Yes another 496 issue. possibly injectors
07-15-2013, 09:40 AM
#1
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Yes another 496 issue. possibly injectors
Ok, I am sure some of you have read my many lenghty posts on my Velocity 260 with a Merc 496 Mag (non-HO). I had the dreaded soot issue, which was partially taken care of with a PCM tune by Whipple and a new intake gasket (with new longer bolts). The boat ran great but still soots up a bit. I have found that a lot of the soot is being caused by Ethanol blends. I have been running Ethanol free 90 Octane gas from the local marinas (at $4.99/gal) and the soot has gone down to a managable level, however still have more than other boats. Now on to the issue....
From the first time I fired it up last season until present I have noticed what I would call a small misfire at about 2000rpm, only while planing off. So basically while under heavy load. The misfire was hardly noticable when just my wife and daughter are in the boat with me. It is more noticable with more people/weight in the boat. Another thing I noticed is while going through no wake zones there is a small spot in the throttle where the RPM goes up as I pull the stick back. Then as I push stick forward there is a flat spot where RPMs go down slightly then back up. All of last year and this year the boat runs great, still runs lower 70s (GPS), idles fine, etc. Just this misfire while gettting on plane. This season it seems to have gotten a tiny bit worse. The misfire is now evident with light load in boat. It also feels like it takes a bit longer to plane off, but not a lot longer. Over the off season I had my prop repaired and labbed again, so my plane time may be slightly affected by the prop work. I actually had someone, that was behind us, tell me he heard the misfire while we were getting on plane and he told me he saw a small puff of black smoke at the same time.. So this misfire is most likely contributing to the minor soot issue I am now seeing. More observations.. Ever since buying the boat (used) I have noticed minor amounts of raw fuel in the water, behind the boat, upon start up. With the bengine off, occasionally, we will smell gas fumes (randomly) behind the boat when swimming behind it. It is not constant, just randomly you can smell gas for a few seconds, then it is gone (maybe a fuel injector leaking???). This doesn't happen everytime out But when it does, you can smell the gas multiple, random times. I put a fuel pressure gauge on the fuel rail and idle and WOT pressure are consistant and where they should be. However, if the boat sits for an hour, the fuel pressure completly bleeds of to zero (not sure if that is normal... I am guessing it is not). So when I start the boat, It doesn't fire immediately on the bump of the key like it had previously. I have found turning the key to on, then back off, and on a few times primes the fuel system and makes it fire as a Fuel injected engine should. There are NO leakes in the bilge, so it isn't a leaky line or anything like that. Also it appears fuel mileage is down a bit this season, although I don't have any real numbers to confirm, other than it just feels like we are putting gas in more often. The boat does not go into Guardian mode, and I haven't experienced any alarms.
Is there anything I should check before taking in for a scan? I just am fearful I will pay for a scan only to find the PCM says all is good. The boat runs so good once on plane, I don't think it is anything major.. Because the Velocity 260 w/496 mag should run low 70s GPS and I have hit the 70 mark on a few occasions this year. I did dump some Fuel Injector cleaner in the tank (enuff to treat 40 gallons) with 1/2 tank of fuel over the weekend. The soot does look like it may have reduced more (but hard to measure soot...)
Before this misfire started to appear, I had the boat winterized by my Merc mechanic. His procedue for winterizing the motor is to hook an external fuel take with a oil/fuel mixture to coat the cylinders. Is it possible this method has pluged an injector? The misfire was not there in the fall, boat was winterized, and the misfire appeard the first time out after winterization.
It appears there is a rich condition as is supported by a misfire, soot, and the visual of a puff of black smoke. So will a PCM scan really lead me in the right direction or is this one of those replace parts one at a time things? Am I possibly on the right track with the injectors? Has anyone every had a positive result by usinf "dump in the tank" fuel injection cleaners? Does this sound like something more simple, like a vacuum line? I have changed the MAP sensor (last season) and there was no difference at all. The IAC was changed a few seasons ago as it tested out bad. Would the IAC have any impact on mid-range/heavy load?
From the first time I fired it up last season until present I have noticed what I would call a small misfire at about 2000rpm, only while planing off. So basically while under heavy load. The misfire was hardly noticable when just my wife and daughter are in the boat with me. It is more noticable with more people/weight in the boat. Another thing I noticed is while going through no wake zones there is a small spot in the throttle where the RPM goes up as I pull the stick back. Then as I push stick forward there is a flat spot where RPMs go down slightly then back up. All of last year and this year the boat runs great, still runs lower 70s (GPS), idles fine, etc. Just this misfire while gettting on plane. This season it seems to have gotten a tiny bit worse. The misfire is now evident with light load in boat. It also feels like it takes a bit longer to plane off, but not a lot longer. Over the off season I had my prop repaired and labbed again, so my plane time may be slightly affected by the prop work. I actually had someone, that was behind us, tell me he heard the misfire while we were getting on plane and he told me he saw a small puff of black smoke at the same time.. So this misfire is most likely contributing to the minor soot issue I am now seeing. More observations.. Ever since buying the boat (used) I have noticed minor amounts of raw fuel in the water, behind the boat, upon start up. With the bengine off, occasionally, we will smell gas fumes (randomly) behind the boat when swimming behind it. It is not constant, just randomly you can smell gas for a few seconds, then it is gone (maybe a fuel injector leaking???). This doesn't happen everytime out But when it does, you can smell the gas multiple, random times. I put a fuel pressure gauge on the fuel rail and idle and WOT pressure are consistant and where they should be. However, if the boat sits for an hour, the fuel pressure completly bleeds of to zero (not sure if that is normal... I am guessing it is not). So when I start the boat, It doesn't fire immediately on the bump of the key like it had previously. I have found turning the key to on, then back off, and on a few times primes the fuel system and makes it fire as a Fuel injected engine should. There are NO leakes in the bilge, so it isn't a leaky line or anything like that. Also it appears fuel mileage is down a bit this season, although I don't have any real numbers to confirm, other than it just feels like we are putting gas in more often. The boat does not go into Guardian mode, and I haven't experienced any alarms.
Is there anything I should check before taking in for a scan? I just am fearful I will pay for a scan only to find the PCM says all is good. The boat runs so good once on plane, I don't think it is anything major.. Because the Velocity 260 w/496 mag should run low 70s GPS and I have hit the 70 mark on a few occasions this year. I did dump some Fuel Injector cleaner in the tank (enuff to treat 40 gallons) with 1/2 tank of fuel over the weekend. The soot does look like it may have reduced more (but hard to measure soot...)
Before this misfire started to appear, I had the boat winterized by my Merc mechanic. His procedue for winterizing the motor is to hook an external fuel take with a oil/fuel mixture to coat the cylinders. Is it possible this method has pluged an injector? The misfire was not there in the fall, boat was winterized, and the misfire appeard the first time out after winterization.
It appears there is a rich condition as is supported by a misfire, soot, and the visual of a puff of black smoke. So will a PCM scan really lead me in the right direction or is this one of those replace parts one at a time things? Am I possibly on the right track with the injectors? Has anyone every had a positive result by usinf "dump in the tank" fuel injection cleaners? Does this sound like something more simple, like a vacuum line? I have changed the MAP sensor (last season) and there was no difference at all. The IAC was changed a few seasons ago as it tested out bad. Would the IAC have any impact on mid-range/heavy load?
Last edited by low_psi; 07-15-2013 at 09:52 AM.
07-15-2013, 10:03 AM
#3
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I have done plugs, when I was dealing with the other soot related issue (PCM and Intake). However I have not since. I was thinking of replacing the plugs next, or at least looking at them.
07-15-2013, 11:16 AM
#5
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