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Surging At Idle On 04 Monsoon 340… Need Help To Diagnose.

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Hi All –

My engine is surging at idle. The motor doesn’t do it all the time but often enough to be annoying. It seems to only do it after the engine has warmed up. The problem occurs most often when the boat sits at idle for 1-2 minutes. If I give the boat just a tiny bit of throttle the problem goes away. The surging is only between about 500-1100 RPM. The problem doesn’t seem apparent under load when the transmission is engaged.

If I disconnect the IAC the problem goes away. Any ideas what the cause could be?

I took the boat to the dealer about 1 or 2 years ago for this problem and they said there was a problem with the Ignition Control Module (which was replaced). However, soon after the part was replaced the motor started doing it again. When the ICM was replaced the dealer said the ICM fighting with the ECM to control the timing and that’s what caused the surging. I’m not sure the problem was ever fixed… Especially since if the IAC is disconnected the problem goes away. If the timing was affecting the surging I would think disconnecting the IAC would do nothing. I probably should have taken it back but the boat didn’t do it all the time so it wasn’t a top priority for me.

Anyway I’m sick of this issue enough now I’m determined to get it fixed. Any suggestions would be appreciated….

Oh yea… If anyone has a scanner I could borrow that would be great. ;-)

Thanks in advance.

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I get the same problem, but it happens most often when coming out of gear. Really gets aggravating when trying to take up slack for a skier or keep their line taut. I was thinking it is the ECMs slow response to MAP changes.

What are you planning to look for with the scanner? What ideas do you have for this problem?

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I'm going to say IAC. I had an idling problem a couple years ago that I couldn't figure out. Pete was nice enough to swing by and fix the problem on his way to TN to pick up some boats (that's what I call service). Anyway, it ended up being the TPS, however my idle problem was a consistent high idle. Since yours is variable, I'd lean toward the IAC. Good luck. Oh and I don't think a typical ODBII-type scanner works on our boats. There is software you need to buy directly from Malibu. IIRC it's a couple thousand dollars to purchase the software and cable.....

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iac controls your idle, not so much your tps, thats more for under load. do as a few others said and pull off your iac and clean it with aerosol throttle body cleaner. they tend to get carboned up and can sometimes stick, or not move smoothly

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I think I’m going to try to clean the IAC first.

Can anyone find any additional info in the Clymer manual about replacing or troubleshooting the IAC? I see there is info on page 410 and 411… Is there any other info anywhere else in the book?

Also I see on step 13 there is a calibration procedure I can run through if the engine is put in to service mode. Does anyone know how to do this? I believe there are 2 pins that need to be shorted on the ALDL. Does anyone know which pins this would be?

As for the TPS I don’t think I could check this without a scanner or a trip to the dealership… correct? This is why a scanner would be nice… just so I can see what is going on with everything in real time. Has anyone ever looked into this mefiscan4 scanner (see link) I wonder if I would miss the tuning functionality. Not bad for $200


For the other poster… The IAC is located on the throttle body.

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It's not uncommon for a TPS glitch to not show up on a scanner or a volt meter. Try cleaning the IAC and also check for vacume leaks and loose air intake hoses.

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iac controls your idle, not so much your tps, thats more for under load. do as a few others said and pull off your iac and clean it with aerosol throttle body cleaner. they tend to get carboned up and can sometimes stick, or not move smoothly

The above statement is not completely accurate. The TPS also can effect the idle, as in my case, however due to the fact that your idle problem is erratic and not consistent, I'm still leaning toward the IAC being the problem.

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Thanks for the advice everyone... I’m going to work on the boat tomorrow and see what I can get figured out.

If I was to look for vacuum leaks where would you suggest?

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  • 4 weeks later...

It turned out to be the IAC for sure!!!

Be sure to run the IAC calibration as detailed in the Clymer service manual (step 13 page 411). The procedure puts the engine in base timing mode and calibrates the distance of the IAC plunger in the throttle body . The surging didn’t go away until the procedure was run even with the new IAC. The boat runs like a champ now. Also be sure to clean out the throttle body really good before installing the new IAC.

I have been running the boat for a few weeks now and the problem is definitely fixed.

Thanks to everyone who provided input on this issue

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