Engine surging

According to the GT 40 trouble shooting chart it could be the IAC- idle air control. I'd google that and see if it can be cleaned or needs replacing.

Thanks for the replies, I haven't had 5 minutes till now to sit and read some posts let alone respond.   I've been going through the boat a little every night since I got it home, first just cleaning and such.   I knew there were several little fix-its to do, but after a water line burst in the middle of the lake last weekend almost swamping the boat, I figured more attention to all the mechanics was necessary. It seems the bilge pump itself was working but the discharge line was clogged with a ton of grassy mulch looking junk.

So, getting it back home, one thing led into another and before I knew it the entire dash was apart and the interior back out so I could run all new wiring. Wow, what a mess - looked like a grade school kid had tried to wire patch about every system on the boat. Only 3 of the reset/ power switches worked at all and I lost count of how many bare wire, hand twisted connections with not so much as some tape over them I ran across. Getting it all rewired is going to take a bit of time so I'm not sure when I'll be able to check on the IAC fix to see if it worked or if I'll need a new one. Shooting for this weekend, but with the wiring, new bilge, new blower and cleaning and detailing the engine, it's taking a bit longer than I'd hoped.

Duane,
How many other boat manufacturer dash's have you been under? In CC's defense, they were all that way !
BTW, the scotchlock isn't original!

Finished all the dash rewiring, though the outside isn't finished yet. I had bought some carbon fiber panels for the dash of my last boat, had quite a bit left over and thought I'd use it to remake the switch panels. Searched the house over and can't find them anywhere so I just re-used the old panel till I can get more ordered.
I've never liked the switch/ reset set-up and these were a mess, only 3 of the bunch even worked. I prefer the Carling type switches anyway, so I've basically gutted the wiring nest in the dash and started from scratch. The boat didn't come with a stereo, guess it had fried years ago and no one ever replaced it so I decided to utilize this area for the new switches and just start with a blank plate, except for the key switch, and cut in my stereo on that side when I decide which brand/ model I'm going with. It's going to be a Bluetooth type with no CD which come in all sorts of configurations, all of which will easily fit on that side.

Along with the new wiring and switches I've added a second battery, battery switch and a good ol' fashioned blade fuse panel - I've never had a problem with them, but I have heard of a few folks having issues with the breaker types. Once they start going bad, they aren't just .49 cents to replace. Anyway, the dash went live late tonight and most all worked! Wouldn't you know, I decided to utilize some of the existing wiring within the looms since I couldn't cut it all out - gauge signal wires, etc. still have to get there and unfortunately they seemed to be bundled intermittently with some of the switch wiring. Of course, this was a mistake it seems, using these wires, as all of my own wiring works flawlessly! Too late in the evening to chase down the problem so its going to have to wait till tomorrow. I'll post some pics when I get it all back together, hopefully by the weekend!

In the service manual in the reference section, page 2A-23 second paragraph states that if the map sensor/circuit fails the idle RPM will oscillate.
I would maybe start there then if that checks out I would look at the idle speed control bypass air solenoid (ISC-BPA) that's what the computer uses to allow airflow to bypass the throttle plate to allow a constant idle.
Read the "theory of operation" it gives a good incite on how the GT-40 works

So still had issues after cleaning the IAC, though it did seem to help a bit. Then I decided to change oil and spark plugs and found that 6 of the 8 were just slightly more than hand tight. Of the remaining 2, one was very snug, almost too snug, and the other was loose enough I took it out without a wrench.   
Hmmm....   
Changed the oil, installed new plugs and gave it a try. Imagine that ... no more surging. Well, at least on the trailer. We'll see what it does when I get it to the lake next time.
Might be noteworthy also that the plugs that had been installed when I bought the boat were Autolite AP104, which after some research weren't on the approved list of replacements for the original Motorcraft plugs, but that part # was found to be one heat range too hot !   Well, hopefully there wasn't any damage since according to the previous owner they had only been in there for a few hours of light duty operation.

I just don't understand some folks. As I read in a post earlier today, we are living in the 21st century - a smart phone google search takes just a few minutes!

It could be too that some more research would tell you a little more info

It seems that the Motorcraft plug would be a AWSF-22 or 22c, but most people on CCF use an Autolite 764 or AP 764.

Those are one heat range hotter than the original Motorcraft and they work just fine.

Autolite 763 or AP763 are one heat range colder if you want a colder plug like the Motorcraft AWSF-22 or 22c

The interesting part is that the AP-104 plugs that you took out are the fully threaded version of the AP-764 plugs. Everything else about them is the same......and they work just fine too.

Here's a link to an old thread with some info on the 104/764

link

If the plugs solve your problem, it's not because the AP-104's were causing it, except maybe because they were loose and now you have new plugs that are tight like they should be.

I've used the 104's for quite a while now with no issues in a GT-40P headed engine.

So.....what did you put in it for plugs?

KENO wrote: The interesting part is that the AP-104 plugs that you took out are the fully threaded version of the AP-764 plugs. Everything else about them is the same......and they work just fine too.

Thanks for the info, it's good to know there won't be any hidden damage from an incorrect plug!   I did do a fair amount of research though, even on the Autolite website and couldn't find the info you provided (also searched this site ... guess I wasn't typing in the right question).   That is why I like forums like this. There's always someone out there that has a lot more information and experience than yourself and you can nearly always get an answer or a solution to your problem.

By the way the 764 is a 'suppressor" plug and the 104 is a "resistor" plug

Maybe somebody will come up with an easy to understand explanation of what the real difference is other than spelling and then they can explain why they're interchangeable.

All I know is that they're interchangeable.

I found that info too, seems a little light on facts from Autolite tech.

Also found the info on suppressor plugs filtering out AC better.

My 104's run just fine with electronic ignition. Never could tell the difference between 104 and 764 in how they ran.

Edit and my truck radio works just fine with resistor plugs

Ha ha ...   104, 764 -- suppressor,   resistor.    This is turning out to be one of those Chevy or Ford things. Or more recently (also evidently instigated by me) Acme 422 or Acme 224 LOL

UPDATE: ( although it's very late )

Addressing the original topic that I'd started here, I did replace the IAC with a Ford Motorcraft version a couple weeks ago. So much was going on and I had a couple other threads going as well that I had forgot to update on this one.   The new module worked like a champ and the surging is completely gone. Thanks again to all for their input.

FYI - Though the factory PCM IAC was somewhat rusty, and paint covering the rust I was able to raise the original stamped part # for a cross reference. The Motorcraft part I replaced it with is not merely a compatible module, it is the exact same part number, just purchased from an online auto parts vendor.