Hear me out, I know this is weird.

My max sat in in my back yard for *years*. Once I decided to try to fix it once and for all I got it started and went for a drive. The gas was probably 1/4 4 year old gas, 1/4 1 year old gas, and 1/4 new gas. It ran, but wouldn't shift at first until very high rpms, like 5000+, it was terrible. Once that tank was lower I put in new regular or mid grade gas, and immediately it shifted better, not great but no more insane rpms.

ffwd a few months, and it was shifting better but at high 2000's almost 3000 rpm and with some force, this was after a few months of mid grade gas only.

ffwd a few months, after replacing my knock sensor and 3 - 4 tank fills of only premium and now it's shifting much better, like low 2000s and very smooth, and today, for the first time in 7 months, I've been able to get it to downshift at WOT and actually go FAST.

So, pure coincidence? or did the better gas at each stage get noticed (eventually) by the ECU and lead it to send better shifting instructions to the transmission?

What do you think?

 

Pure coincidence. The shifting of the transmission is caused/controlled by the TCM (Transmission Control Module) that is mounted on the floor of the car somewhat below the radio.

But the transmission itself plays a part in this, too. It sends signals to the TCM, such as oil temperature, oil pressure, rpm. In your case of the car sitting for several years, I would guess that the oil may have broken down and the various solenoids, spring loaded ball check valves probably are sticking. Driving the car again is slowly freeing up those parts.

I would suggest changing the transmission oil that would have fresh chemicals. ATF oil is a high detergent oil for cleaning the internals. If you drain the oil yourself, you will only get maybe half the oil. This may be good enough to clean and free up those sticking parts. Drive the car for a month or two and change the oil again if you need to/want to. This would effectively change 3/4 of the oil.

To get all the oil changed, you would have to get the transmission flushed. But on higher mileage transmissions, this seems to cause problems, so I would not have that done.

 

Yes, transmission fluid is huge.

 

Sounds good and makes sense. Yeah I think once the foot or so of snow here has melted I'll do the gentle fluid swap.

 

I agree with the others.

The transmission had deposits in the passages. These are slowly dissolving. I would drain and fill the trans 2 or 3 times to replace most of the fluid.

But first, a question.

what is the color of the transmission fluid?
red, redish, redish brown, brown, blackish?

How many mIles on the car? Has the fluid been changed before?

 

I believe it was reddish brown when I checked a month ago.
~260,000 kms, so ~160,000 miles
Not sure if it's been changed, but I have the service records from about 2000 so I'll take a look when I get home. I'll also take a pic of the fluid.

 

Reddish brown indicates that the fluid probably overheated in the past, and might have changed its chemical properties at bit. Seems like that's where your internal deposits came from. The fluid should be changed.

if the fluid was blackish, there might be a risk that the transmission would not last long if the fluid was changed. At the end of a transmission life the fluid has most of the friction materials in it. The transmission would not last much longer anyway, but a fluid change would kill it quicker.

 

Update.

After steadily getting slightly better until about April, it then started degrading again, shifting from 1st to 2nd at 2800 rpm, then 3200 rpm, then I adjusted the transmission fluid level and it immediately would only shift at 3400 rpm, then about a month ago it degraded to 4000 rpm. I thought I had had a throttle position sensor code a year ago but I wasn't 100% sure, but then it came back.
Fast forward to a week ago, suddenly it occasionally wouldn't get out of 1st at all unless I revved it to 5000+ rpm, and a couple of times it wouldn't shift at all, and now I was getting the 16 flash Overdrive thing.

I really thought my transmission was shot, due to the slow degradation, but I read the transmission codes and it too was TPS. So I ordered one and just installed it an hour ago.

It's like I have a new transmission. The car started so fast I thought it hadn't started, the idle was rock solid, the behavior where when in a gear I'd let off the gas and the idle would plummet down to about 400-500 rpm then bounce back up to 1000 was gone, and the shifts were suddenly perfect, smoother than I've ever felt, and at low rpms, like 1800-2000 rpm. But, when I hit the gas hard it had more power than I ever remember it having.

So yeah, if you ever think your transmission is dying, hell even without a code I would highly recommend replacing the Throttle Position Sensor.

 

I always used premium gad. The previous owners informed me that lower octanes made my 96 Max not run as smooth, so I never bothered to try regular gas myself. But hey, if it works for you, it works. I think I'll change my TPS now. Thanks