I have 1998 Infinti Q45t 4.1L V8, It has 202,750 still runs great except you can hear little knocking sound when idling. Is that bad if you hav knocking sound from engine?? So do you think that we can fix it?? So what wrong??

OK let's do a little check. with the car in a clear area and no kid's or anyone around the car get in and start the engine.Now do you hear the noise with the car sitting at an idle.Let's say yes then put your foot on the brake and put the car in gear do you hear the knock if so hold the brake secure make sure no one is in front of the car and apply the gas lightly if the noise stop's then you have a loose torque converter to flywheel the bolt's are not tight..Now if you hear the knock and it's high pitch metallic then chance's are it's a rod or lifter in the engine.If the noise is a thumping dull noise you have a bad crankshaft.My guess is the torque converter because you said the transmission was worked on and a lot of time's without Loctite and the proper tourque the converter bolt's will work loose.GOOD LUCK HOPE THIS HELP'S

It sounds like a valve or valve lifter issue. First check and make sure the oil livel is where it needs to be. Then, I would let it idle for a few minutes and see if the knock goes away when it warms up a little (still idling). If not, its a bit more in the advanced stages of a valve or valve lifter going bad. Either way, I would change the oil with some high quality synthetic oil and get a filter with a non drainback valve (k&n or Mobil 1 has them for sure). Those attempt to keep the oil in the top half of the motor when you shut it off. I would always let your car run for a minute (longer in winter) before you drive it to get the oil pumped up and circulating.

sound like ignition timing off.

Well lift the car up and put it on drive wile you can hear what the knock is. See if is your engine or the transmission. And if ok check your spark plugs make sure you getting fire to them

If there is no "Check Engine" light try a bottle of good injector cleaner in your gas, or cleaning the throat of your throttle body with a good spray cleaner. If the "Check Engine" light comes on then put it on a code scanner and that will point you in the right direction.

It may be piston slap. This is caused by either engine wear with the piston bores being too large for the pistons, or by poor manufacturing tolerances, or a combination of both.

To test for this, start the engine up first thing in the morning and run it for about thirty seconds while you listen carefully and memorise the sound and intensity. Turn the engine off quickly, remove the spark plugs and put two squirts of motor oil into each cylinder. Reinstall the plugs, start the engine up again and listen. If you have piston slap the noise will have been greatly reduced or even eliminated for about thirty seconds seconds and then returns.

The fix is to rebuild or replace the engine, but if it works just fine then don't worry. Many slappy engines keep going for tens of thousands more mile. The fact that your knocking is only at idle suggests your crankshaft is not to blame.

If a knocking sound occurs at idle, a bearing for the camshaft or the crankshaft may be bad.

problem 1 could be a loose wire, but if it is failure in the switch it will need to be replaced cleaning is probably not an option. Problem 2 bad gas can cause the stopping at idle speeds add some stp fuel treatment, read label to make sure it dries up water in gas. Fill up after you add the treatment and you will probably be good. problem 3 is tricky. Try turning off the a/c then turn it back on you should hear a little click like sound which should indicate if it is in fact turning on the a/c. If it sounds like it did turn on then turn off a/c and switch to heater or vent mode you should hear what sounds like little doors being closed. If you don't hear a change or heater doesn't blow hot air or your outside air vent doesn't blow cooler/outside temp. air then you may a have a leaking vaccum line or the vaccum unit is not working right.

the timming chain system i would inspect

Sounds like your idle air control valve is bad. This valve, like it sounds, controls the idle on your vehicle and from what you make it sound like you are having idle issues. The IAC is located on your throttle body and is pretty easy to change out. Let me know if you have any other questions. Thanks for rating my response and for using FixYa!