5.7 Mercruiser Cuts out/Stalls Past 3/4 Throttle. RPM Limiter?

[CrownlineRider92]

Looking for some advise what to check next. Below I outlined what I have observed and the troubleshooting steps taken so far. Thanks in advance for any knowledge.

Boat Specs:
1992 Crownline 196BR, 350 Mercruiser, Alpha One Gen 2 Outdrive, Quardrajet Carb

What Happened:
I was pulling a tube this weekend on the lake. Prior to this issue, the boat was running great with no issues. Throttled up to get on plane and the motor shuttered/died almost immediately at 3/4 throttle. Boat started back up with no issues and idle fine. Tried once again and the boat died the same as the first time at 3/4 throttle. The engine did not stubble or bog down before it dies, it just cuts out. If I throttled back quickly, It would stay running.

Third time, I throttled up and once again the engine dies immediately at 3/4 throttle. I left the throttle forward and pulled the cover on the engine hatch and found the shift interrupter switch was pulsing on/off quickly.

I spent about a hour on the lake trying different things I outlined below with no luck resolving the issue. In the end, I ran the boat at lower cursing speed/rpm the rest of the weekend with no issue. Boat seemed to run just fine but still would shut off if I throttled past 3/4 throttle. I was happy it never left me stranded and did not end the weekend early! Now that I am home, I am looking at what to do next to get this issue resolved. Felt like the boat had a governor or rpm limiter I could not pass.

Question:
What would cause the shift interrupter switch linkage pulse on/off like this at 3/4 throttle? Is this even a shift interrupter issue? Would the shift interrupter toggle on if the engine dies? Ignition System issue maybe?

Troubleshooting done so far:

Shift Interrupter Switch:
First thing I did was to disable the shift interrupter switch. I moved the lever arm off the cam which it sat on. Once again, I throttled up and the boat died at the same exact rpm/throttle condition. I started the boat back up and shifted into forward, then tried to shift back into neutral. Because the switch was not cutting ignition, I was unable to shift out of gear. This told me that I effectively disabled the switch.

Shift Cable:
The the lower shift cable was recently replaced (2 years ago). I removed the shift cable from the linkage and it operated with no binding. I then adjusted the shift cable by turning the brass barrel and repeated my test to see if the motor still died at 3/4 throttle. No matter which direction I adjusted the shift cable, the engine would still die at 3/4 throttle.

Fuel System:
Engine does not bog or stubble. If it was starving for fuel, I do not see the engine cutting out as quickly as it did. This rules out fuel system I think.

Ignition System:
At this point, I think it has something to do with the ignition system cause of the consistency of the engine shutting off. My trouble is most components of the Ignition system tend to fail completely when they do.

Items to Check: 1. Cab & Button 2.Coil (Boat runs so has spark, doubt it is bad) 3. Spark Plugs (Check Burn Color) 4. Distributor Sensor (Reading tell me that it either works or doesn't. Once again the boat runs at low rpm) 5. Thunderbolt IX ignition module.

Thanks once again!!

 

[achris]

Operate the throttle to full (with the engine off) and have a look to see if it's interfering with any electrical wires. It sounds like the throttle cable might be rubbing on a wire and has rubbed through and now when you get to 3/4 throttle it's shorting to ground, killing the ignition system... Other than that, do the instruments go dead as you hit 3/4?

On a 1992 engine, you have a Thunderbolt IV ignition, and they have no rev limiters.

Chris....

Just had another thought... Control box. There's an ignition kill switch in those, the mechanisms in the control box have rubbed through a cable. Worth looking at....

 

[CrownlineRider92]

That is a good point achirs. I will definitely check to make sure no wires are interfering with any of the linkages. I know there is no rev limiter but that exactly what the boat feels like and i felt it best described the condition.

On my boat there is a ignition kill switch on the dash next to the key. I checked the lanyard and everything seems to be fine.

 

[CrownlineRider92]

So last night I checked the wiring on top of the motor to ensure it was not interfering with any shift linkages and shorting out. All the wires were routed clear away from the shift linkages so this is not the issue. I also checked the control box and found nothing wrong there. The only wires I have at my control box is for trim and the neutral safety switch.

Any other Ideas?

 

[achris]

Move the throttle to the position it shuts the engine down and then check for power at the coil + terminal.... (should be full battery voltage)...

Chris.......

 

[Bondo]

and found the shift interrupter switch was pulsing on/off quickly.

Ayuh,..... Welcome Aboard,...... I ain't got an answer yet, but This, ^^^ has me puzzled,.....

I don't think it's an ignition issue yet, I think something is turnin' the ignition off,....

Not sure what ya did to the shift switch, But, I'd rather ya disconnected the wires, rather than playin' with the switch itself,....
Lever arm, 'n cam says it's an older unit that's NLA,....

I suggest ya follow Chris' diagnostic instructions, 'n We'll see where it leads too,....

 

[TurtleTamer]

Stab in the dark: the interrupter switch "pulsing" is a result of the ignition cut-off. Like it's trying to pop out of gear due to the sudden loss of torque on the prop.

My theory is that you have a dead spot in your ignition. I really ought to research the different types of ignitions in these motors because I'm only familiar with the EST type found in my 3.0. But I believe that it's part of the advance curve in the ignition control circuitry that is failing open, or shorted to ground, at that part of the advance curve.

One question I have is, are you sure it's reliant on throttle position, or is it reliant on RPM? If you can nail it down to an rpm, my theory can be tested by retarding the timing by a few degrees and seeing if you can reach a slightly higher RPM.

I hope I'm not way off and leading you astray. These experts can hopefully chime in and let us know if I'm making any sense.

I wonder if it's not also possible that you have a similar dead spot in your tachometer. It has been said by Bondo, I believe, that the tachometer can ground the ignition circuit (grey wire?). Again, this would have been on a 3.0 ignition with EST so I don't know if it's applicable.

Very interested to hear the eventual solution to this though.

Edit: found this post by Achris: https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

Also I re-read when you saw the pulsing in the switch, so I guess there goes my theory.

 

[achris]

The pulsing of the switch is generated by the prop overrunning on the clutch dogs. That's caused by the engine cutting out as the boat's moving forward. The motion of the boat is forcing the prop to turn. That pulsing is a result, not a cause.

Chris..........

 

[CrownlineRider92]

Thanks for the input guys. I have not had time in past few days to work on it but hoping to find time this weekend.

I agree with Chris that the pulsating switch is a result of the prop overrunning the dog clutches after the motor dies. For that reason and seeing no change when the I disabled the switch, I do not think the issue lies in the the shift interrupter switch.

My gut is still telling me it is a ignition issue. The question is do I have a dead spot or is something cutting power to the system. I think my next step is to hook the boat up to the hose and see if the issue occurs when the motor has no load (ie boat out of water). If the motor dies, I will check to see if I still have power to the coil.

TurtleTamer, you had a good point about the tach shorting out. I might unhook it to see if the issue goes away. My tach has acted funny so I would not be surprised if it is causing issues. Since I got the boat, it always seems to be inconsistent to me.

 

[CrownlineRider92]

Alright guys, Sorry took so long to get back to this. Today I was able to hook it up to the hose and run the motor. The motor ran fine and revved up in neutral. When put in gear, It revved up past 3/4 throttle and did not cut out (This probably was not good for either the motor or the out drive but it was for only a couple seconds). I shifted back in neural, waited a few seconds for the prop to stop spinning, then I shifted back into reverse. When I did so, I watched the neural switch kick in and kill the motor.

So next I disconnected the shift cable from the linkage at the motor. Started the motor back up and let it idle. I then shifted it by hand in both fwd and reverse. When doing so, I could feel a force pushing back on linkage every so often.

Now I wondering if my dog clutch is warn causing the force on the cable when the prop is loaded at high rpm and engaging the shift kill switch. I drained some oil out of the lower unit to inspect and there was a fair amount of metallic/black particulate in the oil. The out drive oil was just changed at the beginning of the season and has only a few hours of run time.

Does this sound like a possibility? Is there a method to test dog clutch in the unit? Can they be replaced easy enough without too many special tools?

Thanks again for all your guys help on this issue!

 

[achris]

...So next I disconnected the shift cable from the linkage at the motor. Started the motor back up and let it idle. I then shifted it by hand in both fwd and reverse. When doing so, I could feel a force pushing back on linkage every so often.....

Clutch dog over-run.. Completely normal.

Let me explain. Most engines will not hold an ABSOLUTELY steady speed, they 'hunt', very slightly, it's normal, and it's such a small amount you can't hear it. As your engine is hunting, especially at idle, on the lower speeds the prop is turning (or trying to turn) slightly faster than the input shaft. And because of the ramps on the backsides of the dog clutch, this tries to push the shift mechanism back into the neutral position... And that's what you are feeling....

It doesn't happen when the boat's in the water because the water acts as a huge damper, and as the engine speed drops, the water slows the prop....

HTH

Chris.......

 

[CrownlineRider92]

Thanks Chris for that explanation. That makes me feel better cause I know a outdrive repair can be costly.

I still do not understand why the shift interrupter switch was engaging when I shifted into reverse.

Correct me if I am wrong:
1.The shift interrupter switch should only engage when shifting out of gear.
2.The shift interrupter switch should not be doing anything out of water with no load on the prop.
"The system is NOT designed to work when putting the drive into gear, or when the boat is out of the water and on muffs. Without the load on the prop, there is not enough resistance on the shift dogs in the lower unit to cause resistance to the shift cable and engage the shift interrupt system. " (https://forums.iboats.com/forum/eng...3-how-to-how-the-shift-interrupt-system-works)

Only reason I can see the the shift interrupter switch kicking on is due to a sticking low shift cable. I went ahead and removed the out drive to inspect the cable inside the bell housing and it all seems fine. With the shift cable disconnected from the linkage on the engine, I checked the motion of the cable and it seemed to glide in and out smoothly with no catch points or sticking.

This issue has me stumped... I'm tempted to just replace the lower shift cable and see if the issue goes away.

 

[achris]

I have seen many of those cables get crud up inside the shift slider at the cable end... You have to disassemble the cable to check it...

Chris.......

 

[CrownlineRider92]

I disassembled the cable and pulled the inner core out of the sleeve. I did not notice anything major other than a few rub marks. The one concern I have is the inner core looks to be a steel breaded cable with a plastic low friction jacket. The jacket has some rub small marks that may add friction to the system. Are these style cables worth a dang? I have replaced a few shift cables on buddies boats and they were all metal core. Any thoughts on this?

I am going to go ahead and get an shift cable kit on order and throw it on and see what happens.

 

[CrownlineRider92]

Update: This weekend I installed a new shift cable and took the boat out to the lake to test. When I got out of the idle zone and throttled up, the boat behaved the same exact was as it was before. (Lower shift cable was not the issue)

Cruising around the lake, I noticed if noticed a few things.

1. Off plane acceleration: Boat accelerates till hit 3/4 throttle then bogs, and stutters until throttle is backed off.
2. Cruising just below 3/4, Motor seems to be running normal. If I slowing added throttle, the engine once again shutters and bogs out.
3. Cruising at half throttle, then hammer it to full throttle, motor bogs down for a second then takes off at full RPM.

I'm now confident the issue lies in my Quadrajet carburetor. I believe the issue occurs cause my secondaries. I do not believe the issue is due to the secondary throttle plates but rather the secondary butterflies as they are vacuum controlled. I know there is a spring adjustment that can be made to the butterflies. The carburetor was freshly rebuilt over the winter.

Any experience with similar issues? Do you have any recommendations on adjustment?

 

[CrownlineRider92]

I THINK I FIGURED IT OUT FINALLY!! I pulled the flame arrested off the top of the carb today and sure enough, the secondary butterflies were half open with no spring pressure. The torsional spring that hold the secondaries shut slipped off. Reading symptoms online, this will cause a high rpm bog with seems to be what I am experiencing. Only time will tell if this is the true culprit. Ill report back next time I get out on the water.

Thanks again for all the help on this guys.