Mercruiser 4.3L EFI Rev Limiter

[Fish Bin Laden]

I have a 4.3L EFI Mercruiser serial number OL 034997. It appears that the rev limiter is activating prematurely and randomly. After a short period of use the tacometer needle will oscillate wildly usually between 4500 & 5000 revs although I normally run the engine at cruising about 3800 - 3900. When this occurs the motor will tend to de-rate and although without any input from the throttle or noticable increase in revs, the problem will get worse to the point where the rev counter will touch maximum of 6000 at which stage there will be noticable misfiring and back firing. The only way to proceed when this happens is to reduce the revs to well below 3000 and even then the motor runs erratically, seemingly in sync with the oscillation of tacometer. I have fitted a new distributor module and cap, also HT leads, fuel pump, coil, tacometer etc etc. Ideas please

 

[ENSIGN]

Re: Mercruiser 4.3L EFI Rev Limiter

You may just have a bad tach.disconnect it under the dash and take the boat out and run it.let us know the results.GL,Ted

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

You may just have a bad tach.disconnect it under the dash and take the boat out and run it.let us know the results.GL,Ted

thanks for that Ted, I will give that a go although I failed to mention that this problem only occurs when the motor is under load, it runs fine and the taco needles remains rock steady right thru the rev range when it is out of gear

 

[ENSIGN]

Re: Mercruiser 4.3L EFI Rev Limiter

Most boat engines run fine on a flusher.your going to have to do your testing with the boat backed down the ramp.Do a compression test and look at the plugs for signs of water on them.Even a little water getting into a cylinder can cause your problem. Also put some of your gas in a glass jar and let it settle out. Look for water at the bottom. GL Ted

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

Most boat engines run fine on a flusher.your going to have to do your testing with the boat backed down the ramp.Do a compression test and look at the plugs for signs of water on them.Even a little water getting into a cylinder can cause your problem. Also put some of your gas in a glass jar and let it settle out. Look for water at the bottom. GL Ted

Hope to go to sea this weekend Ted, and try your helpful suggestions. I will keep you posted on results. Just to clarify, when I say the problem does not occur when the boat is in neutral, I mean by that, that I can put the boat out of gear while at sea and the motor will run freely when it will not when in gear I.e. not just with the muffs on.

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

Hope to go to sea this weekend Ted, and try your helpful suggestions. I will keep you posted on results. Just to clarify, when I say the problem does not occur when the boat is in neutral, I mean by that, that I can put the boat out of gear while at sea and the motor will run freely when it will not when in gear I.e. not just with the muffs on.

Took boat out today with taco disconnected. No change. Reconnected taco and removed fuel filter and tipped contents into jar. About 20percent water but filter had been on for 60 hours. Fitted new filter with clean fuel and ran for 2 hours. No change. At home tipped contents of new filter into jar. No evidence of any contaminates. Am still convinced problem is with the rev limiter which I guess originates from the ECU. Is there any way I can disconnect this temporarily for diagnostic purposes?

 

[achris]

Re: Mercruiser 4.3L EFI Rev Limiter

.. I guess originates from the ECU. Is there any way I can disconnect this temporarily for diagnostic purposes?

No...

 

[dubs283]

Re: Mercruiser 4.3L EFI Rev Limiter

have you connected any type of scan tool while running the engine??

an rpm overspeed will set a fault that you can see with a cds

 

[Pete104]

Re: Mercruiser 4.3L EFI Rev Limiter

And will be there forever until the code is cleared.

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

Yep. The original mechanic i took it to said the fault was the rev limiter and i assume he deleted the code. That was with the old tachometer which wasn't working and i guess he just thought i had over reved it. After fitting the new tacho i could see that the problem persisted but i have not got the confidence in that mechanic to take it back to him. I have taken it to another mechanic who has thrown all the aforementioned parts plus some at it before drawing a blank. Do you think i am up for a new ECU which incidently is not obtainable new here in New Zealand

 

[Pete104]

Re: Mercruiser 4.3L EFI Rev Limiter

No, don't throw anymore parts at it!
You have to put faith in whoever has the equiptment to service this problem. Merc's CDS
Go back to the first guy & have him start over. Has anybody done any injector testing?

Try running it on a remote tank. Take a 6 gallon/litre can with fresh gas & connect it to water sep filter fitting. Preferrably with a fresh filter. Check the fuel pressure also. A bad regulator could act like this & no, it won't show any fault codes.

 

[achris]

Re: Mercruiser 4.3L EFI Rev Limiter

... new ECU which incidently is not obtainable new here in New Zealand

Says who!?!??! NZ Merc dealers are supplied out of Mercury Marine in Melbourne, and I know that have plenty of ECUs for your engine available...

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

No, don't throw anymore parts at it!
You have to put faith in whoever has the equiptment to service this problem. Merc's CDS
Go back to the first guy & have him start over. Has anybody done any injector testing?

Try running it on a remote tank. Take a 6 gallon/litre can with fresh gas & connect it to water sep filter fitting. Preferrably with a fresh filter. Check the fuel pressure also. A bad regulator could act like this & no, it won't show any fault codes.

thanks for some excellent ideas Pete. I will try them. As far as I know the injectors have not been checked so maybe I will start with this. Could you please explain how a fuel issue could cause the tachometer to spike with no discernable increase in engine revs.

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

Says who!?!??! NZ Merc dealers are supplied out of Mercury Marine in Melbourne, and I know that have plenty of ECUs for your engine available...

To quote Gary Hatton, Director Auckland Marine Centre: “Unfortunately the ECU for your engine is no longer available through the Mercruiser parts system”
Also Jono Hanley from marineworkz.co.nz : “As far as I can see in my system the ECM you are after is NO Longer available”

 

[Pete104]

Re: Mercruiser 4.3L EFI Rev Limiter

Mercury won't just up & send a new ECM or PCM. They will want it back. But prior to sending it back, they want to communicate with dealer. ECM's don't just up and need re-programming (reflashing in some circles) for no reason. Granted, there are drivers in them that fail & that is what Mercury would change/reprogram.
There are places here in the states that do alot of HP work but also will re-flash certain programs in them.
No matter how you cut the cards, you will considerable down time.

Q#13 Fuel won't make RPM spike to 6000rpm. The way I read your reaction to the throttle to make it better, don't add up. That engine is rated for 4800rpm rev limit kicks in @ 4850rpm (should anyway) and then drop rpm below 3000 to keep it running, is why I voted fuel.

Have you checked and made sure every ground on this engine is clean & tight? Looked at the in & out of the ECM. Especially check the "cannon plug". The main engine harness.

That's another thing to try. If you know how to hot wire your engine, with cannon plug disconnected?! Try it, that will eliminate the dash. You may find that the hour meter is spiking the ECM.

 

[Don S]

Re: Mercruiser 4.3L EFI Rev Limiter

Have you looked at the coil? or the ignition module, cap and rotor? is anything cracked and arching? You may want to believe it's a rev limiter, but I highly doubt it is.
If you don't hear the engine rpm increase, then the ECM is getting a false signal from somewhere.

The boat needs to be run on the water, under load, with the diagnostic computer hooked up and the problem is occurring. The real problem may not set a code. On those older MEFI I and 2 ECM's there are very few codes to set.

 

[Technorunner]

Re: Mercruiser 4.3L EFI Rev Limiter

Where is the ECM getting its RPM signal from is it from the coil, hallsensor in the distributor or an sensor on the flywheel or balancer infront of the engine? The problem should be where it gets its signal from as Don says!

 

[Fish Bin Laden]

Re: Mercruiser 4.3L EFI Rev Limiter

Many thanks for your helpful and knowledgable suggestions. I will spend this weekend checking all electrical connections in and out of the ECM and motor for signs of corrosion or bad contacts and then take the boat for a run. If there is no change I will take it to a mechanic with this thread.

 

[Tail_Gunner]

Re: Mercruiser 4.3L EFI Rev Limiter

There are codes on your ecu post them if you would like to save a few dollars or a pic would be fine. However if your somewhat comp savy you may want to unplug your ecu and under a good glass check out the pins for uniformity and free of corision.Litte note e-proms are a solid state device and are either on or off..I doubt very much you have a bad ecu

Heres a link that will show you how to make a basic tester..if that works you dont or its highly unlikely your ecu is bad

http://www.offshoreonly.com/forums/...ke-marine-efi-code-tool-less-than-1-00-a.html

 

[ewenm]

Re: Mercruiser 4.3L EFI Rev Limiter

could also be the signal sending unit within the distributor??