Mercury 250hp issues… warranty coverage?

[A-a-Ron]

Alright so a bit of a rant here but more so looking for advice on possible diagnosis, and more importantly and I getting a bum steer here regarding warranty policy?

Long story short, I’m having fluctuating RPM’s in my cruising range (3800-4400 RPM). The issue is getting worse now to the point where it will fluctuate 500 RPMs all by itself while cruising in dead calm water when in this RPM band. Once I cruise above 4500 RPM it is absolutely smooth and runs perfect as soon as I drop back into this RPM band the RPM gauge dances all over the place.

Last summer, I had a fuel in water alarm, so I pumped the tank dry and changed both of my fuel filter water separators, which I do every 50 hours anyways highly unlikely to be a fuel issue unless it’s fuel delivery i.e. fuel pump but again it doesn’t happen above 4500 RPM

Google university tells me it could be an O2 sensor, which would be warranty covered. Also could be intermittent ignition issues. My fuel efficiency has gone down from 2.5 MPG to 2.1 MPG now that this issue is consistently happening (another symptom of a faulty o2, apparently)

My main question regarding warranty is I am being told by M&P Marine that even if the issue is confirmed to be something warranty related, they want to charge me $750 for the water test to diagnose. Even if it is a warranty covered problem. That seems ridiculous to me, and I am wondering if that is true or if I’m just being given a story because nobody likes doing warranty work.

Also, any thoughts on what the root of the problem could be would be appreciated. Thanks.

 

[kingblazer84]

I know atleast im my world of automotive and if it's warranty, the diag is always covered by the manufacturer within reason (Diag charge applies but gets zero'd once its been confirmed its a true warranty scenario), so the shop has to be willing to eat some labor to get labor through warranty and they might be trying to double dip if they dont get paid enough for the warranty claim ie diag and repair. Warranty sucks at the best of times. Are you a loyal customer to them? I would talk to the service manager and see if he will wave the fee if its warrantable.

codes? now this is different from my background of automotive repair but that being said it all functions the same way as a vehicle engine. I would love to know what the live data shows from the o2's and also what your fuel pressure is at when below 4500rpm, when you get water in a fuel system it can create havoc.. especially with precise components like injectors etc. The ignition misfire underload is possible but without codes/live data you would just shooting a parts cannon at it..

 

[seachicken]

Ya I would think they can plug something in and see if there is a code. In today's modern world I wouldn't waste time and effort on checking for water in the fuel. Might be cheaper to pay them to plug in the diagnostics computer. Water in the fuel would be more consistent misfire throughout PRM range. Fuel mapping can change with RPM range and the o2 sensor is part of that so you could be barking up the right tree but that parts cannon gets expensive.

 

[ab1752]

That sounds a hell of a lot like an air leak in the fuel supply line. I just went through this with a buddys boat band it was a poorly sealed raccor 90 fitting. 3 cents of tape and fixed. It was pulling air and was a real pita to find. Have you tried running this thing on a Jerry can yet? I had to do this to isolate the raccor.

This boat was bad in the same rpm range, then nail it and it would hit full rpms. But you could feel it was struggling and was bad again once you throttled down.

 

[A-a-Ron]

That sounds a hell of a lot like an air leak in the fuel supply line. I just went through this with a buddys boat band it was a poorly sealed raccor 90 fitting. 3 cents of tape and fixed. It was pulling air and was a real pita to find. Have you tried running this thing on a Jerry can yet? I had to do this to isolate the raccor.

This boat was bad in the same rpm range, then nail it and it would hit full rpms. But you could feel it was struggling and was bad again once you throttled down.

Interesting. I mean, I could try that, simple enough for a test. But with mine, above 4500 RPM it’s fine, like I can cruise 4800RPM at 35 knots for a solid half hour and it doesn’t flutter once, feels great. And even floor it from there to WOT and it’s not struggling at all. You would think an air leak like that would show up there as well.

 

[InterMechanico]

Do you have electronic or cable controls? It could be a throttle position sensor wandering

 

[ab1752]

Interesting. I mean, I could try that, simple enough for a test. But with mine, above 4500 RPM it’s fine, like I can cruise 4800RPM at 35 knots for a solid half hour and it doesn’t flutter once, feels great. And even floor it from there to WOT and it’s not struggling at all. You would think an air leak like that would show up there as well.

I found flow rate increases when you nut it so the issues are worse under less throttle. Ideally do this test under load at sea. You just need a small Jerry can and a buddy to make sure the supply line is in the gas for suction. It isnt enough to just reef on the clamps, you gotta rule it out fittings and that ******* raccor piece of **** that stole too many hours of my life haha.

 

[A-a-Ron]

Do you have electronic or cable controls? It could be a throttle position sensor wandering

Mech/cable controls

 

[tiller]

Very good chance it is the canister style fuel filter in the engine. They can be a total pain. Supposed to be changed every 300 hours i believe, but some operators change them every 50 hours as they have rpm fluctuations. Merc said the issue could be the dye in the marked fuel.

 

[A-a-Ron]

Very good chance it is the canister style fuel filter in the engine. They can be a total pain. Supposed to be changed every 300 hours i believe, but some operators change them every 50 hours as they have rpm fluctuations. Merc said the issue could be the dye in the marked fuel.

I change that little ******* every 100hr and I even changed it again before the last outing.

I trailer so I always fill with unmarked fresh (in theory) gas station fuel. And I run Stabil marine 360.