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MG MGF Technical - Compression Testing

Being of a paranoid disposition, how does one go about compression testing a mk1 VVC F? I've done carb'd minis before but what do I need to disconnect to stop the F from firing up? The coil isnt terribly obvious!

I really intend this just to set my mind at ease, the only symptom is slight water loss (if I fill to the max it'll lose about 300ml over a tank of fuel). It has always used a little water (for the 2 years I have had it) but it seems to be increasing recently. New-ish pressure cap, no water on the alternator or anywhere else obvious, a little dipstick mayo when commuting in winter but gone now, oil looked fine when I last changed it.

Blind optimism?

I have a proper leak down tester you can use if you are passing by SW London (Surbiton)
Derrick Rowe

Pete, perfectly possible to use a gunstons compression tester on the K-series. The spark plugs are down pretty deep channels though, so buying a suitably long spark plug box spanner is a good idea.
Rob Bell

I had a slight water loss like yours last year (just before the HGF!!).

Mine was caused by a small leak at one f the clips on the underfloor collant pipes and a dodgy water pump.

May want to check for small leaks as much as you can in case it goes big time one day like mine.

I have a link on my site for changing spark plugs on MGF so you could use that (although it will be slightly different as yours is a mark 1 with a distributor and mine is 2000 with coils.) ( )

Let us know how you get on as I would like to do a test myself, just for my own amusement.

Sorry, that link was wrong should be
I Brown

Nice website there Broon! :o)
Rob Bell

Did my compression testing with the aforementioned gunson's tester. Biggest problem was the depth of the spark plug holes, because the rubber pipe started slipping on the brass screw thread (if that makes sense) so I did at one stage have the tester stuck in the block! (borrowed my neighbour's tiny needle nose pliers to get the b

Dont mean to scare you but mine was a minor leak and I kept my eye on it too but out of the blue one day ti just went.

I Brown

VVC uses a wasted spark system Pete - no distributor. The only way to prevent a spark is to disconnect the coils...

13bar sounds fine to me (can't find official compression figures sadly) - so it is unlikely that the firerings on the gasket are failing. Unfortunately, this is not the sole mode of head gasket failure: elastomeric bead failure is far more common :o(

As Broon says - keep an eye on things.
Rob Bell

But presumably it does mean that combustion gases are not pressurising the coolant? Although I could be losing coolant into the oil or straight out of the block?

If I dont find a leak soon i think I'd best invest in getting a garage to pressure test the coolant.

Absolutely right Pete.

I presume that you've checked the under body coolant pipes and the radiator? Both are very prone to corrosion - and if you're still on the originals, then there is a very good chance that they're leaking... A good deal cheaper to fix than a head gasket.
Rob Bell

The main thing to look for on a compression test is uniform readings across all 4 cylinders - so no worries there then.
John Thomas

Compression-test is done the easy way via Test-Book...
Measuring the starter current via the current clamp fitted at the battery!

If only there were a cheap and affordable version of Testbook for all us MG enthusiasts to own and cherish Carl! LOL
Rob Bell

Yes,Rob. Isnīt it about time someone did just that.
It is by all means old technology by now ,a suitable PIC-processor would replace the whole trolley full of ink-printer,286 confuser,a magnitude of cables etc. ;O)


Underbody pipes look fine, they appear to be painted black. What's the best way of examining the radiator - from underneath?

Pete, I'm in a similar position with water loss but slightly less and some weeks(400 miles minimum) not at all. I have looked at every thing from on top and underneath on a ramp hot and cold, running and stopped and block tested the coolant.
I read somewhere that the water pump has a fickle habit of intermittent leakage and doesnt show it on the outside.
All I would suggest is if you top up a lot, use distilled NOT our local super hard tap water as this has particulate deposit implications as well as the dilution of inhibitor. Have you a level sensor? Keep watching I guess...

Pete, the only good way of inspecting the radiator is, unfortunately, to remove the front bumper. It corrodes rapidly, particularly on the left.

Charles, yes, the water pump can weep - but you'd expect to see evidence of this on careful inspection of the engine (may require removal of the timing belt cover though)
Rob Bell

Have you followed the 'sticky green stuff' posting on general threads. Carl has a possible solution for your regular leak.........

ive had simmilar problem for a couple of months, MY2000
im loosing coolant 200ml every tankful(its very random some weeks more than others and it varies in distance no pattern just diffrent sizes of wet patches by rear wheel drivers side so.........took it into Browns and Gammons a few days ago they pressure tested it no sign of leak made a note of corroded rad and corroded coolant pipes under car but they are convinced after a good rummage around that its my water pump agreed to have it changed as part of my service which is due soon .
in the mean time its a drop of ready mix 'ala' coolant when needed...
drac jnr

This thread was discussed between 04/05/2006 and 12/05/2006

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