Rage

Does anyone know specifically what wears away fastest (cylinder wall or piston rings) when cylinders are washed with excessive fuel and this shows up as drop in compression? Or do both wear essentially the same. The rings in question are moly type and cast iron cylinder.

stevesxm

in my experience its the bores and the piston skirts that take the beating worst but it really depends on the metallurgy. if it was bad you would see vastly accelerated wear on everything and some vertical scoring in the bores. the ring lands will take a beating as well and you might see some dishing on the top surface of the compression rings if it ran that way a long time.

Smitty

iTrader: (1)

I had this happen to me on my new motors that were programmed overly rich from the start and I had to tear them down and rebuild them this winter.

After about fifteen hours running time, the compression was down 20-30 points per cylinder. What I noticed upon teardown was that the cylinder walls looked like they had been chromed and that the "crosshatch" of the hone was almost gone. No visible wear on the pistons at all. Basically lost alot of the sealing power of the rings due to fuel washing off the oil on the walls, thus causing it to be polished.

The fix is to re-hone and new rings if the motor is fairly new. If not then time to bore out cylinders.

Rage

This is exactly what I was hoping for .

After 36 hours cylinder # 1 is down to 125 psi and cylinder #3 is down to 135 psi from a post breakin of 150 psi which the other six cylinders still retain (compression test on a cold engine).

There are a number of possibilities for this low compression which will be investigated but one is fuel wash down (see recent A/F vs RPM data for my engine on all eight cylinders attached).

How rich was your engine that resulted in your loss in compression and how low was the compression when your rehoned the cylinders? Any similarities to mine?

Smitty

iTrader: (1)

My compression new was 150. After 10 hours it was down to 110-120 . The motors were supposedly set at 13.0 afr on low end to 12.0 afr at top end under boost. Can't prove if the tuners AFR readings were accurate or not

When I put them in the boat I was seeing readings of 10.0 afr at idle to 11.00 at top end under boost. One motor stopped running after 2 hours due to plug fouling issues.


Can't explain why so different in the boat. I paid someone to remap it but the damage was already done in the first 2 hours of running.

I have rebuilt both motors and will install them this spring and start all over. Summer of 2008 was a complete loss for me.

Smitty

iTrader: (1)

I just noticed that you still have 6 cylinders with 150 psi. I thnik you may have another issue besides wash down. When you have wash down on an EFI motor set too rich , all cylinders should suffer equally. If carbed then you may have a carb issue or just a poor set up of the rings in those 2 cylinders.

Rage

Smitty,

Yes, mine is a NA MPFI (Merc PCM555) and the only way it should be a wash down issue is bad injectors in the low cylinders. That is why the A/F test (data attachment above) on all eight cylinders to check for this. Since leak down was good on all cylinders the valve train is high on the list to check right behind a retest of compression on a warm versus cold engine. This was thrashed out in an earlier thread.

Thanks,

Bill

ezstriper

you better pull the whole engine down, if fuel broke down the oil it can kill the bearings or worse as well..Rob

Rage

Thanks ezstriper. The oil analysis showed that there was 3.8% gas in the oil resulting in some reduction in viscosity and additives but no abnormal wear products and that the oil was still functional when changed. So I should be ok for now but need to fix the gas in oil problem for the future.

bobl

Bill, some things to keep in mind when analyzing your A/F data log. These were separate runs that were superimposed onto the graph, right? Any throttle position change or MAP sensor change will result in a spike. So, each time you advance the throttle you get a squirt of fuel. As the attitude of the boat changes or even hitting a wave hard during the run can cause the MAP signal to change slightly. I'm guessing that is what is causing some of the spikes and valleys in your log. Overall the A/F values look to be about right, except at idle. I'm guessing the richness right off idle to 1400 rpm is due to throttle movement and load as the vacuum drop during acceleration. It's leanest at cruise, then drops to mid 12's at WOT, all OK. Seems like a leaking injector would show up most at idle. I would like to see your idle A/F closer to 13:1. That rich of an idle could be causing your excess fuel dilution, but I don't think would wash the rings and still only show 3% oil dilution.