My first thought was a bad vacuume modulator as I don’t see much f a leak. After sitting for a few weeks a saucer size spot develops from seepage at the dip stick tube o ring. I just had the engine overhauled and they must have damaged it. But the amount of fluid I am using is much greater, like 4 qts in a 100 miles.
I crawled up under the truck today and don't see a modulator anywhere. No oil dripping when running that I can see. Are they somewhere different than they were in the 1970's? r/r of trans is what I remember. Did they do away with them?
I had a '79 3/4 ton van, the modulator was on the right side, about the middle of the pan. I don’t know if there the transmission is different between 79 and 88. I think 88 is before they started going electronic, so it might be.
If it isn’t the vacuum modulator, the only other place I can think of is the transmission oil cooler in the radiator. See if it is ending up in your coolant.
Is it possible that it’s leaking out of the dipstick or the vent but only when you’re driving down the road? If it were really sucking in a quart every 25 miles through the modulator, you should be leaving a pretty good smokescreen behind you!
ETA: Unless it’s a diesel I suppose. But then you’ve probably got the smokescreen all the time anyways.
I never see any smoke. I was thinking the same thing about flowing out the dip stick tube when I am going down the road. It doesn’t leak when idling but does leave a small puddle after sitting. Tomorrow if I have time I will inspect the line off of the modulator for oil residue, if I don’t see any I will change the oring on the tube and try it again.
