Help Chasing Erratic Idle Please
 

1996 7.4L LX Mpi, 263 hrs., SN OF886167, all OE, raw water cooled. Wouldn't idle upon purchasing the boat and was told IAC was bad. Replaced IAC (it came out in pieces) and it idled per spec for a few trips. Around the same time the VST was rebuilt, fuel filter changed, oil and filter, plugs, cap, rotor and wires, fuel tank pickup checked and clean, 90* fitting into mechanical fuel pump clean, tank to pump hose changed.

The last 4 trips the idle began getting erratic again. It started with surging when bringing the throttle up to 1000 rpm for a couple of trips. That was also followed by the inability to idle on the hose immediately after pulling from the water and i had to keep it going with the throttle, I can hear the IAC whistling during this. Now the last 2 trips idle is at 1100 rpm in the water but immediately after it won't idle on the hose still, I can still hear the IAC whistling. Just before the last trip the manifolds were changed so there shouldn't be any water leaking into the cylinders.

I scanned with the Rinda Codemate prior to my last run and got "coolant sensor low" and "TPS high" codes on a cold engine and not running. Problem is the 2nd motor runs just fine and I got the same codes out of it 2 minutes later. I know, I know scan it, right?! I've been trying for months with 3 different options in town and keep getting kicked down the road. One is at Merc. And they have no idea when it's coming back. Another says they're 10 days out, want the boat today and want to charge me storage for each day they're not working on it until they get to it (laughed in their face). And the third always has an excuse of why he couldn't keep our appointment.

If anyone knows of a shop on Florida's Treasure Coast who can scan MEFI ECU's, I'd be happy to give them my business. In The meantime if someone could take a shot in the dark at what may be causing my idle issue, I'm all ears.

Thanks for any help you can lend
Keith

maybe your chasing a vacuum leak

Ran your S# but it comes up as an outdrive serial #. You sure that's the engine serial# ?

I found a paper tag on mine back by the coupler. The one on the ecm box was warn and unreadable.

You can scan for codes with a paper clip and a resistor 12V LED (radio shack), but sounds like your new IAC has gone bad.

Concur.

Sorry typo. Here's the correct. OF886167. I edited the OP as well.

As described in post #1 I scanned with a Rinda codemate and didn't receive an IAC code. The codes I did receive are posted in the OP.

Make sure the yellow and black wires going to the coolant sensor have not deteriorated and shorted out , and you have a crisp 5v ref voltage to tps and ect. No green funk in ecm connectors

I'm thinking along the same lines as Airjunky or bad temp. Sensor. If it was a vac leak I feel the idle issue would be of the same kind (unable to run OR high idle) at all times and not so random. Hell maybe I'm wrong, but that's why I asked the community for assistance. I'll check it all.

Keith