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1995 SN Pro-Boss GT40 5.8 HO with Unstable RPM
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PT Skier View Drop Down
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    Posted: June-22-2015 at 8:58pm
Hello guys,

My boat's engine is going crazy... I recorded this video to see if you can give me your thoughts on this...
video

As you can see the rpms are not stable. Only when I reach the 3000rpm does the engine starts working normal.... At the end of the video the engine stops as it cannot stand enough rpm. Can this be related with a faulty hp fuel pump?

I also have a loud whistle on higher rpms. Is this related with the above or should it be a different issue?
Ski Nautique 196 OB (1995)
PCM Pro-Boss GT-40 Multiport EFI 5.8L 310hp
750 hours
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Bri892001 View Drop Down
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Some things to check, would be:
Full tuneup including wires, when was the last time:
The little tube inside the FCC, check for split/crack
Idle Air Control Valve

Check out this thread also:
http://www.correctcraftfan.com/forum/forum_posts.asp?TID=25584&PN=1&title=poor-mans-gt40-diagnosis
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jhersey29 View Drop Down
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Points: 272 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jhersey29 Quote  Post ReplyReply Direct Link To This Post Posted: June-23-2015 at 11:33am
Vacuum leak? Search YouTube for vacuum leak and propane. It might help you find a crack or leak. In the lines. Seems like its also doing at higher rpma so check the line to the map sensor.
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Points: 66 		Post Options Post Options   Thanks (0) Thanks(0)   Quote PT Skier Quote  Post ReplyReply Direct Link To This Post Posted: June-23-2015 at 11:44am
Originally posted by Bri892001 Bri892001 wrote:

Some things to check, would be:
Full tuneup including wires, when was the last time:
The little tube inside the FCC, check for split/crack
Idle Air Control Valve

Check out this thread also:
http://www.correctcraftfan.com/forum/forum_posts.asp?TID=25584&PN=1&title=poor-mans-gt40-diagnosis


Last tune up was 1.5 years ago with new cables, spark plugs and distributor cap.
I already have a new HP fuel pump in transit to see if it solves the problem... I think that everything is OK with IAC valve.
Ski Nautique 196 OB (1995)
PCM Pro-Boss GT-40 Multiport EFI 5.8L 310hp
750 hours
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Originally posted by jhersey29 jhersey29 wrote:

Vacuum leak? Search YouTube for vacuum leak and propane. It might help you find a crack or leak. In the lines. Seems like its also doing at higher rpma so check the line to the map sensor.


jhersey29, this winter I had the engine completely disassembled for a complete overhaul as I was already having this problem before together with a rod bearing failure. I had all the gaskets replaced so it shouldn't be a vacuum leak. Also note in the video that when I go above the 3000 rpm the engine stabilizes and I can also confirm that when I run the boat on the water. The only thing I notice is that when running above 4000 a loud whistle shows up which is very annoying.
Ski Nautique 196 OB (1995)
PCM Pro-Boss GT-40 Multiport EFI 5.8L 310hp
750 hours
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Bri892001 View Drop Down
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Hmmm... This isn't the first time rough running has been accompanied by a whistling noise on a GT-40... I can't, off the top of my head, remember the cause though.
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jhersey29 View Drop Down
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Points: 272 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jhersey29 Quote  Post ReplyReply Direct Link To This Post Posted: June-23-2015 at 12:15pm
Originally posted by PT Skier PT Skier wrote:

Originally posted by jhersey29 jhersey29 wrote:

Vacuum leak? Search YouTube for vacuum leak and propane. It might help you find a crack or leak. In the lines. Seems like its also doing at higher rpma so check the line to the map sensor.


jhersey29, this winter I had the engine completely disassembled for a complete overhaul as I was already having this problem before together with a rod bearing failure. I had all the gaskets replaced so it shouldn't be a vacuum leak. Also note in the video that when I go above the 3000 rpm the engine stabilizes and I can also confirm that when I run the boat on the water. The only thing I notice is that when running above 4000 a loud whistle shows up which is very annoying.


Loud whistle sounds like air moving through a small crack. I'm still voting vacuum leak. Especially since you just took the motor apart versus your idea of you just took it apart so it's not a leak. Torque down the intake manifold again.
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I agree, sounds like a vacuum leak.   Pull all 8 spark plugs and study the burn pattern.
Keep them in order so you know 100% sure which cylinder they came from.
Unless all 8 are affected by your lean condition the plugs should help you find the source.
A lean burn will make cylinders run lean, a lean cylinder will run hot causing detonation and that can pound out a rod bearing causing rod bearing failure.   Fix it or you will need another lower end repair soon.
The propane test may help you find the source of your leak. Good Luck.
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