mercruiser 496 misfire

[mconner52]

I am working on 496 Mercruiser I just had the engines out to do transom assy and put the engines back in and ran it and I had what felt like a dead miss throught the rpm range about every 3 to 5 seconds checked fuel psi to be high, replaced cool fuel because of paint delamination and replaced injectors still had same issue, did lea down test to be good 12% leakage on all cylinders, replaced spark plugs, finally I think it is ignition or electrical related, note I just replaced all battery's and alternators are charging as their are supposed to, I am stumped and looking for some advise on what next to do, I figure I am gonna run the boat on the computer and do a cylinder drop test to narrow down which cylinder is misfiring, also I did a spark test n a spark board and all coils have good blue spark that jump about a 1/2 inch gap, any help is appreciated. thanks

 

[Bondo]

finally I think it is ignition or electrical related,

Ayuh,..... Welcome Aboard,.... Does that motor run the flat distributor cap, with the spark leads comin' out the sides,..??

If so, slap a new cap, 'n rotor into it, 'n Try it,....

Those are a known problem, as they cross-fire sometimes,....

 

[alldodge]

Howdy

Could use an engine serial number to narrow down the motor differences. Those 496's changed from the earlier ones in component changes. Seeing a miss every so many seconds, wonder if all the connections are clean and tight. Check crankshaft position sensor

checked fuel psi to be high

What was the fuel pressure reading?

Would be good to know if there are any codes thrown. Assuming that when fuel module and injectors were replaced that all the lines were cleaned out also

 

[mconner52]

no bondo it has individual coils for each cylinder, and alldodge serial number is 1a053122 fuel psi is in the 40 range and is holding steady it was reading high because of the paint coming off the coolfuel and clogging the regulator but I have teplacedthe cool fuel since then, when I did the injectors I cleaned all the rails an entire fuel system, no codes being thrown currently, im waiting to get a new smartcomms box to run my computer on it and do a cly drop test to try and determine what cylinder is misfiring to try and nrrow down the problem

 

[JustJason]

All you can do is a cylinder drop test in order to see if it's any 1 cylinder that's causing the misfire. If you run through all 8 cylinders turning them on and off with no change then you would simply want to run the engine off of a different fuel supply and see if that makes a difference.

 

[alldodge]

OK, pressure was higher then 40 but now its working within range (39 idle and 43 WOT).
Since you replaced the module you should have the new design that uses the barbed hose fittings on the module for cooling hoses.
Was the fuel rail regulator replaced or cleaned (item 18)?

http://www.mercruiserparts.com/bam/subassembly/31819/12826/90

 

[Fun Times]

mconner52
There's a good probability that one of your coil towers or spark plug wires are leaking spark heading to the nearest ground which typically the ark on these engines will run under the coil down towards the valve cover area. Sometimes running the engine at night may help locate the arc leakage while looking under the coil bracket(s) assembly and the bracket hold-down bolts...You may have to slightly move a wiring harness to get a better view in order to see it arcing.

The following link is a good example...Be sure to read to post number 21 for the final outcome, http://forums.iboats.com/forum/engi...engines-outdrives/51515-mercruiser-496-ho-dif

Let us know, good luck.

 

[mconner52]

ok what I found happening is that while monitoring my computer the electronic throttle body is closing for a fraction of a second making it sound like a misfire but it seem like something is commanding the throttle body to close, tried swaping ecms btween engines no change as well as changing throttle bodys btw engines, also hooked into the helm control modules to see if the dts system was telling the throttle plate to close but that was not the issue, seeing that the throttle body gets its signal directly from the ecm the only thing I can conclude is that I have a problem in the wire harness, but that is an expensive part to just guess on, any input??

 

[alldodge]

Don't think its the ECM, and the throttle plate closing I would assume is from the computer seeing a miss so it reacts.

Try putting a timming light on each plug wire one at a time and see if it is a given cylinder, or random.

 

[mconner52]

its not a miss I did a cyl drop test and all cylinders are firing th ecm is the only thing that controls the throttle plate, a misfire would not cause any change in the ecm or throttle plate

 

[mconner52]

Ok j found the problem what is happening is that while running the vibration ad the engine at is the electronic shift acuator is shorting and telling the engine that it is shifting and making the throttle body close and causing the issue I am having I have tested it with a known good acuator so the problem was the actuator this one was a pain in the ass

 

[camalot]

Ok j found the problem what is happening is that while running the vibration ad the engine at is the electronic shift acuator is shorting and telling the engine that it is shifting and making the throttle body close and causing the issue I am having I have tested it with a known good acuator so the problem was the actuator this one was a pain in the ***

Good find thinks for letting us know!!!