Idle problem merc 2005 4.3l carb

[ShayneA]

Hi everyone, My engine runs fine almost all the time. I have an issue that occasionally the engine will idle extremly rough and stop... makes for interesting docking. I had it on the muffs today and decided to remove the tamper proof device on the idle mixture and adjust it. It runs a whole lot better now. I thought I had solved the problem but during my testing the problem came up again. What happens is I advance the throttle toward 1500 rpm and it will suddlenly drop 200 rpm and run rough. When I bring it back to idle it is running between 550-600 and very rough and will eventually die. When restarted it runs fine and most of the time i can rev it up and return to idle just fine. When it acts up if I can manage to get the revs back up it still won't run right until I restart.

Do you think the ICM could be causing this? I tried to check the timing at idle during one of the incidents and It appeared that it was off perhaps 16 instead of 10.

Thanks

 

[jerryjerry05]

Re: Idle problem merc 2005 4.3l carb

Possible shift cable problems.Possible shift assist switch sticking.Jerry

 

[Coors]

Re: Idle problem merc 2005 4.3l carb

http://forums.iboats.com/showthread.php?t=167561

 

[Don S]

Re: Idle problem merc 2005 4.3l carb

Idle problem merc 2005 4.3l crab

To pass the EPA standards, carbed engines are adjusted as lean as possible, and it does affect idle, dieseling, and engine life. Adjusting the idle mixture richer will absolutely make it idle better, stop dieseling, start easier, and last longer. Illegal in some states, but the state laws don't affect how the engines run.

I doubt you have an ICM problem, but I think I know what it might be. Do you have an Alpha drive? Does this happen when you slow down still in gear, or only when you go to neutral?

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Thanks for the thoughts jerryjerry and coors. Don, I must thank you for the original thought on the idle mixture. I can't believe how much better it runs! Acceleration is also better!

When I reproduced the problem I was on muffs and was simply using the throttle with the shift bypass. It does not seem to matter if it is in gear or not. It is an alpha drive. It is very random. When it is running everything is very smooth and I can go through 20 acceleration/deceleration and not have it happen or only do 3 acceleration/deceleration and have it happen.

 

[Don S]

Re: Idle problem merc 2005 4.3l carb

It really sounds like one of the problems from the new style shift interrupt switch, and being as you have an 05 model makes it even more so.
If you can get this to do it on muffs, that is great, get it to running rough, then disconnect the shift interrupt switch. If it smooths out. Replace the switch.
That same switch is used on outboards and has caused all sorts of wierd problems.

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Thanks Don, I will try it and see what happens and will post the results.

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Hi Don, I just did a test and when the condition happened I disconnected the shift interupt switch and it didn't help... it continued to run rough.

What makes me think of the ICM is that if I turn off the engine and restart it right away it runs fine.

Do you think the Knock retard sensor could be faulty as an alternative to the ICM?

Thanks.

 

[Don S]

Re: Idle problem merc 2005 4.3l carb

What's the SN of your engine?

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Hi Don, it is OW335975

 

[Don S]

Re: Idle problem merc 2005 4.3l carb

The first thing I would do is unplug and look at all the connections to the ignition module and the knock module. Both the module and the harness should be clean and shiny silver. If they are black or white, that is corrosion and not making good contact.

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Don, i pulled the connectors and looked at them and they look fine. Once I reconnected them I tried again and couldn't get it to happen . It doesn't mean it is solved but I will test again tomorrow and hopefully that was the issue.

Thanks so much for your help! I really appriciate it .

One other thing. When I adjusted the IDLE mixture i put it in base timing mode and adjusted in until the engine just started dropping rpm and then did a 1/2 turn out. Is this correct?

Thanks!

 

[Don S]

Re: Idle problem merc 2005 4.3l carb

One other thing. When I adjusted the IDLE mixture i put it in base timing mode and adjusted in until the engine just started dropping rpm and then did a 1/2 turn out. Is this correct?

Not exactly.
Adjusting a carb is not an exact science (like fuel injection) it's more of a feel, and how it sounds and reacts. I adjust by those methods, some adjust by vacuum gauge, I never learned that or tried. But what I do know for a fact is that it doesn't happen instantly.
You have to adjust till the rpm drops, back off the adjustment till it smooths out, goose the throttle a couple of time to clear it's throat, then do it all again. Count the amount of turn each time. Once you find the point it smooths out, back the adjustment screw out another 1/8 turn. Then goose it again and make sure all works like it should.
This method seems to work the best for making an engine run the best, shut off the best and start the best. But it has nothing to do with what the EPA wants, which is to run the engine so lean that it self distructs, diesels when you shut it off, and shakes the boat to death when you let it idle. If that is happening, then you have it adjusted to the PROPER specs.

 

[ShayneA]

Re: Idle problem merc 2005 4.3l carb

Thank you for the detailed explanation. I will follow the procedure you outlined. I find it amazing that there are so many new engines out there running in so lean and poorly... I am sure glad I decided to 'detune' the engine . Never ran so good.