Compression testing , hot or cold ?

[robrado974 1]

I was told the other day that a compression test should be done with a warm engine , presumably because the pistons have expanded ,the rings are nice and tight in the bore ? .as some of you are aware I have a non starting Vr , I need to do a test , am I wasting my time cranking a cold engine ? And will I come up with a true reading or not ? .

Thanks

Rob

[seanl82 23]

If readings are reasonable anyway, it shouldn't make a great deal of difference Rob. I forgot to warm mine up before I tested earlier in the year, yet my readings were all still within tolerance without being great. If yours are borderline, it may be worth getting it warmed up if you can for peace of mind on that front mate. Don't forget it needs to be cranked with WOT too bud.

[robrado974 1]

Thanks Sean , what is wot ?

Rob

[Roger Chatfield 0]

Wide open throttle...

 

What are the symptoms?? Is it turning over really quick? I can't it being a compression problem unless something drastic has happened like the chain has broke.

 

Rog.

[robrado974 1]

Thanks Rog , symptoms are non starting , have a look at robrados non starter thread , I'm sure it's fuel , but I want to do a compression test before I take the injectors out and test them . A quick run through , basically , when I was trying to start the car it would take 5 or 6 cranks and then it would splutter and slowly pick up and run fine , it's gone down hill now to just turning over . The last time It was running , it was sat on the drive warming up and it just cut out . As I said have a look at my thread , I've replaced a lot of parts and checked a lot of things . I think it could be down to blocked injectors .

Rob

Edit chains have done 50k .

[Roger Chatfield 0]

As you know mate I had a few starting problems recently, I'd be very surprised if all 6 injectors have failed, can you here the fuel pump running?

 

Let me know if you want a hand mate.

 

Rog.

[Matt@OCD 0]

Ideally you should do hot and cold compression tests but cold will suffice, it's more about the cylinder differences than them all being low :)

 

As said hold throttle fully open when doing it.

 

Another test is to wet the bores with oil after doing a first test too, this will show up knackered rings

[robrado974 1]

Yeah fuel pump is working , relays checked as well , taken the pipe off at the rail and fuel is there , I thought it was the pump as it was whining , so I replaced it .i have a spark . So my thoughts are is fuel being pumped out of the injectors properly .

Rob

[robrado974 1]

Ideally you should do hot and cold compression tests but cold will suffice, it's more about the cylinder differences than them all being low :)

 

As said hold throttle fully open when doing it.

 

Another test is to wet the bores with oil after doing a first test too, this will show up knackered rings

Thanks mate , will do as much as I can this weekend , next weekend I have off so will have more time to try and sort this out

Rob

[robrado974 1]

As you know mate I had a few starting problems recently, I'd be very surprised if all 6 injectors have failed, can you here the fuel pump running?

 

Let me know if you want a hand mate.

 

Rog.

Forgot to say , thanks Rog , if I don't get it sorted , next weekend I would appreciate help and would love to see the storm .

Rob

[Dox 23]

If you suspect a fueling issue give it a squirt of easi start whilst cranking

 

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_221705_langId_-1_categoryId_255221

[vw rule 19]

Hello Rob,

 

It's best to carrier out a compression test on a engine yes when hot,

but in your case where your's won't start it will be ok.

This is because if there's any internal wear / damage it doesn't always show it's self,

until the engine is up to temp and everything has expanded to normal working conditions.

 

You need to have your foot down full on the throttle pedal to get a correct reading in PSI / BAR,

because if not you'll be getting false low readings as the other guys have said.

 

Just make sure you unplug the ignition coil / coil pack to stop the HT leads from firing (sparking)

or if you don't then at least keep the HT leads as far away as possible from the spark plug ports,

as fuel / oil vapour will be compressed out of the cylinders via the spark plug ports.

 

Plus you can take out the fuel relay (number 67 on Corrado)

just to stop the fuel vapour as fuel pump won't be working then.

 

Note !

When you unplug the ignition coil / coil pack and fuel relay,

it should be ok but it can bring up a fault code in the ECU,

it won't affect the cars running but it's best to check and clear the fault codes if any are present.

 

A good compression reading should be above 168 plus for most 1.6 / 1.8 / 2.0 / 3.0 litre engines,

anything lower than 145 PSI (10 BAR) is on the weak side for most normal aspirated engines listed above.

And 1.0 / 1.2 / 1.4 litre engines should be 150 PSI plus.

But you'll have to check your data book to see what they should be.

 

2.9 / 2.8 VR6 (ABV / AAA) (190 BHP / 178 BHP) should be close to 188.5 to 190 PSI (13 BAR)

 

2.0 16v (9a) (136 BHP) should be close to 195 - 203 PSI (14 BAR)

The 16 valvers have flat face pistons so have fairly high compression

 

1.8 16v (KR) (136 BHP) should be close to 195 - 203 PSI (14 BAR)

 

2.0 8v (2E / ADY) (115BHP) should be close to 174 - 180 PSI (12 BAR)

 

G60 (PG) (158 BHP) should be close to 155 - 166.7 PSI (11.5 BAR)

This engine is low compression as standard,

as it has a supercharger so extra air will be blown into cylinders,

which raises the compression once the engine is running.

 

As long as the results are around the same from all cylinders,

then you should be alright.

If you've got the cylinders readings as 1= 175 PSI, 2=167, 3=159, 4=163 this is normal.

But if they're reading 1=175 PSI, 2=135, 3=147, 4=163 then really it needs looking in to as two cylinders are down.

 

 

As long as you've got a spark / fuel / compression and the engine is turning over fast enough,

i.e battery is ok then she should start up.

If one of the above is not working correctly then it could be the problem.

When 1 or 2 sensors have gone down / stopped working,

this can course the ignition system not to work and the same with the fuel system,

as some sensors work together like the coolant temp switch and the lambda sensor,

just for example.

 

Is your car popping back out the exhaust like it's trying to start up,

or is there a strong smell of fuel ??

Your best bet is to check she's sparking, and listen for the fuel pump buze,

the fuel pump should prime the system once the ignition is turned on, is it doing this ??

Have you got a imobalizor fitted to your car ?? is it arming and disarming ??

 

 

Hope this helps.

 

Si

[seanl82 23]

Those figures are great for a brand new or recon engine, but for a well worn in VR 165 - 185 psi will probably be as good as you're likely to see Rob. These figures are also within recommended tolerances.

[Dox 23]

Hello Rob,

 

It's best to carrier out a compression test on a engine yes when hot,

but in your case where your's won't start it will be ok.

This is because if there's any internal wear / damage it doesn't always show it's self,

until the engine is up to temp and everything has expanded to normal working conditions.

 

You need to have your foot down full on the throttle pedal to get a correct reading in PSI / BAR,

because if not you'll be getting false low readings as the other guys have said.

 

Just make sure you unplug the ignition coil / coil pack to stop the HT leads from firing (sparking)

or if you don't then at least keep the HT leads as far away as possible from the spark plug ports,

as fuel / oil vapour will be compressed out of the cylinders via the spark plug ports.

 

Plus you can take out the fuel relay (number 67 on Corrado)

just to stop the fuel vapour as fuel pump won't be working then.

 

Note !

When you unplug the ignition coil / coil pack and fuel relay,

it should be ok but it can bring up a fault code in the ECU,

it won't affect the cars running but it's best to check and clear the fault codes if any are present.

 

A good compression reading should be above 168 plus for most 1.6 / 1.8 / 2.0 / 3.0 litre engines,

anything lower than 145 PSI (10 BAR) is on the weak side for most normal aspirated engines listed above.

And 1.0 / 1.2 / 1.4 litre engines should be 150 PSI plus.

But you'll have to check your data book to see what they should be.

 

2.9 / 2.8 VR6 (ABV / AAA) (190 BHP / 178 BHP) should be close to 188.5 to 190 PSI (13 BAR)

 

2.0 16v (9a) (136 BHP) should be close to 195 - 203 PSI (14 BAR)

The 16 valvers have flat face pistons so have fairly high compression

 

1.8 16v (KR) (136 BHP) should be close to 195 - 203 PSI (14 BAR)

 

2.0 8v (2E / ADY) (115BHP) should be close to 174 - 180 PSI (12 BAR)

 

G60 (PG) (158 BHP) should be close to 155 - 166.7 PSI (11.5 BAR)

This engine is low compression as standard,

as it has a supercharger so extra air will be blown into cylinders,

which raises the compression once the engine is running.

 

As long as the results are around the same from all cylinders,

then you should be alright.

If you've got the cylinders readings as 1= 175 PSI, 2=167, 3=159, 4=163 this is normal.

But if they're reading 1=175 PSI, 2=135, 3=147, 4=163 then really it needs looking in to as two cylinders are down.

 

 

As long as you've got a spark / fuel / compression and the engine is turning over fast enough,

i.e battery is ok then she should start up.

If one of the above is not working correctly then it could be the problem.

When 1 or 2 sensors have gone down / stopped working,

this can course the ignition system not to work and the same with the fuel system,

as some sensors work together like the coolant temp switch and the lambda sensor,

just for example.

 

Is your car popping back out the exhaust like it's trying to start up,

or is there a strong smell of fuel ??

Your best bet is to check she's sparking, and listen for the fuel pump buze,

the fuel pump should prime the system once the ignition is turned on, is it doing this ??

Have you got a imobalizor fitted to your car ?? is it arming and disarming ??

 

 

Hope this helps.

 

Si

 

To add to that, you need a fully charged battery before you start the compression test, all spark plugs out, disconnect the crank sensor and there will be no fuel injected or any spark to ignite it

[robrado974 1]

If you suspect a fueling issue give it a squirt of easi start whilst cranking

 

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_221705_langId_-1_categoryId_255221

Hi mate , I have tried this .

Rob

[robrado974 1]

Thank you so much for all the help everyone . Once again the forum comes up trumps , I will be able to carry out my task on the Rado , armed with all the info I need . Brilliant !

Rob

[P3rks 11]

The correct readings for a VR6 are:

 

160psi is factory standard with an upper limit of 189psi and a wear limit of 109psi. A 44psi max difference across the range.

 

Hope that helps.

[mic_VR 3]

I'm not overly knowledgeable about these things but if it was running fine and has gradually deteriorated, you're getting fuel through to the rail, got spark, ignition is fine.... would a faulty fuel pressure regulator cause you an issue?

[Kevin Bacon 5]

Nope.

[mic_VR 3]

Fair enough.

[Kevin Bacon 5]

Get a fuel pressure gauge, tee it into the return line, then make sure you are seeing 4 bar when you turn the motor over.

[Roger Chatfield 0]

Won't the return just show zero as it's open ended at the tank.

 

Rog.

[Kevin Bacon 5]

10 points for spotting the deliberate mistake. Should be the pressure side :)

 

It's not quite open ended though as it fills up the swirl pot in the pump housing first.

[robrado974 1]

Thanks for yet more replys and info , fuel regulator has been changed and the pressures were fine when tested . VW rule has said the temp sender runs in conjunction with the lambda sensor , when I had a vagcom on the car it said lambda fault , I have a new one to fit , so will try this .many thanks again

Rob