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Engine stumble

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bthorsen View Drop Down
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    Posted: June-01-2019 at 4:31pm
Happy Saturday boys... I'm working on another issue this weekend.

After fixing 2 issues (starter solenoid and exhaust plug leaking) this spring while the boat was still on trailer, I launched the boat. Of course, there's a now something else needing attention. The engine seems to have a stumble at about the time of getting to around 1/3-1/2 throttle- I can baby it below that and get up on plane, but it's not happy above there. I may have had this issue a few years ago, and I think I just replaced the spark plugs. I just pulled 2 of the plugs, and of course they look pretty new, but they are Autolite. I know that whatever parts I replaced last time (7 years ago?), they came from the Nautique dealer. I am just way overdue for new plugs? The boat is used for skiing only, so gets 20-40 hours a year and last year didn't even go in as we had our 1st child (a boy!!!)

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KENO View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: June-01-2019 at 4:45pm
Do you have points in the distributor.

If so, I think I'd check the gap/dwell on the points as a start.

A lot of the time, if you think it's ignition it turns out to be a fuel system issue and vice versa so look at both systems.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote bthorsen Quote  Post ReplyReply Direct Link To This Post Posted: June-01-2019 at 5:05pm
Sorry, I should have mentioned- the engine is a GT40 in a 1996... it is FI- so fairly sure it doesn't have points, but I'll double check. Last time this issue happened, I remember thinking fuel, but it was spark, so that's why I'm thinking about plugs right off the bat.

I'm an ex-mechanic and now engineer, but actually I'm not really sure the best way to diagnose this and pinpoint the issue on this era of an engine. Any tips/checklist would be appreciated!
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KENO View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: June-01-2019 at 5:11pm
Originally posted by bthorsen bthorsen wrote:

Sorry, I should have mentioned- the engine is a GT40 in a 1996... it is FI- so fairly sure it doesn't have points, but I'll double check. Last time this issue happened, I remember thinking fuel, but it was spark, so that's why I'm thinking about plugs right off the bat.

I'm an ex-mechanic and now engineer, but actually I'm not really sure the best way to diagnose this and pinpoint the issue on this era of an engine. Any tips/checklist would be appreciated!


No points there then

Plenty of other stuff to go wrong though.

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KENO View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: June-01-2019 at 5:20pm
Somebody is bound to come along and mention fuel pumps and checking fuel pressure and fuel filters and somebody else might wonder if it's going into limp mode and somebody else may wonder about distributor cap and rotor condition..

A better description of how it behaves would be helpful

What are the engine RPM's when it starts to act up?

And what's it do when it acts up ?

You could click on the link below to give you plenty of reading that may help you out

link
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: June-01-2019 at 9:27pm
Wow Ken, you kind of covered all the bases in one breath!!    
JCCI
1995 Ski Nautique GT40
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bthorsen Quote  Post ReplyReply Direct Link To This Post Posted: June-03-2019 at 3:34pm
weather was not cooperative this weekend. I'll be out of town a few days this week, so TBD when I'll get back to this and provide more details... although, by then, I'll likely just go ahead and install the Motorcraft plugs I bought and see if the issue remains.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bthorsen Quote  Post ReplyReply Direct Link To This Post Posted: June-17-2019 at 12:03pm
The weather here in SE MI has been pretty crappy, but I finally got to changing the spark plugs yesterday. After that, I drove for about 10 minutes, just cruising, and no problems. Then stopped and did 3-4 "hard pulls" as if pulling up a skier. The first pulls were fine, but then the stumble problem came back. The boat would get up to about 28 mph at light/med throttle, but would stumble at any speed/throttle above that and not go any faster.

I am running with fresh fuel, but did drain some fuel from the FCC, in order to see if there was any water (which there wasn't), and the locknut on the drain plug cracked in half, so now I need to figure out where to get a new one. I think I'll have to remove the assy and install a new filter while I'm at it and take the plug with to find a new nut.

I did some reading and my next move is to get a fuel pressure gauge and watch fuel pressure on the rail when it happens. If it doesn't change, then it's spark, or something with the ECU... (right?).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: June-17-2019 at 2:43pm
Originally posted by KENO KENO wrote:

Somebody is bound to come along and mention fuel pumps and checking fuel pressure and fuel filters and somebody else might wonder if it's going into limp mode and somebody else may wonder about distributor cap and rotor condition..

A better description of how it behaves would be helpful

What are the engine RPM's when it starts to act up?

And what's it do when it acts up ?

You could click on the link below to give you plenty of reading that may help you out

link


I quoted the above to save wear and tear on my typing fingers.

What are the RPM's ?

At 26 to 28 mph they're probably about 2700 ish

At 2700 ish , your SLOW system ( frequently referred to as limp mode ) kicks in if you have a bad signal to the computer from the water temp switch or the oil pressure switch.

Your dash gauges can read just fine since they're fed from a couple of different sensors but a bad switch can cause the engine to go into limp mode.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: June-17-2019 at 2:49pm
Here's a link to a recent thread about going into limp mode.

Maybe it's your problem, maybe not but it's easy to rule it out.

link
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