DJet Frustration, Ever play that Whack-A-Mole game at the arcade? Options

[ConeDodger]

Remember that arcade game Whack-A-Mole? You take a hammer and whomp the moles as they come up through the random holes? McMark and jcd914 and me had that kind of day playing Whack-A-Mole with the DJet on Jim's engine (jcd914) in my car...

Recently the engine started running bad. It had a low speed miss and then would go like stink when you got into the advance curve. I have a bunch of new 914 DJet components so I started throwing parts at it. This is a method used by people who aren't mechanics to fix something they don't want to pay a mechanic to figure out and fix. Take a note if you aren't a mechanic - this method is often more expensive then just paying the damned mechanic. So, I replaced the MPS with one that I bought from George at AA a few years back and never really used, I replaced the harness with a Bowlsby harness, I put a resistor in the CHT circuit... Each thing I did made it run better in some ways but often worse in others.
So, I decided that since I was having the Sacramento Porsche Family Reunion today and McMark was coming up, how about having him take a look at it and paying him for his time. Mark was game. He brought his LM 1 Wideband AFR and we methodically went through the tuning process. Here is where the Whack-A-Mole game began. Everything we did would improve it but unmask some other issue. Some things did nothing when they logically should have. We replaced the distributor with a rebuilt one from Rich Bontempi, it ran worse. We discovered that two injectors were not firing. So the distributor was good but the trigger points, which were new by the way, were bad. Swapped in the trigger points from the original distributor and it ran on all cylinders again but still ran bad. Lean, very lean... Then Jim Dupree pulls in the driveway and steps out of his car with an MPS that he pulled off of one of his spares. 30 seconds to plug in the vacuum line and the harness plug and the problem is solved. Turns out the new MPS from AA is bad or badly tuned for the motor. I suspect the later as it will hold vacuum.
So after three hours of Mark's time and lots of parts. The engine - Jim's engine runs very nicely again... Ahhh... the joys of a 30+ year old car (IMG:style_emoticons/default/headbang.gif)

[ConeDodger]

Well, we finally hit that doggone mole squarely on the head. And not a moment too soon either as I was about to call Jim Dupree (jcd914) and say 'bring your carbs. I give up'. But today we employed serendipity rather than logic and found the problem.

For those of you who are 'Webster' challenged, serendipity is defined as a happy accident. We had one.

Up until now, I have employed logic to figure out why the motor runs lean off idle. Figure out what the variables are and eliminate them one by one. Using logic, this problem cannot possibly exist as all of the variables had been eliminated, some multiple times as the possible cause.

Today, Jim and I were going to switch out the ECU for an early one as my Bowlsby harness is an early one. We thought perhaps there was a harness to ECU incompatibility. That didn't work but Jim fortuitously decided to try a different MPS. I decided to entertain his whim even though I had eliminated that as the cause multiple times. Here is what he discovered by serendipity; when the MPS is mounted on the tin as it was in the case of this engine instead of held off to the side in close proximity to the normal mounting spot, the engine misses and goes lean off idle. On the other hand, when the MPS is held off to the side close to its normal mount location, the engine runs like a top.

We reason that there are a couple possible reasons for this and they are important to note for others who might have the same problems. First, it is possible that when the MPS is mounted on the tin close to where the coil on later cars is mounted, the EM interference from the coil or distributor might mess with the MPS. Second, we are grounding it to the tin and causing an electrical problem. Lastly, the vacuum line should not be coiled up but rather a straightish shot to the MPS.

I teach logic and employ it as much as I can but I will take serendipity if it results in a great running car. It did. (IMG:style_emoticons/default/biggrin.gif)