97 nautique gt40 hard starts when hot, not revving out.

[Notique]

Hi people I'm new here and new to my boats.
I have searched hi and low on the discussions and cannot find my problem.
I have a 1997 sport nautique with gt40 351w and since I've bought it it has given me these problems.
number 1 is it starts fine cold but does have a rough idle seems to surge a little until it pans out with a slight rough idle then when ive run it for a while and it it at operating temp it is hard to start and once it does start it surges badly unless I give it some throttle and then it goes away and will run fine.
number 2 is it will rev out to 4800rpm some days then others 4500rpm and the other day it would only rev to 4000rpm seems to have the power each day but not the revs each day.
I have run the scan tool with engine off and it passes but then when I run the scan with engine on it will initially rev up to 1200rpm for a while then dies and stays at code 12 which means idle out of range high. I'm not exactly sure but I'm thinking it could be idle air control, map sensor or throttle position sensor?
any help would be greatly appreciated. Thanks in advance

[Peter6000]

Do you have the pro boss manual? If not let me know I send you a link.
There are only a few sensors on the engine I would check them all preferable from the ECU connector,
all you need is a multi-meter only the MAP uses frequency better multi-meters can measure frequency.
Check for a vacuum leak.
Check all your grounds,
Check fuel pressure
The idle control valve is not easy to check but never had on causing this kind of issue you can bring the engine up to 1200 rpm in neutral unplug the idle air control valve and drop the rpm to 800 if stops surging and keeps the rpm somewhat stable your valve is bad.
If none of these guide you to the problem it could be a Ignition problem if you have access to a scope its easy to check otherwise
I would just replace the parts one by in this order TFI, Ign. coil, distributer cap and wires.
I have used automotive parts with no problems they are way cheaper.

I had the same problem (surging idle and bad hot start) with my engine after rebuilding it. It was drilled to the biggest size and the bigger displacement required more fuel at idle.
I found this info and a lot of help in some Ford Mustang Forums.
I fixed this by using adjustable fuel pressure regulator and added some pressure. But the higher fuel pressure made the hot start even worth so I added a switch to turn off the fuel pump during hot start. All very unsatisfactory so about 3 years ago I switched out the vintage ford ECU with a aftermarket Megasquirt MS3 a lot of work but incredible results.

Peter

[Notique]

Hi peter
thanks for your reply. Yes It is the pro boss engine and yes I have the manual I think tho it might be missing some parts as it's been printed out and put in a folder.
I will check with multimeter but am not really familiar with using them yet. My engine was rebuilt 200 hours ago before I bought the boat so the problem could have been there the whole time.

[Peter6000]

Hi peter
thanks for your reply. Yes It is the pro boss engine and yes I have the manual I think tho it might be missing some parts as it's been printed out and put in a folder.
I will check with multimeter but am not really familiar with using them yet. My engine was rebuilt 200 hours ago before I bought the boat so the problem could have been there the whole time.

Do you have the documents from the rebuild? Do you know the piston size they put in?
If not you can use a borehole camera and take a look through a spark plug hole the overzize is engraved on top of the piston. At least it is on mine.
It's a while ago I dig out my documents and the manual and post it here.
My manual is a scanned PDF I believe it has all pages.
A breakout box like the one in the pic is a very helpful tool it goes between ecu and engine and makes it very easy to measure all voltages while running the enhine.
Check ebay under ford I thing I paid $50 or so.
To bad you so fare away I have all the old parts and tools.



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[Peter6000]

Pro_BOss_GT-40_Service_Manual.pdf

here is the manual I have

Peter

[Notique]

Hi mate dont have rebuild details as it was the owner before the bloke I bought it. I'm starting to think it probably hasnt been done as alot of people claim rebuilds which have never happened. I trusted the bloke as he was not a person who was short of money and seemed genuine.
also thanks for your help It is hard being in australia tho as parts a few and far between for these boats.
I'll try and post a video of what's happening when warm.

[Notique]

I'm having a hard time uploading video from my phone so I will transfer to my computer and upload