Wow, I can't believe that there are so many people describing the exact same problem. Mine does the same thing. I drive the same route to work and back every day, and it does it on the same downhill grade virtually every day. This downhill spot is about 10 miles from work, and it's almost like clockwork. The "cut" of power happens when I tip back in to the throttle at the end of the hill. The cut lasts about 2 seconds, and if I don't gently blip the throttle gently then it bucks badly if my foot is too deep in the pedal when things come back alive. For a while I thought it was maybe RF interference since it happened in the same spot every day. I thought that the ECU might be wierding out driving through that particular patch of road. Very strange...

 

I guess I will join the club. Mine does the same thing... at the bottom of a hill it is just dead for about 2 seconds after pressing the throttle. Mine is a 91 C2 50,000 miles. This sure would be a good one to figure out.

 

Hi:

I just ran across this thread and it reminded me of a similar experience with a customer's 993TT that exhibited the same symptoms.

After spending several frustrating weeks going over the whole car and changing a good number of parts, it was discovered that the cylinder head sensor connector wires had backed out of the connector housing just enough to be intermittent when going downhill under throttle. These connectors/wires are at the front of the engine on the left side on that steel bracket that holds the other two connectors for crank position sensing.

This was very tough to see and required VERY careful inspection after pulling the rubber boot back and making sure that the AMP tabs on each spade were seating in the connector body. We replaced all the spades and recrimped the whole thing before "torture testing" them and watching the DVOM for any changes in continuity.

I cannot tell you that this will fix your problem, but its surely worth checking out and perhaps this might save someone some MAJOR headaches. BTDT,.....

 

I'll add one more to the roster - my C4 cabrio (50,000mi) used to do this, on the exact same stretch of road, in the exact same spot on the way home from the office after it had been coasting for about 30 yards, the "problem" would occur. It didn't buck, but when the gas was applied nothing happened, a couple seconds later it was back to normal. It did it maybe 4-6 times over a 3 year period... For the record, the other two C4's I have, and a C2 I had never did this...

 

Well I'm still fiddling with stuff. I can't rule out any sort of intermittent electrical connection at this time.

What i can say is I had another look at my afm. I looked at the original wear tracks and they don't make sense. What I mean is the track is longer than the swing of my arm. So what that suggests to me is one of three things. Either the screw holding the position of the arm relative to the door flap allowed it to move, someone adjusted it (which i doubt), or the door flap changed the amount it could move. Now I messed with this screw when i changed the track my arm rode on, but I set it back per the wide open position. (stupidity, I thought I had to to remove the arm)

What I did this time was reset the arm per the minimum position of the old track. I think the problem is mostly gone now. Of course, I've thought this before, so I will see.

My point with all of this is it really could be in the afm for me. And Iw onder if anyone could answer a question about it.

#1 The screw accessible from the outside (under the press fit plug) what exactly does it do. Does it effect the base voltage emitted by the afm, or does it change the position of the arm per the flap (barn door)?

I think its the former, but I havent looked at whats underneath, nor remember much of what i saw when I had the arm off.

 

Guys,

Has anyone got any further with this problem?

Mark 90C2

 

When I disconnect the ox sensor all is well.

When its hot this problem is worse and my car isnt as smooth under partial throttle. With the ox sensor disconnected it is smooth as well as fixing the problem.

Doesnt tell me what is wrong, but it supports my theories on what is going on.

 

I've been living with the problem since my last post but due to be in the shop soon and will be giving a full print of this thread to my mechanic! The one thing I do know is that swapping out the AFM cleared the problem completely so am convinced that its somewhere in this unit. I'm hoping some of Heirsh's investigation helps towards solving the problem - I'm just not sufficiently clever mechanically to go go down the self-fix route on this one without the risk of permanent damage!

There must be an answer out there.

Peter

 

Going back to the post of Steve Weiner, I'm having trouble locating the cylynder head sensor connectoron my car. Has anyone got a photo please?

Mark 90C2.

 

bump

any new ideas on this?

 

FWIW, I used to have this same problem but after having the valves adjusted and the plugs changed, I haven't noticed the problem in the past year. I also replaced the clutch slave cylinder and hose about the same time, although I can't imagine that was related. Now, no more hesitation downhill or anywhere else.

 

I also had the same problem. Mine is a 93 C4 that Adrian referenced in this thread as being the Denver car with the same problem. I worked to resolve this for years replacing lots of different parts but nothing really seemed to work. The problem would also get worse as I gained more altitude. Denver was not as much of a problem as in the mountains where I live at 9,000'. Anyway, I too suspected an aging AFM and related DME program.
I recently converted the car to a Turbo but in the process added a 993 MAF. The DME was reprogrammed to accommodate the MAF. The problem is now gone, totally. It is now much more drivable as all hesitation has now been removed. I would be curious to see if anyone else out there who has switched to a MAF has the same results with elimination of this problem
Hope this helps,
Peter

 

just to add mine to the list. mine does the same thing intermittently on exactly the same stretch of road. is in an area where i am off throttle -- kind of coasting slightly downhill into a small town where the speed limit is low and lots of cops. when i go to get back into the throttle it almost feels like the car has stalled--- but it hasn't. after a second or two everything is back to normal. this has happened maybe a half a dozen times in the last two years. it almost feels like when the car cuts out when rev limit is exceeded. it doesn't happen enough to really concern me, but is more of temporary inconvenience.--tom

 

You can add mine to the list as well. Same stretch of road 60mph, slightly down hill, no power when throttle is pressed at the bottom of the hill to maintain 60mph. Power returns after a few seconds and it drives normally. Happened yesterday with 7/8th of a tank.

 

I still have the problem. Mechanic has looked at it and has not solved it.

Hook up a hammer (or alternative) and watch the trim while this happens. It goes from one end of the spectrum to the other. i firmly believe it is a mixture problem. When you are in the barely-on-the-throttle-mode it adjusts the mixture, but then when you get out of it the mixture is all wrong to the other end of the spectrum and it bucks (or otherwise no power) until it readjusts.

Pull your ox sensor temporarily and see if it goes away. Mine does. It is highly tempting to leave it unplugged. Also do you guys see a cure if you go wot (which causes the system to ignore the ox sensor and go to presets)?

Just my thoughts and experience with my car. My mechanic, unfortunately, doesnt believe my diagnosis so I'm stuck not knowing what the cause is.


/sigh