6bta overheat issue- with an interesting caveat

[Jeffrey Melhauser]

My 2001 6bta has developed an interesting ‘overheat’ issue and I’m pulling my hair out.

When I get in the boat and it’s cold, meaning hasn’t run in a day or two, I can jump in, fast idle until temps come up to operating, get on plane and run ( including WOT) and temps are right where they should be on both the gauge and when taking an IR to the thermostat housing -between 185-190 and are within a couple of degrees of the other engine. The issue shows up after I shut down the motors for a couple hours and it has an opportunity to ‘heat soak’. When I get back going and jump on plane the port motor starts overheating once I get past 1200 rpm or so on my way up to plane (When I say overheating the alarm goes off around 205, I let it get to 210-215 or so then immediately pull down to idle where the temperature quickly corrects to the 190 range)

I took readings today with my IR gun today while doing a fast idle after letting it ‘heat soak’, maybe 16-1800rpm, and the port motor started showing hot on the gauge, stabilized around 210 with the alarm starting to sound, but IR readings still showed acceptable at 183ish depending where I shot the housing. In one last test, I returned to the dock, added a few extra lines and ran the engine in gear at 1150rpm or so (highest I could achieve while stationary) for at least 5 minutes. Guage indicated close to 200, yet IR indications all around the housing were showing no greater than 186 depending where shot- and man was that engine working hard.

I’ve yet to pull the impeller or start chasing down the raw water side because I’m convinced the motor isn’t actually overheating as I can jump in the boat when she’s cold, run on plane for an hour and it’s fine. She has a newer SMX water pump and the HX & aftercoolers were pulled and serviced last season.

So far I’ve replaced the temp sender, thermostat, swapped VDO gauges on the dash, and swapped the coolant caps and she’s still doing it.
I just went and spoke with the people at Lauderdale Speedometer and we both agree it sounds like a ground issue; which leads me to my next question: where does that sender lead actually ground on the block? When I follow it it disappears into a wiring bundle behind the starter (and no this particular sensor doesn’t actually ground to the block from the sensor itself). My next though is to run clean wires from battery to sender to guage.
I’m puzzled why letting the boat ‘heat soak’ would affect a ground. Had anyone ever seen anything like this?

Topic

[Jeffrey Melhauser]

Took apart all grounds from starter, alternator, sender, and main engine ground and gave everything a good cleaning…. problem seems to be fixed.
Interesting that a poor alternator ground would manifest as an engine overheat indication only after the engine battery bank had been drawn down

[Tony Athens]

Jeff,

Post a few pics of “your plugs, connectors & senders” along with a pic you your gauges.. You may have OEM wiring..

Tony

[Jeffrey Melhauser]

Ran the boat this morning with the sender isolated to the battery bank and guage and problem didn’t occur… after reconnecting old ground I was able to replicate the overtemp indication. Where things get interesting is the ground on the starter is clean and checks out on the multimeter. Since the guage is acting up in correlation with RPM’s I suspect something with the alternator. Going to start checking that out once engine cools down.

[Jeffrey Melhauser]

Thanks Rob I appreciate the response.
Where physically on the engine do you usually find the wiring harness cannon plug depicted on the schematics? I’ve found various similar looking plugs in various locations. I want to make sure there’s a good connection on that plug and specifically the tan wire isn’t compromised.
Grounding wire on the starter has been checked and cleaned up on both engines. I’m going to run some new ‘clean’ temporary wires between the battery, sender, and guage in the next day or two. I’ll report back
Jeff

[Rob Schepis]

Wiring Diagram

See attached..

[Author]

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