

[Bob Kontak]

Advance Auto. Under $50 for a decent inductive timing light.

[Bill wjw]

Hi Peter,
The plugs are copper NGK. Their web site gave 28 as the gap for the plugs I used. I member 7 was in the number. Although I got the a plug number a couple of changes ago that had the same set of numbers only with 8 instead of 7. I know that is low end gap according to Bentley. The old plugs looked fine. No oil and a nice clean gray color. I want to replace the muffler ( sent PM0 Ed ) and have our local Porsche garage set timing and CO. I tried a can of SeaFoam last week and will try some techron this week. Pressuring testing the WUR will be next for me. Thanks for the timing light advise Bob. My last one was to fancy and poorly made. A piece of crap.

[T77911S]

i like a simple timing light. i dont like the ones with the dial advance. dont trust them. JMO.

7 and 8 are the heat range. 7 is hotter than 8. 8 is colder than 7. the gap is a bit small.

look at the other things first.

make sure you run the engine at higher RPM's with the techron in it. i would hold the RPM's between 4 and 5k in my 325is. like between stop lights. then sometimes i would just run the piss out of it. the idea is to make the injectors spray a lot of fuel. this is just something to do while checking other things.

when setting the mixture on a porsche, its not good to let it idle for very long. it effects the reading.

[Paulporsche]

Is the backfiring out the exhaust or back through the intake?

[Bill wjw]

It is backfiring through the intake. Only when cold. Once it reaches operating temperature no more popping and missing. If I feather the accelerator and go light on the gas that helps when accelerating my cold engine from a stop.

I timed the hunting with a cold engine last night and 23 seconds was all it lasted before settling to 1400 rpm.

I will have a cold engine tonight and can test the fuel pressures after work.

Techron added on my 12 mile highway drive at high rpm seemed to add response to the acceleration

[TibetanT]

Bill:

Got your PM.

I will send you pictures to the Email provided soon!

Ed

[Bob Kontak]

Originally Posted by Bill wjw

I will have a cold engine tonight and can test the fuel pressures after work.

Unplug the electrical connector to the WUR before starting when you check. You prolly know this.

Get a reading, then plug it in. Document the pressure increase across time.

I am a little rusty at checking pressures. It's been a few years.

Driving down to the bottom of the hill should pretty much have dialed it in to "full" lean. I think on the earlier SC's there was maybe a 90 second rich window and newer ones (emissions tightening) there is a smaller window - maybe 60 seconds. Given my rustiness, the time window may pertain to the fast idle period?

Another thought is the frequency valve is not getting power. I have posted this video several times where I disconnect the O2 (Lambda) relay. You can hear backfiring through the intake but the car still runs. Maybe?

https://www.youtube.com/watch?v=5tdpCYyvqSc

Easy test is to unplug the relay and see if it runs the same.

On a side note, I would suggest replacing the relay even if not the problem. My car literally ran smoother post replacement of a functioning unit. Cleaner contacts was the feedback received.

[wannaporsche]

If Poping gets really bad, you might try holy water...

[Paulporsche]

Just a thought for those guys who know. (I don't have O2). Can his exhaust leak affect the operation of the O2 sensor?

Also, is your dome light working? The light and the O2 sensor are on the same circuit.

Let us know when you get the fuel pressure readings. The hunting right at startup indicates a too rich mixture. The intake backfiring during warmup indicates too lean. It's possible your WUR isn't changing the control pressure and the CO has been set by someone so the engine starts and is OK while warm but not rich enough during warmup.

You have renewed ignition parts. Make sure you have no vacuum leaks.