Does the computer have a circuit that maintains a high idle until oil temp cools down? After a run, motors don't slow down to normal idle When I pull the throttles back, they tend to slow down to about 1500 rpm and then slowly come down to idle. Linkage closed so not a sticky linkage issue.

Also same thing happens when I start them back up when hot. If I shut them down for a few minutes after running, then re-start, they will idle high (1500) and slowly drop to idle in about 10-15 seconds.


Why don't they come right down to normal idle??

I'm with Sutphen30. Could be a number of things, TPS (throttle position sensor comes to mind but the scan tool is the way to approach this or you'll just be hanging a bunch of expensive parts

Scan tool shows no problem. Also a Speedwake member has same thing going on.

As throttle position is increaed the IAC's opening amount is increased. When you shut the throttle quickly the IAC slowly starts shuting to get the motor to the correct idle RPM the software says the motor should idle at. Also before you start, the IAC opens up to a preset amount so you do not have to touch the throttle to start motor. Once motor starts the IAC starts closing to get motor to correct idle RPM.

Now with that said, is this a new problem or has it always been that way? If new, then IAC may be going bad or gumed up & not moving as fast as it should. If since new than settings merc has is not letting the IAC open & close fast enought.

mine does it too,, so does my neighbors i think normal. also when u shift u have to wait a little before idle comes down to shift again around the docks,
yours too??

Idle speed control...every set of 575's I've had do that.

The IAC is the cause but replacing it might not help. Sometimes if battery/alt voltage is a little high (say 15 to 16) the IAC will stay a little deep. Check your voltage when running hard. If it is a little high it can cause the IAC and all of your sensors to act a little weird. If it has been a while since you have done maintenance then remove the IAC and give it a good cleaning. Clean the bore as well. Me? I would remove and refresh the throttlebody. NEVER NEVER attempt to clean the throttlebody with spray while the IAC is installed. Loose deposits could jam it up causing a hard to troubleshoot idle problem.

Roby

The only part that a TPS would play here is that if it were giving too high of a signal at idle the IAC would think you were still under throttle. In this case a code would present itself.

Some people (you got to feel sorry for them) think it so important to get over 5.4 volts out of the TPS. Their reasons are they believe the injector PW will not max out and realize the full power potential. Therefore I have seen them stick small objects between the throttle button and the TPS arm to get that extra half a volt. I know this is not the case here but it was funny and I wanted to share it.

Roby

Hey Roby,I thought the "high tech" way of playing with injector PW was with a lap top ?? You make good points.--Lou

Check to see how many counts the IAC valve is at during a warm idle. Merc says 0 to 40 with 40 being about the middle of the valves range. I have seen engines that do not return to idle very well if they are closer to 0. Unfortunately, the only way to change this relationship is to "tamper" with the factory throttle plate stop.