I have a 91 S5 automatic FC, non-turbo.

Initially, I thought my 5th and 6th ports were stuck, as I had no high speed power, just lots of low speed torque.

I changed my plugs and did the grease on the 5th and 6th port actuator trick and it appears now they are opening. I seem to have some real power coming on after 4000 RPM.

However, with the daytime and night time temperatures varying wilding in central Illinois, the car now exhibits these issues: When it is really cold out at night, the car continues to exhibit strong low speed torque on the way to work, but tops out at 65 miles per hour, dropping down into 3rd gear from fourth annoyingly often, around 4,500 RPM. When it warms up outside, on the ride home, it doesn't seem to do this and very smoothly accelerates past 4,000 RPMs. to a top speed of about 80 mph in fourth, if pushed. If I floor it at 4.000 RPM there is no doubt the 5th and 6th ports are opening as it is like a rocket going off. I'm, not interested in finding the top speed there. LOL.

If I had to guess, I would say there is something temperature related which is effecting the shift points of the automatic transmission.

Could this be a vacuum hose or something internal to the transmission.

Stumped. Any ideas?

 

The ECU changes timing advance based on incoming air through the AFM.

Also poor quality ATF will effect shift points and shift duration in the tranny. When was it last changed?

 

>The ECU changes timing advance based on incoming air through the AFM.

>Also poor quality ATF will effect shift points and shift duration in the tranny. When was it last changed?

___________________________________________


ATF was changed around 20,000 miles ago.

What is AFM?

ECU I assume is the engine computer under the dash?

 

Yep, (as found in the FAQ for FC at the top of this section) ECU is the engine computer. The tranny computer interfaces with it.

The AFM is the Air Flow Meter. The air filter is right before it under the hood.

 

Alright. I just had the air filter changed during a recent oil/filter change. So that would leave a defective AFM, maybe. Cold temperature air entering the intake might close the AFM, a bit? Right? Assuming it was working correctly. Warmer air would open it? If it was stuck partially closed under cold circumstances, I can see where the engine would have reduced intake and less power. But the fact is, torque remains high under these circumstances. That might imply that something on the exhaust side of things isn't opening? Or am I misinformed on the way the AFM works?

I wouldn't think 20,000 mile ATF would not impair the transmission operation.

Could cold solder joints on the ECU be influenced by severe cold weather (below 25 degrees F)? That may be stretching a bit.

 

If the flapper door on the AFM doesn't open, then the fuel pump won't operate. Air temp doesn't have anything to do with how far the door opens. Air temp does affect how much fuel is injected. Colder air is denser, so more fuel is needed to maintain a proper air/fuel ratio.

Icemark said the timing advances or retards based on air temp. Check the timing at idle with a timing light. If its slightly off, then the change in timing fromt he temp could be moving out of acceptable limits. Could be a bad IAT (intake air temp) sensor in the manifold. I don't have any real hypothesis as to what your problem is, just throwing out ideas.