Mercury 115 Hp 4stoke low rpm misfire

[raymor]

I have a 2005 Mercury 115 hp 4stroke. It starts fine but runs very rough and sounds like it is running on two cylinders. It runs this way till I can get it above 2500 rpms. I have replace the Yamaha fuel filter/water separator and the internal fuel filter. Pulled the plugs and all look normal and gaped the plugs to the spec. Are there any sensors that would make this motor run very rough at low RPMs. I can get full power and RPMs once I make it past the 2500 Rpms range. Most people here seem to have problems with this motor on the upper RPMs with bogging down, but I don't just the lower RPMs. Thanks for any help anyone can be.

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Bump

 

[patbalmos]

Re: Mercury 115 Hp 4stoke low rpm misfire

Try cleaning your IAC valve. If you do a search you will find some pics of it. usuall a good quality carb cleaner works well. Also try draining your vst housing and see if you get any water and crud out of it.

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Thanks for the reply Patbalmos. Would the IAC valve make the motor sound like it is missing as I thought all it did was control the air flow. I know where the IAC is located and will give it a try, but it almost sounds like it is the spark is intermittent at low RPMs. Would the MAF have anything to do with my problem? I will take a look at the IAC and report back here. Thanks again for the help!

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Well cleaned the IAC today and no change. There was hardly any carbon on it. I checked the coils with a inductive timing light and one have of one coil seems to be intermittent. Guess I'll be picking one of those up tomorrow. Will report back with the results.

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Well check the coil last night when I got home using a ohm meter and the primary and secondary was in spec. Now I'm not sure whether the coil is breaking down or it is a timing problem. Guess the next step would be to check out the timing.

 

[patbalmos]

Re: Mercury 115 Hp 4stoke low rpm misfire

I would try to rule out the cheap and easy things first.

 

[patbalmos]

Re: Mercury 115 Hp 4stoke low rpm misfire

I would try to rule out the cheap and easy things first. Does it run the same way when the engine has fully warmed up?

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Runs pretty good till it warms up then I get the misfires. Thought it might be the temp sensor so disconnected it, but still had misfires and the idle speed went up about 200 rpms.

 

[raymor]

Re: Mercury 115 Hp 4stoke low rpm misfire

Well found the answer to this problem. It ended up being the plugs. Replaced all four at $7.29 each (what a rip off). These were NGk LFR6A-11. There must have been more than one defective plug as the problem did not move with the swap of the plugs. Don't think I have ever seen a failure like this in a plug unless it was bad right out of the box. So following Patbalmos advise with go with the cheapest part first seem to work as it was either the coils, wires or spark plugs. Thanks again for the help.