Mercruiser 350 w/ EFI idles too high

[dlowell]

I have a 1994 31' Silverton with two marine power 350 V8 inboards with electronic fuel injection. Both engines used to idle fine (below 800 RPM). After a two day, 150 mile trip, the starboard engine began idling at 2000 RPM (port engine is still fine at 750 - 800 RPM). I have checked for vacuum leaks (only see one vacuum line just below the fuel injectors and it seems fine). Any ideas on what else to check?

Derrick

 

[Don S]

Re: Mercruiser 350 w/ EFI idles too high

Disconnect the throttle cables from both engines and see if they slow down. 750-800 is also too fast. They should be around 600 or 650 with EFI for any engine. Might want to check your tachometers accuracy too.

 

[dlowell]

Re: Mercruiser 350 w/ EFI idles too high

Thanks Don.

Tried the throttle cables (no change). Was told by someone else that it could be the fuel filter / water separator (which should be changed often anyway). Does this make sense?

Derrick

 

[dlowell]

Re: Mercruiser 350 w/ EFI idles too high

Changed fuel filter (as it was only $11). Filled new filter with fuel before installing as recommended by numerous people.

Still no change (right back up to 2000 RPM). Any more idease wouldbe greatly appreciated (this is geting frustrating).

 

[Don S]

Re: Mercruiser 350 w/ EFI idles too high

Thanks Don.

Tried the throttle cables (no change). Was told by someone else that it could be the fuel filter / water separator (which should be changed often anyway). Does this make sense?

Derrick

Doesn't make any sense, a clogged filter with either dirt or water will keep the engine from running, not make it run faster.
The first thing you need to do is get a manual from Marine Power so you can troubleshoot the system. Without it, you basically get to throw different parts at it and hope for the best.
Usually there are people that will post here and say "This is what happened to my engine and I did this to fix it" but they probably don't have the same engine as you do, so be carefull, they are not all the same and replacing parts won't help if it's not a faulty part problem.

 

[JustJason]

Re: Mercruiser 350 w/ EFI idles too high

Spray some foaming oil around the peremiter of the intake manifold, the throttle boddy and the throttle plate shaft and see if it gets sucked in. If it does then you found your vacuum leak.

If it doesn't... Your IAC motor probably took a dump.

 

[dlowell]

Re: Mercruiser 350 w/ EFI idles too high

Tried the foming oil, didn't see any obvious suction.

Since I have two engines (one idles fine), I'm thinking of taking the IAC off of the good engin and trying it on the problem engine.

Not sure exactly where the IAC is usually mounted.

I looked them up and found a picture, but not sure where to find it on my engine(s).

Are they simple to remove?

Do they require calibration, seals or any other installation task other than just connect / disconnect?

 

[JustJason]

Re: Mercruiser 350 w/ EFI idles too high

Most of them are plug and play, but I cant speak for marine power.
If you pull the iac off the old motor, make sure you look inside the cavity + blow it out with some brake clean and compressed air. If there is even a little bit of junk in there it will keep the pintle from sealing.
The iac is typically on the throttle plate assembly... but i can't speak for marine power.

 

[Don S]

Re: Mercruiser 350 w/ EFI idles too high

As I said before:

The first thing you need to do is get a manual from Marine Power so you can troubleshoot the system. Without it, you basically get to throw different parts at it and hope for the best.

 

[dlowell]

Re: Mercruiser 350 w/ EFI idles too high

Found the problem.

It was the throttle position sensor.

However, I couldn't tell this until I got some help with a diagnostic computer.

Sensor seemed fine until we checked what it was reading with the computer. At idle it was showing the throttle to be open.

It must have gotten fried on the 150 mile trip.

Replaced it and idle is fine now.

Some tips on the IAC though, these things need to be reset if they have been removed and bumped out of position.

I reset mine with a 30 second idle, then shut down the engine for 10 seconds, then restart.

If you don't do this the engine may still idle fast and you will think there is still a problem (at least I did).

Thanks for all the help.