Oil foaming out of lower unit trim pump reservoir. I know if air was getting in that would cause it but that doesn't seem to be the case. No leak down from full up right position and no water in oil. I use a 20/50 the last time I fill it up. Could that be the issue?

You should be using hydraulic oil or I believe Dextron transmission fluid NOT engine oil . Flush the system out thoroughly and refill with the proper fluid .

Hi TheDeep
In all respect - I used to have a older Mercruiser (165 hp) and the owners manual said to use straight 30W oil for the trim pump. I have no idea when Mercruiser made the switch to hydraulic oil - but I would suspect that's where the foam is coming from -- However, I'm in no way any type of expert

Safe Boating

3pointstar

Straight 30w oil in trim pumps is perfectly normal. All of my boats have used it.

Mercruiser use to recommend 10w 30 motor oil in the trim reservoir along with currently their power trim & steering fluid so either one will work.. Power steering fluid from the auto store works as well.

If you have the older yellow cap for the trim reservoir make sure the pin hole in the top is not plugged. Its there to purge air out of the system. The newer style caps are a lot better plus help keep dripping water on top of the cap from leaking into the trim reservoir .

Oh forgot you mentioned filled the reservoir - how full did you fill the reservoir ? Plastic reservoir only requires 3/4 full when fully trim in (down). See if you filled the reservoir full especially when the trim was fully trimmed out (up), as you trim in (down) the reservoir oil level gets higher, if it is too full, the trim fluid will overflow. Also make sure you do not have any water mixing with the trim fluid.

BUP

I do have the old yellow caps and I have ordered new caps....So do you think that I may have had air in the system that isn't able to get out because of the vent in cap being plugged.......and you don't think the type of oil is a problem?

I am starting to think that it maybe old air trapped that hasn't gotten out yet but my trim works good as long as I have proper fluid level and seems to only loose oil when it foams out during trimming....still scratching my head...

I add with the drives trimmed all the way in... And I fill it to the seem in reservoir.

I wonder if using 20/50 instead of a straight 30 is an issue?

Could a seal be bad in the pump creating air causing the foaming but not cause a leak down on the unit or loose oil level?