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 Midget and Sprite Technical - Engine knocking noise

On a run today, Manchester to Blackpool. Filled up with fuel first thing at Tesco.
After a while there was a slight knock under load, pinking type of sound. Thought maybe rubbish fuel from Tesco.
Gradually noise got worse and happened even under light load.Finished run early.
No noise when free revving the engine.
I have now discounted pinking so what are the possible culprits?
Big end ?
Small end ?
Don't think mains as would expect a heavier knock.
Head gasket related ?
The oil pressure is unchanged at 40psi idle and 60psi driving.

Any suggestions or advice welcome.
Thanks
Les
Les Robinson

Hi Les, sounds like headgasket to me. Had it on Jan's Frog a while back. It started like very light pinking but gradually got worse, then on a trip to Southport to see Jan's sister it suddenly lost power. Luckily only 10 miles from home, limped back on light throttle but in the end it became almost un-driveable.

Whip the head off, have a look.

LIDL have special offer from Monday for fibre-optic camera which might fit plug hole.

Rob

Rob aka MG Moneypit

If it is a 1500, then it could be a bottom end knock ie big end.
A small end would make a lighter tappy noise.

1500s are prone to bottom end problems. I don't think the A series have the same issues if looked after.

You would expect to hear it on start up before the oil pressure builds up, in particular if it has not been run for a while.
JB Anderson

Hi Rob
Bernie also said try head gasket, will try that first. See if I can do a compression test.
JB - engine is a 948cc

Les
Les Robinson

Not Mains - they rumble
Not little ends - they clatter
Unlikely to be big ends as your oil pressure would be well down by the time they get that noticeable.
Could be head gasket - check expansion tank for bubbling when the engine is running. Do a compression test.

As I am an optimist I would look for a blowing exhaust manifold, - if its blowing close to the head it can make a very loud metalic knocking noise, almost like someone tapping on an anvil with a light hammer.

But do try and identify the cause or eliminate problems before you start to take the engine apart. The temptation is to whip the head off to take a looksee, but once you have done that you can no longer do basic checks like compression, oil pressure, misfiring etc
Guyw

Exhauste leak perhaps...ive got a leak still and it sounds like thats


put
prop

^^^^As I am an optimist I would look for a blowing exhaust manifold, - if its blowing close to the head it can make a very loud metalic knocking noise, almost like someone tapping on an anvil with a light hammer.^^^^

yepp

i missed out posting guy by ((( Thatttttttt ))) much

Guy... good discription im keeping a copy of that one for myself

prop
prop

LOL Prop! I was about to say "Keep up Prop!" :-)
Guyw

Ive only got 2 thumbs.... haha

gez the pressure : €

Prop
prop

Well its good to know we are thinking along the same lines, Prop! It's certainly odd just how unlike what one might expect a blowing exhaust to sound like! Its a real metallic hammering. When I first heard that on my car I thought a broken con-rod end was smashing against the cast iron of the crank case. Scarey!
Guyw

I agree it does sound like a mouse with a hammer trying to escwpe inside the engine block luckly ive experianced it severwl times with my work truck

i need to eat a can good of veggies and get a couple of hose clamps so i can fix my leaking exhaust pipe

well i got to twke the car for ite nitly spin... only 97 F at the moment

augh... this is going to hurt

prop
prop

Thanks for the comments.

Blowing exhaust - was short of time last night but quickly held a rag against the exhaust tail pipe and couldn't hear anything blowing down stream.

Going to do compression test next.

Will keep you informed of outcome.

Cheers

Les

Les Robinson

Les, the tapping noise I was referring to is specifically when there is an exhaust leak close to the head - its a quite different noise to one part way down the system. On mine it was the underside of the manifold/ head joint, beneath the carbs. Barely noticeable on tick-over but tapping started when revved up a bit. It had blown out a section of the gasket there, so was an easy fix. But quite worrying when I first heard it.
In your case it may be nothing to do with this, but if it were it would be good news as the alternaives are all much more of a problem!
Guyw

If a head gasket is blown between either front two or rear two adjacent cylinders, a harsh pinking rattle occurs on anything but light throttle as new fuel-air mix on the compression stroke is injected through into the adjacent cylinder on its power stroke and detonates there.
A simple way to detect this is remove the plugs one at a time and the compression leaking through from the adjacent cylinder can be clearly heard in both directions as the engine is cranked over.
Paul Walbran

Doing compression test tonight.
Assuming for now that it is the head gasket then what is the best replacement type - copper?
I see that Moss have a competition one - copper / asbestos.
Where is best place to source one, looking for decent quality.
If new gasket fitted then how many miles before a re-torque? I'm off down to Beaulieu in 2 weeks time (about 550 mile round trip) and wont be able to get many miles on before that.
Of course it might not be the gasket at all!!

Les
Les Robinson

Done compression test.
1 and 2 cylinders are 180psi
3 and 4 cylinders are 135psi
So guess its head gasket blown!
Les Robinson

Sounds like Paul's explanation wins the prize then! Well done Paul in NZ!
Guyw

Les, I'd use this head gasket. If 1275 I use part No. BK450.

http://www.minispares.com/product/classic/C-AEA647.aspx

Re-torque the head down after 1 heat cycle (Get it up to temp, let it cool then re-torque)
J White

135 psi

on both cly?

I got my doulbts... it could be but thats alot of PSI and on both cly plus the same amount of psi on both cly


did you test at WOT with all the spark plugs removed ?

Id put a vacume gauge on it and see what comes up before you pull the head. Heck id also include a leak down test and even one of thise acid strip test

im wondering if you dont have a broken/cracked black spacer block on the rear carb

i just have a problem with BOTH cyl are reading 135 psi and that the psi is a high 135 (for a blown hg) and not something like 20 psi on 3 and 55 psi on 4

Is there any mayo in the oil cap or mayo inside the valve cover ... any sweet smell of anti

or is the oil sludgy sewer green color ?

I just wouldnt be so fast to pull the head just yet id investigate a little further if you pull the head and the gasket is fine it will take alot more work to track down the real culpret

prop
prop

Correction ...i got my doulbts its a blown HG not that the psi read 135
prop

As to head gaskets... payen is considard the best by the old folks

but if you got lots of cash MLS are really nice. I have no complaints on mine ... and for some reason i cant remember the manufacture name... haha

id be leary of copper... in the archives its great for racing but poor for day to day ...that said i have no experiance with copper
prop

What about a worn rocker bushing/bearing on the back post

it could be the rocker shaft has just worn and groved its self

if it was worn im guessing it would not provide the full lift maybe and guessing thst would throw off the psi a few ibars while giving a consistant psi reading for both cyclnders.. the wear would give the sound of big or small end bearing wear banging

agian ... im just guessing

prop
prop

Les
Check what gasket you have fitted as the compressed volume varies and will effect compression ratio. Also some modern composites do not require a retorque.
Alan
Alan Anstead

I'd be surprised that 135 is enough leakage to cause the problem, but on the other hand I've never come across one just starting to go as would be the case here if head gasket is the cause. By the time people bring it in it's zero thanks to about 1/4" missing from the gasket.

It is unusual but not impossible for identically low compressions on adjacent cylinders to be other than a gasket leak. Certainly it's enough of a drop in compression to be worth investigating.

Did you hear any farting via any spark plug holes whilst doing the test? It's a usful test to carry out to help analysis before removing the head:

If testing no 1 compression and the fart is heard via no 2, it is likely head gasket. It will be accompanied by farting heard via no 1 plug hole when no 2 comprssion is being tested. You will need to listen carefully given that the leakage is at the "minor" end of the scale (it is holding 135 psi rather than zero, that make it relatively minor!) You may also need to compare the sound with what's happening in no's 3 and 4 as a control sample.

If testing no 2, and the noise is heard via no 1 AND it does NOT happen the other way (test one, listen to 2 as above) then it is most likely inlet valve - see, these agricultural siamed ports are useful for something!
(This assumes I have my (firing order derived) sequencing correct, I have been known to muddle it in the past ... if not it will be the other way round.)

Finally, if testing no 2 and the far is heard via no 3 plug hole, it will be an exhaust valve in no 2. (Normally I'd say exhaust valve or head gasket, but in this case the compresion in no 3 rules out the latter)

Of course, if it's not the head gasket, then that's back to square one for the rattle.

Wouldn't it be great if the exact tone of a rattle could be transmitted via the BBS ... it wqould make diagnosis much easier ... or perhaps even more debated!
Paul Walbran

Hi Paul

Cant say I heard any specific farting noise although I'm not exactly too sure what noise I'm listening for.
Started measuring compression at no 1 cylinder.
When doing no 4 there was a loudish squeal as if something seizing or running dry. But not whilst testing no 3. This was not the pinking noise heard while driving.
With all plug holes open and spinning the engine the noise wasn't there.
I fitted all plugs and ran the engine and no noise.

End of the day I'm going to have to remove the head to look at the gasket and maybe valves.

Les
Les Robinson

Basically your are listening for any noise that isn't present in other cylinders.
The best way of checking it is to crank it over with one plug at a time installed to provide the compression loading. That allows you to concentrate on listening for anything different and not having to worry about the noise noted below as a possible for no 4.

While dismantling, once tha manifolds are clear and before releasing the head, crank it over with the plugs in. Any leaking valve will be evident by a SHHSSH out the relevant port.

It is worth doing these tests, takes a couple of minutes max and can be very informative.

No 4: Depending on the tester, there can sometimes be a little bit of a leak between tester and plug hole that can make a bit of a noise, that may be your squeal.

Paul Walbran

This is a fun game! I think I have similar issues but I think and hope it might actually be a gearbox problem.

From sat inside the car when driving I can't distinguish where it is coming from, but I have a pretty brutal sounding clatter in first (first and reverse gears coming together due to too much float somewhere). I hope it is just an extension of this issue.

The car has always done it, through head gasket changes, manifold changes etc. etc. Oil pressure is OK and the engine survives being run up to 6000 rpm regularly so I would have though it if it were something sinister with the engine it would have blown up already!

Malc.
Malcolm Le Chevalier

although saying that mine is 1500... so perhaps ignore that!
Malcolm Le Chevalier

Took head off tonight.
Gasket blown between 3 and 4.
About a 2mm gap.
So that's the weekend sorted, fitting new gasket. Gone for a Payen copper.
Les Robinson

"Gasket blown between 3 and 4"
There's me having a senior moment again, muddling the figures around in my head - somehow I got the 1&2 as the low ones, so the 1&2 and 3&4 are round the wrong way in the above ramble. Oops!
Paul Walbran

paul i did notice but it was clear what direction you are going

id love to see a photo of the head gasket... like paul i wouldnt have thought 135 psi would do this ...as he said it must have just started very recently

id rethink the copper HG ... there great for racing but are considard poor for street cars

but then agian i was wrong on the blown HG...haha

congratz and job well done

prop
prop

Here is pic of gasket.

Les

Les Robinson

I have had very similar gasket failures in the past. I think it comes about from overtightening the cyl head nuts. This may sound counter-intuitive but overtightening crushes the fire rings down so the head gasket sealing pressure is spread over the entire gasket surface rather than being concentrated on the fire rings. Not so important where there is plenty of gasket surface to back up the rings, but where it is very narrow between cylinders it creates a weak spot.

The 135 compresion reading is still achieved as the compression still builds if the engine is spinning quite fast. 135 is aftr all only a 5:1 ratio to ambient pressure. When you took the compression readings, did you remove all of the plugs whilst you tested each cylinder in turn? I have seen the test being done with the other 3 plugs still in, but it makes for heavy going for the battery and I don't think it gives as accurate results as the engine turns slower.
Guyw

This thread was discussed between 12/06/2016 and 16/06/2016

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now