Thunderbolt IV issue?

[Ramman1505]

Last year I was out on the water for about 30 min no issue. I shut the boat off and we floated for a bit. When I started the boat back up I couldn't get up on plane. The engine would get up to around 2700 rpms, then it felt a few cylinders had dropped out. I noticed when this happened, the tach went up in rpm to about 4800. If I let off the throttle the engine would return to normal running condition along with the tach readout back to 2700. This occurred at the exact same rpm repeatedly, regardless of throttle position. I'm convinced it's a spark issue and not a fuel one, given the behavior of the tach, and the consistency at with rpm it occurs at. I've tried disconnecting the tach, along with bypassing the shift interrupter switch with no luck. It seems to only happen once the engine is warm. My concern is the thunderbolt IV module is going out. There were a handful of instances where if I would floor the boat, it would come out of this misfire state and haul ass like nothing was wrong. The engine behaves perfectly upon deceleration thru the entire throttle and rpm range. The specific issue is some type of spark issue occurring around 2700 rpm, on a warm engine, upon acceleration only. Anyone have any thoughts to what else this may be? Boat is a 1987 Chris Craft Scorpion with the 5.7 Mercruiser and Thunderbolt IV ignition.

 

[dubs283]

Have done any recent tune up work? Plugs, wires, cap/rotor, check contents of/change fuel filters, etc...?

It possible but not likely the ignition module is faulty. First guess would be an issue with the igniton sensor in the distributor. Been a lot of discussion about those lately here.

Would not rule out fuel supply issue yet either

 

[Ramman1505]

Have done any recent tune up work? Plugs, wires, cap/rotor, check contents of/change fuel filters, etc...?

It possible but not likely the ignition module is faulty. First guess would be an issue with the igniton sensor in the distributor. Been a lot of discussion about those lately here.

Would not rule out fuel supply issue yet either

I really don't see how it would be a fueling issue. When the boat is cool it runs like new with great power. I inspected the cap and rotor and all seemed well. It really seems to be some type of spark issue when warmed up. Perhaps the ignition sensor could be the culprit. I had also considered the coil as well. I'm trying to understand why the tach rises when this misfire condition occurs. Was hoping someone here had experienced this before.

 

[dubs283]

Was hoping someone here had experienced this before.

We have, hence the info/direction.

There is not one magic source for the solution to why your boat runs the way it does. We are here to give you insight/direction to solve the problem yourself based on info you provide. You can inform us with as much or little info as you like but you won't get far without disclosing tests/maintenance performed, proper engine information and items inspected.

 

[achris]

.... When the boat is cool it runs like new with great power. ...

This ^^^ piqued my interest.

When I first got my 4.3LX (1994) it also had running issues, but only when warm. Really, really long story short, as the engine got to fully warm the distributor lost it's good ground contact. (Dissy ali, engine cast iron, different rates of expansion when warm)... Cured my problem by connecting a spare ground in the harness to the dissy body. Now, I'm pretty sure your module is mounted on a plate on the exhaust elbow, but the sensor in the dissy is grounded to the dissy. That's where I'd start, either run a dedicated ground, or better yet, replace the sensor with the new 3-wire style (and run the black directly to the lug on the back of the engine that the battery ground runs to). I'd also have a really good inspection of ALL your grounding points. That means removing them, cleaning them to clean bare metal, and refitting them. Then cover them in liquid neoprene..

Chris......

 

[Scott06]

I really don't see how it would be a fueling issue. When the boat is cool it runs like new with great power. I inspected the cap and rotor and all seemed well. It really seems to be some type of spark issue when warmed up. Perhaps the ignition sensor could be the culprit. I had also considered the coil as well. I'm trying to understand why the tach rises when this misfire condition occurs. Was hoping someone here had experienced this before.

Tach is driven off negative side of coil so that would make one think yes it is ignition related. Wouldn't hurt to measure fuel pressure as a sanity check as a plugged fuel tank vent can take run time before it occurs.

If you disconnected the tach at the coil and it made no difference. Try this TB troubleshooting guide from the stickies at top of merccruiser section https://forums.iboats.com/threads/how-mercruiser-thunderbolt-ignition-systems-work.676476/

You can ohm the coil official specs are in the factory manual- I think .6-.8 ohm primary and about 8500-11k ohms secondary. You need a good meter to see the primary side at that low ohms...

Intermittent ignition issues like this can be difficult to diagnose. If your sensor in the distributor is the style with posts vs the later sealed style I would certainly replace it.

You could also measure spark strength with a inexpensive gap tester or a more expensive ignition analyzer that gives a kV reading. I bought one of these and got base lines on all my engines when they were running well.

 

[Ramman1505]

This ^^^ piqued my interest.

When I first got my 4.3LX (1994) it also had running issues, but only when warm. Really, really long story short, as the engine got to fully warm the distributor lost it's good ground contact. (Dissy ali, engine cast iron, different rates of expansion when warm)... Cured my problem by connecting a spare ground in the harness to the dissy body. Now, I'm pretty sure your module is mounted on a plate on the exhaust elbow, but the sensor in the dissy is grounded to the dissy. That's where I'd start, either run a dedicated ground, or better yet, replace the sensor with the new 3-wire style (and run the black directly to the lug on the back of the engine that the battery ground runs to). I'd also have a really good inspection of ALL your grounding points. That means removing them, cleaning them to clean bare metal, and refitting them. Then cover them in liquid neoprene..

Chris......

Good thoughts here. I will certainly check all grounds. I'm just so confused because if I can get it past this rpm, which at times I can, it runs like nothing is wrong. It never misses when throttling down, not once. It's only right at 2700 rpm, when throttling up, when warm. Very odd. You are correct, the ignition module is mounted to the port exhaust manifold and gets hot. This is what drew me to thinking this was likely the problem.

 

[Ramman1505]

Tach is driven off negative side of coil so that would make one think yes it is ignition related. Wouldn't hurt to measure fuel pressure as a sanity check as a plugged fuel tank vent can take run time before it occurs.

If you disconnected the tach at the coil and it made no difference. Try this TB troubleshooting guide from the stickies at top of merccruiser section https://forums.iboats.com/threads/how-mercruiser-thunderbolt-ignition-systems-work.676476/

You can ohm the coil official specs are in the factory manual- I think .6-.8 ohm primary and about 8500-11k ohms secondary. You need a good meter to see the primary side at that low ohms...

Intermittent ignition issues like this can be difficult to diagnose. If your sensor in the distributor is the style with posts vs the later sealed style I would certainly replace it.

You could also measure spark strength with a inexpensive gap tester or a more expensive ignition analyzer that gives a kV reading. I bought one of these and got base lines on all my engines when they were running well.

I have looked thru the troubleshooting guides before. Like you had mentioned, intermittent spark issues are hard to troubleshoot with those guides. Sounds like a ground check, and new ignition sensor are a good starting place. My guess is the module us the culprit as it gets super hot mounted to the port exhaust manifold.

 

[Scott Danforth]

the new 3-wire trigger that Chris mentions

 

[todhunter]

I have looked thru the troubleshooting guides before. Like you had mentioned, intermittent spark issues are hard to troubleshoot with those guides. Sounds like a ground check, and new ignition sensor are a good starting place. My guess is the module us the culprit as it gets super hot mounted to the port exhaust manifold.

The exhaust elbow should be getting all that hot during operation, and the TB IV should have phenolic standoffs that further thermally isolate the unit from heat. Not saying your TB IV isn't the problem...just that it shouldn't be seeing all that much heat, in reality.

 

[Ramman1505]

The exhaust elbow should be getting all that hot during operation, and the TB IV should have phenolic standoffs that further thermally isolate the unit from heat. Not saying your TB IV isn't the problem...just that it shouldn't be seeing all that much heat, in reality.

I agree with you when the engine is running, however when you shut the engine down, the manifolds heatsoak with no cooling. I have touched the module after sitting for a few minutes and it's to the point where I can't keep my hand on it. This is when the issue seems to arise.