Knocking when A/C is on

coolbrze · Jun 5, 2008

2002 Ranger 4 cyl. 5 speed manual 2wd, truck in sig below... Anyways when I turn the A/C on I hear a knocking coming from somewhere inside the engine bay. Any idea what this knocking could be? I'm not sure if it's only when the A/C is on or if it's just louder when the A/C is on, but it's definitely heard, and slightly felt in the steering wheel. Any advice is greatly appreciated!

reginald fairfield · Jun 5, 2008

Mine does it too, and it is high mileage.. 140,000 +. I think it is the extra strain on the motor causes normal knocking to get worse.

MAKG · Jun 6, 2008

There is no "normal" knocking that can be felt in the steering wheel.

Get a stethoscope and figure out what is knocking, at idle. Then replace it.

CraigK · Jun 6, 2008

You might try this to help diagnose . . .

Try a tank (or a few gallon$ on am empty tank) of premium. If the knocking stops, it's likely preignition. Check timing and your antiknoock sensor. If you have one.

CraigK

MAKG · Jun 6, 2008

If it's preignition, his engine is dead.

If it's detonation (spark knock), a much faster way is just to listen to it. Detonation sounds like marbles in a tin can, and it is VERY often worse when the throttle is stomped. Preignition is a very different beast, and it's extremely serious and far rarer than some people think. And detonation isn't always cured by premium (e.g., if it's caused by oil use, premium + 10W-30 has much lower octane than regular).

If the knock is a regular tap, bang, or thump, it's not detonation. If it occurs at idle or lightly loaded, it's not detonation.

coolbrze · Jun 7, 2008

I've only got 48K on this truck so I wouldn't think it's due to high mileage. It's not something to do w/ the gas/fire (I wouldn't think) as it only does it when I'm driving and not when idle. It's like something is loose and is bumping around when I drive, not when I'm sitting still.

Knocking when A/C is on

coolbrze

15+ Year Member

reginald fairfield

15+ Year Member

MAKG

15+ Year Member

CraigK

15+ Year Member

MAKG

15+ Year Member

coolbrze

15+ Year Member

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