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MG MGB Technical - Mysterious Engine Noise

Car is a recently acquired 72 BGT that is now my daily driver. Occasionally I get a "rumble" from that is definietly the engine and not road speed dependent. This happens only when the engine is hot, only at very low speeds (at least that I can hear), and sort of sounds like the engine is in direct metal contact with the "frame". There is also a vibration associated with the rumble. Checked the engine mounts visually and they appear to be solid and intact. Hope to get under the car this weekend and take a good look for problems. Special attention to exhaust routing and the U-joints.

Any additioal things to look for?

Thanks

Larry
Larry Hallanger

Also check the trans mount. More fun to reach.

Steve
Steve

Larry a couple of things that can sound like that are engine down pipes knocking against the sump rail or gear box or loose in its steady bracket off the gearbox.
Loose alternator mounting bolts. A loose baffle in the muffler.
DENIS4

Larry-
You're wise to check the U-joints. From what you describe, I'll bet that's where the problem lies.
Steve S.

Soft trans mount rubber = tail of trans contacting
the trans mount crossmember.
Daniel Wong

Larry. Hard to diagnose as there are so many things it can be.

Horns loose.
Radiator loose
Fan loose--rumble or stiking radiator.
Harmonic balancer loose.
Water pump pulley loose.
Alternator loose. (Also, air pump if fitted.)
Broken carb heat shield.
Loose air cleaners.
Exhaust pipes hitting steering/frame.
Exhaust system hitting rear of engine mount.
Motor mount loose.
Transmission stay rod loose.
Transmission rubber mounts bad.
Tranny cross member loose.
Bad U joints/ yokes out of alignment.
Bad prop shaft sliding joint.
Steering column hitting frame if front cross member is not tight.

This is a basic list of things which might cause a problem such as you describe. If you can recruit an assistant, put the car up on either jack stands or a lift, and have the assistant "drive" the car as you attempt to isolate the noise, you can focus your efforts in a more directed manner. If not, make a list and start checking off the various areas that may cause such a problem until you can find the source.

Les
Les Bengtson

That's the classic description of main bearing failure. Would help if we knew what "very low speed" means. It commonly means "lugging", which both causes and worsens bad mains.

FRM
FR Millmore

Steve

Checked the U-joints. They are OK.


Les

Lubed U-joints at the same time (forgot to do them when I lubed the car a couple of weeks ago). Took about 15 strokes on my grease gun before old grease started pushing out of the spline. No fittings on the u-joints themselves (permanently sealed bearings I hope).


FRM

Low speed is 3-5 mph making a sharp turn into my driveway in second gear. I am also sometimes using the brakes in the turn. Runs well and smooth at higher revs.


Les, Daniel, Steve

Looks like I need to start with Les's list in hand and start checking off items one by one.


Thanks

Larry
Larry Hallanger

"No fittings on the u-joints themselves (permanently sealed bearings I hope)."

"Permanently" meant "for only as long as any fleet owner is likely to keep the car from new". Fortunately decent quality replacements regained the grease pont some years ago, as did the sliding joint on replacement prop-shafts. Which does only grease the sliding joint, and not the UJs.
Paul Hunt 2

The sharp turn condition would have me looking at engine/gbx mounts letting the exhaust contact the steering shaft or floor first off.

FRM
FR Millmore

Larry,

on my roadster i also had this 'problems' once. On this car, it was a fitting of the oil pipe from the filter base to the cooler, that came into contact to the inner wing at very low speeds.
I fiffed new mounts for the engine and gearbox and turned the fitting of the oil pipe a little upwards and there was no noise anymore.

Before starting this job, check out whether there are no vibarations from cables leading to the carbs and the heater valve (that can contact the hood) and check the rubber mounts of the exhaust system and it's support at the gearbox bellhousing.

Hope this helps

Ralph
Ralph

This thread was discussed between 04/10/2007 and 11/10/2007

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