Rough idle/rough running at low rpm

[River City Water Boy]

I have a 1999 Sunsetter Lxi with a fuel injected 325 Monsoon. Just in the last couple of weeks it has developed a little bit of a rough idle. Not terrible but irritating just the same. As I accelerate, it continues to be a little rough up through 2000 rpm and then smooths out after that. I don't think it is necessarily running better beyond 2000 rpm I just think that as the engine speed increases the rough running gets hidden. The rough idle is something I feel more than hear. I can feel it in the seat and see it in the way my rearview mirror vibrates and shakes at idle. Then again, you can hear it a little too. Kind of reminds me of a garbage truck a little bit. The idle speed fluctuates between 650 and 725 rpms when in neutral and 600 to 650 when in gear. I took the boat out this morning by myself and found my top speed to be barely 45mph at WOT. Seems to me the last time I tried that by myself its top speed was closer to 50.

Last year I replaced the plugs, plug wires, rotor cap, dist. cap, ignition coil and both fuel filters. Two days ago I replaced the throttle position sensor which did not help. Last night I found a vacuum hose from the base of the throttle body to the starboard valve cover that had hardened and was loose. I thought for sure this was the problem. I replaced that vacuum line as well as the one other vacuum line I could find. After taking her out this morning there was no improvement.

I am pretty meticulous about maintenence and care of my boat. I always put Stabil in the tank and run the boat on the river for 20 minutes before putting it away for the winter. I always run injector cleaner through the first couple of tanks in the spring. When I changed the fuel filters last year I found no sign of any water, gunk or rust. I guess my point is I do not think the rough idle is related to any sort of fuel filter/fuel contamination problem.

I am wondering if this problem is the result of a faulty sensor (other than the tps of course) of some kind.

Has anyone else out there run into this problem? If so, what is the magical answer? If anyone has a suggestion they feel confident about I would sure appreciate it! Thanks in advance! If there is anyplace I can find someone who has been through this before this has to be it.

[Surfin247]

I have a 1999 Sunsetter Lxi with a fuel injected 325 Monsoon. Just in the last couple of weeks it has developed a little bit of a rough idle. Not terrible but irritating just the same. As I accelerate, it continues to be a little rough up through 2000 rpm and then smooths out after that. I don't think it is necessarily running better beyond 2000 rpm I just think that as the engine speed increases the rough running gets hidden. The rough idle is something I feel more than hear. I can feel it in the seat and see it in the way my rearview mirror vibrates and shakes at idle. Then again, you can hear it a little too. Kind of reminds me of a garbage truck a little bit. The idle speed fluctuates between 650 and 725 rpms when in neutral and 600 to 650 when in gear. I took the boat out this morning by myself and found my top speed to be barely 45mph at WOT. Seems to me the last time I tried that by myself its top speed was closer to 50.

Last year I replaced the plugs, plug wires, rotor cap, dist. cap, ignition coil and both fuel filters. Two days ago I replaced the throttle position sensor which did not help. Last night I found a vacuum hose from the base of the throttle body to the starboard valve cover that had hardened and was loose. I thought for sure this was the problem. I replaced that vacuum line as well as the one other vacuum line I could find. After taking her out this morning there was no improvement.

I am pretty meticulous about maintenence and care of my boat. I always put Stabil in the tank and run the boat on the river for 20 minutes before putting it away for the winter. I always run injector cleaner through the first couple of tanks in the spring. When I changed the fuel filters last year I found no sign of any water, gunk or rust. I guess my point is I do not think the rough idle is related to any sort of fuel filter/fuel contamination problem.

I am wondering if this problem is the result of a faulty sensor (other than the tps of course) of some kind.

Has anyone else out there run into this problem? If so, what is the magical answer? If anyone has a suggestion they feel confident about I would sure appreciate it! Thanks in advance! If there is anyplace I can find someone who has been through this before this has to be it.

I have never worked on your specific motor, but if you have one, check your IAC. It acts like a choke on carb motors. Take off air filter and see if the butterfly is opening completely after your engine is warmed up. Other things mentioned on this sight include the other sensors: O2 sensor if applicable,temp sensor,knock sensor and MAP sensors. These all help the ECM to control your fuel/air mixture. Also, you might want to check for any vacuum leaks around your intake manifold. Edited September 11, 2008 by Surfin247

[River City Water Boy]

Thank you for the info. Pardon my ignorance but what do the letters IAC stand for? Any idea where it is located? I would assume it would be on the throttle body. Is it the the sensor that is located right next to the TPS?

Also, you mention checking to see if the butterfly is opening completely. Wouldn't the butterfly be completely open only under full throttle?

I have ordered a Clymer manual for my boat but it will not arrive until next week. Once I receive it I am sure I will be able to identfy all these sensors and parts. Until then I'm in the dark.

Thanks again.

[VinRLX]

IAC- idle air controller, IIRC