1987 LeSabre 3.8. 2 months ago started making grinding sound at idle . Goes away with acceleration or increase of 300 (or so) rpm. The car is running well. Water pump is one year old and seems fine. Replaced belt-tensioner (serpentine belt also) as this seemed the obvious problem but no change. Replaced 38 year old power steering pump still no improvement. This car was available in 1987 without AC so bought the belt for non AC (essentially bypassing the AC compressor) and still no improvement. Alternator checks out OK and the electrical system is working well. Very difficult to discern the origination of the sound even using a stethoscope. Any thoughts/suggestions?
A grinding sound at idle that goes away with an increase in RPM can be a frustrating and difficult problem to diagnose. You've already done some excellent troubleshooting by replacing the belt tensioner, serpentine belt, and power steering pump, and by checking the alternator and bypassing the A/C compressor. The fact that the sound is so hard to pinpoint with a stethoscope suggests it could be coming from a component that's not as obvious.
Here are a few possibilities and suggestions to consider:
Engine Mounts: A worn or broken engine mount can cause the engine to sit lower than it should at idle. This could cause a part of the engine, or an accessory, to lightly contact the frame or another component, creating a grinding noise. As you accelerate and the engine torques, it may lift just enough to stop the contact. This would also explain why the sound is difficult to locate with a stethoscope, as the sound is a result of contact rather than a failing internal component. Visually inspect the engine mounts for any signs of cracking, separation, or collapsing.
Harmonic Balancer (Crankshaft Pulley): The harmonic balancer is a weighted pulley on the front of the crankshaft that dampens engine vibrations. It has a rubber ring in the middle that can deteriorate over time. When this rubber fails, the two metal pieces of the balancer can start to rub against each other, creating a grinding or rattling sound. This sound is often most noticeable at low RPMs and may change or disappear as the engine speed increases.
Timing Cover Gasket/Water Pump Impeller: While you said the water pump is a year old and seems fine, it's worth a second look. Sometimes a newly installed part can be faulty. A grinding sound could be caused by a failing water pump bearing, which might not be obvious at a casual glance. Another, less common but possible, issue is a failed timing cover gasket. A grinding sound could be from the timing chain or gears rubbing against the timing cover itself. This is a more involved diagnosis and would likely require removing the timing cover to inspect.
Idler Pulley: Although you replaced the belt tensioner, sometimes there's a separate, non-tensioning idler pulley on the accessory drive system. If your car has one, its bearing could be failing. These are generally less expensive parts and are easy to replace. Even if it's not the primary tensioner, a separate idler pulley can make a lot of noise if its bearing is shot.
Exhaust System Contact: Similar to the engine mounts, the exhaust system could be making contact with the frame or a heat shield at idle. The low-frequency vibrations of the engine at idle could be just enough to cause this contact. An increase in RPM smooths out the vibrations and may cause the engine to lift slightly, ending the contact.
Next Steps for Diagnosis:
Re-check all pulleys and accessories: Even with a stethoscope, it's tough to pinpoint. With the engine off, spin each pulley by hand to feel for roughness, looseness, or a gritty feeling in the bearings.
Visual inspection of engine mounts: Get under the car and visually check the condition of the engine mounts. Look for cracks in the rubber and any signs of separation.
Check for contact points: With the engine running at idle, carefully look and listen for any part of the engine or accessory system that might be brushing against the frame or another component. You could also try gently prying up on the exhaust system with a pry bar (when the engine is cool) to see if you can recreate or eliminate the sound.
Get a second opinion: Sometimes a fresh pair of ears can hear something you've missed. A mechanic with experience with older vehicles might be able to offer a different perspective.
