Why does my 89-90 SOHC 240SX make a rattling sound right after I startup which then stops about 2 or 3 seconds later?
The rattling you hear coming from under the hood right after start up is the timing chain in your SOHC engine. This problem frequently occurs in just about every SOHC 240SX out there sooner or later. It also happens in Nissan’s truck with the KA24E engine. Why does this happen? It happens because on both sides of the chain there are chain guides. Its kinda like a bike with a chain guard. These 2 chain guides keep the chain inline as it spins. However, for some odd reason the guides in the KA24E engine are made from plastic. What happens is usually the right chain guide starts to break apart from stress cracks that can be due to alot of things (heat, oil contamination, and/or vibration). This only happens mainly with the right guide. The left one has a rubber coating over it which protects it. So, the chain guide cracks and finally gives out. This chain guide is about 8 inches long if I recall and is held to the block by 2 metric allen bolts. One on the top and one towards the bottom. Well if the chain guide cracks somewhere in the middle of these 2 bolts, which is most likely will because that’s its weakest point, the guide is free to swivel around these 2 bolts as the chain rubs up against it and chews it away. It doesn’t take long for the chain to finally chew just about all of the plastic guides away and there is virtually no guide left to support the chain (just 2 pieces still connected to the block by the bolts). When the car is cold and the chain is at its loosest, it will rattle when the car starts because it is rubbing up against the inside of the front cover. I’ve heard that the chain tensioner is hydraulically actuated by engine oil but, I’ve changed the tensioner myself and it seems to have just a strong spring in it which forces a piston type piece up against the left chain guide which in turn tensions the chain. Anyhow, once the engine starts turning the chain tightens and the rattle stops. This all happens in a matter of seconds. Now, you may wonder where all this plastic that has been chewed away goes. Well it goes down into your oil pan and stays there for years and years until someone realizes it and cleans it out. Don’t worry though. There is a fine strainer on the oil sump that should block any of the pieces from going into the engine’s internals. Keep in mind my car started making this rattling noise at 55,000 miles and I changed my chain at 104,000. Obviously the rattling wasn’t doing much damage and I didn’t see any damage done to the front cover or chain when I got in there so I don’t think the annoying rattling was too detramental.
You will ask, “So how do I fix this?”. Well there are a few options. You can do it yourself. The parts for the job cost around $135-150 depending on where you get them. You’ll definately need a service manual for torque values and for step by step instructions. You’ll also need a pretty good tool set with metric sizes, including metric allen keys.You’ll also need a pulley puller to get the crank pulley off the shaft.The job took me about 12 hours to do myself.