1990 stock 240sx rough idle

[nightheron]

I have a stock 1990 240sx with only 250,000 miles. It is idling rough, sometimes worse than at others. I've cleaned throttle body and mass air sensor, replaced spark plugs and wiggled wires and connectors, no change. Any advice on where to start? Timing chain makes noise when I start the car most of the time, but has been doing that since I got the car 50,000 miles ago.

[2fizzleforshizzle]

Hummm? Only 250,000 miles? At that mileage you should be happy with a rough idle and a running KA24E! Does that thing pass smog?

[moso]

if you haven't run a compression test do so. remember to remove all the spark plugs and hold open the throttle when you crank the car. write down the numbers and post em up. most likely your rings are shot, or maybe bad valve seats. but a compression test is definately in order for a car with 250k.

[nightheron]

Thanks for the comments. I recently checked the compression and it was inbetween 170 and 175 lbs on all four cylinders, about the same as when I bought the car. I disconnected the negative battery cable for a while to reset the modem and it improved things a bit. I checked out the factory manual and there are about twenty things that can cause a rough idle, oh well.I don't know if it will pass smog, here in Eureka you only have to smog a car when you sell it.I thought that single cam motors are just getting broke in at 250,000 miles. Seriously I was thinking about pulling the motor and rebuilding the trans, replacing the motor mounts and timing chain, and getting the car painted, if I can get it done reasonably. Is this stupid? Would it be better to just get another car?
Modified by nightheron at 2:16 PM 9/9/2009

[2fizzleforshizzle]

IF it still runs, run it! As soon as you dig in to it you will open up a huge can of worms! Witrh 250,000 theres probally like 5 sets of plastic timing chain guides in your oil pan!

[allenms240]

My car has the same problem, although its not 250,000 miles. Yea dude, that engine is a damn tropper to still be running. Be glad its still firing. But can it be a vacuum leak? Those are the worsdt to deal with o.O

[nightheron]

I took your advice and checked the vacuum lines. I guess after 20 years and 256,000 miles I'd break down and spend seven dollars on vacuum lines. I found one that was rotted out and some were hard and probably not completely sealed. I replaced all the lines I could see and get to. I had to plug up one of them, car ran smoother when it was plugged. Now the car idles and runs super smooth. I guess I'm good for another twenty years. Changing your vacuum lines on an older car is definately a good thing to do. Easy and cheap too.