'90 535i plug fouling problems

[Bob Knight <bob@xxxxxxxxxxxxxxxx>]

Hi - My mechanic's stumped and if he is, therefore I am :). I have a 
sharked '90 535i with ~140K on it. The car is maintained well and
generally is a joy. During the past couple months, however, a 
problem's cropped up. The problem is stumbling during hard 
acceleration in first and second. The first time it cropped up, the 
throttle position sensor was determined to be bad (showing full on
and idling at same time) and replaced. Soon after the replacement,
the problem surfaced again. Plugs were replaced. A couple of weeks,
and the problem came around. Plugs again were replaced, with a 
recommendation to switch gas (I was running Phillips 66 premium),
so have switched to Chevron premium. Problem came around *again*.
Again, the plugs were changed. Now, three weeks later, it's starting
(albeit more slowly).

My mechanic's analyzer shows absolutely no faults from the computer.
He says that absent faults, there's nothing really that could cause
this. He would see throttle position sensor problems on his analyzer.
John's *real* good (has umpteen years BMW experience) and I've used
him for over 10 years. But obviously we're missing something.

So. I suspect (for some reason) it's a marginal sensor that isn't 
getting detected by the computer. I would like to ask the list if
people have seen problems like this or if they have advice on where
to look.

Thanks in advance.

Bob

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