Indmar engine dropping RPMs under power

Hi, I have a 2001 Indmar 5.7L TBI Chevy. Last time out on the lake when pulling a wakeboarder the engine suddenly dropped RPMs way down. Sometimes it would last a maybe 4-5 seconds and then rev back up and other times it would stay in the low RPM mode long enough to bring the boat to a halt so I shut off the engine. On two occasions there was some sort of clunking noise as I turned the engine off. It made me think maybe a cylinder fired at the wrong time.

The problem repeated multiple times when pulling wakeboarders, but it ran fine when in neutral and revving up or when driving back to the ramp. This wasn't a thorough test, so it may not have any significance.

The MEFI ECU is not reporting any codes and repeats the 1-2 normal code. My first thought was that the engine was overheating and the ECU dropped down to safe RPMs, but the temperature gauge shows normal.

The two prior times taking the boat out it was sometimes hard to start after sitting during rider changes. Because it smelled gassy when it finally did start up, I thought it might have an injector leaking when the engine was off, but this problem didn't show up again and the injectors weren't leaking.

Anybody have any ideas what the problem might be or things to check? I plan to watch the fuel rail pressure the next time out to see if it's fuel starvation. I checked the distributor and it looked pretty good, but I cleaned the contacts anyway.

Ironically it did turn out to be overheating from a clogged raw water intake/transmission oil cooler. Apparently the ECU doesn't throw a code when it overheats and drops the RPMs down to a safe level.

Glad you got it figured out. My Chevy 350 has done the same a few times. I finally had my dealer install a strainer before the trans cooler to prevent it from happening again.

Great idea. Thanks.

I would check that temp gauge since you were overheating and I'd change the impeller since you surely cooked it.