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Bill[_12_] Bill[_12_] is offline
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First recorded activity by BoatBanter: Jan 2017
Posts: 4,553
Default Mercruiser 5.7L Idle Issue

On Mon, 14 Jun 2021 11:59:01 -0400, "Mr. Luddite"

On 6/14/2021 8:01 AM, Tom Greene wrote:

Drove boat to a cove and floated for an hour. When I started it, it
started right up and sputtered and stalled when I accelerated. After
being towed home, I changed the IAC filter and the boat started and ran
fine for several cycles. Then it happened again. It started and ran
fine. We floated on the lake and died when we accelerated. It starts
but stalls after a few seconds. I changed the IAC valve and it started
and ran great until we turned it off and restarted. Same thing. It
seems to run and idle fine until we accelerate or rev engine. I am
back where I started. Any suggestions of what to try next?
Thank you!

Just a guess. I had an older 350 ci I/O that exhibited some of your
symptoms. It would start but die under load. Then it stopped running

Turns out it was a bad, leaking coil. It finally blew the connector
right off the top. It would start again but die as soon as I put
it in gear and tried to move. Towed back to the slip and it started
right up again.

Replaced coil. All was fine.

The fact that he said IAC valve has me thinking this is a new EFI
engine. It probably doesn't have a regular coil.
Is there an OBII2 connector? maybe try scanning for codes.
The problem might be the HP fuel pump. Is there a port to monitor the
rail pressure. That won't usually throw a code.

There is a Shrader valve on the rail. The pressure tester is not that
expensive. Should be around 44 psi. Also check the water separator
filter. Since it seems to be heat related, could also be coil.