Compression Test Advice

[jackflash 1]

hey all,

 

I thought I might try a compression test but I've never done it before.

 

As I understand I need to remove the plugs and coil/dizzy lead then turn the ignition over looking for the max reading on each cylinder. I also think i need to stop the fuel pumping, by maybe removing the fuel pump fuse.

 

Could anyone give me some pointers please> I dont know which fuse is for the fuel pump(i dont have a handbook etc) also I could do with some advice on what sort of pressure readings I should expect--see to remember reading 150 to 180 psi for a 1.9 engine.

 

Anyway as I say ive never tried to carry this test out before so any help would be great

 

Thanks

[SurGie 54]

HERE may help you.

 

[jackflash 1]

Thanks for the link,

 

I carried out the test and got the following results, looking at the engine from left to right

 

1--- 160psi

 

2--- 155psi

 

3---170psi

 

4---170psi

 

cylinder 1, 3 and 4 shot straight to their respective figures whereas 2 went straight to 130 then afew extra turns took it to 155.

 

Do these figures seem about right? it was on a cold engine-- i just fitted the tester and turned the key until max pressure showed., the engine has done around 218000 miles thats why im checking it

 

 

***edit*** ive just spotted on the forum that i should have had the throttle pressed(fully open) --will this make much difference?!

 

Any help or thoughts greatly appriciated

Edited July 6, 2011 by jackflash

[SurGie 54]

When i did this test on my friends MR2 mk2 GT Turbo i forgot to have the throttle pressed in, so i did the test again with it open and found no real difference in the pressures.

 

Those figures look low for a gti6 engine, my results were on that thread. I think you should try it with the throttle open again. They do seem quite consistent with each other though which is a good sign. There should be about 15% or PSI amount difference iirc between all the cylinder pressures. Its when there is a bigger difference between them that could be a result of something not quite right.

 

I did do mine in the cold winters temperature and still the results were very high, so i think it depends on the engine tbh.

 

The cylinder numbers start from the gearbox onwards, BTW.

Edited July 6, 2011 by SurGie

[DamirGTI 342]

Well , yes ...

 

You need to do the measuring on fully warmed up engine , and with throttle fully open when cranking ..

 

If the pressure builds up fast on a few initial strokes , but slower on last ones - leaky valves , carbon deposits buildup at the back of the valves , bad head gasket , cracked head .

 

Opposite of the above , if the pressure builds up slowly on a few initial strokes , and gradually increases on last ones - bad rings , liners .

 

Add some oil on the cylinder which is lower PSI and redo the test - if the pressure builds up with adding oil = bad rings .. if not = leaky valves , head gasket , cracked head ..

 

You'll need to crank the engine about seven to ten compression strokes while testing .

 

Damir

[stefan 4]

A rebuilt/new 1.9 should show 185 psi/13bar (9.6:1 ratio), so your figures are probably reasonable (if not excelent) for that kind of milage. As stated above, do the test on a warm engine,throttle fully open, coil disconnected, as for the fuel you can just disconnect the wiring from the injectors,and it's always good to repeat whole cycle from cil. 1-4, just to be sure you got the correct reading.

But 218 000 miles?!-I belive a rebuild should be on the cards in the near future.

[jackflash 1]

ok guys thanks for the replies and advice --i will retest on a warm engine and see what happens. ( sorry i dont think i mentioned its a standard 1.9 8valve engine)

 

What difference does having the throttle open make? Sorry if thats a dumb question but surely that just pumps in more fuel?(i honestly dont know)

 

I just left the plugs off for ten minutes and figured the fuel fuel would evaporate off.-think i will take that advice and disconnect the injectors.

 

218K is high but car seems to have been well looked after over the years. I was reasonably happy with the results except for the 155 result.

 

dont know if the pressures dropped off as the tester locks at the highest recorded pressure.

 

Ive no idea how many times the engine cranked over --i just had my kid turn the key for 5 odd seconds and watched what happened!

 

anyway thanks again --i will post back the results for this 'vintage' engine

 

regards

[DamirGTI 342]

Dizzy signal cable unplugging is best/fastest way of disabling both ignition and fueling for compression testing ..

 

Damir

[jackflash 1]

thanks Damir thats what i did --just disconnected at the coil which seemed ok!

 

I'm abit concerned with the engine because it just doesnt seem as quick as i remember them being( i owned two 1.6 GTI's in the 90s) so i'm looking for any reason, maybe it just needs a good setup!

 

could be because its running lean? sparks are very white --other than that ive been cleaning hoses and just trying to check everything out

 

anyways i will report back later and thanks again--thank goodness for this site!

[DamirGTI 342]

Try to advance ignition timing a little bit .. that should make it more responsive (could clean the injectors as well if you haven't done them before ..)

 

.. and book a rolling road session for air/fuel measurement and AFM adjustment if necessary .

 

Damir

[jackflash 1]

i am assuming i advance the timing by just rotating the dizzy body?---i seem to remember doing that many years ago!

 

as for the injectors what would i clean them with? and i also seem to recall they need a set of replacement washers when i refit them--sorry its been ages since i played with these engines.

 

cheers Damir

[DamirGTI 342]

Yes , mark the current position of the dizzy in relation to the casting , undo the nuts and move ti (while standing at the LH side of the car i.e. left wing , looking at the dizzy directly in-front) = clockwise to advance , anticlockwise to retard ... so , mark the position and turn it clockwise (advance) for say 3mm. off the marked spot .. and go for a spin .. it's best to do it that way "bay ear" , few times , advance and drive .. when it starts to pink - back off 3mm's of the last mark , move on retard side and job done ..

 

Fuel injectors - you'll need to take them to the garage for cleaning on ultrasonic cleaning machine (everything else is .. well , waste of money - cleaning additives etc. ..) , they have rubber O-rings and little filters inside - if wanted , could ask them to replace all complete with the cleaning like complete re-con .

 

Someone from over here do the injectors cleaning/refurbishing via post service .. (MadProfessor ? the boys will know , can't remember ..)

 

Damir

Edited July 6, 2011 by DamirGTI

[jackflash 1]

I will try all this over the weekend--cant today cos its off to a sprayer to renew the underseal......

 

post back with results etc

 

unfortunatley the car is needed everyday for my work run (40mile round trip on country roads) so no chance of posting the injectors to anybody!

 

thanks again

[marksorrento205 80]

i belive Madprofesser who does the injector servicing is based on the IOW too Drop him a pm

[marksorrento205 80]

Clicky

[jackflash 1]

thanks for the clicky

[P_Monty 2]

A rebuilt/new 1.9 should show 185 psi/13bar (9.6:1 ratio), so your figures are probably reasonable (if not excelent) for that kind of milage. As stated above, do the test on a warm engine,throttle fully open, coil disconnected, as for the fuel you can just disconnect the wiring from the injectors,and it's always good to repeat whole cycle from cil. 1-4, just to be sure you got the correct reading.

But 218 000 miles?!-I belive a rebuild should be on the cards in the near future.

 

Just to hijack slightly, why doesn't 13 bar equate to a 13:1 ratio, given that its 1 bar going in?