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5.7L MIE w/TKS changing idle problem.

sandkicker

sandkicker

Outstanding Contributor
This problem plagued me last season on and off. I'd go out for a run after a normal TKS start up and be returning to my slip. I'd be idling thru the no wake into the marina proper and just about the time I want to dock (stern to) the idle would pick up to 1100 RPM (from 650). Since I run a 15x21 prop on a 2 ton boat, this is not funny. Several other times, while in long no wake areas, I noticed that the engine would go from idle up to 1100 and then return to idle all on its own. This year I intentionally took it out, ran it at cruise and then did an extended run at idle. What I found out and finally noticed is that when the idle picked up I could hear the TKS hiss! Then the his stopped and the idle dropped back to normal. The engine , if run long enough, would "hunt" on and off the TKS idle. The engine is FWC with a 165 degree T'stat and when this hunting occurs, the engine is running near the T'stat set point. At cruise it runs at 180+ degrees. Since the engine will, when started at the slip, start at the higher RPMS and then drop to 650, I don't think its a T'stat problem. Is the sensor for the TKS so critical? Is this sensor an on/off or an analog sensor? Is this sensor available in different temps? Since the engine is a factory MIE that's raw water cooled with amechanic (it's a re-engine) installed FWC, was the TKS sensor changed for FWC?
 
Remove the spark arrestor and pull the muffler/filter out of the throttle body where the IAC motor pulls its air from. If this is clogged up it will make the IAC work at 100% over heat and fail. Chances are you need to replace the IAC.
 
mmmmm..... this is a 2bb old school carb, not throttle body. Uses a "enrichment module" P/N 88148 which is just a solenoid which opens another internal passage all the way or closes it (normal idle). Not an IAC motor which controls the air flow around (bypassing) the throttle plate. In any event, no muffler/filter in the carb at that point, hence the trademark "hiss" of TKS carb'd engines at idle. I'm thinking whatever t'switch keeps the solenoid closed when the engine is warmed up is flooey.... now all I need is to find it and check it out. Anyone know where that thingy is... and is it a switch or an analog sensor?
 
Sorry Bob I miss read your post. Thought I read EFI. There is a thermal switch that goes open when the engine heats up above 110 degrees taking power away from the enrichment module.

Did you try disconnecting the enrichment module after the engine was warmed up to see if the surging stopped?
 
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No... Last few times out had company on board. Will take it out by myself this week and do some trouble shooting. It's really annoying. One time it kicked up just as I was backing into the slip and was just coming up on the outer pilings. :-( I really hate looking like a "benny with a boat" while docking.
 
I had a similar problem that maybe is related on a 5.7l 2bl carb when the carb was removed and rebuilt the mechanic just tightened the carb bolts down, what happened is this put uneven pressure on the throttle plates all I did was loosen carb and tightened them back down in sequence and needed to loosen the throttle plate scews to let them seat when engine was warm. It did the same thing idled fine when cold after up to temp the expansion in materials wouldn't always let them come back idle. Good luck.
 
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