DIY Engine Work

I'm curious how many GT-40 experts are on this site. There a folks who I know can tear apart a boat, rebuild the floor and engine. There are people that can troubleshoot basic issues and make necessary repairs with confidence. And then there are people who change the oil and drop their boat off at their local Correct Craft dealership for work, like me.

I've studied the PCM manual and read tons of postings on CCF regarding GT-40 engine issues. What I have learned is that symptom could be ten different fuel or electrical parts. Some parts are $1.00 and some can be $500+.

I know the rule of thumb is to not start throwing new parts into a motor, as it can get costly. I've had one minor gremlin with my GT-40 I've been trying to square away for five seasons. My boat has had two tune-ups by Nautique and the flipping issue is still there. Here's my issue and this is what's been done:

After about 20 minutes of use, I put the boat in gear. The RPM's will drop and almost stall, but is never has stalled. Sometime under this condition I gun it (while in gear) and there will be a minor hesitation then it accelerates perfectly fine. This doesn't happen all the time. Only maybe 3-4 times during a two hour period. The engine always idles smoothly and I have no issues with top end, approx 44.5 MPH. Starts right up, no issues.

This is what's been done / replaced by Correct Craft:

Spark plugs
Spark plug wires
Distributer cap
Fuel relays
Fuel filter
Fuel hose in FCC
Throttle position adjustment
Fuel pressure and vacuum checked
Battery & battery connectors replaced

From what I've been reading, I think I may have found the possible cause. I plan on ordering the following parts:

Fuel Pressure Regulator
Throttle Position Sensor

Not sure if the engine is running lean, My old man and I pulled the plugs in the fall. He thought they looked like they were running hot. I took a couple of photos and I'm not sure if anyone can tell by looking at photos. Let me see if I remember how to post! Any advice would be helpful.

Thanks -Ken

I kind of like the IAC idea. Your problem is a little tough to solve because it's kind of minor. Your marina isn't throwing major parts at it but I don't think they're as knowledgeable as the folks here.

I got the stalling vapor lock bugs out with LP fuel pump. Once you get the bugs out you're going to love the reliability. It's an awesome engine with a beautiful torque curve.

The Nautique tech told me this engine doesn't have any way to send and receive trouble codes. Is this true? Can I buy something to hook up and receive codes that may diagnose this issue?

No fuel smell during hesitation. I think it's starving for fuel when it's hesitating. The plugs look a bit like it's running lean.

Thanks for all the ideas and feedback. I'm determined to fix this!

That's pretty cool. I've never used one before. Will it somehow tell me what's happening, whether it's lean, rich, electrical issue, etc? Will it help determine what part I need to replace? I'll definitely order one. Ken

The issue I'm having only happens maybe 3-4 time per Lake outing. It's something to do with idle speed and engaging into gear. Prior to engaging it into gear, the engine is idling a smooth and steady 680 RPM. Load on the engine will drop it to about 400 RPM, at which point it's still running but sounds like it may stall. I'll just click it back to neutral and the RPM's rev right back up like nothing happened. Feels like my old vapor lock days with my 1980 Nautique. Either a sensor is out of whack telling the engine more fuel please or there's something happening with fuel pressure in that moment. I was thinking fuel pressure regulator or throttle position sensor. I have zero issues with the engine starting, idling smoothly or acceleration to higher speeds. I was thinking if it were an injector issue, the problem would be consistent. Bottom line is I think she's starving for fuel under load when this happens. Hope to find that right replacement part that makes her happy again!

Ken

Intermittent Vacuum leak?

Looking at a Ford truck forum with people who are complaining of hesitation on their 5.8 liter. One guy mentions a bad throttle position sensor. Also mentioned an O2 sensor. Do inboards have O2 sensors?

The spark plug in your picture looks like it has very few hours on it. It also looks like you are running lean. The should be just off white to light gray or light tan.   Not brown/dark gray or black.
Lean burn conditions do run hot so your father is on track.
Lean could be vacuum leak.
If this plug only has 5 or less hours on it, you may just be seeing it is still new.
Injected engines are leaner at idle than a carburetor for sure.

Ken, I see in your diary that you brought it to MacCallum's a few times. Mark there is in my experience an extremely good mechanic.

I had the same problem when I first got my boat 6 years ago. I didn't figure it out, nor did Mark. It seemed to resolve itself to some extent as I ran the boat more. It's much more pronounced when I've been running for a long time, and less so during ski sessions when the boat runs for 10 minutes then is shut down for a bit as we switch. At it's worst it would almost stall, recover and surge a bit. It never does it in neutral, never does it unless the engine is very warm. I've found it to be a non-issue the last few years and don't really notice it much.

If your engine was Carburated i would be looking at ethanol problems or water in the fuel. Does Fuel injection suffer from the same stuff? off idle bucking ,hesitations and up and down rpms at an under load condition. quickly clearing up is a sure sign for normally aspirated engines

Ken, Do you have perfect pass?

Kind of reaching but the servo can mess with your throttle a bit. My PP has a few quirks that drive me crazy. Anyways if issue occurs when PP is turned off then it is likely not PP related.

Hey Joe, good point. Thought that was kind of a part that either works or doesn't. Do you think I should try replacing this first or do you think I should order additional parts to replace all at once? On a scale of 1-10, what's the difficulty level replacing this? 10 being the most difficult, 1 being most of you all can replace it in your sleep or after 2 cases of beer. Ken

Got it, thanks Joe. It's on my shopping list along with the testing device. I have heard those suckers can go bad and cause a world of hurt. Just need a little warm weather and we'll be good. Ken