[ryan hegarty]

Hi again , have been carrying out compression testing on my 1997 520i and note that each cylinder (while hot) is giving a reading of around 105 psi (+- 2psi) does this sound right?
When I'm testing compression sometimes the pressure drops and then jumps up again as if i'm losing pressure elsewhere, can this be caused by a vacuum leak?
I intend to check it again when the engine is cold.

Thanks in advance :thumbup:

 

[Fast Bob]

Hi again , have been carrying out compression testing on my 1997 520i and note that each cylinder (while hot) is giving a reading of around 105 psi (+- 2psi) does this sound right?
When I'm testing compression sometimes the pressure drops and then jumps up again as if i'm losing pressure elsewhere, can this be caused by a vacuum leak?
I intend to check it again when the engine is cold.

Thanks in advance :thumbup:

Make sure the battery is fully-charged and in good shape (it`s also a good idea to have a battery charger on hand). The engine should be *warm*. Make sure you`re using the correct sparkplug-hole adapter. Take your reading when the gauge needle stops climbing, record the highest number. Proceed front to back. "Good" compression should be between 150-170 psi, with no more than 10 psi variation between cylinders. Abnormally low compression could indicate worn piston rings, a stuck valve, or worn cam lobes. How many miles are on that engine ?

 

[ryan hegarty]

Thank Fast Bob for the reply - to update compression about the same when cold (may be slightly lower) I have checked them again after adding about a capful of oil and note an increase of between 25-40 psi - worn piston rings?
The car has done 165k miles and has the problematic Nikasil block.
Also new battery fully charged and connected to charger/starter with a new compression tester.

 

[edjack]

Also, make sure to prop open the throttle all the way, and that all spark plugs are removed before testing.

THe very best repeatable compression tester is one that actually screws into the spark plug hole, and seals with an O-ring.

Adding oil to cylinder with an attendant compression increase certainly points to rings or cylinder wall wear.

We've had problems in the USA with Nikasil blocks, but only with the 4 liter V8, not the six.

 

[ryan hegarty]

US didn't get the M52 Nikasil block - unfortunately the UK did this wasn't resolved until week 10 1998.
I'm almost 100% sure now it's rings or cylinder wear can anyone give me a rough idea as to whats involved to repair the problem?

 

[Fast Bob]

US didn't get the M52 Nikasil block - unfortunately the UK did this wasn't resolved until week 10 1998.
I'm almost 100% sure now it's rings or cylinder wear can anyone give me a rough idea as to whats involved to repair the problem?

I don`t know enough about that particular block to know whether it could be bored or sleeved, but those would be two possibilities. Another would be to locate a scrapyard motor that`s still in good shape.

 

[ryan hegarty]

Can a vacuum leak also cause lower compression?

 

[Fast Bob]

Can a vacuum leak also cause lower compression?

No

 

[ryan hegarty]

Does anyone have any other thoughts as to what could be causing the low compression other than worn rings/cylinders and if any of the engine restorers work eg engine restore?

 

[Jase007]

ryan:

If you are down 40+ psi in each cylinder ... I would suspect pistons / rings and valve guides / seats, etc...

I don't think a liquid "engine restore" will chemically bond enough to the cylinder walls to compensate for the apparent wear to the block / head.

Sorry.

 

[ryan hegarty]

Hi Jason, does this engine restore stuff work or is it just a load of rubbish?

 

[Fast Bob]

Does anyone have any other thoughts as to what could be causing the low compression other than worn rings/cylinders and if any of the engine restorers work eg engine restore?

All that stuff is *crap*....remember, if it builds up on the cylinder & valve surfaces enough to make any appreciable difference in compression, it`s *also* building up in critical oil passages, reducing vital oil flow.

 

[ryan hegarty]

Cars a bit old to be spending thousands on think i'll drive it till it dies - is there anything you can recommend to increase the life of the engine?
I did the compression test to try and diagnose the rough idle problem I was having when cold, could the low compression be the root cause of this - car does seem to drive ok though no noticeable smoking either.

 

[poolman]

With that low of compression numbers--I don't think it would run--if they are that low--there must be something off with the timing to cause it to be down that low--the pressure should be up to around
170 per cylinder or so--my car with 170k miles is still pushing 180 per cylinder--somethings off

 

[ryan hegarty]

Car drives well, no smoke (Plenty of smoke after wet test) and is doing around 35-40 mpg but it doesn't idle well when cold.

 

[poolman]

Sounds like a vacuum leak to me--have you replaced the Vanos seals? Replacing the seals helped my car greatly.

 

[ryan hegarty]

Yip Vanos done recently have been looking for vacuum leak and cant find any - starting to think compression tester not reading right although pressure did increase by about 25psi when wet testing.

 

[ryan hegarty]

Can a timing issue cause low compression readings as well as a rough idle?

 

[poolman]

Yep

 

[Fast Bob]

Can a timing issue cause low compression readings as well as a rough idle?

If your cam timing is off, it`s possible that the valves are remaining slightly open when they should be closed. This would contribute to a low reading.