I recently purchased a 1998 Civic EX coupe (manual) with 120,000 miles that didn't have service records. Car is is good shape inside and out, unfortunately not getting the gas mileage it probably should get: 22 City and 29 Highway. Other issues are:
1) When I rev the engine in neutral, at about 5K RPM's it starts to knock. I tried a motor mount test and they seem okay.
2) The car is very jerky especially in 1st gear (not my driving) and can be in higher gears, but to a lesser extent. Not like a severe engine misfire or stuttering, more like a rocking motion.
3) Engine idles well, seems to run well, gets up to operating temp in a normal way (within a few minutes). Just has seemingly poor gas mileage and the knock above 5K RPM's.

I have given it a complete tune-up including timing ,oil filter,air filter change. Cleaned all mechanical valves, except fuel injectors (couldn't find how-to's on the net).Cleaned all grounds. I've never diagnosed,tested or changed a knock sensor or possible bent valve, but after reading up on poor gas mileage and jerking (don't laugh) issues with civics it seems that Knock Sensor, Bent Valve,Jumped Tooth in timing belt, could all be a possibility.

I purchased a complete timing belt kit yesterday and hope to get it by next week. Is there a way to clean a knock sensor (possibly with electronic cleaner/lube)? and is it worth buying a cheap E-bay one.

 

Did you set the ignition timing using a timing light?

Have you replaced the plugs, wires, cap, and rotor?

Any CEL codes?

Any exhaust leaks?

Engine running cold?

 

Yes

Yes

No

Maybe . New exhaust manifold and gasket just after purchase, however there seems to be a faint puttering sound coming from near driver's side rear tire.

No (according to temp gauge. have not had a digital reading yet)