Re: 30 HP Mercury EFI
I have seen a bunch of these. The plastic intake manifold/air plenum warps and develops a small air leak. The IAC valve which normally governs air flow for idle, and hard deacceleration winds up having a stroke trying to adjust air flow while the leaky plenum is letting all kinds of extra air into the engine. Result--- the engine is slow to deaccelerate. Since you are way out of warranty, the first thing to try, is to get the throttle plate to close completely at idle. There is a non-adjustable stop for the throttle plate that you can take a flat file to. If you unbolt the ECM from the front of the engine, you should be able to access it. Try filing some material off of the stainless stop. This will futher limit air flow at low speeds, and this should help the situation. If the running condition is now bearable, run with it. Second option is to file a complaint with your local dealer. I have made calls for customers that were out of warranty in the past, and had great luck with getting Merc to send out a new intake assembly. Usually the parts were covered, but the customer is responsible for the labor. New intake is updated with an adjustable throttle plate stop. Where you are 5 years out of standard warranty, I don't know if you will have any luck going this route. Worth a shot, and if you really feel motivated, you could buy the intake to solve the problem. I believe it retails out for around 4 or 5 hundred. Also, you didn't mention it, but typically the other symptom that goes along with this issue, is an idle that's about 4 or 5 hundred RPM to high. Not good for the gearcase when shifting into gear at these speeds. Most of the engines that I found with this problem, were engines that had high idles, and the customer wasn't even aware of it, as most boats that take a thirty horse, were not rigged with a tachometer. Check yours. Hope this helps you.
