MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Engine knocking

I took my 1973 MGBGT on a run a couple of days ago 200 miles + there and back with a break, on the motorway running between 60 and 70 mph on average. She ran fine until I got to within 3 miles of home when I had a sudden loss of power with knocking at the top of the engine. I limped home and discovered that the oil had dropped below the minimum level despite my having checked it a few days previously. After it had cooled down, I refilled with oil and started the engine again with the knocking and rough running still very much apparent. I took off each spark plug lead one by one and discovered that the knocking appeared to come from the top of the engine 3rd cylinder from the rad! Oddly enough the knocking stopped when I took the lead off and it started again when I replaced it. There does not appear to be any water in the oil or vice versa or any oil leak from the engine. She was running warmer than usual but I was driving at a sustained engine speed of around 3000rpm. I would be interested in any possible diagnosis you folks could give. My initial thought was either small end bearing or valve failure as a result of the low oil. I did think about a broken con rod, but would have expected a rather more cartastrophic failure. I don't know why the oil had dropped so low either in the first place unless it was because it was running a tad warm. Any ideas anyone?
Thanks
Paul
Paul

How many miles are on the engine? I have visions of a burnt piston.

Just my first impression.

Wayne
Wayne Pearson

Its done around 70K!
Paul

It sounds to me like a cracked piston, probaly at or near the wrist pin. This would explain why the noise dissapears when the sparkplug lead is removed from that cylinder. Also, with the piston failure, one would expect an increase in oil consumption due to greater clearances between the piston and the cylinder wall. Most likly the cylinder is not damaged yet. RAY
RAY

Or just a sticky valve as it was running hot? Easy to check that by removing the rocker cover and turning the engine by hand.

Can check the burned valve theory with a compression tester, less than a tenner on ebay.

N
Neil

Paul,

I've had the knock come and go with pulling the plug wire on two cars, a 59 Rambler and a 51 Ford. Both times it was a rod knock. In fact, I drove quite a few miles with the plug wire removed until I could repair it.

RAY's guess would explain the oil loss and rough running, while mine doesn't. However, a loose rod could be caused by low oil level. Any chance a dropped or jammed valve could cause this?

I suppose you are going to have to start at the valve cover and work your way down. Maybe you should run a compression test before taking anything apart.

Charley
C R Huff

Looks like you have a piston problem. Certainly check the compression but was the engine smoking heavily when it was running and is there heavy fumes emitting when you take off the oil filler cap as that would be the give away sign.
Iain MacKintosh

There was no smoking noticeable at all from the exhaust or when I subsequently opened the filler cap.
Paul
Paul

It might be the head gasket blown between #3 and #4 cylinder. Try removing all the spark plugs then crank the engine while blocking #3 with your finger and feel or hear what happens in #4. If that wheezes when #3 should be compressing then it could be cause. The reason I suggest that is it happened to me last year - the head gasket burned out between #3 and #4 and although the engine ran at idle , there was a really bad knocking under even the slightest load. Also, the burn extended to one of the oilways allowing the crankcase to get pressurised. I noticed a small drip for a few days before I heard the knocking, and I think that over any distance it would have dumped quite a lot of oil with no apparent smoke.
Miles Banister

Oh, I should also have said that when I disconnected the #3 plug lead there was no knock at all.
Miles Banister

Sounds like the little end of the con rod/gudgeon pin. on number 3 cylinder is worn.
When you remove the HT lead, it is taking the pressure off so you do not hear the noise.
Trevor Harvey

Did the oil pressure change when this noise appeared. If there was no change then it very well be a piston or wrist pin noise.This means that there is not a problem with the crankshaft.
Sandy
Sandy Sanders

I thought I saw the pressure gauge flicker down momentarily when it happened, but other than that it was at or near normal when I nursed it home!
Paul

There's a distinct difference between "knock" and "tap". The knock is a low frequency sound that you "feel" as much as you hear. Also, if a connecting rod bearing is going, the "knock" will be more noticeable on deceleration than on acceleration. Main bearing failure is more noticeable on acceleration. I recall some notes in the archives about connecting rod bolts over the years and some of them break (never happened to me, but recall the articles). It's actually not easy to "scatter" an MGB motor (not the 5 main engine anyway). Trust an old west Texas boy that has "scattered" everything from flathead V8s to 256 Chevy blocks. Any way you look at it, you're in for a complete engine teardown. 70K shouldn't result in bearing failure, not even the rear main bearing that bears most of motor's "working load", unless there is a problem with oil level or oil delivery to the bearings. In the late 60s and early 70s, overhaul intervals were about 75K, but the first overhaul usually only resulted in crankshaft polishing, possible knurling of pistons, rings, bearings, gaskets, etc. Thus, something has most likely "failed" rather than "worn out". It's not difficult to drop the oil pan and inspect the connecting rod caps and bearings. A worn out one will be obvious (worn and exposed to the bearing's copper core) and your connecting rod and crank journal will now be "egged" as we use to say.
Rick Penland

I don't think that this is a cylinderhead gasket problem but it is ceertainly worth doing a compression check before you remove the head. If the gasket is gone between three and four the compression will be low but equal on both cylinders.
Iain MacKintosh

Just by way of an update and as a thanks to all of you who responded to my woes. It may also come in useful in the future on the archives. The upshot of it all is that the head gasket had gone between cylinders 3 and 4. There was no damage anywhere else (phew!!) and she is now purring like a kitten! I have been told since that B series engines have a tendency to do this, but maybe someone out there knows more. Anyway I seem to have got off relatively lightly this time!! Thanks again all!!
Paul
Paul

This thread was discussed between 14/05/2009 and 29/05/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now