Misfire code P0303, 2006 MDX, 180k miles. Previous owner had same issue about a year back, valves were adjusted by dealer, coil packs / plugs checked and/or replaced. Eventually diagnosed (?) as very small breach in head gasket. Never ate coolant and never blew any white smoke though. He added some Barrs to it, which actually 'fixed' the problem. I bought it fairly cheap, figured I'd deal with anything major when the problem returned.

It's back as of a few weeks ago.

Could be a bum gasket as originally suspected, but I don't know. It's sure idling like it has one. Missing, but won't die. Still does not seem to be eating any coolant, still not smoking.

Odd gurgling in the heater core though, which I don't suspect is coincidental. But could be. It just cooled off in Cali and I just turned the heater on for the first time about 2 weeks ago. So could be symptom, could be coincidence.

What is the low hanging fruit I should look at? Would not surprise me if it's the gasket or head, but I would think there would be a bit more sign of that somewhere . . . coolant loss, smoke, stalling out, etc.

 

And just got word from the last owner . . . coil packs were replaced but with non-OEM . . . so that could be a possibility I guess. Have heard these MDXs are fairly finicky about the coil packs.

So I guess the question is can a failed gasket or head be so minor as to send a P030 code (and intermittently trigger the VTM-4 light as well) but not cause any detectable coolant loss?

If there is enough air pressure getting into the coolant system to cause some percolation in the heater core, I'd think there would be enough coolant getting into the ignition chambers(s) to show smoke.

Engine has not overheated. No sign (yet anyway) of coolant in oil. No foam.

Just kind of a mixed bag of symptoms here.

 

Always, ALWAYS do a Coil Pack test every time you have a Cyl Misfire code.. Its quite easy and can be done on the fly.

EDIT: P0303 means that the misfire is located in the cylinder number 3 this cylinder is the FRONT furthest to the RIGHT (Closest to the battery)

PROCEDURE:
1.- Remove the Engine Cover so you have clear access to the Coil Plug connectors
2.- Start the engine and let it idle
3.- 1 by 1 Disconnect a coil plug and listen to any IDLE fluctuation. If you disconnect a working coil you should hear and feel the engine bogging down and vibrate, Do not disconnect more than 1 coil plug at a time.. Since you got P0303 start with Cyl number 3 coil plug.
4.- Repeat until you find a Coil Pack that does not change the engine behavior what so ever, This will by an indication of a faulty coil plug.
5.- Change the faulty Coil Plug!

If all coil plugs pass this test then you can move on to new diagnostics, Will wait for the results for a follow up.

 

OEM coil > *

 

Thank you, will do this tomorrow - no time to wrench on Mondays.

Just 2 questions . . . if CYL 3 is already misfiring, will removing that coil pack cause any further degrading of idle or RPM drop? And . . . can codes be cleared from the DMX memory just by removing the negative terminal for a few minutes? I'm asking because it was part of the last owner's diagnostic procedure, but I am starting to suspect that he might have been chasing an old code the last few days he was working on the car. Just curious. But yeah, main question being will removing that #3 coil pack degrade that idle at all if it's already showing misfire?

Thank you for your help. Much appreciated.

 

Removing negative terminal will indeed clear the code.

and no. that's how people test coil packs If unplug = no change, then problem!

 

 

Well I finally got an afternoon off that overlapped with a dry day.

I checked as suggested, removing one coil pack at a time while idling. I really saw NOTHING until the engine came up to temperature.

But once it came up to temp, I clearly saw drop of RPM and rough idle when I removed 5 of the 6 coil packs. One coil pack I could remove with no change in the way the engine idled.

But here is the odd part: it wasn't for just #3 (the front-most, furthest to the right, closest to the battery). It was the cylinder directly across from that one, backmost, furthest to the right, closest to the brake cylinder.

Letting the engine cool down for a few minutes, and then will try to reconfirm. But yeah, no change at cool idle, definite change at warm idle, but the coil that "appeared" bad in the test was not the cylinder sending the code.

I assume this is semi-encouraging?

 

Oop, meant to say "JUG #3 . . ." not "JUST #3 . . "

 

That is great!
It was a mistake on my part, I called like this:
4 5 6 Brake MC
1 2 3 Battery
Front of the Vehicle

But the correct cyl placement is like this:
1 2 3 Brake MC
4 5 6 Battery
Front of the Vehicle

So the code is still correct.. You have a bad coil plug on Cyl#3! My bad.

 

#3 definitely seems to be the culprit as confirmed by unplugging the proper cylinder and the code! Switch coil packs to see if you throw another code in another cylinder and then buy a new one. It might be the plugs as well but my money is on coil packs. Remember, OEM > *.


While you're there, I would suggest torquing down all the plugs to the proper torque specification to ensure none of them come loose (#5 is notorious for doing so in these engines) I've personally had an ejected #5 plug in my J30A1 6th gen Accord

 

Well, that did the trick! Swapped the coil with one from a "known good" cylinder and the problem migrated with the coil. So I was confident enough at that point to spend the money for a replacement coil . . . and BAM. Problem solved.

I noted the coil I removed was a cheap aftermarket one, so I just replaced it with a Borg/Warner . . . if I keep the MDX I will upgrade those all at some point, but at 178k miles, I might just sell this MDX and move on. I think maybe a Land Cruiser or LX 470 at these mileage numbers might be a safer bet. But anyway, just VERY happy to have my MDX back for only a $40 repair cost. I do like it an might just take the risk of keeping that odometer rolling.

Thank you for your help with this, guys. It was much appreciated.