Brickboarders,

Has anyone else noticed a funky exhaust resonance that starts at 2,800-3,000 rpm? The engine is smooth from idle up to 2,800 or so, and then the tone changes to what could be described as a loud drone. Only thing is that, from the driver's seat, the "droning" seems to come from behind me instead of from under the hood.

Would a larger exhaust fix this, or is it an inherent problem with the 240 exhaust system? The engine also sounds "stressed" over 3,000 but that is probably a separate issue.

I had a drone like that on an older Dodge at one time.
Get a piece of heavier metal say around 3-4" long an inch wide and a exhaust clamp that will hold the piece of metal to the exhaust pipe.
Try clamping the metal to the exhaust, start off on the exhaust pipe around the tail of the trany. continue on different areas until it goes away.
You will find the sweet spot somewhere from the Cat back to the baffle (muffler)

Since your car has a two piece drive shaft, their is acenter carrier bearing surrounded in a rubber in the middle. This will drone when it' starting the fail. Check this. If needed I have a new one that the owner brought right before someone wrecked the car.

From the experiences of other posters and my own as well, a droning center bearing may be cured by injecting grease using a needle fitting on a grease gun.

Mine has been quiet for a year and 15k after doing it, and it's a quick and easy job. I imagine checking the bearing requires dismantling the driveshaft.

Peter

Hi John McPhail,

How old is the exhaust piping on your 1986 240? What is the configuration? Stock, with the front resonator downstream of the catalytic converter, and the rear muffler?

If you have inspected the exhaust for faults and found none, the 240 exhaust can make a harmonic in final drive, like in the highway, at around 2800-3000 engine RPMs. It may have been a set up with the manual transmission (more so M46 with over drive engaged) and differential. These can also make produce an aural harmonic like a tone. The tone may be intermediate or even high pitched. Even with clean lubrication fluids at proper levels.

Though low levels of differential or transmission oil fluid can make these make a gear whine or bearing noise.

Inside the (if equipped) front resonator and rear muffler, you have metal baffles welded to the muffler interior. These baffles can come unwelded and produce a louder than normal exhaust, can change the exhaust note anywhere through the engine ROM range, as well as obstruction preventing free flowing exhaust.

The engine sounding stressed at 2800-3000 RPM can indicate poor tune. If the valve clearances are wide, the engine can sound more stressed at an RPM (can even make a piston-slap or, if really bad, diesel engine sound).

A clogged catalytic converter can cause an engine to run weakly and sound stressed such as when you merge into highway traffic.

Hope that helps.

Duff.

Jack up the car and wiggle the exhaust to ensure no part of it could touch anything, then try tweaking the hangers for more clearance.
--
1992 745, >500k km

Might just be badly hung. Hanging an exhaust is the term, and it should be pretty loose. Check all of the rubber mounts, they can start to split before they break and drop off

Hi,

Check header pipe and/or cat. converter heat shields.

I would also take a look back at either your over or under axle pipe (whichever one you have) that goes into the rear muffler-the pipe can sometimes get shifted and vibrate against the thick shield that is beside your gas tank.

Travis

That happened to my '86 wagon years ago!

On the lower turn of the header pipe check for a cracked bracket or or missing nut that holds the pipes to the transmission bracket.

Also check for loose heat shields if you still have them on there.
They are spot welded together as two halves.

They are filled with insulation from the factory and will hold water splashed against the bends in the pipes. Not all of the water gets boiled off all the time. This causes rust that weakens everything around there.

At first I tried welding them to the pipes to keep them tight.
Later, I heard exhaust leakage and found cracks running the length of the bends in the two pipes.
I welded the cracks up and made a thicker bracket brace with a heavier nut.

It's just best to get rid of the shields all together.

Phil