D_jet trouble shooting question - car is home, journey continues, June, update- no more problem-- the fix was simple.... Options

[DRPHIL914]

OK. car has been in shop for the past week, and today Bob spent several hours on it but what he found was when the MPS is disconnected from the vac hose it will idle and run, but when it gets connected it goes immediately lean and dies, and will not run with that hose on there- the MPS holds Vac no loss of pressure, but that doesnt mean that its not bad.
1- i thought that if an MPS went bad it defaulted rich? but when this is getting vac it goes lean right away and dies. but it will run and idle with out the vac , the AF meter then shows 9.8-10.2 or so which is of course rich but not flooding out.

so the question is why is it going lean when hooked up? when this issue started about 2 weeks ago, it had been running just fine and all of a sudden it quit running.

2. could there be another issue we dont see that would be making it way to lean other than the MPS?

might need to test another MPS, my 2nd one i just bought from someone here and it has never been opened but will not hold vac (slowly looses vac pressure) so it probably need to be opened and re-sealed. not a good candidate to trouble shoot with. ------

FYI- 1975 d-jet 2.0. newer wiring harness , CHT is newish(replaced 2 months before this issue started and was running perfect at that time)

[DRPHIL914]

so yesterday i decided to try and figure out where my other issues were coming from and then tune the MPS a bit more;
- ended up recheck ing the TPS setting because it was off after being at the mechanic and reset that, and i had to pull that temp sensor to make it easier to get the cover on and off the TPS, not sure why but in my box of parts i ran across a brand new one i had inmy stash so i put that new one in there because i noticed that if that temp sensor is disconnected the car will not run, same as a CHT or MPS AND it is the one part i had not tested or tried a replacement of. IT is possible there is another short somewhere in the system and i had a few ideas from reading other similar theads. so i went back thru the grounds- all good. one post mentioned the 3 red wires that attach to the positive terminal of the battery. No corosion there but that whole connector could be re-done so i might do that.
-anyway yesterday i could not get the car to idle , or stay running , but after replacing that sensor, car ran perfect with no sputtering or cutting out at all, perfect idle with out surge or variation. Yes we had a bad MPS but car ran when the vac hose was off due to it being super lean other wise and i now think it was this sensor causing that lean running condition.

How common is it for that plenum temp sensor to go bad? i am going to spend some time over the next week driving the car and seeing if i can get it dialed in. Because i have spent weeks chasing my tail on this car and have had it running only to find another part that was part of this problem coming and going, i am optimistic but cautious. i feel like i could get in the car 2 days from now and find it not running again. I hope this is it, time will tell.