miss/stumble

[Mike Miller]

Seeking a little list wisdom here.

About 50 miles from the end of a 575 mile drive today, I go to pass a car
and notice a stumble or miss at about 1.6 bar boost. Car is not chipped. It
seemed I could slowly get to 1.5-1.6 bar and then I would get the
miss/stumble. My first thought was fuel pump/filter. This was very
consistent in 5th gear. Then it started doing it in 4th gear - it seemed
throttle postion related. WOT or near there would miss, but easing the
throttle down worked ok til about 3/4 throttle. This is definately not the
ECU killing the fuel pump ( over rev or over boost) - I have felt that and
it is no where near as violent. I can hold the car at 1.6 boost in 5th and
it does keep going - just missing. I was able to do a WOT run to 6500 rpm in
1st thru 3rd gears OK. As soon as I hit 4th, the miss was there. That would
seem to me to indicate that the fuel pump/filter are ok. The miss got much
worse after doing this though, requiring maybe 1/4 throttle to cause it in
4th or 5th. Anything over about 1.2 bar boost in the top 3 gears - Sounds
like limp home mode, but I could still get 1.5 or so in 2nd. At this point,
I was just trying to get home and abandoned further troubleshooting.

My current thinking is maybe a knock sensor retarding the timing or possibly
a MAF sensor starting to fail. Any other suggestions? Outside temp was sub
20F and the colder it got, the less throttle input required to casuse the
miss. Not sure of relevance as the farther I drove the colder it got. As it
is dark and 10 below outside, I have not tried to pull any codes.

I had filled up with gas about 200 miles earlier and ran 75-90 mph over a
couple mountain passes in ID and MT, so I don't think it's bad gas. That
would have shown up soomer, IMO

thanks,
mike miller
helmville mt