I filled my sea water pump with sand and had to be towed. My normal mechanic is no longer around so I had to take it to another merc mechanic. They back flushed and put new sea water pump on and I told them to put on new thermostat also. Went to the water ran ok except when I got on plane, the temp hand kept jumping back and forth between 170 and about 190 for awhile then dropped to 120 a would not come back up. Took it back to same mechanic, he said found more sand and backflushed again and test ran. I get it home hooked to hose and it came up to normal(170) for about 5 minutes then temp gauge starts going all over the place between 120 and about 185. So I call this mechanic back this morning and he is out for the day, getting leary of going there anymore. Any thoughts on the problem ?? It is a 1999 502 m.p.i. with low water pickup on the drive... Thanks in advance for any help...

it is due to the muff on the pickup the water is mixed with outside air i have same setup. This has happened to me before it may a couple of flushs to get rid of sand in block .When temp goes way down like your did sand stuck in themostat keeping it open

After you get this solved, I highly suggest a sea strainer. You will never have this problem again. Sea strainers are not just for ocean runners.

My temp guage always fluctuated a LOT on the hose between 100 and 160. It would only do it on the hose. Test it in the water.
Also, if you sucked in that much sand, it will take a long time to get it out of the block and several flushings.

I had a similar problem once... it was the ground path for the temp sensor. The sensor was on the thermostat housing and the housing wasn't making good contact with the manifold. He may have goober'd up the contact in the gasket. Easy test with an ohm meter... I'd also invest in an infrared thermometer just for peace of mind.

Thanks guys. It sounds like I need to run it in the water to see what it does. I never beach because of the low water pick-up, I got in some shallow water and when I realized it instead of turning it off and walking it out to deeper water, my dumb a** put it in reverse and backed out, stirring sand for several feet! Never had an issue on the hose before with the temp gauge though? The guy who fixed it did tell me he backflushed a sh** load of sand out on the first flush.

Thanks again for the help on this issue, I ran it yesterday in the river and everything seemed fine.. This is the first time I have had the gauge go wacky on the water hose, so now I know...