Help me diagnose this...

[weinie]

Went out on my 84 catalina 27 w/ universal m18 diesel today. After a couple of hours of motoring, my RPMs seemed to drop significantly at max throttle. While I normally get about 3000 at full throttle, i now seemed to max out at about 2200. It also stalled out when I brought the throttle all the way back, so I'm pretty sure it's not a problem with the throttle cable or lever

Also, although I didn't put 2 and 2 together until several hours later, I did note a very strong exhaust smell it the cabin; strong enough that I opened a hatch to vent it out.

Here's what I am thinking....

Water sucked up through the exhaust. The flap on my thru-hull is missing, and although there were no following seas (it was dead calm), perhaps water was sucked back up from the wake of passing motor boat if my anti-siphon valve is clogged.

Does this explanation fit?

If so, do I now have water in my cylinders?!?!?!??!
I would think that if water did get that far up, I would have 0 RPMs.

[jimd333sail]

I don't know about the exhaust, but is it possible you picked up something around the prop which is lugging the engine down?

[imagine2frolic]

Prop wrap would probably cause a shake. The water would be shoved out if it runs. Sounds like a fuel problem possibly.......i2f

[stam22]

Once I had some irratic rpm problems that would come and go. It turned out to be an old fuel hose that was dry rotted, so when it flexed slightly at speed, it sucked in air, when I slowed down, it straightened out and didn't leak and ran normal. I changed all the fuel lines, problem solved.
If not fuel line related, I would change the fuel filters before anything else. Or have they been changed recently? I guess they could be clogged again if the fuel was contaminated.

[imagine2frolic]

check your filters for fuel, and air......i2f

[jrd22]

If you weren't seeing a lot of black smoke at full throttle it's probably not the prop. I'm with I2F, fuel filter(s) or clogged screen on the pick up tube in the tank. Can you get full rpm in neutral?

[dabnis]

Plugged tank vent? Open the filler cap to tell, or blow through the vent outlet.
Air leak on the suction side of the lift pump? Check all fittings and the filter housing. Pressurize the system right out of the tank, the leak should weep fuel. If pickup tube has a screen maybe partially plugged? As mentioned, check or replace fuel lines. Some pumps may have a built in screen or filter that may be plugged. At the worst you can replace everything from the tank outlet to the high pressure pump. If nothing works you could remove the HP pump assembly and have a diesel shop check it, but suggest you start with the simple things first. Let us know what you find.

Dabnis

[weinie]

Problem solved!!!

Fuel pump filter in lift pump (which i never knew existed) was all crudded up.
Never knew it was even there!

[dabnis]

Thought that might be a possibility. Thanks for the feed back, good to know.

Dabnis

[weinie]

Sometimes one forgets the obvious. Just before I set out I noticed my exhaust flap missing so immediately my mind veered towards a highly unrealistic scenario rather than the most likely.