When at an idle, I hear a knocking noise coming from the top of the engine, when standing in front of the car. Could this be a loose valve or are they auto adjust? This is a 2003 MPV with 68,000 miles.
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Are you sure you are not hearing a popping noise from upper intake and throttle body because of a intake valve burnt or sticking open. Do a compression test on all six cylinders to see which one is low.
Knocking in the engine varies from simple failure to detonate to you are about to throw a rod. This is something that can be difficult to diagnose and if misdiagnosed, costly to repair. I would take that in to a mechanic as soon as possible. Barring that, I would check all your spark plugs.
OK let's do a little check. with the car in a clear area and no kid's or anyone around the car get in and start the engine.Now do you hear the noise with the car sitting at an idle.Let's say yes then put your foot on the brake and put the car in gear do you hear the knock if so hold the brake secure make sure no one is in front of the car and apply the gas lightly if the noise stop's then you have a loose torque converter to flywheel the bolt's are not tight..Now if you hear the knock and it's high pitch metallic then chance's are it's a rod or lifter in the engine.If the noise is a thumping dull noise you have a bad crankshaft.My guess is the torque converter because you said the transmission was worked on and a lot of time's without Loctite and the proper tourque the converter bolt's will work loose.GOOD LUCK HOPE THIS HELP'S
This is hard to say with hearing the noise but try to pin point the noise , is it coming from the top or bottom of the motor , the knocking noise could be a bearing in the lower motor or may be something else making it sound like a bearing,
Depends on the noise you are hearing and how the engine is performing with the noise versus before the noise came along. It come be something as simple as an idler pulley making noise which is replaceable in 10mins or crankshaft main bearings knocking which would be more cost/time effective to replace the engine. Need a better description of the noise to diagnose online. Top/bottom, knock/tick, does it go away when idling/under load etc... the more details the better.
After it's warmed up to operating temp, if it makes the noise at idle but goes away reving the engine, I would guess that a lifter has gone bad. Rod bearings as a rule knock all the time. It's hard to diagnose these things over the internet, because there are so many variables when it comes to noises.
You need to warm the car up and then while it is running, open the hood and try to pinpoint the source. lifter noise will sound like a "tick" and come from somewhere near the top. Rod bearings will sound like a 'knock" and will come from the bottom of the engine.
It may be piston slap. This is caused by either engine wear with the piston bores being too large for the pistons, or by poor manufacturing tolerances, or a combination of both.
To test for this, start the engine up first thing in the morning and run it for about thirty seconds while you listen carefully and memorise the sound and
intensity. Turn the engine off quickly, remove the spark plugs and put two
squirts of motor oil into each cylinder. Reinstall the plugs, start the
engine up again and listen. If you have piston slap the noise will have been greatly reduced or even
eliminated for about thirty seconds seconds and then returns.
The fix is to rebuild or replace the engine, but if it works just fine then don't worry. Many slappy engines keep going for tens of thousands more mile. The fact that your knocking is only at idle suggests your crankshaft is not to blame.