engine compressions
02-18-2015, 07:19 AM
#1
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engine compressions
Help a6 1998 Quattro 2.8
when engine is cold 5 cylinders compressions are reading 200 + one 50 resulting in rough ticking over when engine is hot all cylinders are 200+ and the engine idle is perfect can you help please with this problem?
when engine is cold 5 cylinders compressions are reading 200 + one 50 resulting in rough ticking over when engine is hot all cylinders are 200+ and the engine idle is perfect can you help please with this problem?
02-18-2015, 08:03 AM
#2
AudiWorld Super User
Why did you do a compression test?
What are to miles on the car?
Are you losing fluids or mixing oil/coolant?
Possible items would be valve seals, worn-out valve seating, leaking head gasket or piston rings.
Thermal expansion is the only good thing on an old engine running as it nears the end of the road and stays that way, so the most logical answer would be how much money and time do you want to spend "?" pulling a head to inspect the cause with the least being a valve seal or head gasket....the most being a rebuilt head or a piston ring-job.
02-18-2015, 11:36 AM
#3
AudiWorld Super User
Questions are in order to even determine if this is a concerning problem.
Why did you do a compression test?
What are to miles on the car?
Are you losing fluids or mixing oil/coolant?
Possible items would be valve seals, worn-out valve seating, leaking head gasket or piston rings.
Thermal expansion is the only good thing on an old engine running as it nears the end of the road and stays that way, so the most logical answer would be how much money and time do you want to spend "?" pulling a head to inspect the cause with the least being a valve seal or head gasket....the most being a rebuilt head or a piston ring-job.
Why did you do a compression test?
What are to miles on the car?
Are you losing fluids or mixing oil/coolant?
Possible items would be valve seals, worn-out valve seating, leaking head gasket or piston rings.
Thermal expansion is the only good thing on an old engine running as it nears the end of the road and stays that way, so the most logical answer would be how much money and time do you want to spend "?" pulling a head to inspect the cause with the least being a valve seal or head gasket....the most being a rebuilt head or a piston ring-job.
To me, this sounds like the cylinder is being washed out from a leaking injector. I would swap the injector from that cylinder and move it, and see if the odd compression loss moves with it. Fuel will wash the oil away from the cylinder walls and reduce compression substantially, even to 0 if enough gas is washed into it. As soon as the engines running oil is splashing underneath, exhaust valve opens pushes everything out, plug can now fire and it runs normal and compression is back.
If you rule out the injector and you say it ticks very loud after starting, possible failed lifter not opening a valve much untill its ran awhile, would cause a somewhat low reading on a gauge.
Last edited by Prospeeder; 02-18-2015 at 11:42 AM.
02-18-2015, 01:38 PM
#4
AudiWorld Super User
Sorry to nit pick lol but Your referring to valve Seats not valve seals. The seals are the rubber ones at the top keeing oil from running down the valve, nothing to do with compression. The valve seat is the sealing surface on the head the valve seats against and the valve would quickly burn and be 0 compression if this was the case. You will not get a 150 psi increase of compression just from thermal expansion of worn internal parts like rings or valves.
To me, this sounds like the cylinder is being washed out from a leaking injector. I would swap the injector from that cylinder and move it, and see if the odd compression loss moves with it. Fuel will wash the oil away from the cylinder walls and reduce compression substantially, even to 0 if enough gas is washed into it. As soon as the engines running oil is splashing underneath, exhaust valve opens pushes everything out, plug can now fire and it runs normal and compression is back.
If you rule out the injector and you say it ticks very loud after starting, possible failed lifter not opening a valve much untill its ran awhile, would cause a somewhat low reading on a gauge.
To me, this sounds like the cylinder is being washed out from a leaking injector. I would swap the injector from that cylinder and move it, and see if the odd compression loss moves with it. Fuel will wash the oil away from the cylinder walls and reduce compression substantially, even to 0 if enough gas is washed into it. As soon as the engines running oil is splashing underneath, exhaust valve opens pushes everything out, plug can now fire and it runs normal and compression is back.
If you rule out the injector and you say it ticks very loud after starting, possible failed lifter not opening a valve much untill its ran awhile, would cause a somewhat low reading on a gauge.
I'm not trying to pick fly poop out of pepper either but I do have some questions
A fuel washed cylinder would or should foul a plug beyond firing?
How could a failed lifter cause a lack compression when the valve should seat out of the lift area of the cam lobe?
02-19-2015, 12:39 AM
#5
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Questions are in order to even determine if this is a concerning problem.
Why did you do a compression test?
What are to miles on the car?
Are you losing fluids or mixing oil/coolant?
Possible items would be valve seals, worn-out valve seating, leaking head gasket or piston rings.
Thermal expansion is the only good thing on an old engine running as it nears the end of the road and stays that way, so the most logical answer would be how much money and time do you want to spend "?" pulling a head to inspect the cause with the least being a valve seal or head gasket....the most being a rebuilt head or a piston ring-job.
Why did you do a compression test?
What are to miles on the car?
Are you losing fluids or mixing oil/coolant?
Possible items would be valve seals, worn-out valve seating, leaking head gasket or piston rings.
Thermal expansion is the only good thing on an old engine running as it nears the end of the road and stays that way, so the most logical answer would be how much money and time do you want to spend "?" pulling a head to inspect the cause with the least being a valve seal or head gasket....the most being a rebuilt head or a piston ring-job.
Thank you for your response in answer to your questions
1 The reason for test because of rough running until engine reaches normal temp
2 190,000
3 No loss of fluids or coolant
02-19-2015, 12:55 PM
#6
AudiWorld Super User
In all fairness Prospeeder did bring up valid points and he is a current seasoned auto technician, maybe you should pull that low compression cylinder spark plug out a few minutes after a cold start to see if its gas soaked.
Last edited by jcman; 02-19-2015 at 03:55 PM.
02-20-2015, 11:36 AM
#7
AudiWorld Super User
Correct not a valve seal, valve seating would be the least.
I'm not trying to pick fly poop out of pepper either but I do have some questions
A fuel washed cylinder would or should foul a plug beyond firing?
How could a failed lifter cause a lack compression when the valve should seat out of the lift area of the cam lobe?
I'm not trying to pick fly poop out of pepper either but I do have some questions
A fuel washed cylinder would or should foul a plug beyond firing?
How could a failed lifter cause a lack compression when the valve should seat out of the lift area of the cam lobe?
The lifter idea is more about if the intake valve cant open, or open very much is will not draw in much air and would lower the compression reading, less air to compress would mean less compression. What is really really odd is I have gotten cars in broken timing belt, and they sat at theer house untouched for a year and its towed in. You pull the heads, and the lifters can collapse so much the valves actually close!
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02-20-2015, 02:05 PM
#8
AudiWorld Super User
Your right the plug does foul, but when the engines running higher rpm it can clear out the fuel pushing it out the exhaust and dry the plug enough to start firing and then the miss goes away.
The lifter idea is more about if the intake valve cant open, or open very much is will not draw in much air and would lower the compression reading, less air to compress would mean less compression. What is really really odd is I have gotten cars in broken timing belt, and they sat at theer house untouched for a year and its towed in. You pull the heads, and the lifters can collapse so much the valves actually close!
The lifter idea is more about if the intake valve cant open, or open very much is will not draw in much air and would lower the compression reading, less air to compress would mean less compression. What is really really odd is I have gotten cars in broken timing belt, and they sat at theer house untouched for a year and its towed in. You pull the heads, and the lifters can collapse so much the valves actually close!
Ah yes the old hydraulic lifter not pumping-up right away makes sense, we have three intake valves and the OP's seems to be a very substantial intake volume loss unless 2 or 3 are acting up not just 1, back in the day when I was a pup working a Vette resto shop the tech used to soak the lifters in oil before put a head back together and I remember asking him why, even though most of the rebuilds had solid lifters on the HP 327, 396 and 427's and really none on the 350 or 454's....the good old days
02-21-2015, 04:37 AM
#10
AudiWorld Super User
I've never cleaned a lifter out of the car…if they can't be cleaned with an engine flush it's a tedious procedure to clean them outside the engine. Might just consider buying new ones.
That being said, here's a link to some poor sod who cleaned VW lifters: