5.7 INDMAR MONSON THUMPING SOUND

[Jarrell5]

I am in need of some help The end of last year my 2004 VLX (Indmar 5.7 Monson) started bogging down while surfing, making a thumping, missing sound. I replaced the plugs, wires, rotor cap & inline fuel filter at winterization. I took the boat out for the first time yesterday and it started right up, idled great, but as I pulled away from the dock and pushed throtle 1/2 way it started again....the thumping, missing, bogging sound. If I give it full throtle it seems to work thru it and smooth out. I have noticed at full throtle the speed will sometimes vary on occasion without the thumping sound.

Before going out I put in 30 gallons of fresh gas, but am wondering if I could have water or trash in the lines.

We were out on the lake all day and the boat cranked everytime and never shut down.

What could cause this?

Fuel pump?

Fuel Rails?

Water in tank?

Trash in tank or lines?

Maybe I am wrong by thinking it's a fuel system problem. Any help recieved would be greatly appreciated.

Thanks,

Matt

[areamike]

Almost sounds like timing.

[tvano]

is the engine misbehaving or could it be the damper plate giving you a warning?

[sundaydrivers]

Read my post ***Before you replace your fuel pump****

I had these symptoms.

Whining from fuel pump

Hesitation/bogging down when trying to accelerate quickly.

bogging down above a certain speed.

misfire

There is a small filter on the intake side of the fuel pump that could be clogged. As soon as I cleaned it out the boat was fine. Takes only a few minutes to clean it out with some brake cleaner or PB Works. Sounds like your fuel pump is starving from the clogg and it would cause all of your symptoms.

Edited June 1, 2010 by sundaydrivers

[Lance B. Johnson]

sounds to me like a single cylinder misfire (you described thumping). If so, you can rule out the things that would cause issues in all cylinders like timing, fuel pressure etc.

Start by removing the plug wires one at a time while the engine is running and note how the engine runs (use high tension pliers or )

If the engine runs worse, than that cylinder is working. If there is no change than that cylinder is the problem.

Things that can cause that misfire:

Bad/ clogged injector

Low compression (do a leakdown test to confirm)

Bad plug

bad wire

Bad Cap

If two or more cylinders (not adjacent) it could be carbon tracking in the cap

IF two cylinders adjacent could be blown head gasket.

This one is tough because I rely on my ears so much to tell me. It could be a single cylinder or then again maybe not.

[Jarrell5]

sounds to me like a single cylinder misfire (you described thumping). If so, you can rule out the things that would cause issues in all cylinders like timing, fuel pressure etc.

Start by removing the plug wires one at a time while the engine is running and note how the engine runs (use high tension pliers or )

If the engine runs worse, than that cylinder is working. If there is no change than that cylinder is the problem.

Things that can cause that misfire:

Bad/ clogged injector

Low compression (do a leakdown test to confirm)

Bad plug

bad wire

Bad Cap

If two or more cylinders (not adjacent) it could be carbon tracking in the cap

IF two cylinders adjacent could be blown head gasket.

This one is tough because I rely on my ears so much to tell me. It could be a single cylinder or then again maybe not.

Thanks for the advice. Is this something that I can test at idle out of the water since it only seems to do it while 1/4-1/2 throtle?

[Jarrell5]

is the engine misbehaving or could it be the damper plate giving you a warning?

Not sure what a damper p[late is, but I am about to look it up. Thanks for the response!

[wienrdog]

The thumping & stumbling sound like a backfire (or bad knock) to me. Could be timing, could be a lean condition (partially clogged injector) in a cylinder (or two).

The damper plate goes between the engine & transmission to absorb some of the shock of putting it in/out of gear. It will typically rattle at idle speeds if its starting to go out. It won't cause the engine to stumble.

Could you have mixed up one or two of the wires when you swapped them out?

[Pistol Pete]

Not sure what a damper p[late is, but I am about to look it up. Thanks for the response!

Damper plate

[Lance B. Johnson]

Thanks for the advice. Is this something that I can test at idle out of the water since it only seems to do it while 1/4-1/2 throtle?

You will have to pay close attention. Guess what I am trying to say is that with a V8 the difference can be subtle. I have had many people come in and say that their car idles fine but when I listen I can hear a dead cylinder. Just pay close attention, you could have someone watch the tachometer and record the rpm drop as well.

But without hearing it, I will have to say it could be almost anything that the others mentioned. Good luck to you

Edited June 2, 2010 by Ruffdog

[Jarrell5]

Update----Last night I pulled the fuel line from the bottom of fuel pump (thanks sundaydrivers!!!) and with a mirror and flashlight found a black ball of something blocking a potion of the screen. I used carb cleaner and some Q-tips to remove it and is now clean as new. I am crossing my fingers that this resolves my issue. Heading to the lake in a few hours and will let you know if it worked.

Thank you for all the responses!!!!!

[dalt1]

Well you should be back home by now, did it work?