496HO Surging?
 

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.

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.

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?

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

Fuel Filter or fuel pump screen

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

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

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.

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.

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.

Thats exaclty what happened to me, thats why i was asking if it was a wot, my problem was only at wot. I posted about this a few times in other threads, the problem drove me nuts for a few days. They sell and in-line stainless filter from merc for this problem. I also used a scan tool and showed no problems......I glad you found it........

I'm interested in the gizmo you purchased to check the engine. Give us some more details like website, model number, and cost. I'd like to have one myself...

The software is called Diacom and I got through Rinda. http://216.230.203.183/marine/marine.htm It was $579 They also have the regular Mercruiser handheld scan tool which is I believe $399

IT'S BACK
 

Well, back to the drawing board. I hoped that I had found the problem in the fuel pump having crap in it but I guess that wasn’t it. I was able to go out twice and have no surging problems but this past weekend the surge came back and seemed worse than ever. Thankfully (sort of) I had my scan tool available. I was able to record the ECU parameters as the engine was surging. Basically the engine will drop anywhere from 300-1000rpm until it pops, then stalls. I can re-fire it and rev it in neutral all through out the rpm range and everything is fine mechanically. Below are some screen shots from the ECU recording that show a 700RPM drop but minimal to no change in any of the ECU parameters. The ECU is showing NO fault codes either. I am basically at a total loss here. I’ve talked to 2 different people who believe that there could be an electrical problem such as a bad wiring harness. Has anyone experienced this or have any ideas? Thanks for any help you can offer.

Sorry for the shots being a little small. OSO is resizing them.

I suspect you may have a clog in the fuel system again. What you are describing is consistent with fuel loss, not spark. Whatever it was that you cleaned out of the boost pump is still in the tank and it has likely clogged the pump again. Or it made it through to the next choke point. You may need to do a tank cleaning and thorough inspection of the entire fuel route to find it. Before doing that if you can hook up a fuel pressure gauge, watch the pressure as it occurs-if it drops down you have another clog.

Thanks for the feedback Ted.

Had same problem
 

De-ja vu, It took me a couple seasons to figure out.
Actually dixie doug figured it out for me. Same thing
bunch of crap clogging the screen. Metal shavings and
bugs. Theres an old thread on this subject. I thought it was just fountain putting a tank off the shelf with junk in it. Great quality control. Every one with a 496 should
remove this pump and clean the screen. Mine was only
at wot, I was racing a 27 on cumberland and it started
missing, And I was winning, really John

Are you reading your scan tool when the surging begins under load? I would look for lean fueling condition which would back up Teds belief which is also my feeling that you are getting a fuel starvation condition under load that when you back off the throttle the computer is trying to overcome with a richer mixture setting and hence the engine begins to surge heavily or "hunt".
I would invest in a fuel pressure guage, connect it to the shraedder valve on the fuel rail and watch fuel pressures at idle and under load. Use the ranges on the good engine to crosscheck pressures on the surging motor. This will pinpont if your fuel system on the surging motor is having a problem.
Good luck, troubleshooting skill is what makes great mechanics!
Raylar

Thanks for the feedback guys.

Raylar, yes the scan tool data logs in real time then I can go back and replay the file. The screens above are actually two back to back frames. The 700rpm drop was instantaneous.

It looks like I'll be taking a look at the fuel system.

Gentlemen, thanks for the suggestions. Hooked up a mechanical fuel pressure guage and sure enough......fuel pressure was all over the place.....40-20-30-10-20-5-20 etc.

I pulled the boost pump off again and it had a bunch of crap in it again. I never thought that was the problem since I just cleaned it out but I guess there is something in the tank. It is like a brown paper substance that is clogging the screen in the boost pump. I may rig some inline filters before the boost pump until I can get all this crap out of the tank.

Any suggestions on a good inline filter?

We used Racor RC-S3228UL AND HAVE NOT HAD ANY PROBLEMS

I had the same problem with my "new" 2002 496 Mag HO - boost pump screen filled with junk. I think I found a real promising answer tonight on the Mercury Parts Express website.

Under engine accessories, they list a fuel filter kit that says "Prevents blockage of the boost pump inlet from debris that may be in the fuel tank. This filter is installed in-line between the fuel tank and boost pump. Standard equipment with all current engines using a boost pump. This filter will backfit older Mercruiser 8.1S/496 Mag engines."

The part number is 35-864572A 1. Depending on who you enter as the closest Parts Express dealer, the prices in my area ranged from $84.05 to $110.86. Also, they list a replacement filter, part number 35-864572T. Again, prices varied from $47.70 to $57.24. I'm in a hurry, so will probably pick this up from a local dealer, but BAM in Florida sells these two items for $73.54 and $45.30, respectively.

In addition, I like the idea of using a Racor filter in the normal water separating/fuel filter location, which is after the boost pump. It has a 10 micron filter rating, which is much better than the Mercury filter that has a 25 micron rating.

All these filters are kind of expensive, but well worth it if they eliminate that awful feeling you get when the engine bogs down when you're out on the water.

I'll get my fuel delivery problems resolved so that when Raylar gets his heads/intake/cam/ECM upgrade sorted out I'll be ready!

Thanks for the part numbers gentlemen. Those are rather pricey for an inline filter.......oh that's right........it's Merc. :)

Here's a question. Can a regular automotive carb inline filter work? Or would it be too restrictive?

I'm not qualified to answer the question about an automotive filter, but I thought it was interesting that Mercury includes the Racor brand filters on their parts store website. I take that as a strong endorsement for their product.