GT40 High Idle

Thanks for adding me to the forum! I’ve been able to do a lot to my boats with the help of reading threads here. However, I have run into an issue I haven’t been able to find a solution to.

I recently purchased a 2000 sport nautique from a dealer outside of my area (this is my second boat with a GT40). On the test drive the engine started well and appeared to be in great shape. After the engine was warm the it would not come down from 1300-1600 rpm. They worked on it for a couple of hours and took a video of it idling properly, They said they made an adjustment and replaced the coil, we purchased it and brought it home.

I was installing a new cap and rotor and spark plugs and noticed the idle air/speed control was unplugged. I plugged it back in and took the boat to the launch to run some fresh gas through it. To my surprise, the high idle issue was back. I unplugged the IAC and it idled around 690-700, but with it plugged in it is between 1300-1500.

The coil does not appear to have been replaced and the adjustment was to the stop screw for the throttle.

I am upset with myself for not catching it before leaving the dealership, but I can’t do anything about that now. I have not had the opportunity to plug it into a code reader, so I’ve replaced a number of parts:

Idle speed/air control
Map sensor
ECT sensor (to computer not gauge)
Vacuum lines
Act sensor
Throttle position sensor
Cap and rotor
Spark plugs
Thermostat
Impeller

I unhooked the battery for an hour in an attempt to re-set the computer. It has a rather new battery and I have cleaned and greased the connections.

Fuel pressure at idle is about 33psi and 41psi with the vacuum line to the regulator pulled off. The engine runs great otherwise.

As far as I can tell the computer is commanding the IAC open for some reason. Could this be an issue with the computer? Thank you for any insight, I am about out of ideas.

I don't have my book with me but one thing to keep in mind is that these are very similar to early 90's 5.0 Mustangs. Here is it's procedure might be worth a try-

From Ford Performance: 1986-1995 Mustang Idle Setting Procedure

Please Note: These steps MUST be performed in sequence!!

Get engine to operating temperature.
Disconnect IAC (Idle Air Control) Solenoid. If engine stalls, turn throttle-stop screw in one full turn and restart. Repeat as necessary until engine will idle with IAC unplugged.
Set idle using throttle stop. Idle speed will vary depending upon modifications made to the engine. Set to where you are comfortable with idle speed.
Set TPS (Throttle Position Sensor) anywhere from .96 to .98 volts. This is done by loosening the mounting screws, and lightly rotating TPS. If you cannot obtain the desired range, you may need to make the mounting holes slightly larger. This can be done by drilling them.
TPS wire colors to be probed are generally BLACK and GREEN wire or the GRAY/RED and GRAY/WHITE, depending on your vehicle.
Stop engine. Disconnect negative terminal battery cable for 15-20 minutes. You may wish to tap the brakes or turn the headlights on and off a few times as this will help to ensure that you have purged any remaining electrical current from the vehicle.
Reconnect IAC Solenoid
Reconnect battery cable to the negative terminal
Restart engine.
If problem persists, inspect for mass air meter problems and possible vacuum leaks.

Thank you both for your help! I will read through the poor-mans guide again and see if anything corresponds with what is going on with mine.

Gary, thank you for the setting-idle procedure, I will give that a try tomorrow and get back to you with the results. I think I remember seeing something similar on one of the Mustang forums but disregarded it after reading something about our tps not being adjustable, but enlarging the holes is a great idea (if out of range).

Something interesting that may or may not be useful with diagnosis is after re-setting the ecu the engine started and idled at around 900 rpm with a slight lope until slowly creeping up to 1300-1500 rpm. This initial idle seemed reasonable for a cold start at 50 degrees.

Thank you again for the help, I’ll be back tomorrow with the results!

Alex

I was able to adjust the throttle and the tps ended up at .98 volts without having to drill the holes larger. I unhooked the battery and waited 40 minutes but after starting it again the problem was the same.

I was able to find a scanner and some codes came up as follows.
12 (RPM not within upper band limit)
13 (RPM not within lower band limit)
21 (ECT out of range)

I have tried two different ECT sensors, but i have not verified with the meter that they are working.

I started test 3E10 (Idle speed control) in the GT40 training manual and have gotten as far as I can without a breakout box. The both of the ISCs I have test good and I am getting just over 12v from the connector.

I took the computer out of the plastic shroud and everything looks good, there is no corrosion on any of the pins.

I have replaced the vacuum line to the MAP sensor, it was in good shape but I already had it out, and have taken the throttle housing to clean it, again it wasn’t too bad.

It idles perfectly with the idle air control unhooked, so that is what is opening up causing the elevated idle. As soon as I plug it in it opens up and doesn’t fluctuate open more or closed more at any RPM. I have a second one I was able to test this with. I can try to upload a video if that helps.

Thanks for all of the help and suggestions!

Splinter96 wrote: I was able to adjust the throttle and the tps ended up at .98 volts without having to drill the holes larger. I unhooked the battery and waited 40 minutes but after starting it again the problem was the same. I was able to find a scanner and some codes came up as follows. 12 (RPM not within upper band limit) 13 (RPM not within lower band limit) 21 (ECT out of range) I have tried two different ECT sensors, but i have not verified with the meter that they are working. I started test 3E10 (Idle speed control) in the GT40 training manual and have gotten as far as I can without a breakout box. The both of the ISCs I have test good and I am getting just over 12v from the connector. I took the computer out of the plastic shroud and everything looks good, there is no corrosion on any of the pins.

Since the only dumb question is the one that didn't get asked, here's a question for you . .

Are you sure you changed the ECT sensor and not the ECT switch that also feeds the computer?

With the ECT out of range, it seems that if it was low like it is during startup and warmup the computer being fed that info would tell the ISC BPV to open to raise idle speed

It sounds like maybe that's what's happening since as soon as you plug it in, the valve moves open some and speed goes up because the computer tells it to open.

I think I have the Ford part numbers -none of them were ever marine rated just motorcraft branded parts. I can get them in a couple of days if you can wait . On second thought the Ford/motorcraft part number may even be on each part already

Gary S wrote: I think I have the Ford part numbers -none of them were ever marine rated just motorcraft branded parts. I can get them in a couple of days if you can wait . On second thought the Ford/motorcraft part number may even be on each part already

Wait 2 days Gary, that's absurd

Here they are courtesy of an old post of yours

Gary S wrote: You should have said something Joe some of those are just Ford parts. NP gets 79 bucks for the water temp sensor,nos oem on ebay is 18 shipped Ford part # F2AZ-12A648-A. Air charge sensor is F2DZ-12A697-A. Ebay nos oem 18.50 shipped. MAP sensor E7DZ-9F479-A, NP is 259.00, ebay has an nos oem one for 80.00

   

Splinter96 wrote: The sensor I replaced is located behind the alternator and pretty much below the MAP sensor, the end is cone shaped and screws into the intake manifold.

Sounds like the right sensor to me   

Make sure it was the ECT not the air charge temp sensor.

After a lot of troubleshooting and replacing parts, I am happy to report I have fixed the issue. I replaced the ECU and the engine works perfectly. I purchased a used one on eBay, it’s a Mustang ECU with the aftermarket chip, I figured I couldn’t go wrong for $250. I don’t know what tune is on the chip so I may have it re-burned at some point, but it seems to work perfectly right now.

I also found four cylinders were running lean based off of the color of the spark plugs, so I replaced the injectors with rebuilt units, these also work great. They are for a 460 Ford.

Would this be a good fix to add to the poor mans diagnosis thread?

Thank you again for all of your help with this, it really helped with the troubleshooting process.