496 HO MAG Running Rough Missing
 

I have replaced the plugs and checked compression. All cylinders between 162 and 172 PSI. I have replaced the 3-relays up on top as i has a FUEL PUMP RELAY CK code. I checked fuel pressure at the rail with engine off and turning the key on and have 43PSI. I will be replacing the wires and awaiting their arrival. something on the scan is that the IAC duty cycle is all over the place like between 10 and 40 (I guess %?). the "Whistle" of the intake sound is surging up and down and not smooth and steady like the other engine that seems to run fine. on the engine that seems to run fine I scanned it twice. the IAC was first all over the place while watching the first time and then was a steady 0.0% when I checked it the second time. what's this mean? also on the engine that is acting up the problem seems to somewhat come and go about that fast as the tach will move up and down 200-300 rpm with the throttle set at second notch about 1,800 rpm. this is all being done on the trailer in the driveway. I first started all of this because I am only getting 72mph for a top speed and is about 6-8mph off. What's your thoughts?

Should I install the fuel pressure gauge on the rail and leave it there while running it on the trailer to check to see if the fuel pressure holds steady? the pressure gauge I have has a release valve on the side so I don't know if this gauge has a check valve and holds the pressure or not kind of like a compressions gauge does.

Never looked in the throttle body behind filter/spark arrestor before but the engine that is not running correctly has no muffler. the port engine does. I installed muffler in the star engine and the surging of the intake air whistle is now gone and is constant, but still not running correctly. the idle is low. when moving throttle to first click it seems to struggle to get just above normal idle speed. second notch it struggles to get to about 1,600 rpm and then drops to about 1,000rpm. The other engine will hit 1,500 at first notch and about 2,300rpm at second notch.

I exchanged the IAC from the other engine.....same thing. I have approx. 40psi on the fuel rail while engine running. the thing surges like clears up a bit then goes back to stumble then will clear up some and pick up some rpm then drop back and about that fast. it does not sound like one cylinder.

engine serial # OW008002. 2005

Have you rescan the motor - if you do - does it show 100 % available power ? Any other codes / faults. Also check for vacuum leaks especially around the IAC area. Also have you checked for good solid spark from each coil. Use completely sealed inline spark checkers.

IMO you might need your fuel injectors checked out & serviced. 496 engines did have problems with fuel rail corrosion internally. The later ones hold up a lot better as I want to say 2001 - 2005 could be the problem years. When testing your fuel pressure, does your gauge stay steady. These motors really like 42 - 43 steady psi fuel pressure. Have you done an injector balance or dropped test ?

Does your surging happen all the time at all rpms or just @ idle speed ? Can you even reach max rpms ? How old is your fuel ? Run motor on a clean outboard gas tank and new fuel line with fresh fuel to see if you have a fuel delivery problem. With any problems you always want to separate the fuel tank / anti siphon valve out of the mix to pin point at least the problem lies on the motor side or the fuel tank area.

Have you rescan the motor - if you do - does it show 100 % available power ?

Always shows 100% power available and no fault codes.

Any other codes / faults. Also check for vacuum leaks especially around the IAC area.
I WILL CHECK FOR VACUUM LEAKS

Also have you checked for good solid spark from each coil. Use completely sealed inline spark checkers.

NO I HAVE NOT AND I DO NOT HAVE ONE OF THESE TOOLS

IMO you might need your fuel injectors checked out & serviced. 496 engines did have problems with fuel rail corrosion internally. The later ones hold up a lot better as I want to say 2001 - 2005 could be the problem years. When testing your fuel pressure, does your gauge stay steady. These motors really like 42 - 43 steady psi fuel pressure.

FUEL PRESSURE IS LOWER ON THIS ENGINE THAN THE OTHER ONE. JUST SITTING ON THE TRAILER N WHACKING THE THROTTLE TO 3,000 RPM I HAVE SEEN THE PRESSURE DROP TO THE AREA OF 37PSI WHEN BACKING OFF THE THROTTLE IS WHEN THE PRESSURE WOULD DROP...OF COURSE THIS IS WITH NO LOAD. I HAVE A NEW SPARE BOOSTER PUMP.

Have you done an injector balance or dropped test ?

NO. I DO SEE THIS ON THE SCANNER BUT DO NOT KNOW HOW IT IS CONDUCTED AND DO NOT HAVE A MANUAL.

Does your surging happen all the time at all rpms or just @ idle speed ?

I DO NOT KNOW FOR CERTAIN. I KNOW WHEN I MOVE THE THROTTLE TO THE SECOND NOTCH IN NEUTRAL THE RPMS WILL RISE TO APPROX 1,800 RPM THEN DROP TO APPROX 1,200 AND SET THERE AND SURGE UP AND DOWN.

Can you even reach max rpms ?

THIS IS A PROBLEM TO ANSWER AS WELL. I LIVE 1 1/2 HOUR'S FROM THE NEAREST LAKE AND WHEN I WAS OUT THE LAST TIME WITH KIDS, TRAFFIC AND CHOP AND SUCH IT WAS A PROBLEM TO EVEN ATTEMPT TO WOT THIS THING LET ALONE STARE AT GAUGES. I NEED TO GO AND TEST IT I KNOW. JUST TRYING EVERYTHING I CAN HERE TO HOPFULLY CURE IT BEFORE I DRIVE ALL THAT WAY AND HAVE TO BRING IT BACK RATHER THAN LEAVING IT IN THE RACK WHERE IT BELONGS.

How old is your fuel ?

I HAVE RUN A FOR CERTAIN FULL TANK OF FUEL THROUGH. I WOULD SAY IT WAS 1/3 FULL FROM LAST YEAR AND WAS WELL STABILIZED AND I ALSO RUN LUCAS TREATMENT ALL OF THE TIME. ALL FUEL THAT WAS PUT IN THIS YEAR WAS MARINA FUEL THAT IS 91 OCTANE AND HAS NO METHANOL/PURE GASOLINE.

Run motor on a clean outboard gas tank and new fuel line with fresh fuel to see if you have a fuel delivery problem. With any problems you always want to separate the fuel tank / anti siphon valve out of the mix to pin point at least the problem lies on the motor side or the fuel tank area.

OK I CAN DO THAT

......Also......this weekend I flip flopped the PCM from engine to engine and the IAC. no change. I replaced the items to the good sounding engine on the port side. Now the port side engine is running exactly the same (Bad) as the problem engine on the starboard side. Not certain I have the IAC's correctly placed back and expect the new one from Summit Racing will be here today. The scanner shows the IAC duty cycle to hold at 99.9%. then another time I looked at it, it was moving around between like 14 and 45%, and ran the same as in bad or at least not as it was running. before throttle at second notch it would run at 2,200 RPM now it goes to 1,700 and drops back and surges like the other one.

The other thing with the port side engine that is now seeming to run like the star engine that has seemed to be a prob. when I removed the IAC from the engine that seemed to run best it had obviously been replaced at one time and the gasket was spun 90 deg and only one of the bolts was through the IAC. this would for surely cause a vacuum leak and the increase in higher RPM and maybe giving me the idea that this engine was running better than the other, and now with the gasket in correctly curing a possible vacuum leak dropping the RPM down slightly showing me what I have, but there's still the mystery of why I cannot pull what I should for MPH. I am taking it to Chevrolet dealership here in town to have some of the above checked as mentioned by BUP as they have the diagnostic equipment to do this, as he is a friend and can get it in. every where else is out a month......and as mentioned I do need to check fuel pressure while driving it and engine is under load.

Thanks much for the help. let me know if you can think of anything else they need to check. are there any links that you would have to a shop manual?

Like I said check for leak(s) around the IAC area. Also a simple check is to remove your boost fuel pump and then look at the bottom screen to make sure it is not restriction with junk. Its a simple check. Also fuel vacuum test in which
Merc spec is 2 hg MAX. If it is higher then you either have a air leak or restriction or fuel pump problem like cavitation.

Also I was referring to having your injectors serviced /check because that will knock your speed down, cause running problems as well. I think you have a vacuum leak ie: IAC area or a problem IAC and fuel related issues.

As far as OEM service manuals I have them all in book form. I suggest you get one for your app or any other apps. 496 is # 30 - you want the later version of 2006 and the PCM manual # 33 - you want the 2007 version here 4 sure. The 2001 version stinks not enough material covered. You need both manuals. Good luck. The car shop has Diacom by Rinda or Merc CDS ? I hope so or else they will not be able to scan it.

I have the Mercury/Rinda scan tool. There's a screen on the "Bottom" of the fuel pump"? I looked on the blow-up and didn't see a screen. The in side of the pump as it sits in sideways behind the water separator? I have a spare and was going to replace it tonight and I will check for screen. Thanks again

As in the "IN" side of the pump?

Screen is in the inside of the bottom end of the boost fuel pump. Was it very clean or restricted ?

Funny to see this post. I had a friend call me yesterday with the same issue. Turned out it was a plug. Out of the blue a plug just crapped out. No codes on the 496 but with a new plug not even a set it is all better. Local shop wanted 16+ a plug so he only got one and will hit up the auto shop out of town this week to change the rest. Said he can get them for around 6 dollars each. Cheap thing to try. If some one already suggested this Im sorry for reposting it but I do not have time to read the entire thread. Just trying to help out. Good luck.

it was very clean. I replaced the pump and going to try it some time here probably Thursday. Jobbie Nooner going on Friday and traffic will be getting bad on Thursday but will let you know. Thanks again BUP

Keep plugs, IAC and fuel pumps on board at all times and the tools to change them I guess....is what I will be doing.

Ok you are side tracking here, U said in post 1 that you already changed plugs and checked compression. Correct. Also If I recall I think you said on your scan that U were showing 100 % available power when running. I have never seen a motor showing 100 % power while running especially thru out the all RPM ranges and have a dead hole or completely dead spark plug or a dead coil(s). A dead coil(s) will also show a fault code. Good luck with it.

yes changed all of the spark plugs and took compression checks first thing. just stating I was going to keep spares on board. ok great to know that a dead hole such as coil or wires/spark/spark plugs will show a fault code.

.....and I am assuming this to be the case with bad wire(s), or all of the above that would cause a miss-fire?

To catch up.....Boat runs 74MPH top speed on GPS. Port Engine runs 4,900 and starboard runs 4,600rpm by the tachs. when gassing the throttle in neutral from idle to approx. 2,000rpm and back real quick the engine pops/backfires out the exhaust when almost back to idle. any thoughts?

Any thoughts?

Reving in N on the water hose or in the water by THROTTLE ONLY only will set the overspeed code hence drops ignition but should set off the warning beeps to - so is your warning horn active while this backfiring is taking place and or throttling in persay N . Your test here with the back firing - does this happen when you are truly driving the boat on the water in gear running forward - throttling up and back - ( accelerating and deceleration running in gear) ?

Overspeed usually sets at 2500 rpms give or take a few here when throttle up in N (persay) - I say persay because it really is called reving in throttle only .

I had a crankshaft position sensor that went south, but was throwing no codes. It was causing RPM loss and weird issues that I thought was a fuel problem. I did a LOT of troubleshooting like fuel/pressure/quality/filters/boost pump; and spark plugs/wire/spark test/misfire tests...
...but BUP aka John helped me narrow it down to the CrankPS.

BUP will know for sure, if you can provide your serial numbers (if you have them), but I have also had to replace my fuel boost pumps due to bad fuel issues (several years earlier than aforementioned problem). I bought fuel in FL that was was labelled and advertised as non-ethanol; but turns out it was after we tested it. I left it sit in the tanks (untreated) for a few months in Florida humidity and it ruined inline filters, f/w separators, and fuel boost pumps on both of my 496s when I fired up the boat for the season back in Virginia.

Thanks for the post Brian. Will add why I posted in post # 23 that response was because in post 21 OP says running at 73 mph plus has some rpm differences from one motor to the next as that can be anything for the cause of that. It could be prop related, engine related problem, sensor related problem, fuel related problem, and including is the engine throttle plate able to achieve wide open position. Anyways his posts ONLY states "while gassing in N " never any mention of back firing while running. Just saying

IMO if a clear description of running issues and a list in order of things that has been checked along with parts changed out, sure would help to determine what the real issue is. Also OP mention from the scan motor(s) are showing 100% available power or am I not correct about this ?.

If all possible post your scan report. I would think this boat is put to bed being in Michigan but the engine still could be scanned during this timeframe without running the engines. . Again just saying and sticking with it, trying to help.

This would be stabbing the throttle quickly in N in the water no alarms when doing this. Timing of alarms does not seem to be related and while just cruising across the lake. The other engine does not do this pop/backfire. It's not real loud as I guess it doesn't matter my true concern is dealing with alarms practically every time I go out. Sometimes scan shows something and sometimes not. I have throttled back, not turned the engine off, scan right there on the spot and show no codes. I have a whole host of replacements and the boat spent the winter with boat mechanic. It was me that requested the boat back for the beginning of boating season here in MI prior to the icebergs being gone from Hubbard lake, so he had no chance to water test. I would have to look but believe it was the crank sensor, water pressure sensors and plugs, batteries, alternator, spark plugs, compression tests, got an alarm in there for an iMac and it was bad and replaced that out on the lake last summer as I keep them in stock, low pressure fuel pump, filters, and then some. I have not checked fuel pressure through rim range under load and need to do that.

There was a CPS replaced last winter. I belive it was on this engine. I will look up paper work when I get home. On the engine that I have the lower rpms that have had the fault codes of OVEREV and FUEL PUMP RELAY CRK, the burden portion of the wiring harness that was repair (? There's a lot of wiring harness that is covered and and cannot get to or see), has the popping sound at when coming down in RPM off a quick stab, I have replaced CPS (TO VERIFY), fuel pump relays and checked all connections there of, replaced alternator, batteries, fuel filters, fuel pump (I guess low pressure or prime), spark plugs, taken compression checks, visual check of spark plug wires, rebuilt sea pumps, replace pressure sensors and plugs, I burn ethanol free gas, treat it and use Lucas fuel treatment, replaced IAC mufflers, checked that throttle cables open throttle all the way to WOT, removed drives and serviced them, replaced gimbal bearings, aligned engines, replaced exhaust manifolbs and risers on both engines, had an IAC alarm this past summer and replaced that. Thanks for the continued help.

No back firing while runwind that I am aware of. Just this pop from one engine and not the other. I will get better info of what I have done and checked when i can get back home to my notes. Boat is 1.5 hours away locked in storage. Thanks again for the help.

Remember, you have a CAM position sensor and a CRANK position sensor.
Cam on front.
Crank on back.

You could also have a prop issue causing RPM difference.
They could not be exactly the same (did you replace one?) or you could have damage that is affecting one's performance vs. the other.

Will add, the lingo from Merc and Merc techs - just in case and any written work orders from the service shop of you need to check out what sensor was really replaced.

CPS = CRANK POSITION SENSOR

CMP = CAMSHAFT POSITION SENSOR

Stick to BUPs advice and change one item at a time.

A surging IAC is indicative of vacuum leak or bad injectors, or a failed IAC. Your injectors could conceivably cause both problems, surging idle and poor top end performance. Surging idle in that the flow/spray pattern is erratic causing variable fuel amounts and thus the IAC is trying to adjust air to keep idle stable. Poor top end performance (lower WOT rpm) for the same reason, the fuel flow is insufficient to achieve rated power.

I once swapped injectors from 24 lb to 42 lb (nearly doubling) and changed NOTHING else. IAC went NUTS. Full throw from 0 to 150 cts back and forth. It was trying to do its job but couldn't keep up with the exceedingly excess fuel flow. There was a method to my madness but that is a side bar.

Point being injectors can alter IAC behavior as can vacuum leaks.

Props are stock original 28 pitch and appear to be in perfect shape they are ten years old should j send them in to be checked or labbed? Who/where?

I will verify what and which engine. I know the CPS can throw an alarm without code, what about CMP?

I have a plan of sending injectors in to have them cleaned n byno'd this spring.

Does anyone have reasoning for not usino MDS spark plug wires rather than merc, or other suggestions?

Sorry this stupid tab isn't like usino my pc MSD wires

Looks like the port side Cam sensor was the only thing replaced of the CPS and CMP, after reviewing the repair report. I will put 2-CPS and 1-CMP on the list and verify their operation prior to install. I saw someone here installed a new one and it did not work.

Hi I was just readying this post and was curious if you figured out the issue because I'm have the same issue with mine. I just replaced the fuel injectors and when I went and fired it up its still idling right and surging up and down with a sucking sound. Thanks

Hi I was just readying this post and was curious if you figured out the issue because I'm have the same issue with mine. I just replaced the fuel injectors and when I went and fired it up its still idling right and surging up and down with a sucking sound. Thanks

MANY Multiple issues just the jist here - he sent me all 16 fuel injectors - contaminated with rust extremely bad - they were really bad to the point not worth even keep cleaning them., also completely rusted internally fuel rails somewhat a problem with the 496's early versions more so than latter versions. They are weak for corrosion protection internally. He even sent me the fuel rails - still to this day can not even believe the engines started and ran.

All new fuel rails and injectors and other fuel components were needed after he sent me them to examine. - I had worked with him over the phone extensively last year along with his techs. He still had some other small issues on top of lurking but that was going to require a inperson test drive with multiple testing tools to monitor many output specs and running conditions.

Found out after the fact the wiring harness has a tiny bit of meting spots - possible for his current state of issues. New harness replaced I think. We talk once in awhile currently.

His app really needed someone in person to look really indepth at because like many boats it surely had alot more than one issue causing problems. I got him very close over the many phone calls as his boat really came around running actually the best since he bought it plus what I provided in proper new parts, proper info and proper installs, like I said got him real close to running perfect.