5.0 coughing/popping at high rpms

[MBAKER]

Need some help on a 2000 carbureted 5.0 Alpha one.



At the beginning of the season I went through the carb, and put in new plugs, wires, dist cap and rotor. Took it out for a test and to dial in carb and it ran perfect. Went out a couple more times with no issues. Then took it for a week long lake vacation and started having problems. It would start and idle fine and run fine up to just under 4000, but if you pushed it passt 4000 it would 'cough' or 'pop' a few times like it was blowing back through the carb, but wasnt constant just 3 times or so and then stop. Im not sure how to describe it either. Its more like a coughing sound because it doesnt really have a sharp crack to it like I think of a pop but it was pretty forceful sound.


First thought since we had just filled up from a different pump that we might have had some bad gas. So we changed the fuel filter, and dumped in some Merc fuel treatment. Ran through most of that tank refilled and thought it acted better. But then the coughing came back.


Next since we had replaced most of normal ignition parts in the tuneup I decided to go ahead and change the coil. Again, seemed better but the coughing is still there.


Something seemed to help as its not as bad as when it first started. But im kinda stumped now And nothing changed other than going to a different lake. It ran perfectly fine before this earlier in the season and it never acted like this before the tuneup either.


Any suggestiong or thoughts?

 

[TilliamWe]

Re: 5.0 coughing/popping at high rpms

Is the bottom of the boat dirty? Fouled with growth?

 

[MBAKER]

Re: 5.0 coughing/popping at high rpms

No its clean.

 

[Don S]

Re: 5.0 coughing/popping at high rpms

Never just stick a new filter on an go. You have no idea what the problem was or could have been.
Get a new fuel filter, remove the old one, empty the contents into a clear container and see if you have water in it. If it does, then you need to get the water out of the gas tank or you are just going to fill up another filter. And never re-use a filter after you empty it.

 

[Failproof]

Re: 5.0 coughing/popping at high rpms

Leaning out for some reason i would guess. Partially plugged jet? Does it pop as the load increases? Eg when above 4000 running into the wind vs with it?

 

[MBAKER]

Re: 5.0 coughing/popping at high rpms

I was going to dump out the filter and look at it, but managed to drop it on the way back to the truck. Ill pull this one off and look at it now this weekend.

If it was leaning out wouldnt it do it more consistently, not just 2 or 3 times and keep going? Pulling a skier or tube from idle to 3500 never misses a beat but as soon as it goes to about 4200 is when the problem comes up. And its not everytime 1 out of 5 times it may run up wihtout issue, but 4 out of 5 it will cough around 4200.

 

[Failproof]

Re: 5.0 coughing/popping at high rpms

Which carb do you have? I would say to hook up a timing light and see if you have a loss of ignition (so slight that the tach does not pick it up) could be pick up in distributor, coil, cross arching, tach shorting at that rpm, and so many other things. Knock sensor dropping the advance, water in fuel, a clump of sludge in carb bowl, and on and on.

 

[MBAKER]

Re: 5.0 coughing/popping at high rpms

Its is a Mercarb...also no knock sensor.


If it was indeed a fuel problem, would it be more sustained not just a few times at a certain rpm?

 

[Failproof]

Re: 5.0 coughing/popping at high rpms

Which carb do you have? I would say to hook up a timing light and see if you have a loss of ignition (so slight that the tach does not pick it up) could be pick up in distributor, coil, cross arching, tach shorting at that rpm, and so many other things. Knock sensor dropping the advance, water in fuel, a clump of sludge in carb bowl, and on and on.

 

[MBAKER]

Re: 5.0 coughing/popping at high rpms

Ok been doing some searching and came across several threads about problems with Mercruiser replacement dist caps. Is there a certain cap style or number that had problems?

 

[dingdongs]

Re: 5.0 coughing/popping at high rpms

have a look under the engine in the pitch black to see if theres any arcing going on.you may be suprised.

 

[MBAKER]

Re: 5.0 coughing/popping at high rpms

Im gonna check it with a vacum guage and see if that tells me anything about the valvetrain operation and try to rul out a sticky valve or weak springs. Then Im gonna check over the wires, and check the sensor in the distributor and pull the plugs. If it still does it Im gonna try to the old cap again. I read several other posts where a new Mercury cap was installed and it was fine for a few outings then all of a sudden starts acting up so its worth a shot.

Ill still check the fuel filter, but I feel like if it was fuel it would be affected at all rpms not just a 2-300 rpm window.