1998 4.3L RPM Drop

[Johnnymc]

I have replaced most

engine parts on my new remand 4.3L and the engine currently will drop the RPMS when the boat gets up on plane.


The problem starts at 4,000 rpm but will still drop at 3,000 to approx. 1200 RPM. The engine is a new remand, as is the carb, ICM, Dist Cap, Rotor, and coil. I was going to see if this problem stops if I disconnect the tach or - coil feed to the ICM and run the boat without over rev protection..


Is there something else I should consider before this move.

 

[Rick Stephens]

Crappy fuel
Fuel pump
Anti syphon valve on tank plugged up

 

[wahlejim]

Fuel Vent, all of Ricks suggestions.

Isolate the problem. Take a run on a remote tank. If it runs great, start looking at fuel supply issues. If it still has the same problem, start looking at the engine.

 

[Johnnymc]

Thanks for the ideas. I should have noted that I have installed a new fuel pump, removed all (most) old fuel, new anti syphon valve on tank, new water filter. The old gas had about 40% water in it and I was able to pump all but 1/2 litre out of the tank. Still your idea of a remote tank has merit as I might have collected the remaining bad fuel in the separation filter.

When it first went into the water it idled fine, once I accelerated the engine it quit and wouldn't restart, that's when I started to look at fuel as the culprit, now it starts and idles fine will accelerate but will not hold RPMs. IF water is the culprit (remaining bad fuel) I would expect the engine to be rough throughout RPM operating range.

 

[Rick Stephens]

As you work on it, and clean odd and ends, the problem can change to plugged ports in the carb instead of issues with the feel lines, filters and pump. A reman carburetor is a very likely source of issue from the beginning. Starting out with bad fuel and getting it cleaner as you go is the only direction you can go, of course, but it doesn't take much to plug up a carburetor and can't begin to count the number of people who needed to re-rebuild a rebuilt carburetor. You may need to recalibrate the carb as well. No way to know what a rebuilt carb was calibrated to.

You might post your engine serial number and which carb it has. Make it easier to follow along.

Also would do more diagnostics instead of just throwing more parts at it. Not picking on you, but you've replaced too much already.

 

[Scott Danforth]

how long did the boat sit with old fuel in the tank.

by getting most of the old fuel, you meant that you got it all, correct?

it doesnt take much sludge to make it to a carb to gum things up.

 

[Johnnymc]

Thanks for the valuable input. The original serial number 0L322808. It's was a neglected Trophy 2052FD out of Louisiana that needed a total redo from front to back. The original engine block and intake manifold was cracked because of frost. I knew this before buying it, so that at least was not a surprise.

Have no idea how long it sat before I bought it, but judging by the amount of water in the tank it must have been for some time. The original tank fuel sender was rusted through to the tank, so that was how the water leached in. I pulled all but 1/2 litre (guess) of the old gas out, it's tough for me to say that 100% was removed, tilted the boat up and it was on an angle so all the fuel eventually stayed in a corner where I got everything I could see.

Fortunately I get dealer pricing on Merc parts, so that made this project worthwhile, otherwise I would have chosen another boat.

It has a new Mercruiser 2 bbl carb.

The engine runs well at idle and accelerates up to 4,000 rpm without hesitation then it backs off on its own to 1200 but will continue to run smoothly. Since the ICM reads over rev situations I started to look there.

I think at this point, it's relook at the fuel supply, put a new water separator in it and then test out fuel from a remote source. At least this will take out the question of fuel.

If there are some other ideas out there I am all ears. Cheers!

 

[Rick Stephens]

New or not, might check float level and needle valve. The weird one is I'd expect the engine to run rough at 1/2 throttle when it drops to 1200 RPM on its own. If it does it again, pull a spark plug and see what it looks like at that moment.

 

[Bondo]

I think at this point, it's relook at the fuel supply, put a new water separator in it and then test out fuel from a remote source. At least this will take out the question of fuel.

Ayuh,...... Welcome Aboard,..... When ya pull the filter on it now, check it's contents by pourin' the contents into a clear container, to see what's in there, besides clean fresh gasoline,.....

 

[Scott Danforth]

did you clean the tank, or just remove the little bit of goo left in the bottom.

most likely the sides of the tank were coated with dried goo, adding new fuel simply loosens the crud.

as Bondo stated, pour the contents of your fuel filter into a clear container and look (replace with a new fuel filter)

then pull the carb apart and clean that

 

[Johnnymc]

Thanks every one. Will have a look at fuel system on Monday and report back.

 

[Johnnymc]

Well ran the motor with a new water separator and fresh gas from a separate tank. The 4.3 did the same thing and dropped RPM's. My local tech says the next thing is to measure the fuel pump pressure before and during the fault to see if the pump is working properly, as well I will check the voltage to see if there is a VD at the same time. He says the pump should show an increase in pressure when the motor is running at 4k rpm.

Does anyone know where I can find the specs for fuel pump pressure. I have gone on the web and have been unable to unearth any information.

 

[Rick Stephens]

Carbed engine 4-7 psi.

 

[Johnnymc]

Thanks Rick

 

[Johnnymc]

Latest update: Fuel pressure at idle is just over 7psi, voltage 11.5. Fuel pressure stays at approx. 7psi while the motor rpms build to 3500, as we approach 3500 there is a drop in RPMS to 1000 to 1200 fuel pressure is just below 7psi, with the vibration from the engine the needle was moving quite a bit, but there was virtually no change in fuel pressure when you get RPM drop as well voltage was at 13.2v.

I have chatted with my local Merc. tech and we are at a loos as to our next move, so any help here would greatly be appreciated.

 

[Johnnymc]

Latest update: Replaced the Distributor Sensor Assembly per Mercruiser, same problem exists. Another call to Mercury.

 

[kulle]

Dont rule this out, happened to me...boat ran like $hit, no power, low power, would always run fine at idle or very low acceleration. I went down the path with the fuel, carbs, etc. ruled it all out only to take my out drive off to replace u-joint bellows and found the attached lodged in my exhaust - CULPRIT.

 

[Johnnymc]

Thanks for the information. Just by chance am I looking at a piece of the shutter assembly from the exhaust. I found one piece in the leg and was unable to locate the other three pieces.

Thanks,

 

[havoc_squad]

If there is a spot on the motor for direct intake vacuum, you should be able to put a vacuum gauge on in line and do a vacuum test for on the water in gear to identify an intake or exhaust restriction.

 

[Johnnymc]

Many thanks I will look at this.