29 Sep 2023
Please read all the way to the end. I have been fighting a persistent drivability problem in this car for quite some time. It is related to what the E55 crowd will call the "hiccup" problem, where between 1000-2000 rpm, a temporary lean condition manifests, and the car stumbles a bit. All M113K cars seem to do this, but if you have a supercharger pulley change or a larger throttle body, then the problem is exacerbated. My E55 has a 168mm lower pulley, and it does this. It is an annoyance, but livable, as you learn it is just a quirk and the car is fine. This CF had this problem for the better part of two years. It was far more severe than the E55, but it was livable, also. Passengers always noticed it, and I would shrug my shoulders. More than one tuner took a stab at fixing it, but nothing seemed to work. Now enter the MCT transmission. This uses a clutch rather than a torque converter, which gives a more or less direct connection to the driveline, much like a manual transmission does. It has no fluid connection that can dampen these hiccups, like a regular automatic will do. In this case, the hiccup went from an annoyance, to a total pain in the rear! The car was borderline unmanageable on the street, especially in traffic. The stumble was a violent jerking in the driveline that made the car difficult to manage. The decision was made to leave the car home during the Crossfire Fall Rally 2023, mainly due to this!

Throughout the course of things, several gaskets had been replaced, another 82mm throttle body, another set of fuel injectors, and several other items were replaced. Tests were made with the supercharger belt taken off, to see if it was an engagement issue - it wasn't. Spark plugs changed, no effect. Fuel pressure was changed, no effect. Different ECU was tried, no effect. CAM and Crank sensor were replaced, no effect. RCM was changed (yeah, we were throwing the long ball!), no change. Air leak checks performed, no result. After my tuner made several, and by several I mean on the order of 30 tunes later, we figured we had one of three variables remaining:
1. Bad MAP sensor
2. Bad supercharger bypass valve
3. Air leak somewhere
Preliminary checks showed no issue with #3, that I could find, and #2 was checked in various other ways. #1 was more difficult. The MAP sensor on an M113K is notoriously hard to change in the car. I decided to take a shotgun approach and replace everything at once. My son came home from college to work on a few niggles on his SRT6 after the absolutely-incredible-and-you-better-not-miss-it-next-year Fall CF Rally, so of course I put him to work on my car. Last night we pulled the supercharger assembly off the car to replace all gaskets, a new MAP sensor, and a replacement bypass valve. The bypass valve is actually a throttle body from various MB four cylinder cars. I bought a nearly new one for a M111. We had the supercharger off in just over an hour (team work is awesome!) and found that the "donut" rubber seal on the bypass valve had not been seated, and was pushing itself out from one side severely. We could not tell if it was leaking, but if it wasn't yet it was going to before long. It had to be leaking under boost at the very least. From earlier diagnosis the bypass valve seemed to be OK, but not taking chances! Replaced it and all of the gaskets and a brand new MAP sensor from MB. Put it all back together.

Took the car out for a ride and sure enough, the hiccup problem was back to "E55 levels" and none of the severe jerking/bucking bronco business from earlier. I acquired a pressurized smoke tester from Amazon and will smoke test for leaks Saturday just to be sure.

It should be assumed that if you take on a modding project, or just own unique car like a CF, that your time making repairs is donated to the cause as part of the hobby. However, we sometimes will not fix a problem because we think it will cost too much, or take too much effort. Let me give a breakdown on costs for what was done last night:
- Used M111 throttle body, eBay, $30 shipped
- Gaskets, seals, new from MB: ~$50
- MAP sensor, new from MB: $51
Total: $131

My point here is that the cost of this fix was less than $150, and about 4 hours of time. Which leads me to the next point:
Problems like this can be difficult to trace, no doubt. As a long time tuner myself, I used to hate being used by the customer as a diagnostic tool, by writing tunes that I knew full well meant that the car had problems. The tuning process was a waste of my time and would not fix the root problem. The shoe is on the other foot in this case, and I owe my tuner a great deal of gratitude and an enormous apology. He stuck with me and kept working on tunes, as I was insistent there was no problem with the car. There were problems! Was it a single item that was replaced or a combination? Who knows? But the fact remains that a lot of valuable time was wasted by the tuner on problems that were not of his making, and that is downright abusive. I am publicly apologizing to him (he knows who he is). I also want this to stand as a monument to others that no matter how clean, no matter how perfect you think your car is if the tuner tells you there is a problem with the car, then you need to go back and dig deep. This problem was in the car for almost two years. Two years! It was fixed in one evening. Don't be me!

I want to thank @Dave2302 , @CL770 and @Deplore , who all three were telling me that they thought there were issues with the car that needed to be addressed, and what the fixes should be. I appreciate all of your advice! Thank you, guys, I'll move on advice much faster in the future!