You should be able to get over 3,000 rpm out of your engine in calm water
, no wind
conditions. Believe max continuous cruise
rpm for the 3GM
is 2800 rpm but you should be able to get close to redline rpm in flat conditions. You have a problem somewhere, either with the prop, boat, or engine.
The exhaust elbow on the 3gm30f is a notorious failure issue. They seem to clog easily and kill the performance. I may not be correct on this, but believe they restrict the water flow in the exhaust as they clog up. If you aren't getting any water out of the exhaust or very little, I'd start looking at that. Check the no load max rpm, first but then proceed from there. Check that the intake water supply isn't clogged at the thru hull
, replace the water pump impellor, and clean out any broken impellor vanes and other large crud out of the heat exchanger
. If you still don't have a lot of water coming out the exhaust and can't get beyond 3,000 rpm, it's time to start looking at the exhaust elbow, prop, etc.
My engine began overheating
and had limited water out the exhaust. Power and rpms were still pretty good. I didn't check the water intake when my engine started to overheat. Everybody said "Exhaust Elbow" so dumb me didn't do the basics and the cause turned out to be a clogged intake. After much swearing and eventually having to call a diesel
Mech to get the elbow off the exhaust pipe, we found the elbow still relatively clean. Decided to replace it since we'd gone to all the trouble. The exhaust is easy to disassemble from the engine but a bitch to unscrew the mixing elbow from exhaust riser pipe. To get it apart, you need a BIG pipe wrench; a cheater bar; a big, well anchored a vise; and a torch. The threads really get stuck together with the salt water
, exhaust gases and heat. The elbow itself is one of the most expensive pieces of cast iron that you'll ever buy. Seem to remember it was close to $200. If you don't wreck the rest of the exhaust plumbing
in getting it apart, the rest of the exhaust should be reusable.
After putting the exhaust back together the mechanic
did what I should have done in the first place. Checked the intake and found it clogged. He reamed it out with a steel
rod and then replaced the impellor as it was short a few vanes. Found most of the impellor vanes in the heat exchanger
, he burped the hot water heater heat exhanger as air in that can cause overheating
. After everything was back together, we had lots of water out the exhaust the engine ran like a top.
Check the easy stuff first before you get into the hard and costly items.