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MG MGB Technical - dodgy engine noise diagnosis game
|any one up for a game of guess the engine issue?|
after a successful run from london to bristol (including two hours of traffic jams coming out of london) the next day i tried to go from cheltenham back to london.
On the first A road hill a noise a lot like a very tappety noise begins, it is almost a tinny clicking sound, it is dependant on engine load, so with the throttle open only a little it is not there but whilst moving an trying to pull the car it is. The noise gets louder over the next mile. It occurs in all gears. I pull over to the side of the road to open the bonnet and have a look. At this point the car starts to falter at tick over and peters out. At which point she will not restart.
Soooooo. I check oil, water, spark presence, spark plug colouration, fuel to the carbs, and all is fine. I clean the sparks and reset the gaps, and with a lot of persuasion i manage to get her to start once she is cool.
so i start of again gingerly trying to reduce the number of miles between me and london, and sure enough one mile down the road on a busy multi lane roundabout, the engine starts to falter and no amount of gentle throttle persuasion could keep it from dying.
now the RAC feller reckoned it was to do with the timing chain but I implore the massive wisdom of the mg car club brain trust to put in their two pennys worth.
|My first thought while reading the symptoms was that the timing chain tensioner pad could have worn out allowing metal to metal contact with the, now loose, chain. This could cause the noise you heard varying with engine load. The slack chain might also cause the starting and running problems.|
I dismantled an early B engine recently where the tensioner was so worn that most of the metal backing was gone and and it was hanging out of its guide at an angle. I had never heard this engine running so don't know what it sounded like.
good to know your view is similar, was it a difficult thing to replace once the block was out of the car?
|I don't see why changing the load on the engine changes the load on the timing chain. The symptom sounds more like pinking to me, with timing slipping more and more towards advanced until the engine could no longer run. That *could* be from timing chain problems as much as the distributor, but I'd go for the easy one first.|
|I tend to agree with Paul|
Tickimg from the timing chain isn't really load dependant
To me it sounds like yet another headgasket
Time for a compression test
|Another vote for the dizzy. Is it standard or electronic?|
Ticking from the timing chain could be slightly load dependent as under load it will be slack(er) on one side and when cruising, flapping on both sides!
Still go for the easier and slightly cleaner option!
If you do need to do the tensioner its a while since I've done one but I remember it as a piece of cake! It should be accessible without taking the engine out. Just take the radiator off to get to the front pulley and chain cover on the front of the block. You might need to remove the engine from the mountings and jack it up a little to clear the steering rack.
|Did you fuel up after the first journey. You could have got some poor petrol.|
Did the RAC connect a strobe, or even check the static timing.
|If you had a relatively long successful run the day before it could be that you sprung a head gasket leak? Certainly sounds like 'pinking'. When you put your foot down, a rapid tinny metallic clicking, off when you ease of. A timing chain problem is going to make a noise all the time, just might get louder the faster it moves. Check for an 'emulsion' viewed through the oilf filler cap, although on a long run, even water might evaporate off. A cylinder head gasket leak means the combustion temp has gone up hence pinking, but then again you might have got some bad fuel? Mike|
|Silly me... lifting to clear the rack is a midget thing! Cross member might be in the way on a B.|
|But along these lines; Is it possible to change a cam while engine still in car? I can see that removing the radiator and possibly the rack in order to gain good access to the cranckshaft bolt, perhaps jack up the engine, but what about the rear of the cam? Does it just pull out? and certainly the cam bearings must stay in situ? The more I think of it the more I think, why bother, lets get the whole lot out? Not that I am thinking of this just an idle thought. Mike|
|Mike, changing a cam with engine in is possible but unless you are doing it just to change the profile, why would you?|
If you need to change a cam for other reasons the rest of the engine will (most likely) need work eg crank bearings, clutch plate, seals, rings etc. Best get it whipped out for such work. On your specific questions..
1, rear of the cam is not locked and just runs in the rear bearing
2, cam bearings are press fit in the block and will say put.
Jon, not saying you should do that yet! Well, not until you work out what is actually going on....
|hey, blimey thats a huge response overnight, thanks everyone.|
I was wondering about pinking because in the past I had a car that drained itself of coolant on the motorway and had to get to the services on empty and the pinking got bloody loud. It certainly had that same feeling of loss of power when it was happening.
I'm not seeing emulsion under the oil cap, but it s had a persistant brown sludge in the radiator cap, that still appears even after multiple flushings ( i put this down to gunk and rust due to it having sat for 10 years before I bought her and finished her.)
It is an electronic distributor system. The RAC guy didn't check timing and I haven't either yet, but as soon as i get hold of my gun again I ll give it a look. The dizzy was sound and secure when i checked her though.
Also the RAC guy said that he could hear a noise even at tick over when he was listening.
I did fill up on the first journey, but after having accidentally drained my tank on a trip three weeks ago and cleaned everything up after she got a bit gunky, It didnt feel like that kind of issue, to me bad fuel and sludge always feels more intermittent and less mechanical in noise.
I think I'll follow the advice of checking timing and compression before going any further, especially as she s now stranded outside my house
thanks again everybody
|ok so off the back of the last round of lovely replies I have done some more diagnostics to the engine.|
After charging the battery fully, i checked the compression, the timing, used a colourtune and checked for presence of spark.
The compression was awful, with cylinders 2,3, and 4, showing only about 70psi (book value says it should read close to 170 psi) whereas cylinder 1 showed a ridiculous 20 psi.
the timing showed the pulley mark to be stable and holding at 9 degrees before top dead centre. (which was a total surprise)
the colourtune, showed no combustion at all.
The spark indicator, showed healthy sparks on all four cylinders.
This would suggest either valves, piston rings or head gasket. I am yet to do a wet compression test and to check the valve gaps, which would help show up whether its a piston ring issue or whether i have a stuck valve.
But please world wide web of MG lovers, what does this sound like to you?
p.s. have had some suggestions of doom, when taking the tappety noise and lack of compression into account, which might sugest some metal knocking around in the cylinder which obviously will be a very large job to sort out.
|Jon, is the head an unleaded conversion. Has it ever been overheated? It's also not unknown for the inserts to drop, but all of them??? Take the rocker cover off, check the gaps, the collets, the valve heights when closed and while turning the engine over with a spanner, watch what's happening. Anything looks wrong, take the head off, not a big job. A worn bottom end is unlikely to suddenly go like this. More lkely head or ignition. Have you checked the dizzy cap and rotor arm. They can operate reasonably well in the warm and dry, at idle. But a bit of damp air and demand for power, shows them up.|
|Jon, yank the head. It's got to come off whatever. Then we will see more.|
Did you do the compression test with the throttle wide open and all plugs out at all times? The cylinders need to fill and the engine needs to spin to give a good result. Anyway do the wet compression test and get back with the results, it may that surfacing the block and head and new Payen gasket will sort this out, quick DIY job and not too expensive.
This thread was discussed between 24/10/2011 and 08/11/2011
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