Surging idle on GT40, especially when cold

[Fast351]

My 2001 Ski Nautique has this annoying throttle surge when it's idling, and it seems to be more pronounced when the engine is cold. Last year I thought it was just old ****ty gas, but it's still there this year and the fuel is new. It seems to idle down OK, but it'll surge from 650 RPM baseline to 1000+ RPM, which makes docking and slow RPM maneuvering a challenge.

I know back in my days of playing with Mustangs and EEC-IV, this was usually an indicator that the IAB was plugged (idle air bypass) or the throttle position sensor was out of adjustment. But seeing as this is a boat, I don't know how applicable this information is.

Just looking for some quick things to check...

[Blamey]

This is a common issue with several possible causes. As you guessed most common is the IACV/IAB. Search for a 'Motorcraft CX1826' as a replacement, I got one from Amazon for $50. Unfortunately it didn't solve my issue.

Other possible causes are bad ECM or Fuel relay or a bad map sensor.


Search from the 'Poor Man's GT40 Diagnosis' over at CorrectCraftFan, its a great post linking to common GT40 problems and solutions.

[vortech347]

It may not be the same scenario but here is what I had happen with my 99 Sport with GT-40. We bought it new in 99 and had the surging issue from day one.

The dealer looked at it 3 or 4 times during the first 3 years and never could fix it. I dealt with it for 11 years and many times of worrying if it would die just as I approached a dock.

I finally, got so frustrated that I contacted PCM asking them for help as I had given up on my dealer years earlier. PCM contacted the warranty rep at my dealer and they agreed to look at the boat well out of warranty.

So I took the boat to the dealer and a technician tried to duplicate the issue in their test tank. Of course, like before they could not duplicate it. The problem was that the surging would typically not start for almost an hour.

The dealer assigned a senior technician and met me at the lake to test it. We drove the boat around the lake duplicated what we did when out on weekends. Sure enough, about 50 minutes in the boat started surging and dying. The technician had already tried several things but nothing made it better, only worse.

After the lake trip he took it back to the dealer and wanted to do some more testing.

In the end he found the issue by tapping on the ecm with a wrench. He said it would start surging, he would tap the ecm and it would stop. Then tap the ecm again and it would start surging.

A new ECM fixed the problem permanently. PCM and my dealer charged me 50% of the labor and 50% for the part which saved me about $700 on the repair.

Since then (6 years) it has not surged a single time when we go out.

So keep that in mind if changing the IAC, checking the fuel pressure and changing the TPS do not make any difference. A new ECM is an expensive part so I would make it my last option if I were you. But that's what fixed mine.

FYI, even though my dealer had a pretty poor reputation for service at the time we bought our boat, they have turned their service department around and are top notch now. So if you live in Central Texas you will know who that is and trust I'm not bashing them. They stepped up in the end when I needed it.

Good luck.