Irregular power 911 sc

[911carl]

Hi all,

I have a 911 SC of '83 (USA version) and I experience some irregular power when the engine is warm. When I'm standing still you can clearly see the rpm go up and down and hear that the engine runs irregular. This problem starts only when I'm driving for a while and the engine is warm. I've already replaced the 02 sensor but that didn't work. I've read a lot about CIS problems on this forum but the "strange" thing I'm experiencing as well is that this problem is solved when I turn off the engine and start again. But after a while is starts again. Can this information perhaps limit the checklist of CIS problems a bit?

Thanks a lot!

[tirwin]

Carl,

Ok... First you gotta realize that CIS can work fine, but it's kinda like a stool with three legs: vacuum, fuel pressures and ignition. If one of those 3 is out, you don't have a stool that holds up. So you need to verify everything is working like it should or you'll be chasing your tail.

So, we'll need more info. First thing is you're saying your major symptom that the idle is surging up and down when the engine is warm. You replaced the O2 sensor with no effect, so let's see if your
O2 sensor relay circuit is working.

Two questions:
Do the interior lights work when the headlights are on and one of the doors is open?

If you look under the passenger seat there is a silver or black cube about 1 inch on each side. That is the O2 relay. Start the car and let it warm up for a minute. Pull that relay. Does the car run like crap? It should. If it doesn't that is suspect.

Let's just check that before we move on to other stuff since you started there.

[schoward]

I would even say replacing that relay may be good measure. It could be fizzling out after it is energized for awhile. I have one that, while it didn't fail, it was tired. Replacing it gave me better running/smoother power. You should always have a spare one of those anyway IMO.

But proper answer is probably to start with mixture check and manual/expert procedures.

[Flat6pac]

A faulty relay under the seat that was mentioned makes the engine feel like it has a bad warm up regulator giving you bad fuel pressures. You can get the relay at an auto store.
Bruce

[911carl]

I had to order the O2 relay, so it took me some time to report back. During the first test drive everything went quite well. When he reached working temperature, the rpm fell down a bit and he hesitated for like 10 seconds. At that moment the power was irregular again but afterwards I didn't experience any problems again. On the highway I had sufficient power and went easily to 100 mph. Does anyone else experience this irregular power for a moment when it reaches working temperature?

[Bill Douglas]

I'm not saying this is what the problem is, but it's an easy one to get out of the way...

My SC gets vacume leaks and that causes strange things to happen. I snip about 10mm off the end of all the little air hoses around the engine and reconnect them. You feel a much tighter fit. And it's easy and free, and may just fix the problem. Also research warm up regulator and auxilary air valve, but guys who know more about it than me will chime in will more info.

[911carl]

I've checked the vacuum lines and there don't appear to be any leaks. They also looked like they were already renewed. Since this problem only appears when the engine is fully warm, I start thinking about a problem with the WUR. How can I check if it's malfunctioning? It's a bit too expensive to replace without being sure that this is causing the problem. When a pull over and restart this problem is solved for a minute or five. Can this be an indication as well?

[Bill Douglas]

It might be a fuel starvation problem.

[timmy2]

Have you checked or replaced the relay as mentioned earlier?
Could be bad solder joints in the relay board as well.

[911carl]

Yes I replaced the 02 sensor relay as well. I ordered an original Porsche one, and have been driving without the passenger seat and it felt warm during driving so I guess it's working.

[psalt]

Carl,

The first step in troubleshooting CIS lambda is to hook up a dwell meter to the test port and read the duty cycle in open and in closed loop. The system will tell you what is going on with no guessing. If this is meaningless to you, try searching for CIS lambda and read up on some of the posts. It is normal for an 83 911SC to have a fluctuating idle when hot in closed loop.

Paul

[gerben1986]

Originally Posted by arhadtrha

Do the interior lights work when the headlights are on and one of the doors is open?****************

Could you explain me how this is related to the running/fuel delivery of the car?? (or do I miss an internal forum-joke ).

[timmy2]

Ignore it, the poster was banned, your thread was spammed...

[911carl]

My interior lights work when a door is open, but my headlights don't have to be on. I also notice that when the engine is warm and I push the throttle in neutral a bit I hear some popping noise. Can this be the pop-off valve? I guess, I'll get a tachometer and deepen in the CIS mysteries... :s

[Bill Verburg]

Originally Posted by gerben1986

Could you explain me how this is related to the running/fuel delivery of the car?? (or do I miss an internal forum-joke ).

He's a guy that quotes at random and hides spam links therein, H'es been banned multiple times but keeps coming back w/ more fake or hijacked IP addresses.

The spam can be seen when quoting him. My theory is that it's a scheme to build false hits for sites. Do not quote him, Do Report it.

[Bob Kontak]

Here is a video I made a few years ago when I pulled the discussed relay. Note the popping when I blip the throttle from the engine compartment.

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

Three years ago a guy in Dallas had the same symptoms. Out of nowhere the car would go to hell on hot days after full warm up. Ran but totally wimpy.

The problem was that the connection between the relay and frequency valve was corroded on the 12 pin connector that sits on the rail just to the right side of the rear shock mount. When it got hot the connection opened and when it cooled it reconnected.

Essentially, the frequency valve had no power and did not work. The valve is integral to the proper fuel pressure being delivered.

Pull this connection and scope it out.

Edit: Beforehand, when you get the symptoms, shut down car, turn the key on (not start) and lift the air sensor and if the freq valve is working you will here it buzz. You may be able to catch if this (or similar issue) is the culprit before ripping things apart. Of course, assuming this is the problem.

If it does not buzz, you could tap the connection and perform other wiggle tests to see if it connects.

[tirwin]

gerben,

The spammer posted a snippet of a post I made at the beginning. It was an honest question though. The O2 sensor relay is on the same circuit as the interior lights. If that fuse is blown then the O2 sensor relay won't work. I realize you've replaced it and that hasn't solved the problem.

A low power problem can be a too lean condition. As psalt suggested you might want to look at dwell and idle mixture.

Follow-up questions. You start the car in the morning after sitting overnight. Car starts ok? And you drive around ok? At some point the car gets "up to temp" and then the problem starts. Now, what is your definition of "up to temp"? And it sounds like low power only happens in a certain RPM range? If you can't consistently reproduce it, then I'm thinking fuel delivery. Cooties in the fuel system? If you can consistently reproduce it then I'm thinking something is "off" - dwell, timing, mixture, etc.