The starboard tach and oil pressure gauge on my 500 efi fluctuates very bad. The tach does it above 3800 rpm and when it fluctuates it move up probably 500-700 rpm, and the oil pressure gauge pretty much does it all the time other than idle. I’ve checked all the ground connections, battery connections, main engine plug, and so on. Does anyone know which pin on the main engine plug is for the tach and gauges? It seems when I’ve taken the main engine plug apart and then put it back together that it has gotten better before. I just wonder if I might have a bad connection inside the plug.

tach is grey, oil pressure is blue. The can spread the pins and spray with crc or wd 40 in connector. do not over tighten clamp on plug.
You should also check oil pressure sender connection. it too has a blue wire going to it.

this sounds like a loose connection problem. They are very tricky to locate and get rid of.

Thanks for the reply. I will check those pins and make sure they are making good contact.

You are not kidding, I've been chasing this one for over a year now. It does not hurt anything it's just aggrevating.

I had a wiring connection problem with my Scarab, the tach would go nuts every once in awhile. Whenever it did this I would notice the voltmeter and oil pressure guages act strange as well. One day I tapped on the tach and the engine died. It started right up and ran okay. I got tired of this crap happening and pulled the dash and found a lose connection on voltmeter. I tightened it up and coated the connections(all) with a couple of drops Liquid tape. I haven't had that problem again.

Have you replaced your plug wires recently? I've seen old arching wires do the same....

Yep, I replaced them earlier this year and it did it before and after. Thanks for the reply though.

Cool!

My mentor when I started out as a marine mechanic always taught me to check the stupid stuff first because it's usually the stupid stuff that cause the problem so it's the best place to start!

I have had the same problem for the last 3 yrs. I changed plug wires, cleaned wires on the back of the gauges, checked grounds and still no change but no worse either. Strange thing is after I run for a while and shut down the motors when I re-start they jump more for a few minutes and then settle down to normal range until I push over 3800 rpm's and the bounce is more prevalent.

I was trying to avoid replacing the tachs because they are gaffrig and I did not want to have a mis-match with Livorsi tachs. Just a little annual.... but I think most of us are with our boats.