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496 HO MAG Running Rough Missing

Old 01-19-2015, 06:54 PM
  #31  
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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.
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Old 01-20-2015, 09:01 AM
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Originally Posted by Sydwayz
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.
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?
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Old 01-20-2015, 09:04 AM
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Originally Posted by BUP
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

I will verify what and which engine. I know the CPS can throw an alarm without code, what about CMP?
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Old 01-20-2015, 09:06 AM
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Originally Posted by Trash
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.
I have a plan of sending injectors in to have them cleaned n byno'd this spring.
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Old 01-20-2015, 09:07 AM
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Does anyone have reasoning for not usino MDS spark plug wires rather than merc, or other suggestions?
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Old 01-20-2015, 09:15 AM
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Sorry this stupid tab isn't like usino my pc MSD wires
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Old 01-21-2015, 06:49 PM
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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.
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Old 06-08-2016, 10:14 AM
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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
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Old 06-08-2016, 10:15 AM
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Originally Posted by CATF800
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?
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
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Old 06-09-2016, 02:00 PM
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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.

Last edited by BUP; 06-09-2016 at 02:08 PM.
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