I just replaced all my gauges on my 97 fever and they were shot. Sometimes they showed some ridiculous temps

I also found that even though I back flushed the oil coolers there was a ton of the impeller stuck in there that had to actually had to be removed with tools. I think after the agony of replacing the impeller in that engine compartment I gave up and forgot to check where the parts of the impeller went.

all carb mercs have 140 and fuel injected have 160,merc never used or shows 180 (FYI)

I looked into the oil cooler, and it was clear. I got really good waterflow out of the backfush, But I'll completely pull the line and check to be sure...i'm hoping it is just a guage issue, so I might try swapping the leads between motors and see if the issue stays with the same guage or not. I'm hoping it's just a screwy guage, I just wanted to exhaust any other possibility. Thanks for the info guys! Keep it coming, I'll keep updating as I try suggestions.

Don't forget to check (assuming you have) the power steering cooler.

Actually, I haven't. I'll try that tomorrow.

If you have twins why don't you test out the malfunctioning sender / gauge theory first. Swap senders to see if it does the same thing then you know it is not the sender. After that swap gauge wires to see if it does the same thing and eliminate the gauge. Then go back to the motor and diagnose. If you have bravo ones and this is a salt water boat you may have the dreaded bravo one cooling hose blockage.

that's what Im thinking

I had a 1998 32' Fever with Twin 502 mag MPI's, had many overheating issues.
Veeeerrrry imortant, do you have low water pick up lowers???
If so, it will suck up all kinds of crap if you are in the same zip code of shallow water. Any grit at all would cause the thermostats to stick.
1) check P/S cooler for debris, that the first stop after the RWP.
2) I tried running them without T-stats to see if that fixed my problem which it did. I installed 140 deg T-stats and I drilled (4) 3/16" holes in them, if they get stuck it will still allow some water to pass.
3)while you have the T-stats out, swap housings and see if problem goes to other engine.
4) I sold it in 2011, while I've heard of the hose at the transom clogging, I never had that problem but, I boat in brackish water.

No low water pickups... I think I have it narrowed down to 2 issues. 1) the boat has 180 thermostats in it, wich is wrong, and 2) after taking it out this weekend, and running it hard, the minute I come off plane, the temp on the port motor drops below 100 after a few seconds, then slowly crawls back up. No way that the motor is cooling off that fast, so I'm going to assume it's either a gauge issue, or a "new" bad sending unit. Also, this was the 4th time I've had it out since I bought it Sept. last year, so there might be something I need to get used to. Thank you all SOO much for the help!

I'm thinking the coolers could be full of rubber from the impellers as well. But I would switch the port temp gauge with the starboard gauge. See what temp the gauge reads you then. It could be just a bad temp gauge.