I changed the cylinder head and gasket
Check for an air/vacuum leak in the manifold gasket area. Check that the accelerator cable/ linkage is connected properly and adjust the idle speed up to around a minimum of 750 rpm
SOURCE: differance between nissan largo and nissan serena engine
The Engines aren't the same for these two cars.
NISSAN SERENA RUNS EITHER: THE "SR" FAMILY ENGINE or THE "QR" FAMILY ENGINE. Find out which is yours.
NISSAN LARGO RUNS THE "KA" FAMILY ENGINE. IT IS THE SAME ENGINE FOUND INSIDE THE FAMOUS '240SX', I HOPE THIS HELPS YOU.
SOURCE: how to repair nissan serena 2002 model qr20 engine
Things to check
Plugs;
Remove the spark plugs and read them, they should all have a dry tip and be clean from any carbon, oil or fuel.
Usually if burning fuel properly they will have a brown or whitish looking tint from burned carbon.
If any of the plugs look wet or black or dirty looking different from the others the problem is with that cylinder, change all the plugs and see if that helps, the next lease expensive thing is replacing the plug wires.If your fouling plugs ask your parts store for 1 number hotter plug ( heat range)
Plug Wires;
Open the hood at night with the engine running and see if you find any blue looking arcing around or in the wires, if so replace the wires.
Fuel Injectors
If all above looks good your fuel injectors might be dirty or one might not be working. You could try an injector cleaner mixed in with your gas
A rough idle comes from one or more cylinders not firing as hard as the others or not at all, there are so many causes for this so you have to troubleshoot with the lease expensive fix first.
Another thing is water in the gas, you can pour plain ole rubbing alcohol in your gas tank if you suspect water in your gas. alcohol mixes with the water and makes it burn instead of mis firing.
Valves
One more thing that can cause rough idle is your valves, this requires a compression check on each cylinder. If one cylinder has less compression than the rest remove the compression tester and squirt some engine oil in the cylinder then re test, if the compression increases it is bad rings or a bad cylinder bore, if not it is a valve problem.
Oil seals around the piston increasing the compression temporarily while testing but makes no difference in testing if the valves are leaking.
Good Luck,... troubleshooting and patience, or a good mechanic is the solution if you are working with neither
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