Several months ago my 6-cylinder Buick automatic experienced transmission problems, shifting into 1st and 2nd gears. Each time it shifted with a “clunk”. Pulled over and checked the tranny level, lots of foam on the dipstick but the shifting problem disappeared and the shifting was normal.
Eight months later exactly the same problem. Once again did the tranny check and found foam again and behold, once again the car shifts normally.
What’s up?
If I remember correctly, foam as you describe, is an indicator that the AT sump is overfilled. First check and confirm whether I am correct or not. Next time you drive your car, check the fluid level at the end of the drive with the vehicle running and see what the level shows you. Check it hot, with car idling. If it is overfilled (or underfilled), you may have found part or all of the problem.
When was the last time you did an ATF pan drop and filter change on this car? I use a 30K interval on all my cars now, regardless of what the owner’s manual may or may not say.
If this is a truly intermittent issue, I would consider putting in some transmission conditioner to see if that mitigates the issue.
In your experience at what level of overfilling do problems start? half quart,full quart, more? Some have posted they keep their trans overfilled to prevent fluid pickup issues, I know customers did not like it when after a trans service the fluid level was not exact. I conclude on your own car you can do as you wish.
I don’t really know. I have never had a AT foam up like the OP is describing, or I don’t necessarily understand what he is telling us. I have seen some small bubbles, etc, mostly on our Subarus, but I have also noticed that the ATF I am using bubbles up coming out of the bottle fresh, so I don’t put any stock in the assessment.
I got the info about foaming from a transmission specialist I trusted many years ago. His comment was that, given a choice between overfilling and underfilling, a slight underfill (between the OK lines on the dip stick) is preferable to overfill and the potential for foaming. As I understand it, foaming ATF doesn’t lubricate properly, so you are at risk of the transmission not performing correctly, and as the OP indicated, create some intermittent problems in shifting, etc. Consequently, I try to do a slight underfill when I do my services, and then top up carefully after the car has been driven a number of miles and the ATF is warm.
From a consumer perspective, I would prefer the slight underfill to trying to suck out fluid from an overfill situation. Topping off is much easier than taking out ATF. Even my Subaru is a relative pain to drain ATF if you overfill, and it has a drain plug.
What year and model Buick are we talking about??? If OBD-II, did you scan for codes?? The foaming could be from overfilling, could be from Coolant in the trans, clogged vent…
transman