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!

 

With a rebuilt engine and the unknowns regarding the cam choice and installation and timing, the question about the ECU's programming, the crankshaft position sensor, there are a lot of things to consider.

Almost too much to consider.

Boiling it down the stall concurrent with rough idle suggests an intake air leak.

Now it may not be the intake manifold per se, but a hose or fitting that is marginal and only leaks once in a while.

A classic case can be a hose that develops a crack on its bottom surface right at the connector, which is generally where a hose fails. The hose is normally leak free but once in a while forces can work to move the hose and the crack opens up and there's a leak.

But almost as quickly as the leak appears it goes away and is very hard to find.

Before I considered something deeper I'd be darn sure there is no intake/intake hose leak.

 

Macster,

Thanks for the response. You are correct. Rebuilding an engine represents changing a lot of variables at the same time. And, the first thing we suspected was a vacuum leak. Having said that, I have replaced all the Varioram vacuum fittings and piping. I also replaced the hoses that connect the ISV to the intake. What I did NOT touch was the myriad of hoses and fittings on the back side of the Varioram manifold. There is a jet pump, check valves, and multi-connector fittings back there.

And, other than O2 sensors, I did not change any of the engine management sensors as a portion of the rebuild. When the stalling was noted, I changed out the ISV (including all the ISV hoses as mentioned above). Then when my tech was attempting to trouble shoot the issue he changed out the distributor caps and rotors and the cylinder head temp sensor. He also tried swapping out a lot of things such as coils and fuel injectors, and found no issues with any of the parts I had installed.

So, that leaves the MAF, TPS, and flywheel speed sensor as the only sensors original to the motor. Of those, I degreased the MAF with solvent without touching the internals of the MAF. I also had to reinstall the speed sensor and it was set at 1 mm of clearance from the flywheel ring gear. The TPS was not touched other than to be involved in the overall degreasing of the engine.

All three of the remaining sensors are a PITA to change out with the motor installed, but I may have to bite the bullet and do it.

The other major change made was the installation of the Web Cam RS cams. But, I find it hard to imagine the cams or their timing as the reason for an intermittent condition such as stalling. I do think that they have roughened up the base idle of the engine, which was expected. But, prior to stalling, I noticed a step change in idle roughness.

 

Yes, a lot of variables. But fortunately most of them do not account or are not likely to account for a simple engine stall.

For the engine to suddenly stop running with no drama -- other than a short lived non-persistent rough idle -- reads like the engine either lost fuel -- rather suddenly -- or it lost spark. Or experienced a sudden air leak. (I can't give up on this just yet.)

If as the engine is running rough the engine spits back any through the intake that's a loss of fuel. If the engine backfires through the exhaust that's an overly rich condition.

I'm not familiar with the 993 engine but does the fuel pressure regular require engine vacuum to control fuel pressure? If so if there's a bad vacuum line this can really affect fuel pressure.

Have to mention there is also the possibility of a bad fuel pump or fuel pump relay. IOWs, the stalling doesn't have to be engine caused.

For ignition the 993 engine IIRC uses two distributors. Are you sure the rubber timing/drive belt that drives the other distributor is ok? What about the ignition switch unit in the distributor?

There is IIRC a single coil. While the coil might be ok what about its connection to the distributor caps, its connection to the engine wiring harness (if it even connects to this)?

My point is there is a common failure point that I believe cuts either fuel or spark.

For the engine to run rough before stalling suggests to me if there is no intake leak -- and I still like this as this is very common when an engine has been out and back in a car all those old hoses just can't take the movement even if one is very careful -- then the problem is a fueling problem.

But a flaky ignition switch unit or a bum coil or coil connection can kill an engine just as quickly.

 

Macster,

Thanks again for your careful thoughts about my stalling issue. I am in full agreement that the issue is a sudden loss of fuel or ignition. One thing that we did not thoroughly run down was the fuel pressure regulator. I will have to check with the tech to find out what he did in that area.

So, I NEVER have a stall when the engine is under load. The stalls always occur at a time when, under load, engine speed drops to 2000-2500 rpm such as when I am decelerating as I approach a stop sign. And, then when I push in the clutch, expecting engine speed to drop to idle.......the engine actually dies.

Not sure that that scenario fits a bad ignition switch.

On the characteristics of my rough idle. There is never any backfiring or spitting that I can hear or feel. Frankly, the onslaught of the rough idle feels like a vacuum leak. As soon as I accelerate, the affects of a vacuum leak (if that is what is occurring) are masked and the engine accelerates smoothly.

Regarding ignition, the 993 has a dual distributor with the secondary distributor driven by a belt off the primary distributor. As a portion of the rebuild, I had the distributor rebuilt which included new bearings and a new belt.

The dual distributors are each fed from two coils. We did try substitution of coils during diagnosis, and were able to eliminate the coils as a culprit.

And, just prior to the rebuild, I had replaced the spark plug wires.

During the rebuild, I replaced the engine wiring harness that resides on top of the engine. I replaced it since the original harness was then 16 years old, and the connectors were becoming brittle from age and heat. So, I guess the new wiring harness could be suspect......except that the stall only occurs at one specific operating parameter.

 

First the ignition switch I was referring to is in the distributor, not the main ignition switch.

With more info reads like as the engine moves around it pinches something.

I'm less in favor of an intake leak, even a cracked hose leak, unless you find the line to the fuel pressure regulator bad.

I'm thinking it could be a pinched fuel line.

It so easy with these newer engines with all the lines to get something misrouted and all it takes is a pinched fuel line to account for the behavior.

Have someone observe the engine while it is idling and when you give it some gas. Better yet make a video so you can watch it yourself. If you can work it out have someone watch the engine while you try to pull the car away from a stop with the emergency brake on. Don't overdue it -- you don't to ruin a clutch -- but you want to put the engine under some kind of load, cause it to torque on its mounts.

What I'm thinking is the engine is moving around too much for some reason -- bad motor mount? -- and pinching/kinking a fuel line.

 

OK. First off, there is no switch in the distributor. It's more, or less, like a traditional distributor. All the timing is handled by the ECU, so all the distributor does is to act as a device that routes the spark to the proper spark plug.

And, the 993 is really engineered as a "layered" engine. All the lines and hoses, are just long enough to go from point A to point B. And, when you lay all the hoses and lines out as you rebuild the engine, it becomes apparent that there is usually only one way to put things.

Regarding engine movement, I replaced both OEM mounts with RS mounts. If anything, they are stiffer than the OEM mounts and constrain the engine from movement. As a consequence, shifting the transmission is now much easier than before installing these mounts. So, I don't think that engine movement is causative. And, when you think of it, my issue comes with the engine attempting to drop to idle, which is not a stressed position.

With the discussions you and I have had, I am thinking that I need to chase down the fuel pressure regulator. And, then I want to probably check out the installation of the flywheel speed sensor. I may change that part out and actually set it a little bit closer to the flywheel. Spec on the adjustment is 1 mm +/- 0.2 mm. So, I may attempt to set it close to the 0.08 mm gap.

 

I googled a parts list of the 993 distributor. It lists a ignition switch of some kind inside the distributor housing.

However, I'm not at the car I don't have the distributor in front of me and if you say there's no such thing in the distributor then that can't be the problem.

If the lines are routed as they were stock then the odds are a line is not pinched. Often when an engine is dropped and installed by a DIY'er he can't resist leaving hose/line routing clips off and let the hoses/lines be loose. This can be compounded if the DIY'er replaces hoses with new and cuts his own. These are cut long and well, complications can arise.

If the crankshaft position sensor gap spec is 1mm +/- 0.2mm and you put it at 1mm that to me is ideal. However, what can happen is when the hardware is tightened down the sensor can move and that 1mm becomes less than 1mm or more than 1mm. When (if) you end up back the crankshaft position sensor check its clearance and tightness *before* you put a wrench on it just to know.

So we are back to the fuel pressure regulator.

 

The distributor does house a Hall Sensor which identifies the reference mark on the flywheel. That may be the switch that you are referring to.

I did some research on the fuel pressure regulator. Apparently the symptoms of a defective regulator are a hard to start condition. Since the engine always fires off immediately, I think that would be indication that the regulator is working correctly.

 

Well, then we've come full circle to your 993 engine having a stalling problem and no suspected causes. Everything, at least on paper, has been eliminated.

At some point you are going to have to get your hands dirty. I don't mean you need to start throwing parts at the behavior, but you need to think about monitoring some vital signs before, during, and after the stalling occurs.

What vital signs? Well, I'm sure you have a list. Off the top of my head, mine would be: Fuel pressure I guess, though I hate to mess with the fuel system. Battery voltage. Ignition timing. Intake vacuum. IIRC the 993 has an ICV. If there's some signal from this, or to this, you can monitor from this it might be useful.