MPS Strangeness - SOLVED!, Doesn't like being properly installed Options

[ThinAir]

At WCR I asked McMark to drive my car and give me some advice on a bucking issue that I had. He suspected I may have a bad MPS so we borrowed a known good unit and just installed it loose in the engine compartment - sure enough, the problem went away.

So now that I'm home I arranged to get a known good MPS and installed it loose in the engine compartment - my engine is so smooth I can hardly believe it's the same engine. I'm a happy dude!

Well... until I actually installed the MPS onto it's regular location on the passenger side of the engine compartment. The bucking behavior came back again and as soon as I unbolted it and laid it loosely on top of the engine things went back again to smooth running.

I'm running a 2 year old Bowlsby wiring harness and 1 year old vacuum hoses so I don't think either of them are the problem. Any ideas? I've never seen anything quite so weird!

[brant]

Ernie,

apparently the spark plug wires (em field?) electro fields mess with the MPS. Your old MPS may be good but affected in the same way this one is.

If move the spark plug wire a few inches away it should clear up. I think folks were tucking the #4 wire under the intake manifolds....

see if you can move the wire away (even temporarily with a zip tie) and if that clears things up.

brant

[Cap'n Krusty]

Ernie,

apparently the spark plug wires (em field?) electro fields mess with the MPS. Your old MPS may be good but affected in the same way this one is.

If move the spark plug wire a few inches away it should clear up. I think folks were tucking the #4 wire under the intake manifolds....

see if you can move the wire away (even temporarily with a zip tie) and if that clears things up.

brant

I can see the #3 wire under the manifold, that's where the factory put it. The manifold is beyond the #4 spark plug, and the wire would be too short.

The Cap'n

[brant]

I might be remembering the firing order wrong
yeah... the rear most wire on the passenger side

[ThinAir]

Thanks, guys. I'll give that a shot tomorrow and report back.

[Dave_Darling]

I found that holding the MPS vertical and tapping on the end will change the way my car idles. There is the metal core in it that moves in response to engine vacuum, and that is what sets the injector duration. If you move the rest of the MPS, you can move the core relative to it, which changes the mixture. It was an odd realization...

Could something similar be happening?

--DD

[ThinAir]

Brant and the Captain get the gold star for this one! I moved the #3 wire under the manifold and remounted the MPS, sure enough it is as smooth as can be.

It's amazing what a difference such small things can make. When Mark suggested that it might be a MPS issue it never occurred to me that it could be anything except a bad MPS. Of course the MPS as the source of the bucking had not occurred to me either. I kept thinking it was something in the distributor.

I sure don't know what I'd do without you! Thanks!!

[rick 918-S]

I'm going to start the 2.0 in Sandy's car for the first time soon. I went out and moved the #3 wire under the intake runner. So your suggestion likely helped me avoid a similar issue.