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Ask GearheadBrown Your Own Question
GearheadBrown, ASE Certified Technician
Category: Car
Satisfied Customers: 68
Experience:  10 Years various auto repair and diagnosis. Nissan Specialist technician. ASE Master certified.
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95 isuzu has a knock when idleing, when i quickly rev it up

Customer Question

95 isuzu has a knock when idleing, when i quickly rev it up it goes away. but when slowly acelorate it the knock is still there
Submitted: 5 years ago.
Category: Car
Expert:  GearheadBrown replied 5 years ago.

I hate to be the bearer of bad news, however this is a classic indication of a spun connecting rod bearing. The condition being that your rod bearings have enough slack to "bounce" around when the engine is not under a heavy load. When you rev it up quickly, the engine is pushing the crankshaft hard enough to take up any slack that the bearings may have, and eliminate the sound temporarily. Then, when on a slow acceleration, again you do not have enough load to remove all of the bearing gap, and the sound returns.

Your best bet at this point is to take the vehicle to a repair facility that you trust. If the damage is not too severe, it may be possible to repair the engine. If you continue driving the vehicle without having it properly diagnosed, the problem will only worsen, until the engine fails completely.

If you need any further assistance, please ask!
Customer: replied 5 years ago.

i replaced the timing belt today and it seemed to help alot... sounds like the noise is coming from the passenger side head. could it be a lifter? or just a rocker arm out of adjustment?

Expert:  GearheadBrown replied 5 years ago.
Generally speaking a lifter tap that goes away when the engine is revved, is not dependent on the speed at which you accelerate the engine. A noisy lifter that gets quiet with the increase of RPM is due to the increase in oil pressure, taking up the extra gap between the camshaft and lifter.

Since your vehicle is equipped with hydraulic lifters, this also rules out an out of adjustment rocker. However, one of the lifters may have collapsed completely, though this is more likely to cause a ticking noise, rather than a knock.

Isuzu does state that if you notice a noise from the lifter, to run the motor at 2500rpms for 30 minutes. This will bleed any air trapped in the lifters, and should allow them to properly close the gap in the valve train.

Your best bet a this point will be to use a stethoscope to check exactly where the noise is coming from on the engine. This will tell you if the problem is occurring in the top or bottom end.
Customer: replied 5 years ago.

one more question i think lol... been talkin to a couple of people, do you think it could be the timing belt tensioner slapping around? concidering that the noise got quite a bit quieter when i changed the timing belt

Expert:  GearheadBrown replied 5 years ago.
If there is not enough tension on the belt, it is possible to hear a sound from the belt hitting the timing cover.

Your vehicle is equipped with a auto tensioner, so if you take it apart and find excess slack, then I would loosen the tensioner pulley, rotate the engine a few times, and re-tighten the pulley to see if it improves.

It is always a good idea to replace the tensioner and pulley when replacing the timing belt, to ensure you don't have problems down the road.
Customer: replied 5 years ago.

how do u go about retensioning it? from what ive noticed it was spring loaded.(not a very strong spring, i can push it with my thumb) and then after u start the vehicle the oil preasure should take over and keep the belt tight. right or wrong? or is it that the little piston that pushes on the tensioner pully defective?

Expert:  GearheadBrown replied 5 years ago.

The tensioner pulley is on a cam-style adjuster.

To check adjustment, measure how far the piston is coming out of the "pusher". The piston should not be any further than 6mm out. If it is further, then remove the adjuster and loosen the tensioner pulley, and adjust by turning the center part, then reinstall/retest. Here is an image to help: Photobucket I looked over the procedure, and it sounds like you may just need to adjust the position of the pulley if it is not tightening the belt enough.

Customer: replied 5 years ago.

ive had that tensioner off. the bolt that your saying that should adjust the tension is not offset at all. there is no adjustment there. is there any way of testing the pusher

Expert:  GearheadBrown replied 5 years ago.

The only thing the service manual has to say about the tensioner is the protrusion of the rod, 4-6mm. If find that the belt is hitting the cover, then your best bet is to replace the tensioner pulley and adjuster.


Also, ensure your oil level is correct and you are using the correct weight. If the oil is too thin, it could possibly have an effect on anything controlled by oil pressure, such as the tensioner.



Customer: replied 5 years ago.

alright... well i will take a look at it again tomorrow night.. and i will let u know what i find.

Expert:  GearheadBrown replied 5 years ago.
Sounds good! Hopefully it is just the tensioner and not anything internal.