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MG Midget and Sprite Technical - Compression Test Diagnosis...

Hi All,

My 1500 had been overheating, which culminated in some pre-ignition. Turns out that the cooling system was down 1+ litre of coolant. I could find no obvious leaks in the system.

I have just conducted a compression test. Unfortunately this is the first I have done, so have nothing to compare against. The results are below.

Dry: Wet:
No 1: 170 No 1: 180
No 2: 170 No 2: 185
No 3: 160 No 3: 175
No 4: 150 No 4: 170

Additionally while testing No 4 with the overflow pressure cap off, I think I heard a bit of gurgling. With the coolant filler cap off and engine cold & running the coolant level rises over the filler and flows out (reasonably slowly).

While I have no experience, the compression readings seem OK, No 4 is certainly a bit low but not terribly so, and the wet measurements are not drastically higher.

What do you all thing? Is coolant flowing out with the engine cold & running normal? Could this be a head gasket leak?

I guess the next obvious test would be an exhaust gas test…

-- Josh
Josh L

EDIT: I should add that the test was conducted on a cold engine.

-- Josh
Josh L

Josh,
The figures don't look bad to me. All are showing rather higher with added oil which is an indication of a bit of piston ring blow-by. But not bad. If they were low, and still low with oil added, one might suspect valve seats were leaking a bit.
I wouldn't think there was any indication there of HG beginning to go.

But a couple of other points.
You should really do the test on at least a warmed up engine, not cold.
The test should be done WOT (with wide open throttle).
You do need to be consistent. Do the test with all the plugs in, and the tester in each cylinder in turn.
Disconnect the LT to the coil and turn the starter for the same length of time for every test (I count 10 engine revolutions).
You need a good battery so that you get the same effect for each test.
Write the figures down in your notebook so that you can see any changes if you redo it at some future date (I test my compression every 6 months - its a good health check)

As regards the coolant needing 1 litre to top it up, you do know that the expansion bottle should only be around 1/2 full when cold don't you?
Guy W

Thanks Guy,

I didn't do it warm as I was concerned about messing with hot plugs, I will redo it in the coming week.
The measurements were done with all plugs out and throttle wide open.

I am aware that the bottle should only be half way full. The coolant was low at the thermostat and it took 1l to refill back above this.

-- Josh
Josh L

Josh,
Sorry for seeming to patronise. Its difficult to judge how much is already known by other owners, and these things are not necessarily obvious!

Engine temperature may not make that much difference. I would expect the figures to all rise, but as what one is looking for is variance between cylinders this may not matter.
Guy W

No problem Guy!

I think we can conclude that the loss of coolant is probably not head related... not sure what else to look for!

-- Josh
Josh L

Hi Josh,

Good to hear from you. Not tempting providence, but I still have your engine steady in the garage. Are you going to the Ace cafe cafe on Tuesday? If so I can give it back to you. Great bit of kit by the way.
frogeye Gary

Josh

I experinced similar issues. Your comp test looks OK to me, so therefore do the easiest things first.

Check all valve clearances are OK - esp exhaust.
Check ignition timing is OK and not advanced or retarded.
Check carbs are not running too lean
Check thermostat is operating correctly - remove it and dunk it in some boiling water to see if it opens.

All the above can lead to overheating if set or operating incorrectly.

Sign of a HG fail is bubbles/fumes in the expansion bottle when the engine is warm, usually a loss of coolant, and sometimes poor performance.

Fairly simple job to replace - just use a decent gasket, don't skimp on that one.
Mark O

I have to disagree... sorry always got to be one idiot in the room...haha

1 liter is alot of coolant with there being a differance of 20 psi between 1 and 4 (170 vs 150) ....my guess would be a crack in
the head between the valves on #4

Or a blown head gasket between 3&4... that May have affected the water portion of the head gasket

Try putting a vacume.gauge on he intake manifold and google how to read a vacume gauge...and that will give you all the info You could.want

Prop
Prop and the Blackhole Midget

I guess it is whether you are an optimist or a pessimist! Cracks in the head are definitely a pessimist's diagnosis. Head gasket is verging on pessimistic with those compression figures. A fully blown HG would be showing much lower figures, especially if it had gone between two adjacent cylinders.

But OTOH maybe I was being too optimistic! Perhaps a first step at this stage would be to re-torque the head and reset the valves.

But looking for gas bubbles in the coolant is a good suggestion. That and mayo in the rocker cover.
Guy W

2X Guy...

I know my thought of a crack between the valves in #4 is very pessamistic...sorry about that



But with the compression still being decent in #4 and 1liter coolant has to be going somewhere, its not evoperating ... and fluid running out the cap and a gurggling sound..I think it just has to be a crack and the most common is beteen the valves

The good news... my machine shop fixed a simialr head for a toyota using a hardened seat...it worked great

Soooo I guess im a pessiamist that perfers to drink full glasses of water ,(prefabbly with ice)

Prop



Prop and the Blackhole Midget

I would re check the compression test when the engine is warm. Run the engine up to temperature so that the cooling system is pressurised and check for leaks before stripping the head. Check the water pump for moisture, and all the hoses and jubilee clips. Also check the exhaust for smoke or moisture this can be a sign that the engine is ingesting coolent. Check all the simple things first before removing the head. Work logically and methodically and you will find the answer. Don't assume anything check and recheck.
C Carter

C. carter Wins the voice of reason and common scence award...

sorry no cash awards.or pretty girls to kiss ... just a community pat on the head

Definatly thumbs up advice

Prop

Prop and the Blackhole Midget

Can you still buy the exhaust-gas-in-the-coolant test kits? That would tell you if there's anything dodgy going on.
Growler

Most garages in the UK with engine test equipment (as needed for modern MOTs) will have a "sniffer" probe that is put into the expansion tank and will detect combustion gasses in seconds.

The home DIY test kits sold at one time were nothing more than litmus paper, packaged up and priced to look like something special. They detect the acidity of the coolant which increases when CO and CO2 are pumped through it.
Guy W

you can also get very easy to use home block tester kits -
http://www.frost.co.uk/block-testers.html

go halves with C L Carter
Nigel Atkins

This thread was discussed between 12/05/2013 and 14/05/2013

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