Weird misfire

[BlackedoutQX4]

Hey guys, new to this forum (posting), but have been a reader for a year or so. I have this strange problem on my 02 QX4 I’ve searched all on the forums about to see if anyone has had an identical problem. So I’m getting a misfire with no code at idle, only when in drive or reverse with the foot brake on. Also will only do it about 35 minutes after being driven. Drives fine when driving, but once stoped at a light idling, about every 7-10 seconds it has a small misfire I believe. It doesn’t misfire when in park or neutral, just drive and reverse. I pulled the plugs, changed them to the specified NGK laser platinums, fixed it for a little bit, but started again, it has codes for P420 & 430. Mechanic I brought it to said that the cats could be so bad that it’s causing back pressure buildup and causing the misfire? I’m just not sure which direction to go in, I definitely don’t want to spend $2000 for new cats without fixing the misfire issue if it’s something else, because I don’t want to foul out new cats with unburnt fuel or oil. I think it does need valve cover gaskets and thought maybe my misfire is being cause by that, but he took all the spark plugs and coil packs out and said no oil was found on them. So I’m not sure. Can o2’s cause that code and a small misfire like this? Or should I bite it and just put new cats in? I appreciate any of your guys suggestions/advice!

[AZhitman]

The brake pedal is the clue for me.... Check your brake booster vacuum line. I suspect a minor vacuum leak (or a bad check valve) is allowing unmetered air into the intake system (but only while the brake is pressed and engine under load).

[VStar650CL]

The brake pedal is the clue for me.... Check your brake booster vacuum line. I suspect a minor vacuum leak (or a bad check valve) is allowing unmetered air into the intake system (but only while the brake is pressed and engine under load).

Good call.

[BlackedoutQX4]

I wouldn’t have thought of that, I will take a look tomorrow and see, thank you! I’m mechanically inclined, but what should I look for obviously beside cracked or old rotted lines or things like that, how can I tell if the check valve is bad? Just by looking or should I do some other tests?

[AZhitman]

I'm a big fan of simply replacing questionable bits. You can visually inspect for cracks, but air can pass thru voids that aren't readily visible.

The check valve can be tested by removing the entire line and blowing on each end... but if you're gonna remove a 20 year old hose, just replace it.

[BlackedoutQX4]

So, I went out and looked today, and I think I found the correct hose you were talking about. It’s the one that’s pretty thick, runs off of the power booster, to the top of the intake manifold if I’m right? It looks okay, a bit questionable on the ends, it’s frayed, so I’m going to replace it tomorrow while at work and see if it helps. Could it be from the cats do you think? If it’s the cats, I don’t want all of that pressure that can’t escape to blow the rings or valve seats, etc, just be extra careful. I’ve read that if it’s an o2 it will almost always set a code, so I guess I can rule that out. It’s really odd, I’ve never experienced a miss like this before. Before I changed the plugs, I would get on it, and you’d feel that cylinder miss, but after I changed the plugs, it only has a small miss when your idling like I said.

[VStar650CL]

If it’s the cats, I don’t want all of that pressure that can’t escape to blow the rings or valve seats, etc, just be extra careful.

The backpressure will only limit your power, that isn't the real risk with bad cats. The risk is that a bad cat can disintegrate, and because most Nissan engines use Miller cycling to eliminate the EGR valve, the engine will "inhale" the metal particles if the cat melts. That's basically the end of your engine, play Taps and call the junkyard. If that really is the problem, don't mess around.

[BlackedoutQX4]

Thanks, VStar650CL. I’ll probably give the mechanic a call tomorrow and see when I can get it in to get them changed. Does $1300 sound like the right price for OE cats from Nissan? That’s the quote he gave me, plus $377 for putting them on. I’m not sure if aftermarket ones are any good, so I figured I should probably go with Nissan ones.

[VStar650CL]

Price sounds about right for OE's. Aftermarkets are okay if you stick with a reputable brand like Bosal or Eastern Catalytic, just stay away from the deep-discount stuff.