I have posted this message on the technical problems forum as well.

I have a stalling issue with a recently rebuilt 3.6 that I have not been able to solve in over a year. Now looking for suggestions and discussion that might help me sort through this problem.

About 18 months ago, I pulled the 3.6 N/A motor out of my 98 993 for what I expected to be an upper end refresh (i.e. a valve job). As I disassembled the engine, I ran into issues that lead me to opt for a total rebuild of the motor, including splitting the case. Though I'd torn down older 911 motors, this one was more complex, mainly due to all the peripherals that are now installed on these engines versus the pre-smog 911 motors I was familiar with (pre- 1972). So, I sought the advice of a "mentor" who runs an active speed shop for Porsche cars on the West Coast.

The build went well. I renewed or replaced all wear components and opted to upgrade cams to Web Cam RS hydraulics, went to RS 52 mm inlet valves, and installed "performance software" in the ECU to allow for some performance increases.

Prior to my ownership, the past owner had installed a LWF. I also added a Motorsports close ratio gearbox to eliminate the huge span between the 1 and 2 gears of the 993.

Following the rebuild and a proper break-in of the engine, I was pleased with the additional power, the wonderful close ratio box, and the fact that I had no oil leaks! However, right out of the box, I experienced a new intermittent stalling issue. Allow me to describe it. The engine starts easily and idles solidly. No problem with that. Engine acceleration is very good. But, every so often, the engine will stall as I pull up to a stop sign. The engine operates as expected until I de-clutch from about 2000-2500 rpm as I brake to stop the car. And, every 5th or 6th stop results in a stall. Watching the tach, normal operation on de-clutching shows the tach drifting from speed down to idle "slowly". Then, on the occasion when I experience a stall, I'll watch the tach plummet from speed to zero without hesitation as I push in the clutch.

The engine will instantly restart and return to a solid idle after any stall.

Also, the stalling issue seems to occur when I have an intermittent "rough" idle. My 993 always had a slightly rough idle. However, it is now rougher and seems to vacillate from rough to what I had before I did the rebuild.

In attempting to diagnose and troubleshoot this issue, we have tried all sorts of things. And, none have worked.

The first thought was that I had to have a vacuum leak somewhere. So, we smoke tested the engine. Nothing showed. Then we attempted spraying carb cleaner on all suspect plumbing areas looking for the vacuum leak to ingest the carb cleaner which would increase engine speed. That test was uneventful.

When I did the rebuild, I chose NOT to replace any of the engine controls. I made that decision based on economics and the fact that the engine ran well prior to the rebuild. Having said that, in troubleshooting the stalling issue, we have replaced the ICV, O2 sensors, and the cylinder head temp probe. None of those replacements has resolved the stalling issue.

Then, we suspected the software that had been installed on the ECU. So, I pulled the ECU and sent it off to a respected Bosch ECU firm for evaluation. They responded that the ECU appeared to be fine (hardware), but they did not have the ability to evaluate the performance software installed on the the ECU. They suggested replacing the software with their package which has been used for LWF's and stalling issues. I agreed to that (at a cost). When I installed the ECU earlier this week, my conclusion was that the stalling, if anything, was slightly worse. However, I am aware that the engine needs to go through an adaptation, which I am currently doing. I plan on putting on 100+ miles to complete the adaptation. But, since the stalling is still there, I am not encouraged.

Sensors that have NOT been replaced include the MAF, TPS, and the flywheel speed sensor. Unfortunately, with the engine installed in the car, replacing these items is problematic, not to mention the fact that they are expensive pieces of hardware.

So, I am nere to elicit comments and discussion that will help me unravel this issue.

In the rebuild, one of the things that I did do was to back date the cam timing hardware from the system installed on 993 motors. So, that may be an area that is suspect. And, with the flexibility allowed by the backdated system, I elected to advance the cam timing slightly to enhance low speed torque. For that move, I feel that the motor pulls stronger off the line. But, I have also changed out the gearbox to a close ratio. The gears, alone, make the car feel like it has an increase in horsepower; so, it is difficult for me to distinguish between all the changes made to the car.

At any rate, comments, questions, and suggestions from the pros on this forum would be greatly appreciated. I need to resolve this stalling issue!

 

You may want to check/clean the idle control valve (not sure of the technical name). I was getting the same stalling as you describe. I removed the valve assembly and sprayed it full of carb cleaner and let soak for several days. Rinsed it out a few times and sprayed again, rinsed. When dry, you should 'hear' the valve nove back and forth as you rotate/rock it back and forth in your hand. That shows it is not clogged up and sticking. My car runs and stops at stop signs with out issues!!!

Good Luck!!!!

 

I would agree with mr_bock, except that it's called the idle stabilization valve. It's the 2"silver cylinder looking thing right in the middle on top of the engine. Careful of the electrical connection and be sure to put it back on in the right direction (some people have reported putting it on 180 out).

ChuckJ
http://www.pistonheads.com/gassing/topic.asp?t=977858