Monsoon running rough

I've already posted this on the Malibu crew website and I got some great suggestions. I thought I might get some fresh ideas here. Boat is a '99 VLX with the Indmar Monsoon with around 500 hours. I have had it for a few months and it started running rough not too long after I got it. The boat will start up and run fine initially. You can take it up to around 3000-3500 and run with no problems. When you bring it back down to idle after the initial run, it starts idling rough, backfires if you give it gas and eventually will quit. If you start it back up, it runs rough and quits. If you disconnect the battery and then reconnect it (essentially resetting the ECM), it will start up and run smooth again for the first run, but after you bring it back to idle, starts running rough again. I have it at the mechanics now. He has replaced cap,rotors,plugs, and fuel filters. Ran a diagnostic check, the engine had no codes set. He replaced the Throttle Position Sensor and the Idle Air Control and checked the timing and neither of these fixed it. The mechanic is stumped. Anybody got any other ideas?

Anybody? I know these engines are common. Seems like on the Malibu site there were a lot of people having similar problems with the Monsoon, but with numerous different causes. I am going to talk with my mechanic in the morning, so I was hoping for suggestions.

My 96 Monsoon just started to run a little rough around idle when I got it out of storage this year. I've only had it out once, and have been working on a bunch of little odds and ends on it at the house. I have new plugs and some Seafoam for the motor, and I'm going to clean the IAC. I'll have it out this weekend, and you can bet I'll be checking back in if it doesn't clear up. This is the first hiccup of any sort that I've had with this motor in ~650 hrs.

I tried some Seafoam too, I know the boat has some old gas in the tank. The funny thing is, every time you reset the computer it runs like a dream until you come back down to idle the first time, and then it does the same thing consistently, runs rough, backfires and eventually quits. It doesn't seem like bad gas to me as it would probably run rough intermittently or right after you crank it up. I will keep you posted on my progress, be it ever so slow. Thank goodness for friends who are on the water and always have a seat for me in their boat.

Based on the resetting behavior... I am not sure if Monsoons use or even have a crank or cam position sensor, but that would be worth checking too. It could be the ignition module too, but that is usually heat-related.

hey trace, where is the IAC on our boats ? I will occasionally get a slight stall when bringing it back down to idle and figure something like that might be the cause.

Crank sensor was checked and did not throw a code, so we are assuming it is functioning properly. If you unplug it, it throws a code. The IAC is mounted on the throttle body, correct?

Yes, but I don't remember exactly what it looks like. It should be a roundish black cap with an electrical plug and a couple screws holding it into the side of the throttle body. I'm pulling mine out tonight or tomorrow night, and will post again.

That is essentially what it looks like, I looked at the old one at the mechanic's place today, I didn't look to see where it was located on the throttle body though.

That era Malibu had vapor locking issues.

I don't think it is vapor lock, isn't that usually heat induced? This problem can be remedied by diconnecting the battery for 1-2 minutes and then hooking it back up. Then it will run smooth.

Have you toyed with replacing the ecm out of another monsoon to see if it cleared the problems?

I don't have access to another one and ECM's aren't cheap. I did talk to somebody who does ECM tuning and flashing and he said he has seen ECM's "lose their programs" before. This mechanic is getting until the end of the week, then it is going to the Malibu dealer. I took it to this guy first because I have known him longer and he has always provided good customer service and he was much closer.

Yes, vapor lock is from heat. I've also never heard of vapor lock problems on Indmars, only Mercs.