79 Porsche 928 hesitating/missing when I apply any throttle

hesitation
porsche
928

#1

Title says it all. I was in the process of combing over the car to find/fix all neglected and failing parts when I decided to steam clean the engine to make it a little easier to see where all the oil leaks were coming from. This was a bad idea as it has not ran above idle since: whenever I give it any gas, it stumbles and misses until I let off the throttle (been going on for 3 months now).



According to my results the K Jetronic (CIS) fuel system passes all pressure tests. The next suspect was the Bosch TSZ ignition system. Ended up replacing all stock ignition components with MSD 6a, new custom taylor wires, & a blaster ss coil. Seems to run a little better, but still not fixed; now it just runs terribly rough missing and sputtering anywhere above idle.



I’m completely out of ideas. Car’s at a mechanic who seems to be in the same spot I am; clueless as to the cause of the problem. Anyone have ideas what could be causing this?


#2

While I’ve had a lot of CIS experience, recent history has caused me to have some doubts.
(long story)

The things I would examine first would be:
Vacuum leaks, especially between the throttle body and the air sensor plate.
Sticking plunger in the fuel distributor.
Sticky air sensor plate.
Problem with the pressure relief valve in the fuel distributor.
I can’t see that any of this would be affected by water so maybe the cleaning and the problem are just conincidence; unless the cleaning dislodged a vacuum hose or clamp. (see first suggestion)

If the fuel distributor is removed to check the plunger for free operation keep this in mind. The plunger must be handled with extreme care; no dropping it, swapping it out with another one, or marring it in any way. Not even near microscopic damage is acceptable.
The plunger should fit snugly but should move as smooth as silk.

And the system did pass both the cold and warm control pressure tests, right?


#3

Warm and cold control pressures are within spec, air sensor plate has a little resistance when running, but mechanic says its normal. We found a few minor vacuum leaks, but after fixing the problem is still there. Wierd thing is if you blip the throttle instead of slowly applying it, there’s no noticeable miss. Its only when gradually applying throttle that it misses (badly).

I’ve since taken the car to another mechanic, who suggests that it may be the ignition pickup inside the distributor; being an electrical component it could be affected by the water. They are working on confirming that theory now…


#4

ok4450 was close with his suggestion of vacuum leak; the problem turned out to be a faulty EGR valve. When they fail, often times the symptoms are very similar to a vacuum leak (a nasty issue for CIS cars). Hope this post helps someone in the future!