I'm running straight 30 weight oil in the trim pump, the oil was full of air changed the oil and within several minutes the oil appearts foamly. The cylinders were drained and the boat is not in the water . What would cause this to happen. This is not the first time I had this problem two previous boats had the same problems.

I have found that this condition can be caused by several different problems. I speak from very recent experience, as I just got done solving a similar problem.

1. The oil level in the pump sump is too low, causing cavitation and aeration of the fluid.

2. You have air bound up in one of the cylinders. What can happen here is that when you go to trim, you have to push a bunch of fluid into the piston as the air bubble compresses, which can deplete the sump, causing problem #1, and, when you stop pumping, the air bubble expands again, forcing fluid back to the pump, making it barf back up aerated fluid.

3. This is the sneaky one. Your pressure relief spring, especially in the trimming "down" direction has gone soft. The relief valve for the "up" direction is supposed to be set for like 3500-4000 psi, while the down direction is set for only 350-700 psi. So, if the up spring starts to wear out, you get maybe 2500 psi let's say, and that's still enough for everything to work fine. But, if the down pressure has dropped to maybe 200-300 psi, and your rams are a little sticky, that's not enough to get things moving. What happens then is that instead of pumping fluid to the trim rams, your pump just sits there and relieves internally over a very short circuit. Voila, more aeration. In this case, the pump isn't really making any pressure, so it won't bog down and sound like it is working.

I had all three of the problems above, plus, after dissasembly of my trim rams, I found that I had one ram with a shock type piston, and three with solid pistons. Some idiot some time ago put in the shock piston, but did not install the check balls that are supposed to be there. I could trim up against the main slug, but not down.

I did some troubleshooting, and reversed the trim hoses, to see if I could trim in using big pressure. I checked out the rams independently, and found that one was fine, and the other was messed up.

So, I disassembled my old offending Prestolite pump. Despite the fact that the service manual will tell you that if anything is wrong with the pump, you have to replace the whole valve body (crooks), you can actually adjust the spring pre-load on the relief check balls that control pressure. I figured out which was which, and reset it a little higher. Put everything back together, and I also welded up the ports in the shock piston, so I now have all solid pistons. Everything works perfectly now. I haven't had an Oildyne or the Parkers apart lately, but they may have similar adjustments. The service manuals don't tell you about this.

Sorry, long story with a lot of twists and turns, but I couldn't really find any good information anywhere on troubleshooting these things, even in the forum archives, so hopefully this will help more than a few people down the road.

If your rams were recently drained, and not bled, you may just have air in them. I've found that it helps to disconnect the tailrod end, and let the ram hang vertically. Fill the pump sump up with oil, then trim for a bit, but stop before your run out of oil. Refill, and repeat until you have gotten all the air out of the system, as indicated by no more air sounds in the pump, and sump level not dropping anymore after trimming.

Use ATF. Being a light "hydraulic" fluid, it won't foam, as long as its a high quality product. ie. Mobil, Amsoil, Castrol.

How about the Mercury power trim and steering fluid ?

tcelano I had a piston/valve in the pump sticking,like you I took it apart,to out the valves and polished the valve and bore,it must of had a little bur in it, worked fine after that, nothing to loose if

I uses the Merc 3-5 and it still aerates some,I think its normal.

Do you think your trans in your can doesn't aerate the oil,On my Chevelle at the full level it would puke out the vent tube,foaming/aerating,I now run it at a lower level,no probs,

Also the merc 5 has anti foaming agents.

Your engine oil does the same thing.same as your fuel system,

Rob

in the trim rams pulling and pushing them with no movement would ya consider no air in it at all???? no sponge feel.

Rob,

Good point. Also it is possible to get a little piece of gunk fouling one of the check balls, which might wreak havoc.

I have hard sumps, so I can't see any minor aeration that might be going on unless the pump completely pukes. If all is well in the system, and everything is bled, it won't puke up, even when run for extended trimming with the fill screw off. If your pump is barfing, you definitely have some issue to attend to.