98 Ski Nautique GT-40 feels like limp mode, need help troubleshooting

[JTFrost]

Hi Everyone,
Glad to be part of the Nautique community. My overall issue that I need help with troubleshooting is with my 98 Ski Nautique(with a GT-40 and EFI) which feels like it is stuck in "limp mode". The max forward speed I can achieve is 8 mph at 1500 rpm's. I purchased the boat in early July this Summer, was not able to test drive the boat, but did test the motor with a fake a lake before purchasing and my mechanic friend looked through the entire boat. Everything looked good and the motor sounded great. I can achieve full rpm's in neutral and in reverse gear, just not forward throttle when the boat is in the water. Also, the past owner has been very involved trying to help solve the issue, he said he had the boat out on the Delta 5 days before I bought it and everything worked perfectly.

After lots of research, talking with multiple mechanics on the phone and in person, below is what we have done to try and fix problem.
We have replaced the following sensors: Throttle position sensor, heat temp sensor, knock sensor, oil pressure switch, oil pressure sending unit, and checked the transmission fluid to make sure it was full.

A few mechanics mentioned it could be the ECM is bad or the forward gear in the transmission is bad. We have been testing the transmission and we have not felt it slip once.

The previous owner said he replaced the oil pump, starter, impeller, battery, and fuel filter before before I purchased the boat.

Our next step for troubleshooting is to take the boat to the lake with the previous owner and a Rotunda Star Tester to see if the ECM is faulty. Back up plan to this is we have an appointment with Inland Marine Marine Sales & Services on Sept 7th to look at the boat.

Please let me know if you have any recommendations to help troubleshoot and fix my boat?

Much appreciated

[NautiqueJeff]

Very simple. Only two things can activate the check engine light / Limp Mode in a GT-40. One is an overheat condition being sensed, and one is a low oil pressure condition being sensed. If you really are experiencing either of those conditions, you need to figure out why. Sometimes the switches that control the limp mode fail though, and activate limp mode even though there is no real issue. This would cause your gauges to read normal readings, but the switches for the SLOW system are signaling a problem, even though there is none. These switches are SEPARATE from the sensors for the gauges.

My bet is that the oil pressure switch has failed. You can disconnect the wire from that switch, and that should allow you to run the boat normally until you can get a new switch. It may also be the water temperature switch, and same deal with that, just disconnect the wire until you can get a new one and the boat will run fine.

Here's the oil pressure switch:
https://www.nautiqueparts.com/produc...nd-gm-to-2006/

Here's the water temperature switch:
https://www.nautiqueparts.com/produc...-mpi-r020014a/

We're talking about the SLOW system here. See diagram below.

These are the ONLY things that can trigger limp mode. Nothing else can do it.

[JTFrost]

Hi Jeff,
Thank you for your prompt reply. We just replaced the oil pressure switch and the temp sensor. The max rpm I can achieve is 1500, so do you think its something else?

[mpost]

Does your check engine light CEL in the center of the dash come on when you turn the key to the on position? It should and then go out when the engine starts and gets oil pressure. If it is doing this then I suspect the oil switch is OK. I do not see the water temp switch in your parts replaced. This can also cause the Limp mode but the CEL should come on, and as your question above implies that is 2700 RPM not 1500.
Have you verified the plug wires are correct for your firing order. When I bought my 98 GT40 I did a compression check on it and brought it home. First time I took it to the lake I had now power. turns out I had mixed up some of the wires when replacing them. Sounded good in the driveway but no go on the lake under load.
Also what about fuel pressure?

[Peter6000]

Hi Jeff,
Thank you for your prompt reply. We just replaced the oil pressure switch and the temp sensor. The max rpm I can achieve is 1500, so do you think its something else?

Does the red check engine light go on when you turn on ignition and does it go off when the engine is running?
If so you not in limp mode.
If it does disconnect oil and overhead switch.
If the really replaplaced the oil pump the engine had to come out take a good look at the wiring.
Otherwise start with the basics
-check fuel pressure
-check ignition timing
-check ignition power
Limp mode should raise the idle rpm to 800 and limit rpm to 2500.
Also if you have perfect pass make sure it's not stock check if the throttle valve opens all the way.
Is the rpm also low if you go full throttle in neutral?

Sent from my SAMSUNG-SM-G870A using Tapatalk

[JTFrost]

Hi Peter6000 & Mpost,
The CEL does light up when turning on the motor, but turns off once the motor is running. We did replace the water temp switch, not the sensor after looking at the link from Nautiquejeff. Also excuse first post, the previous owner replaced the fuel pump and fuel filter, not the oil pump. Regarding fuel pressure we need to buy the parts to check the fuel pressure. Our current idle is 600 rpm.

Peter6000, how do you check the ignition power? Also in neutral we can achieve above 3000 rpm. The perfect pass is working properly after multiple tests.

Questions:
How can we test if the ECM could possibly be faulty?
How can we test if the transmission forward gear is faulty?

Any other recommendations for troubleshooting would be greatly appreciated?

[Peter6000]

Hi Peter6000 & Mpost,
The CEL does light up when turning on the motor, but turns off once the motor is running. We did replace the water temp switch, not the sensor after looking at the link from Nautiquejeff. Also excuse first post, the previous owner replaced the fuel pump and fuel filter, not the oil pump. Regarding fuel pressure we need to buy the parts to check the fuel pressure. Our current idle is 600 rpm.

Peter6000, how do you check the ignition power? Also in neutral we can achieve above 3000 rpm. The perfect pass is working properly after multiple tests.

Questions:
How can we test if the ECM could possibly be faulty?
How can we test if the transmission forward gear is faulty?

Any other recommendations for troubleshooting would be greatly appreciated?

Ignition tester:
https://shop.advanceautoparts.com/p/...SABEgLm0_D_BwE

For timing any light will do should be -10 with spout connector pulled go to 36 or so on high rpm .

For fuel pressure I use a cheap harbour freight tester.

I don't think the transmission could put that much load on a engine if so it should get extremely hot.

If the cel goes of you not in limp.

How does the exhaust look like any smoke or smell?

I can bench test your ecu if you can ship it to VA. But I would do that as last resort.

Check your spark plugs are the wet or dry post pictures if you not sure.

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[James]

Just a thought since you said you can get full RPM’s in neutral and in reverse. Have you checked the throttle lever in the forward position to be sure it is actually opening in throttle body from idle all the way to WOT.

[JTFrost]

Hi Peter6000 & James,

Peter6000,
We checked the timing and its good.
The temp on the transmission was not hot.
Exhaust, no smell thats funky, sometimes under load we could see some small amount of exhaust. But looked normal like a car in idle.
ECU, how can we bench test the ECU? I appreciate the offer btw, but we do have the tools to test. Please let me know.
Spark plugs, we will check this weekend and follow up. Btw, what does it mean if the spark plugs are wet?

James,
We checked the throttle at forward full position and looked at the throttle body and we have full open.

Let us know if you have any other recommendations?

[JTFrost]

Ignition tester:
https://shop.advanceautoparts.com/p/...SABEgLm0_D_BwE

For timing any light will do should be -10 with spout connector pulled go to 36 or so on high rpm .

For fuel pressure I use a cheap harbour freight tester.

I don't think the transmission could put that much load on a engine if so it should get extremely hot.

If the cel goes of you not in limp.

How does the exhaust look like any smoke or smell?

I can bench test your ecu if you can ship it to VA. But I would do that as last resort.

Check your spark plugs are the wet or dry post pictures if you not sure.

Sent from my SAMSUNG-SM-G870A using Tapatalk

We just checked the fuel pressure and we were getting a consistent 39 psi.

Do you guys have any other idea's other than taking the boat to a mechanic?

Were taking the boat to the Delta this weekend with the previous owner and his mechanic friend, so hopefully we can find out whats going on this weekend.

Any other feedback would be appreciated?

Thanks for all your help!

[Peter6000]

so to sum this up you did the following tests and they are OK
-Ignition timing and power
-Inspected spark plugs for good burn color (not wet)
-fuel pressure
-checked vacuum lines
-tested all sensors (MAP,MAT,TPS,CLT)
-checked throttle vale opens all the way
-disconnected oil and temp switch

the next thing I would do is the mechanical stuff
-do a compression test
-test all fuel injectors

if still no luck bench test the ECU

Peter

[gary s]

There is a possibility that the engine has suffered an overheat at some time that has now internally collapsed the exhaust hoses resulting in a restricted exhaust.

[Peter6000]

There is a possibility that the engine has suffered an overheat at some time that has now internally collapsed the exhaust hoses resulting in a restricted exhaust.

Never heard of that, if the engine was that hot I would expect more issues. Also easy to find just take the elbows off and take a good loo. Now I would diffenitly perform a compression and leak down test.
Peter

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[gary s]

I have heard the reason is that rather than the hose burn though and possibly sinking the boat that they are designed to collapse causing loss of performance that makes you check whats going on. Not sure if it's true but it's something to check.

[dshack]

I'm experiencing a very similar issue and am curious if you ever figured it out?