Good luck Rob......

 

I'm not sure you can cure a problem like this without taking the heads off the motor.

Your best hope is that carbon deposits on the piston domes in the bad cylinders are closing the plugs. That way, nothing would be structurally wrong with the engine. It would seem like it would be out of balance, though. Does it vibrate more than usual?

If that is the case, you might be able to get some kind of engine cleaner that will dissolve the carbon deposits. I've never used anything like this, so I can't recommend anything to you. Maybe someone else here can recommend a product to you. It would probably be something you would add to your gasoline.

The only other choice is to keep spacing the plugs out and hope that they don't get closed again. That seems like a bad plan to me. Sooner or later, this problem is going to bite you. Whatever is growing in there will continue to grow.

I can't remember of hearing about a VQ with this problem. It seems unique to me. It seems odd that it could be carbon deposits when it started happening after you missed a shift. I don't know. If it were me, I'd start planning to take it somewhere and have the heard removed. It might be worthwhile to take it to a Nissan dealer and let them look at it. They see a lot of engines with problems.

 

It is overwhelmingly likely you have stretched the rods. Over reving put an end to my motor.

JET

 

Do either you or your mechanic have a Borescope? if so, you can take the plug out and feed the borescope down into the chamber for a look-see!

Just thought of an easier method to check if the rods have stretched.

Get a TDC checker. I bought one from eastwood tools. set up a dial indicator to measure TDC on all cylinders. the 2 that you are having trouble with will push the dial indicator up further than the others.



This morning, I was just working on making a platform, that bolts to the heads, for holding a magnetic stand (for the dial indicator.)

 

You can try Seafoam for the carbon deposits via a vacuum line. You can get it at most auto parts stores. I've used it on alot of different high milage motors and it works really good. You'll need to change the plugs after you use it but that sounds like something you're used to now. Best of luck! Hope it's something this simple for your sake.

 

g.m. top end cleaner,available at dealer.inject directly into intake,make sure there is nothing you like behind the car.with dbw you might need someone in the car to modulate the gas pedal.

 

Wow. This is why a continue to be a part of this forum. So many members with a great understanding of the Z and it's parts. Thanks to everyone. The mechanic has been using the borescope yesterday and today to try to learn more. We'll see. Thanks again.

TDC Checker sounds like a good device too. What is TDC?

 

Top Dead Center. It refers to when a piston is at the top of it's travel aka fully compressing the mixture. When we talk about ignition we talk about timing and BTDC or degrees before top dead center.

 

Thanks. Now is that the degrees of the cam BTDC?

 

It is in relation to the piston not the cam timing events. The VQ has cam angle sensors so it could measure it there but since it has variable intake cams(now both intak and exhaust) it wouldn't be accurate. That's why there is the infamous(at least it used to be for not being shielded well enough) CAS or crank angle sensor. Other cars measure timing events with the cams but it is always just a way to track the piston position.