Engine Operating Sequence

I've been chasing down a serious stumble that occurs around 1500 RPM in my 2007 Cayman S when the engine is hot. It currently is approaching 250,000 miles. The engine was rebuilt and sleeved to a 3.8 at around 195K. I'm looking for a description of the changes that occurs in the engine controls as it accelerates to try to focus where to look. I'm thinking some sort of vacuum leak but to date haven't had any luck.

I guess I'm not telling you anything you don't already know but that is a lot of miles and there are multiple potential causes. Just a wild *** guess but I think Variocam + goes from low lift to high lift in that range (I can't really pin this down in the literature). Just as a test, you might go up one grade of oil viscosity, on the assumption that your hot oil (and the resulting reduction in viscosity) is the trigger. Tell us what components you replaced during the rebuild.

Excellent suggestion from @harveyf - there is a control solenoid on the rear/front of the heads that controls the + action (change of the valve duration) that is known to get funky and cause this sort of symptom on the H6 engines. I had one start doing this on my Cayenne - same sort of symptoms you have - talked to the shop that did the work for me - and they said it was rather common on H6 engines, and they had a factory replacement and a much lower priced replacement valve. I went with the factory replacement valve figuring labor was the majority of the cost, but probably would have been fine with the lower-priced valve. What does your diagnostic tool have to say? Mine was quite clear pointing to it on my Cayenne (Foxwell NT530).

Thanks, the main items replaced on the engine was valve guides, intake and exhaust springs, main bearings, pistons, connecting rods, timing chain and tensioner, thrust bearings, fuel injectors, plugs, AOS, air mass meter, cats. I don't see anything related to the Variocam so your thought has potential. What do you use for literature for info on how the system performs?

I have After Sales Training Documents. I don't think they are commonly available. But I checked and found a graph that shows a lot going on with the variable valve timing and the valve lift in the 1000 to 2000 rpm range. "Tired" solenoids would be the easiest to test and replace. I don't recall but there may be PIWIS /Duramentric/ Foxwell "activations" for the variocam solenoids, as a test. Hopefully the varicam unit on the end of the inlet cam is not getting tired, as that is a bigger deal to R&R.

Thanks, I don't have anything more than a basic code reader and this doesn't trip any codes.

Well, you only have the basic code reader but if you had a device that read Actual Values, I would be looking for deviations between the requested cam position (advance) and actual cam position (advance) during a drive in the rpm range where you get the stumble. If the deviation (normal is less than 1 deg) shows up just during that rpm range, that might explain your causation, if not the repair. A brief deviation, in the great scheme of things, is probably not enough to throw a code. But it could be a host of things. You replaced the air mass meter, which would have been another guess of mine. I would not suspect ignition components at such a low rpm but plugs and coils are always a suspect when it comes to a stumble. Only happening when it is warmed up is a clue but again, there are host of things that could drift bad electrically when heated up. Sorry, it hard thing to diagnose from afar.

There appears to be two solinoids in each head. They are different but both impact the camshaft. Most of the articles I've seen are about changing #45. Any input would be appreciated. I can't seem to find any definitive description of that each does.

Your '07 has Variocam+ right? One solenoid controls the cam timing, the other one controls valve lift. Either one or both could be an issue - but you can narrow it down with a diagnostics tool by looking at the cam timing.

@deilenberger is correct, any one of the 4 in question could be the issue. The sensors are not outrageouly expensive, in Porsche terms, that you couldn't buy one of each and, if you like to be methodical, switch one in at a time and see if the problem gets fixed. But to answer your question, I have not (yet) found an official Porsche explainnation of which goes to which system. Mayby it is a "tribal knowledge" thing that the old techs pass down to the new techs! Whatever, under the adage of teaching a man how to fish, I will direct you to a video linked below where it should become clear which is which, and how they function. My entire series on the Cayman 987.1 oiling system is linked below. Porsche M97 Engine Oiling System Explained My article on the Oiling System of a Porsche 944 engine has been very popular on my website. I suspect that occurs mainly because folks have a problem with their 944 oil pressure and via a Google s… newhillgarage.com

Great video. Awesome and perfectly detailed. The complexity of the Variocam system is astounding and confirms the critical nature of the oil.

Thanks for the compliment on the video. I learned a lot about Variocam while "reverse engineering" the oiling system. The entire article and video series is linked below. Porsche M97 Engine Oiling System Explained My article on the Oiling System of a Porsche 944 engine has been very popular on my website. I suspect that occurs mainly because folks have a problem with their 944 oil pressure and via a Google s… newhillgarage.com