ccarloss

496HO surging

Took the boat out for the “frostbite” cruise Saturday when I felt something strange. The port engine would occasionally surge. I’d shut it down for a minute or two and then it would run fine for about 10 minutes and start surging again. The tach would show a drop of 200-500 rpm’s and then it would pop. On one occasion it dropped about 1000rpm, popped and stalled. If it was in neutral, I could rev it cleanly at 2k RPM’s constant and blip it to 4500 and it was still very clean. When I put it in gear it would start surging again after a few minutes. The engine has approx 280 hours.

Has anyone experienced this or have any ideas?

The 496’s don’t have distributors but 8 individual coil packs and a crank position sensor on the back of the engine and a cam position sensor on the front of the engine (according to Mercruiser diagrams). I am planning to order a scan tool today (any recommendations?) Hopefully whatever it is will throw a code because I don’t want to just throw parts at it. My initial thought is that it is the cam position sensor that is going bad. I understand what the cam position sensor does but I am not sure how the crank position sensor fits into it and I want to make sure I replace the right part.

Any ideas????

Thanks.

Ric232

Probably not this, but check to be sure your breather and PCV lines are connected properly on both ends. Usually this will cause a high idle condition instead of low.

Fuelish Pleasures

I have heard that the surging is usually caused by the timing advancing and retarding...hunting so to speak. Is your gas fresh gas or was it in storage through the winter?

ccarloss

It was fresh gas. I filled it up on the road prior to going to the ramp. New fuel filters also.

Dixie Doug

Fuel Filter or fuel pump screen

BajaRunner

i had one do the EXACT thing. fuel filter. It would rev up fine, but at WOT it would surge. start there.

Tricky919

Was it doing this at full throttle or all throughout the rpm range?

ccarloss

It was happening all throughout the rpm range. I first noticed it at about 4200. I shut it off waited a few minutes while a looked things over and then it was fine for the next several minutes. The next time it started at around 3500 RPM's then it did it at 2700 RPM's. I shut it down and after a few minutes was able to fire it up and run the rest of the way home at about 2800. Each time I refired it the engine would rev freely but once under way it would surge after a few minutes. All gauges read normal.

ccarloss

Ok, I believe (hope) I found the problem and thought some people may want to know what it was.

I purchased the Diacom scan and data log software from Rinda (nice package) but showed no faults. However, I did remember reading about people having a problem with the boost pump on the 496's. So I pulled mine apart and sure enough, the inlet side of the pump (metal screen) had all kind of crap in it. It is amazing that the engines ran at all. Basically once the fuel leaves the tank it goes through a boost pump before entering the fuel/water separator, cool fuel setup and fuel pump. It looks like Merc later added an inline fuel filter before the boost pump on later models to prevent this problem. My setup is a 2001.

The reason I say I hope this is my problem is because the good engine appeared to have more crap in the boost pump than the one that was surging. I did take it out for about an hour on Saturday and ran it up to 4000 - 4500 and once to 5000 and no hiccups. I also recorded the engine parameters that the ECU monitors with the Diacom data logger and everything looked good so hopefully my problem is history.

rv

Good to hear you solved your problem. Thank you for following up with the results. It is always nice to know what the problem really was.