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Compression Test Do's And Dont's


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#1 ryomini


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Posted 09 June 2015 - 11:58 AM



I guess I have decided that it's time I got off my boney rump and started out with a clean page - starting off with a fresh compression test

I have the 1.3i injection mayfair year2000


I did a compression test 2 years back - have to look through my mess and see if I can find the figures

but they were all almost identical


my question is I know you have to be careful about current in the plug that you have removed (when spinning the engine over)


if my memory doesn't fail me don't you need to ground the removed plug to the motor

what is best practice for doing that - does someone have a litle trick


I don't want to damage the ignition - but I have to stop playing guessing games and get some numbers on paper so that I can keep this amazing car in tip top shape


help much appreciated

here's a pic of what's under the bonet right now

Attached File  CIMG2456.JPG   267.39K   12 downloads

#2 RedRuby


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Posted 09 June 2015 - 01:26 PM

When I do a compression test I remove the plug leads (I have them numbered 1-4 prom the radiator end so I don't put them back on in the wrong order) remove all four plugs and put them safely on the bench and then with the accelerator fully pressed turn the engine over on each cylinder in turn trying to turn it over the same number of times for each cylinder, roughly half a dozen times. Obviously making a note of each cylinder in turn. I just leave the plug leads hanging loose making sure they cannot foul any moving parts, never though there was a need to ground them.

Edited by RedRuby, 09 June 2015 - 01:26 PM.

#3 Cooperman


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Posted 09 June 2015 - 02:22 PM

To do a compression test all 4 plugs must be removed.

#4 spiguy


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Posted 09 June 2015 - 10:06 PM

Good idea to disconnect power to the fuel pump - easiest way is disconnect the connector at the inertia cutoff switch on the passenger side bulkhead, or if that is knackered and been bypassed, then pop the connector off at the tank end. Otherwise you will be squirting petrol into the exhaust system and CAT. CAT's don't like that.

#5 Spider


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Posted 09 June 2015 - 10:15 PM

It's actually not bad practice to fit the plugs back in to the leads and earth the body of them when doing a test. Without plugs in there, the HT will try to spark across or through anything it can, so it can puncture things like HT leads, boots and coils (coil packs). It also reduces the possibility of getting a boot from the HT system.


Shorting the leads to earth is usually OK, but some ignition systems don't take kindly to that.

#6 ryomini


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Posted 10 June 2015 - 12:55 AM

Wow thanks

I remember vaguely, too vaguely now that I disconnected something,


I remember now that on other cars I used to ground the plug

but if I remember rightly on the mpi mini you can just pull the power lead to the

distributor (it's under the distributor) - what I can't remember is if this

can or cannot potentially damage the distributor (big volts are going in there)


just checked under the bonnet and remembered that I  disconnected

the power lead going to the distributor cap (iIt's sort of under the distributor - is that good practice ???????)


@spiguy thanks - funny thing I have never looked at that until now - so thanks alot for that info

(I do hope that everybody does disconect the fuel as there is a big risk of fire if not!!!)


so to repeat what I have understood most people :

- remove all 4 plugs

- remove all 4 leads and do not ground them

- do not disconnect the distributor power supply

- disconnect fuel pump

- what

- take readings in each cylinder


I will post my results soon

any more imput is welcome

Edited by ryomini, 10 June 2015 - 01:46 AM.

#7 ryomini


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Posted 10 June 2015 - 03:19 AM

Boy oh boy oh boy am I glad that you people with experience, knowledge, eyes and ears are out here,


actually I see some people like 'cooperman' for example who have built and raced and been in and out of a mini like Dr Who in his Tardis, what a wealth of information and experience, thanks so much for sharing and helping out - gotta keep my mini on the road and gotta get through the next mot


so * I did my homework*




Millwall 1 - Burnley 0 


ok seriously here we go


I unplugged the distributor from the power source from the bottom as the fuel cut off also -  seems to be good practice

no fuel, no spark, no fire - more mini


I used a bought from new Gunson Hi-guage  I tried my best to count 7 crank rotations for each reading


cyllinder 4

14 bar

200 psi


cylinder 3

15 bar

220 psi


cylinder 2

13.9 bar

195 psi


looking good so far right ???????


cylinder 1

8.0 bar

110 psi


cylinder 1 second reading

8.2 bar

120 psi


added one tablespoon 10w30 fresh oil to cylinder 1

cylinder 1 plus tablespoon oil

11 bar

160 psi


then upon initial start up exhaust puffed white smoke for about 20 seconds

then black smoke and now it puts out a constant light, quite hard to see if you don't look hard, stream of black smoke


additional information - I cleaned the plugs yesterday and drove about 20 minutes in traffic.  Took out the plugs today for a compression test and they were heavily fouled, sooty and a little oil all 4 exactly the same

I put an old toothbrush in the 4 spark plus holes and wiggled it around - same it comes out very sooty and a little oily for all 4 cylinders


hope this helps the experts out here as I really need help for the NEXT STEP


my conclusions as an inexperienced sunday avo tinkering type are that it seriously looks like I have a broken or stuck oil rings or ring or a scored piston or chamber or something that essentially means I will have to have the HEAD OFF


the car actually runs very smoothly and there is little tappet noise

starts first time, no accelerator and idles perfectly

Edited by ryomini, 10 June 2015 - 03:25 AM.

#8 Guess-Works.com


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Posted 10 June 2015 - 06:28 AM

Make sure the throttle is open when you do a compression test...


however from your results and the 'oil test' number 1 cylinder does seem to have failing rings, probably top compression ring.

#9 jaydee


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Posted 10 June 2015 - 08:32 AM

Check valve clearance at cylinder #1 first, but it does sound like worn rings.

Unplugging the dizzy cap on a spi it useless, disconnect the black wire at the coil if you want to avoid overcharging the coil.

#10 ryomini


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Posted 10 June 2015 - 01:23 PM


ok will check valve clearance in cylinder 1 but I do not understand why all 4 plugs are fouled

and only after 10 - 15 minutes of driving

I have never had this problem before

by the way my mini is a 1.3 injection not  a spi model

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