Mercr 5.0 thunderbolt V gen. 3 limiter

Jeronimo

Recruit
Joined
May 18, 2010
Messages
3
dear readers,

A mercruiser 5.0 1998 with thunderbolt V gen 3 ignition has the following problem. (s/n: 0L064457) alpha tail

The rev limiter kicks in at 3500 RPM. Always ! very consistant.

I have checkt the temp sensor and the oil-pressure switch.
They seem to be oke. I checkt the ignition timing is oke.

I replaced the hall-sensor in the distributor. And it started to run much better. Is was the old type without casing. But still have the rev limiter at 3500rpm.

Anybody a suggestion before I have to replace the ICM.


Thanks
Jeroen
 

Fun Times

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Staff member
Joined
May 16, 2009
Messages
8,776
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

Yes, You need to confirm that your tachometer (RPM gauge) is adjusted and working correctly, By using a different gauge or tool.

There should be an adjustment dial setting on the back of the gauge that you should try resetting, making sure it's adjusted to work with a V8 engine.

Few Questions for you.

(1) How fast does your boat go when on the rev limiter? (MPH);)

(2) what style boat do you have?

(3) how much does the boat weigh?

(4) what size prop are you running? Pitch & Diameter?

(5) what is the Name Brand / part number of the prop you have now?

(6) Is the prop stainless steel or aluminum?

(7) what is the gear ratio in your sterndrive?

(8) Do you have mercruisers service manual #24?
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

The rev limiter should only be kicking in around 5K. 3.5K sounds more like an issue with the knock sensor, knock module, or wiring. Use the diagnostic path for it in the service manual.
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

How do you know it's the rev limiter kicking in early that is causing your problem?
 

Jeronimo

Recruit
Joined
May 18, 2010
Messages
3
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

How do I know it's the rev limiter ?!

- Well you know how a rev limiter sounds well that's one reason.
- The engine starts to produce miss-fires
- The exhaust smells like unburnt fuel.

Cables , distributor cap + rotor and bougies are replaced.
No difference


Could it be the ICM "thinks" the gear leaver is not in gear ?
If I look at the schematic is doesn't show anything about a "in gear" or somelike switch.

The boat runs 70km/h and used to do 85 km/h.
Nothing is changed except for the limiter that kick-in at 3800 rpm and then stabilize at 3500 rpm. Always !!! without any exception.

Strobe light also shows it's running at 3500 rpm so the gauge is checkt.

If you idle the engine and push the throttle in neutral. It's easy to reproduce.
at 50% throttle we reach 3500 limiter and if you push it further is stays the same only running becomes more rough with back-fires or miss-fires.

If we take the throttle a little bit back just before the limiter kicks-in everything runs fine.

I don't know what to do anymore.
I'm gone connect a scope to have a look at the puls from ICM.

Thanks
Jeroen
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

Have you put it in base mode and mapped the timing curve upto the 3500 rpms?
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

Have you put it in base mode and mapped the timing curve upto the 3500 rpms?

If you put it in base timing mode, it's not going to move at all.

I'm gone connect a scope to have a look at the puls from ICM.

And you would compare your reading to what?

If you idle the engine and push the throttle in neutral. It's easy to reproduce.
at 50% throttle we reach 3500 limiter and if you push it further is stays the same only running becomes more rough with back-fires or miss-fires.

That pretty much tells the story, and YES, it's probably the module, or the wiring to it.
 

Jeronimo

Recruit
Joined
May 18, 2010
Messages
3
Re: Mercr 5.0 thunderbolt V gen. 3 limiter

Well guys,

Problem solved !!!

Such simple solution. The wrong spark plugs

BR6FS in stead of BPR6EFS

I don't know why I didn't see the length difference.

It should be the EFS plug with long shaft.

So stupid :-( But now we know how the wiring works.

Always handy

So not a limiter but just a spark plug with to short shaft. Won't stick in the chamber.

Thanks
Jeroen
 
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