Is there a way to check the TPS voltage on the bench. I have a variable power supply that i can supply 12 v in, and check the voltage out. Is this a plausible test?

I just got off the phone with the mechanic i have been working with. He thinks the MAP sensor is the culprit. He hasn't seen intermittant tps sensors, and it sounds like the IAC is doing what it's supposed to. He told me to remove the MAP and make sure everything looks good there, then reinstall it. Other than that, it's hook it to a scanner and check some values when it's idling at 1000RPM's. Any thoughts?

Well, I had decided to take this in and get it scanned by a local shop. I showed them everything it was doing. They ran it on the hose, got it to temperature, and verified the high idle.

I was told they put it into service mode and ran some test on the IAC that should have lowered the idle. It didn't. He said they tested the harness that goes to the IAC, and it was good?? I guess they also called Merc, and they said change the IAC. So that's what i just did tonight, put in a new IAC, and it has the same failure. Guess it will be going back. Sounds to me this Merc scanner is pretty much a go or no go thing. Don't know if they will be able to pinpoint the problem or not with it? Does this scanner give voltages for the TPS and such? With the IAC out of the picture, guess it could be the TPS, MAP, MAT, or the CPU itself..........

I was doing some reading on some old posts about adjusting the TPS voltage and IAC sensor..... It said something about changing the secondary plate position to give a certain amount of counts???? From what i've learned it doesn't seem there is any sort of IAC sensor, it is just a stepper motor that changes idle air intake according to what the computer tells it. Is this not correct?

Just to throw something out here..... broken intake valve spring or intake valve sticking in the guide. Could still be sticking secondary butterfly but that would be too easy :P

I wonder if it is a vacuum leak?

its efi!

as a test,,the tps is non adjustable..you can open up the mounting holes of slot the holes so you can turn it slightly and see if the idle comes down..Take it back to the people who put the iac into it and have them scan it and see where the tps % is,,it should be a 0% at idle..If its even 1% it will raise your idle..somtimes you can just loosen the tps mounting screws and it will let you slightly turn it and in some cases it will let the idle come down..

i think you said you had the intake off? another thing to check is the injector orings if you had them out and didnt replace them they will also leak causing a vacume leak..just the lower orings on the injectors..if you didnt skim coat them with greece whyle installing the injectors you could have tore one of the orings..

I checked the base timing this morning by shorting pins A and B in the diagnostic plug. Had the engine warm, and kept the rpm at about 1700. From what i could tell, the timing was about 14 degrees. Now assuming the distributor didn't move since it was set at the shop 3 weeks ago at 8 degrees, what sensor would change the timing to 14 even with those pins jumpered? If i brought the timing down to 8 degrees, the idle would come down, but still seemed about 50 rpm higher, but also a big rougher than usual. Take the timing back to where it was, the idle goes up, but is smooth.

I ended up pulling the boat back up to the folks that have been working on this project for the last two years. Figured they were the best to know what is going on with it.

I have no idea why the "book" would tell you to rev above 1600rpm. All I can tell you is the module when in "set timing mode" raises the timing by 10 degrees above 1600 rpm, from my memory. Raise rpm to 1200 and set timing at 8 btdc if this is a merc cal.

I could have been wrong about checking it at 1600-1800. I thought i read from a previous thread to check it there. Either way, i moved it so it was at 8 at that rpm, it would bring my high idle down, but then it wasn't smooth anymore. It's at the shop right now getting diagnosed, i believe i have a sensor or something that is not allowing my idle to come down/and or keep the timing high at idle................

I don't know if this has been discussed yet, I don't have time for the whole thread.

Sounds like Idle control valve is stuck open. Beat the throttle body with a rubber mallet, and if that doesn't work, pull the flame arrestor and jiggle it with a screw driver etc before you start screwing with anything els. I had this problem and it was literally that easy to fix.

If I have missed the mark, then I appologize for wasting everyones time.