Help re. Low cylinder compression

[tvrheart]

Following a new head gasket my BMW hasn't ran completely right. Before I take it to the garage next week for an MoT and investigation into this problem I thought I'd arm myself with some knowledge. I did a cylinder compression check today, all bar one of the cylinders read around 11 bar, with number 4 reading about 8.5. The car is a 1989 BMW 325i Touring with 152k miles, it also needs regular topping up with coolant. Any ideas on what my problem may be, and an idea of how many £££s?

Is now the time to sell it?!

Thanks for your help,

Chris

[lotusguy]

Following a new head gasket my BMW hasn't ran completely right. Before I take it to the garage next week for an MoT and investigation into this problem I thought I'd arm myself with some knowledge. I did a cylinder compression check today, all bar one of the cylinders read around 11 bar, with number 4 reading about 8.5. The car is a 1989 BMW 325i Touring with 152k miles, it also needs regular topping up with coolant. Any ideas on what my problem may be, and an idea of how many £££s?

Is now the time to sell it?!

Thanks for your help,

Chris

Chris,

Sorry to hear of your problems. Because you have replaced the head gasket, this is really suspect. What does the spark plug look like from the #4 cylinder? What does your oil look like? Is it cloudy? When your head gasket was replaced, did you have the head milled? With an aluminum head that has 152k mi. on it, it's sure to be warped to some degree. If you didn't mill it back to 'true', then a new gasket will not seal properly, especially if torqued to factory spec. I suspect you're leaking into the #4 cylinder. Hope this helps...Jim '85TE

>> Edited by lotusguy on Tuesday 7th January 17:34

[tvrheart]

The head was skimmed. The oil looks fine with no cloudy or milky effect on the dipstick or filler cap. The spark plug looked the same as the others too. This is why it is so strange as it is using coolant, but doesn't appear to be entering the engine. Someone said it could be to be do with the valve clearances but again this is where my understanding ends!

[lotusguy]

tvrheart said: The head was skimmed. The oil looks fine with no cloudy or milky effect on the dipstick or filler cap. The spark plug looked the same as the others too. This is why it is so strange as it is using coolant, but doesn't appear to be entering the engine. Someone said it could be to be do with the valve clearances but again this is where my understanding ends!

Hi,

Your answers definitely deepen the mystery. At what rate are you consuming coolant? Do you know your compression before the head gasket was replaced? Had to diagnose with such limited information. As far as valve clearances, this should have no effect on coolant loss, but could play a part in the compression mystery...Jim '85TE

[danny hoffman]

Your valves do need to be set correctly to do a compression test. If you squirt oil down the offending cylinder and the compression improves then it is a piston/ring/bore problem, if it stays the same it is a valve/head/gasket prob.

Sounds like a head/gasket prob because of the water loss

Sorry (but cheaper the piston tyoe prob)

Early BMs were susceptable to warped/cracked heads

[lotusguy]

danny hoffman said: Your valves do need to be set correctly to do a compression test. If you squirt oil down the offending cylinder and the compression improves then it is a piston/ring/bore problem, if it stays the same it is a valve/head/gasket prob.

Sounds like a head/gasket prob because of the water loss

Sorry (but cheaper the piston tyoe prob)

Early BMs were susceptable to warped/cracked heads

Danny,

I thought of recommending a leak-down test, but your method will work just as well... Good thinking...Jim '85TE

[calseight]

tvrheart began this thread with a post which included "cylinders read around 11 bar, (snip) The car is a 1989 BMW 325i Touring."

The 325i engine has a compression ratio that is significantly less than 11:1. And when cranking on the starter, cam overlap will eat into static pressure built up in the combustion chamber.

So how does tvrheart's compression guage see 11 bar?

I don't doubt that it does; I've seen the same result, but I've found it puzzling. Can anyone help clear my fog?

[egomeister]

I had a problem with my 205 loosing coolant. Turned out one of the hoses had a tiny split in it that would let the coolant seep out with the vibrations of driving around, but not obviously when parked up. It may be worth checking all your coolant hoses, moving them around a bit and running your fingers along them to check for any drops of coolant.

Hope this is of some use. (I have no idea about the compression though...)

[greenv8s]

calseight said: tvrheart began this thread with a post which included "cylinders read around 11 bar, (snip) The car is a 1989 BMW 325i Touring."

The 325i engine has a compression ratio that is significantly less than 11:1. And when cranking on the starter, cam overlap will eat into static pressure built up in the combustion chamber.

So how does tvrheart's compression guage see 11 bar?

I don't doubt that it does; I've seen the same result, but I've found it puzzling. Can anyone help clear my fog?

In a word, adiabatic heating. Ok two words. As you compress the charge it gets hot, so the pressure increases by more than the nominal compression ratio. I don't know what figure the compression test should show for that engine, but no doubt the answer is available somewhere.

If you're doing compression tests remember the effective compression will drop quite quickly as the bores dry out, you need to run the engine between testing each cylinder. Otherwise the ones you test later will read low. But no idea about the water loss, sorry!

Peter Humphries (and a green V8S)

[accident]

my two peneth on the compression test.
i tend not to look at the actual pressure of each test but look for any significant difference in results on the same engine.
ie:no 4 reads lower then glug some thick oil down the plug hole(tends to seal all holes for the test only)as was recomended further up this post.
if coolant is lost then you need to know where its going.simple as a hose leak or nasty as a foil thin cylinder bore that only leaks under load.
have you got combustion gasses in the coolant?
if the plugs all look the same (after how many miles?)chances are compression probs are valves but the coolant thing would then be unrelated.
so many variables try more tests then come back

[lotusguy]

accident said: my two peneth on the compression test.
i tend not to look at the actual pressure of each test but look for any significant difference in results on the same engine.
ie:no 4 reads lower then glug some thick oil down the plug hole(tends to seal all holes for the test only)as was recomended further up this post.

acc,

Exactly what I was thinking...good advise...Jim '85TE

[calseight]

greenv8s said: In a word, adiabatic heating. Ok two words. As you compress the charge it gets hot, so the pressure increases by more than the nominal compression ratio. I don't know what figure the compression test should show for that engine, but no doubt the answer is available somewhere.

If you're doing compression tests remember the effective compression will drop quite quickly as the bores dry out, you need to run the engine between testing each cylinder. Otherwise the ones you test later will read low.

---

Does that suggest always doing compression test on a cold engine (with a strong battery)?

[lotusguy]

calseight said: greenv8s said: In a word, adiabatic heating. Ok two words. As you compress the charge it gets hot, so the pressure increases by more than the nominal compression ratio. I don't know what figure the compression test should show for that engine, but no doubt the answer is available somewhere.

If you're doing compression tests remember the effective compression will drop quite quickly as the bores dry out, you need to run the engine between testing each cylinder. Otherwise the ones you test later will read low.

Does that suggest always doing compression test on a cold engine (with a strong battery)?

---

Hi,

GreenV8s has been visiting another thread where adiabatic heating has been mentioned in depth, but in this case, it does not apply. 11 Bar is not an unreasonable compression number for a naturally aspirated car. At 14.7 PSI/Bar, 11 Bar is less than 162 PSI. Factory spec on my low compression turbo motor is 145-155 PSI so 162 PSI is not out of line at all.

But as accident stated, it's the difference between cylinders which is important. These differences should be within 3% of the highest cylinder reading at worst. Any difference greater indicates a problem. This is because factory specs cannot be maintained on an aged engine. But, if it is in proper order, all cylinders should age at the same rate and therefore post similar compression numbers.

A compression test should be conducted on a cold engine with all spark plugs removed and the throttle plates in the wide open position. You will want to see the engine rotate twice on each cylinder at least, 4 times at the most. Be sure to disable the current to the coil and pull the fuse on the fuel pump and injectors to eliminate the threat of fire from spilled fuel and accidental spark.

You do not want to turn the engine between cylinder tests as this will affect test results. The cylinder bores will not suffer any ill effects from this as you are turning the engine at a low rpm (usually 250 rpm or less), and with no load as the throttles are wide open and the spark plugs are removed.

With all the spark plugs out, the engine will turn easier than it otherwise would, so a stronger battery is generally not needed...Jim '85TE

[kevinday]

If the car has a cat beware of dropping oil in the bores as it may kill the cat.