HI,
I have a 93 mitsubishi mirage.A few months ago I noticed a knocking sound coming from around the 3rd or 4th cylinder.As I accelerate it get's louder,but what get's me is sometime's while idling normally you can hardly here it.What type of problem's should I be looking for in a situation like this?Anyone that can give me any advice i will GREATLY appreciate it.
Depends on the engine. Best case its just a loose tappet or a worn hydraulic lifter. The 1.5L 12v 4G15 engine has adjustable roller tappets. If this gets loose, the noise is a metallic ticking sound from the top of the engine.
The 1.8L 16v 4G93 has I think hydraulic lifters. These give up when old and get noisy. One way to check is hold the revs to 3000 for about 30 seconds, then SLOWLY let the revs drop back to idle. If the knocking sound goes away, its a lifter.
Worst case its a big end bearing. Starting at idle, blip the throttle. A worn bearing will sound like a hammer hitting an anvil, and comes from the bottom of the engine. Often theres a no load rev point (about 1800 rpm usually) where the knocking stops. Blip and it knocks, reduce revs and it starts again.
If it is big end, it needs to be addressed immediately, as severe crank journal wear occurs very quickly. The "good" news is that its possible to drop the sump and look at the big ends. If a bearing is shot, its simple to replace if theres no crank damage. If there damage, the best option is to get another engine from a wrecker and rebuild it, then swap it into your car. Or throw the car out if its a rust bucket.
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