Rough Idle on 2007 236 with 6.0L

[Chris Parker]

Popping my first cherry post,
I bought a 2007 236 with the ZR6 motor in it a month ago, and have been out 5 times with no problems with engine. Last weekend I went out and the boat just stopped cold in the water. Got back home, pulled the low pressure filter(which is a aluminum filter with a screen that threads into low pressure pump) and the screen had very little gunk in it. Pulled the FCC filter housing and it too was pretty clean. I then checked power to the pumps when start button was pressed. No power to either pump. I then checked fuel pump relay, and it was bad. I bought a new FCC filter, cleaned the lower pressure filter screen, bought a new fuel pump relay, and boat started right up on the RV pad. I went to the lake yesterday and right away I could tell the engine was running rough. I check the engine for fuel leaks and I could not find any. I tried to board anyway, and with 10 people and full ballast the boat would only do 12mph. Any ideas on what I may have done wrong with the filter change?

[Paxdad]

When you started the boat out of the water did you have it connected to a water hose/ fake a lake/ etc. ??

[Chris Parker]

Yes I have a fresh water flush connection on transom and it was hooked up. Why do you ask?

Next thing I am going to test today is what ohms the fuel pumps read. Anybody know what the specs are?

[Paxdad]

I saw a person with the raw water cooled 409 think that they could run motor out of water and that the closed cooling system would keep engine temp normal.The result is that it cooked the sea water pump and melted the impeller which in turn caused the motor to lug itself down while the serpentine belt was spinning on the pump's pulley.

[Chris Parker]

Good call, I may take the belt loose and try to spin it by hand to see if there is resistance.

After work today I ran the boat at home with fuel pressure gauge hooked up to fuel rail. 60PSI at idle. I could bump the throttle and there would be no drop in fuel pressure. It probably wont be till Friday till I can get it on the water with the fuel pressure gauge hooked up to see if it drops fuel pressure at WOT.

[Paxdad]

Good call, I may take the belt loose and try to spin it by hand to see if there is resistance.

After work today I ran the boat at home with fuel pressure gauge hooked up to fuel rail. 60PSI at idle. I could bump the throttle and there would be no drop in fuel pressure. It probably wont be till Friday till I can get it on the water with the fuel pressure gauge hooked up to see if it drops fuel pressure at WOT.

The pump will have some resistance if the impeller is in good shape. Keep in mind the pump housing has a cam that compresses the impeller vanes allowing it to generate suction from the through hull intake. The pump I saw as stated in my earlier post would not even turn one bit and the belt was spinning over the pulley.

In a way it almost sounds like the boat is in the limp mode based on what you are saying is max of 12mph.

[Chris Parker]

Took the boat out friday with just me and the old lady,check fuel pressure, its right at 60 psi at wot. The boat seemed to have good power, still the idle seems a bit rougher than when I first bought it. I also noticed a slight belt squeal. Tonight I pulled belt off, and tried to turn pulley for raw water pump. It seemed like it took to much effort and was not smooth. I pulled the impeller out and attached photo. Is this impeller bad? Could this be causing light belt squeal and a lower than normal idle? Thanks for the help.

[Quinner]

I would replace that impeller keeping the old for a spare. Impeller will create some resistance when installed, does the pump turn easily with no impeller? When is the last time plugs and cap were changed? Would inspect them, the cap for sure.

[Chris Parker]

I will check that the pump spins freely. The zr6 is a coil pack ignition, not cap and rotor

[Paxdad]

The vanes on the impeller are all intact so that is a good thing however, from the photo it has seen some heat build up as the vanes are orientated over. Most of the vanes should straighten out when removed from the pump except a few where they were resting on the cam plate. I would replace it as suggested above and keep only for a spare.

[max 1.6]

It seems I have the exact same problem. I have the 6.0 409 and it was running perfectly fine last year.
I started it this year and it now idles rough and dies. It starts fine but goes rough after 10 sec.
I have old fuel in the tank with stabilizer (6 months old).

[Quinner]

Definitely add 5 gals of fresh fuel, todays fuel goes bad very quickly, mine has run poorly on the old fuel even when treated, I now always lay it up with minimal fuel in the tank.

[max 1.6]

The problem was the transfer fuel pump, it was fried.