496HO Pops and Backfires
 

Alright guys, I need a 496 wizard.

Dual 496HOs. They are the older style without the cool fuel box on the side. Both were running fine. Port motor started to overheat, killed it and limped back on starboard.

Changed both sea water pumps and went ahead and put all new NGK TR5 plugs(it had delcos). Got everything back together and port motor fires right up and does everything it’s supposed to. Starboard motor fires up for a few seconds on what sounds like only 3 or 4 cylinders. Might pop out the intake or backfire out exhaust and dies.

Hooked a computer up and checked codes, NONE. All the data I can see on the computer seems to be reading correctly (not sure if it’s actual live data or just commaded values).

Double checked all plug wires and they have no visible damage, put old plugs back in it. I have swapped pretty much every sensor that would make the engine run from port motor to starboard. Map sensor, cam, crank sensor, idle air control valve, ecu’s have been swapped. Fuel pressure is over 40psi and holds while cranking and doesn’t bleed down past 35psi after shut off. Port motor fires up and runs still.

I have checked I am getting 12v on the power going into 2 of the 3 plugs on the ecu. Not sure if there is another power wire to check. I have check and am getting 12v on the ignition wire at the engine plug. Main power relay is getting 12v. No blown fuses. Checked and made sure all plugs were still plugged in, didn’t pinch any wires in the 2 inner manifolds when I re installed them.

I have put an inline spark tester on one cylinder and a noid light on another, it doesn’t get consistent spark or fuel. Spark light will flash a few times here and there and noid light will light up one time and not again.

So, has anyone had a similar issue? Anything else I could check?

Does the tach do when the engine is barely running?

Yes tach reads rpm while cranking and running around 800rpm jumps around while barely running

Since it was running fine, 98% chance that something you did caused it. Not saying you did anything wrong, though. I would closely inspect all areas of the engine that could have been touched/moved when you did the seapump and plugs.

You say you came back on the starboard engine. Do the engines feed off separate fuel tanks??

try disconnecting the tach on that side.then look at live data and see if something is a miss.I'd say cam or crank sensor but they usually work or not.

I agree, and have looked everything over. Not even really much down by the sea pump to cause this. Engines run off the same fuel tank.

I’ll try that, it’s acting like a cam/crank sensor problem but I have swapped both. And I feel like if the wire got messed up somehow and it’s not getting signal from these sensors, it would throw a code.

BBaker,

I'm no wizard, but I have had my fair share of experience with the 496HO.

A Good Read

Do you have a Rinda TechMate scan tool? You can use it to do a cylinder delete sequence for both injectors and ignition to help determine if you are actually missing cylinders and which ones. It sounds like just about everything else in is order, so I'd start there.

Thanks. Brad.

Have you swapped the plug wires from the good running engine over to the bad running engine? Plug wires are something you obviously touched, maybe causing one or 2 to fail internally. These 496s are at the age where the plug wires are failing.

engine should still run on 2 dead cylinders, hell even 3

I do not have one, maybe I know someone local with one to try this. But doesn’t the engine have to stay running to do this? Thanks

BBaker,

Yup. Mine ran on 6-1/2 for an entire season. I knew something was wrong, just from the lack of power. I just couldn’t figure out what it was. To listen to it, idle through “WOT”, I couldn’t really hear that it was running wrong.

Thanks. Brad.

BBaker,

Yes. You set the throttle at about 1500 RPM, in gear, and delete a cylinder, one then the next, and listen for a lack of a drop in engine RPM. That's a dead cylinder. Do this with both injectors and ignition, and you'll root out any issues with these systems.

If the engine just won't stay running at all, I'm not sure what to tell you other than to have the ECM tested. They DO go bad. I'd think this would throw a code, but I don't discount ANYTHING when it comes to electronics. A faulty component could also prevent it from throwing a code. Electronics are just voodoo. John Mosetti at Boats Unlimited can test and repair them. This is all assuming there isn't some other, simpler solution; Occam's Razor and all. You might also send the injectors to Smitty (ArcticFriends here on OSO) and have him test and/or clean your injectors. Wouldn't hurt to send them from both engines while you're at it. Eliminate this as a potential cause. It's not like you're going to be using the boat in the interim.

Thanks. Brad.

Thanks. Brad.

I've had the tech mate scan tool get fault codes with just the engine barely firing for a few seconds before on a 496 FYI. In this case it was a crank sensor having issues.