96 SN EFI well after warm up problem

So now I have it running after replacing the coil and distributor. Go out for a ride. Maybe out for about half an hour. All good just puttering along.

Then I decide to cross some short distance with some more speed. Back comes the problem that started this whole friggin' adventure. It starts to miss and cut and be all kinds of trouble. I pull back the throttle and let it putter along at idle if not a little more and it settles down and runs fine like that. Add throttle again and a few moments later it misses and cuts and even a few popping sounds as I bring back the throttle.

It seems to be a warm problem. I can run it for a bit after it has cooled. Have a ski and all. But to use it after it has become warm...dang.

I doubt it's gas as this has happened now across a couple seasons with newer gas. And I pump out the tank at the end of the season. Would regular gas make this difference? I've run many years on regular.

For other over-heating awareness I don't have a thermostat in. Would that cause this nasty miss/stutter? Why? It has straight lake water flowing through. I watch the exhaust and there's flowing water coming out so the water is circulating.

I'm just frustrated as all get out.

What parts should I be throwing at it now? (other than a chain saw :) ) I do still have a new LPP on hand that I bought for all this nonsense but I haven't installed it yet (whether I need to).

Thanks for any insights.

Gary S wrote: Well you do need a thermostat in it,a 160. The computer thinks the engine is cold so it's going to make it run rich. Then just don't go replacing parts on a guess. Go get a fuel pressure gauge set and start there. Sounds as if the pumps may not be able to keep up with fuel requirements. Read through the gt40 trouble shooting manual here on the site to get an idea how everything works together.

Thank you for the reply.
At this point I don't really care about running rich. I'm not guessing things. I read a bunch of stuff here and decided on a coil. Runs now. I read about the engine cutting out and that it could be the distributor. And I also read about the relays so I replaced those too. This happened last year as well and I put it off. It is after all a 23 year old boat. I've managed to barely put any money into it in the 13 years I've had it. So I guess it's all catching up to me now.

I'm obviously hoping for you and all the other experienced people to chime in and say oh yeah happens all the time it's this. Replace that and you're golden. :)

MrMcD wrote: My 95 did the same thing 4 years ago, I did some reading and got advice on this forum to replace the Oil Pressure sensor and the Coolant sensor. At the same time in installed a new Thermostat and impeller. The boat has purred ever since. The computer has a limp mode if it detects low oil pressure or hot coolant. As a test it has been reported on here that you can remove the wire attatched to these to sensors and see if this fixes your issue prior to replacing any parts. My boat only had 99 hours on it when the sensor failed but was 20 years old. In my case, I wanted it to run right away so I just changed them both. Certainly put a thermostat in your engine. 160 is correct temp.

Thanks for the reply.
What does the engine do when it's not getting a signal from these sensors?
My oil pressure gauge works...how would that indicate that the sensor is in trouble?
Coolant sensor...well...it's straight lake temp so the temp gauge does move up but barely.
Are you saying you had these after-warm miss/stutter/cut/idle-fine issues and it was these sensors?
I'm a little over 800 or so hours now. I've probably put just 150 or so in my ownership. Yes...barely using it.

96SNEFI wrote: My oil pressure gauge works...how would that indicate that the sensor is in trouble?

Part to far right is pressure sensor only for the gauge. Part in the middle is the low pressure sensor for limp mode- it signals the computer that there is low oil pressure. The computer then limits rpm. Engine uses same type setup for water temp too.

Gary S wrote: 96SNEFI wrote: My oil pressure gauge works...how would that indicate that the sensor is in trouble? Part to far right is pressure sensor only for the gauge. Part in the middle is the low pressure sensor for limp mode- it signals the computer that there is low oil pressure. The computer then limits rpm. Engine uses same type setup for water temp too.

Ah. Well then that explains that. One analog for the gauge and one sensor for the computer. Got it. Awesome. Thanks for that.

Edit: Ok so I did a little more searching and found threads about these sensors. I can't say the miss/stumble started at 2800 revs. When I had pulled back the throttle to let it putter at idle or so and let it settle down I would then add throttle again. It seemed to me that it would start to miss/stumble again at around 1500 or so. Not very much but I was aware of it.

In case it was missed above...are these the same sensors used in the automotive side like say a 95 F250 5.8? Kinda like the distributor?

Limp mode limits your engine speed to about 2700 ish rpm if the sensors think your oil pressure is less than 5 psi or water temperature is over 200 degrees (some places list it as 210 degrees but let's just say it's getting pretty hot)

If one of the sensors is bad but your oil and water temp are normal, you can drive around all day long at under 2700 ish rpm and everything will be normal.

Hit that magic number of 2700 ish though and it starts stuttering and misfiring until you reduce speed and things will go back to normal

So............if you're sure that oil pressure and water temp are normal (in your case the water temp seems to be low but that won't put you into limp mode) then you find the oil pressure sensor and disconnect the tan/gray wire and find the water temp sensor and disconnect the tan/gray wire (tape the wire ends so they don't contact the block or any other metal on the engine) and the boat will never go into limp mode.

You want to be sure you get the tan/black wires because there are 2 sensors for water temperature along with the sending unit for the gauge on the dash. One sensor feeds the computer to tell it what engine temperature is and the other is for activating the limp mode and has the tan/black wire hooked to it.

Now, take the boat for a ride and see if your problem still exists

This may or may not fix your issue but it'll at least rule out a limp mode issue.

If MrMcD's engine ran like crap at any speed, he had more problems than just a bad sensor   

Based on your edit a few posts back, it's probably not your problem, but it's easy to rule out.

Thank you KENO. Those were threads that I found. There's a story on the other site as well with an Apex engine..
I think I am closer to MrMcD's general cut-out problem than the magic 2700 problem.
But I'll have a go with pulling the wires today and see what happens.
Even a couple few years ago now I had what I thought was an overheat but maybe it was this sensor limp mode thing.
I was confused earlier with the wire removal. I wondered what would happen removing them and 8122pbrainard quoted it would go into limp mode. But then I read here and elsewhere that the system will work without those sensors connected and that they're just a defense mechanism.

Any thoughts on whether the sensors are standard automotive or do I need to get the PCM ones?

96SNEFI wrote: Any thoughts on whether the sensors are standard automotive or do I need to get the PCM ones?

I'd get the PCM sensors if you have a problem

They may have a Ford part number associated with them, but they're only for the marine/industrial division and weren't used on cars and trucks since they didn't have a limp mode

96SNEFI wrote: KENO wrote: 96SNEFI wrote: Any thoughts on whether the sensors are standard automotive or do I need to get the PCM ones? I'd get the PCM sensors if you have a problem They may have a Ford part number associated with them, but they're only for the marine/industrial division and weren't used on cars and trucks since they didn't have a limp mode Thank you. Wouldn't the limp mode be up to the computer with these as just dumb binary sensors?

Something has to tell the computer that it needs to go into limp mode and that's what these 2 sensors do. At 5 psi decreasing the oil sensor completes a circuit to ground and at 200ish the same happens with the coolant sensor.

When that happens the smart computer limits speed to 2700ish rpm at which point it stutters and runs like crap but under 2700ish it'll run OK again.

One other thing about those two switches.

When either switch activates it'll cause that red warning light right in the upper middle of your dash to light up telling you that you have high coolant temperature or low oil pressure.................if the light isn't burnt out

I connect one of my AC hoses from my freon gauges, these hoses connect to the fuel pressure test gauge and to most fuel pump test ports on the fuel rail top of your engine.
The long 5" hose allows you to route it forward and observe pressure while you drive the boat. You need to know pressure as you idle, accelerate and durring your failure mode.
Edit, I worked on a Ford 5.4L recently and I needed an adapter to connect to my AC hose to the fuel rail.
I will have to look at my GT40 and verify what connector it has.
Did you check the anti siphon valve at the gas tank, it can choke out fuel flow when it fails.
I think the 96 GT40 uses the fuel pump module, there is a short fuel hose inside the module that can split and cause pressure issues also.
Once you know if you have a good fuel supply it will at least rule out many things.

Running all the time in an efi engine without a thermostat could leave your engine in cold start mode all the time. This would leave you over rich and waste fuel. Not a good idea.

96SNEFI wrote: Disconnected water temperature switch on top forward starboard with the two wires.

This has me wondering. Do you have any pictures? The water temp sensor your looking for only has 1 wire.......

This is what it should look like

I agree with Gary, if it had 2 wires, it's the wrong one

There's been some misleading stuff posted over the years

Look at the second post in the link below with the picture from the manual, Each sensor has a tan/black wire. The link was posted earlier in this thread but here it is again.

But, like you're thinking, it's probably not your issue since the red light doesn't come on when you have the issue and the engine speed is lower than 2700 ish.

As far as the light , it comes on when the switch thinks there's a problem, and with the wires disconnected, it'll never come on.

At startup it lights up because you have no oil pressure when you first turn the key to the RUN position on the way to the START position

link

The link below has some pictures of a clogged/restricted suction line. His anti siphon valve was clear but the suction was a mess, so look at both.

There's a link in there to his thread on Planet Nautique too with more info

link

We had something similar on a friends 1995 GT-40..

Once some 20 years ago the low pressure pump had a bolt missing and was sucking in air, therefore not filling the fuel control cell fast enough to maintain high speeds for prolonged times. When the issue came up and rpms were lowered for a while all was good until some longer high rpm runs.

This season the low pressure pumps vanes were not smooth enough anymore to suck fuel from the tank through the anti siphon valve, we turned all the vanes around so the running surface was new again, primed the pump by sucking the air out trough a clear plastic hose by mouth and all is good for a couple of years ( hopefully another 24 since it's the oem pump from 1995...)