Mercury 496 Mag HO question
 

Ok, at idle, the engine will run for about 30 to 45 seconds (idle at about 650), then just as its about to fall flat on its face, it will pick itself up and surge to about 1300 or so rpms, then back to 650. Anyone seen this before?

Re: Mercury 496 Mag HO question
 

Look in the search forum under 496 ho I think I saw this last year had to do with the fuel screen being clogged or fuel filter issue....

Re: Mercury 496 Mag HO question
 

The 496 Merc engines uses a stepper motor to control the idle speed that mounts at the back of the intake manifold and is connected to the throttle body by a rubber hose that runs back to front over the manifold under the merc plastic motor cover. You may have a bad merc idle motor, loose connections or the problem could be fuel pressure related. Before you start digging to deep, put a fuel pressure guage on the schraeder valve on the injector fuel rail, you should see about 43-45 lbs at idle with the vacumn line off the regulator. If the fuel pressure is there around 43-45 lbs and the engine will rev but it is still surging at idle, I would suspect the idle air motor or connections. If you have access to a Merc scan tool or a dealer with one it should show a code that would help locate the problem.
Good Luck, I don't think its serious!
Raylar

Re: Mercury 496 Mag HO question
 

That hose on mine was loose on both ends. Check it and lock tight and put back in. Hope its that easy. JC