Question about knocking that coming from the motor

[TQs30z]

Hi Guys, my 260z has some weird and loud knocking noise coming from the motor. I don't know whether this cause it or not, here you go: I take my car for a spirit ride I'm doing 70mph in 4th gear in 4k rmp for for about 7sec and slow down after that. The car run fine after that. However, after a while, the car start making weird knocking noise, and it keeps backfire at the exhaust when after I release the gas pedals. At first, I thought it is some kind of exhaust leak or something, but after I pull over and poops the hood, the noise is coming my the motor, especially from cylinders that closer to the firewall. Any suggestions what's going on?? I'm thinking about the valve hitting the piston, but not sure though, any helps are appreciate, thanks.

Ed

[FastWoman]

Not sure about the backfire, but a loud knocking noise (sounding like a valve going very bad) CAN occur when you've blown out the exhaust manifold gasket, and the exhaust manifold is banging/clattering against the head. It happened on our boat once. I thought the salt had finally gotten the best of its ancient engine, but a replacement of its exhaust gasket made everything right. See if there's a massive exhaust leak between your head and manifold. If so, I bet that's the source of the noise.

[Gary in NJ]

It's hard to diagnose a noise from typed words, but that wont stop us from trying

First off, I doubt the valves are hitting the piston. That usually only happens once, and then the engine stops working.

An exhaust gasket would be my first guess, followed by a head gasket.

It could be a bad/worn main bearing. That noise usually presents itself under light load at mid-rpms.

If you have access to a stethoscope you can listen to the engine in various places through the valve cover and the head itself.

[TQs30z]

Thanks for the suggestion. So, pieces of the exhaust gasket that get into the combustion camber and causing the knocking? It leads some unburn fuel leak into the exhaust manifold and cause backfire? Sounds logic to me. Any more suggestion are welcome. And how hard is it to replace the exhaust gasket? When I remove the gasket, I need to remove the intake as well well right? since they are one piece. I only has normal tool, wrench and screws, any special tool to do the job? Thanks guys.

Ed

[Gary in NJ]

So, pieces of the exhaust gasket that get into the combustion camber and causing the knocking? It leads some unburn fuel leak into the exhaust manifold and cause backfire? Sounds logic to me.

Doesn't sound logical to me The bad gasket doesn't get drawn into the combustion chamber.

A bad exhaust gasket "sounds" like tapping. The backfiring is caused by air being drawn into the exhaust. To confirm a bad gasket look for "splooge" (dark oil) around the exhaust manifold. You can also see the effects of a bad gasket by putting the car in a dark garage; if it's real bad you'll actually see flames (very slight) coming from the head.

While an exhaust gasket is a simple repair, the age of the car almost guarantees a frozen nut or stud.

Edited November 23, 2010 by Gary in NJ

[SteveJ]

Do you still have the smog pump installed? If not, is the inlet to the exhaust manifold where the smog pump feeds plugged? Have you checked the timing lately? How about your spark plugs? The mixture?

Here is where I'm going. If your car is sending too much unburned fuel into the exhaust while it is adding air to the exhaust, you can get a backfire. Typically, there is more unburned fuel immediately after you let off the gas. If the timing is off or if the plugs are fouled, that could lead to more unburned fuel in the exhaust. If the car is running rich, you get more unburned fuel in the exhaust. If the timing is too advanced, you could get knocking, too. If there are hot spots in the cylinders (usually caused by deposits or corrosion) you can get detonation.

What octane are you running in your car?

[FastWoman]

Ed, I doubt any pieces of the exhaust gasket could fall into the cylinder. If your exhaust gasket is the problem, the noise would be from the manifold banging against the head. An exhaust puff flexes the manifold outwards, and then it swings back in and bangs the head. You definitely want to be SURE you have an exhaust leak before attempting a replacement of the gasket. I just did mine, and it's not for the faint of heart. All I needed was ordinary tools. And yes, you have to take the intake off as well. You are likely to encounter frozen bolts and studs. You may need a blow torch to break some of the corrosion loose, vice grips to turn any broken studs, and possibly even a drill and bolt extractor set. You just have to do whatever it takes. If you're a religious person, prayer might help too. If you're not, and maybe even if you are, you'll have a great opportunity to expand your four-letter vocabulary.

Once you've gotten everything off, you'll need a metric tap and dye set to clean out the threads and a new stud/bolt kit from MSA. Also try to find some nickel anti-sieze compound to apply to the studs.

If you opt to have a mechanic do the work, it will cost. I was quoted $800 by the local Z specialist. It might have been worth it in hindsight, except that he wouldn't have changed out the old studs (which is a lot of the work). I ended up with a much better job by doing it myself.

You can read about my adventures here:

http://www.classiczcars.com/forums/showthread.php?t=39833

... and then here:

http://www.classiczcars.com/forums/showthread.php?t=40234

But again, make CERTAIN that's what you need to do before unbolting your manifolds! Make certain you have an exhaust leak where the manifold bolts to the head.

Good luck!

[TQs30z]

Thanks for the reply. Is there anyway to check whether there is leak on the exhaust manifold beside using stethoscope? I try to use a hose to pin point where the knocking noise comes from, but the knocking noise just too freaking loud even in idle, it literally hurt my ear just stand around the engine. So, any other suggestion to check exhaust leak???

Steve J, my car is running way rich before, getting like 10mpg and holley 4 barrel setup. So, I can't really tell you whether the mixture is good or not, assuming it already mess up on the mixture, but it ran fine before beside the freaking bad mileages and after the driving the car, I get the hot rod smell.........haha.

Just hopefully is not serious damage like valve or bearing job, any suggestion are welcome. Thanks everyone.

Ed

[Zed Head]

FW, I have never heard the "exhaust manifold banging off of the head" explanation for the knocking sound that a bad exhaust gasket makes. Do you have any background reading for that? I'm curious. Cast iron or cast steel flexing that much would be an interesting thing.

[Gary in NJ]

FW, I have never heard the "exhaust manifold banging off of the head" explanation for the knocking sound

I have to admit Sarah, that was new to me as well. To my ear, the sound is the uncontained exhaust partially bypassing the manifold. If you ever heard even a small engine like a 125cc dirtbike without a pipe you could understand how painfully loud the exhaust can be.

[SteveJ]

Thanks for the reply. Is there anyway to check whether there is leak on the exhaust manifold beside using stethoscope? I try to use a hose to pin point where the knocking noise comes from, but the knocking noise just too freaking loud even in idle, it literally hurt my ear just stand around the engine. So, any other suggestion to check exhaust leak???

Steve J, my car is running way rich before, getting like 10mpg and holley 4 barrel setup. So, I can't really tell you whether the mixture is good or not, assuming it already mess up on the mixture, but it ran fine before beside the freaking bad mileages and after the driving the car, I get the hot rod smell.........haha.

Just hopefully is not serious damage like valve or bearing job, any suggestion are welcome. Thanks everyone.

Ed

Ed, fix the known problems first. You say the car is running rich. Get the mixture and timing right. Eliminate the easy causes for detonation. Then see if you still have a problem. Otherwise, you'll empty your wallet trying out everything and hoping something works.

[TQs30z]

Cool, I will take care of it ASAP. By the way, the stock timing for L26 is set to 8 degree right? What bother me is that, I send my car to a shop and the shop doesn't how know fix datsun and end up keep throwing money away. I tried it once, the shop said this is old car, what you expect son? It will run rich no matter what, especially for an old car like this and I was like.......well forget it. So, may I ask is it easy to get the timing correctly on a Speed Shop and take care of the mixture??? And how much it usually cost to get the car tune in term of fuel mixture? I will look into the knocking tomorrow. Thanks everyone.

Ed

Edited November 24, 2010 by TQs30z