Cylinder compression test
03-04-2007, 09:49 PM
#1
Three Wheelin'
Thread Starter
Cylinder compression test
Did a compression test on my 86 951 today. Trying to determine the strength, or lack of... my engine before i dive into a full or partial rebulid with several performance upgrades. There are some minor oil leaks that will be fixed as well as some power streering line leaks. The test went as follows... I let the pistons turn 5 revolutions per cylinder and marked the second and final turnover.
#1 - 92/135, #2 - 90/131, #3 - 85/131, #4 - 92/135. what's a good reading for compression? thought it was around 150? Planning a leak down test on the engine and vacuum system sometime later this week. Haven't done a leak down before any suggestions? What's the best way to get TDC and secure the engine so it won't turn? Thanks again for everyones input on this forum.
#1 - 92/135, #2 - 90/131, #3 - 85/131, #4 - 92/135. what's a good reading for compression? thought it was around 150? Planning a leak down test on the engine and vacuum system sometime later this week. Haven't done a leak down before any suggestions? What's the best way to get TDC and secure the engine so it won't turn? Thanks again for everyones input on this forum.
03-04-2007, 09:55 PM
#2
Rennlist Member
When I did a compression check on my motor (and found out I had to do a headgasket) I got 152, 152, 154 dry on my three good cylinders.
Not sure on how that was measured though, as KuHL951 was holding the gauge and I was just sitting in the car cranking the engine.
Not sure on how that was measured though, as KuHL951 was holding the gauge and I was just sitting in the car cranking the engine.
03-04-2007, 10:09 PM
#3
Burning Brakes
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The numbers of 130+ are not up to the max but are well above the lower limits stated in the manual. Did you have the throttle wide open during cranking, was this cold or warm engine?
For the leak down, get a long breaker bar with the socket for the crank nut, I believe it is 24mm. Take all the spark plugs out and take off the distributor cap. Find TDC on #1 then back it off a bit to before TDC. Screw in the leak down fitting hold the breaker bar, add pressure then move the piston up to TDC. Just make sure you hold on to the breaker bar. Then listen for where the air is leaking by, if it is. Take the oil filler cap off to make it easier to hear if it is leaking by the cylinders.
For the leak down, get a long breaker bar with the socket for the crank nut, I believe it is 24mm. Take all the spark plugs out and take off the distributor cap. Find TDC on #1 then back it off a bit to before TDC. Screw in the leak down fitting hold the breaker bar, add pressure then move the piston up to TDC. Just make sure you hold on to the breaker bar. Then listen for where the air is leaking by, if it is. Take the oil filler cap off to make it easier to hear if it is leaking by the cylinders.
03-04-2007, 10:48 PM
#4
Originally Posted by Fluidplay
Did a compression test on my 86 951 today. Trying to determine the strength, or lack of... my engine before i dive into a full or partial rebulid with several performance upgrades. There are some minor oil leaks that will be fixed as well as some power streering line leaks. The test went as follows... I let the pistons turn 5 revolutions per cylinder and marked the second and final turnover.
#1 - 92/135, #2 - 90/131, #3 - 85/131, #4 - 92/135. what's a good reading for compression? thought it was around 150? Planning a leak down test on the engine and vacuum system sometime later this week. Haven't done a leak down before any suggestions? What's the best way to get TDC and secure the engine so it won't turn? Thanks again for everyones input on this forum.
#1 - 92/135, #2 - 90/131, #3 - 85/131, #4 - 92/135. what's a good reading for compression? thought it was around 150? Planning a leak down test on the engine and vacuum system sometime later this week. Haven't done a leak down before any suggestions? What's the best way to get TDC and secure the engine so it won't turn? Thanks again for everyones input on this forum.
03-05-2007, 01:08 AM
#5
Three Wheelin'
Thread Starter
Dan - the car was driven for 20min+ and the throttle was wide open during testing. With the cylinders testing so close in #'s, i was thinking... rings? can't imagine the gasket would be blown in all 4 cylinders. On the leak down, holding a breaker bar sounds dicey. You've done this? How much torque is on the bar? Also, Don't you need to find TDC for each cylinder? Thanks for the help. Dennis
03-05-2007, 01:36 PM
#6
Burning Brakes
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The torque on the long bar created by the air pressure was easy enough for me to hold you just have to be ready for it. Yes you do need to find TDC for each cylinder and for me I used the cam #1 cylinder indicator then with the distributor cap off you can then follow the firing order to find TDC on each cylinder. With all the plugs out you can use a long wood dowel to place in the spark plug hole and as you rotate the crank clockwise you will see the dowel come up and then start down. Caution make sure the dowel rod is at least 12" long you don't want to watch it dissappear in the cylinder. Yes I have done this much more than once.
Your compression numbers are all even which shows even wear in each cylinder. When you do the leak down test run the piston up and down the cylinder to see if the leakage gets worse at close to BDC.
On my first overhaul my compression #'s were each 125 at 2550 ft altitude and at TDC zero leakdown according to the mechanic who did the check at the time. When the engine came apart due to oil coming out of all the gaskets, the machine shop checked the bores and found taper to be out of tolerance for these engines in particular at the bottom of the cylinder.
The reality was that prior to me taking it apart my engine was running very well and not using but about 1 qt in 2000 miles. So from my past experience I would say you have a good chance that your bores are in decent shape but that you may find you have more taper than allowed for our engines so you will then have to decide how far to take the rebuild.
My suggestion would be to carefully mic the bores and your pistons when you get them out and depending on your budget you can then decide whether or not to go with a first over bore and new pistons or have the bores honed with proper tools and paste as a clean up and put new rings back in.
Your compression numbers are all even which shows even wear in each cylinder. When you do the leak down test run the piston up and down the cylinder to see if the leakage gets worse at close to BDC.
On my first overhaul my compression #'s were each 125 at 2550 ft altitude and at TDC zero leakdown according to the mechanic who did the check at the time. When the engine came apart due to oil coming out of all the gaskets, the machine shop checked the bores and found taper to be out of tolerance for these engines in particular at the bottom of the cylinder.
The reality was that prior to me taking it apart my engine was running very well and not using but about 1 qt in 2000 miles. So from my past experience I would say you have a good chance that your bores are in decent shape but that you may find you have more taper than allowed for our engines so you will then have to decide how far to take the rebuild.
My suggestion would be to carefully mic the bores and your pistons when you get them out and depending on your budget you can then decide whether or not to go with a first over bore and new pistons or have the bores honed with proper tools and paste as a clean up and put new rings back in.
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03-05-2007, 03:42 PM
#8
Three Wheelin'
Thread Starter
The tester is a "actron" ...yes made in China and sold thru Pep Boys, part# 1JGO (5PZAS). www.actron.com
03-05-2007, 04:03 PM
#9
Rennlist Member
I'm interested in this too ST ...
I have an 'Actron' tester I got from Pep Boys years ago in a kit (compression tester, timing light, multimeter). Mine is model # CP7827, and says made in USA on the gauge.
I did a compression test about a year ago and got almost identical results (~90, 135 +/-1~2 across all cylinders). This just got me wondering if the engine was worn, or if the gauge wasn't up to snuff, or ??
Pic of gauge below:
I have an 'Actron' tester I got from Pep Boys years ago in a kit (compression tester, timing light, multimeter). Mine is model # CP7827, and says made in USA on the gauge.
I did a compression test about a year ago and got almost identical results (~90, 135 +/-1~2 across all cylinders). This just got me wondering if the engine was worn, or if the gauge wasn't up to snuff, or ??
Pic of gauge below:
03-05-2007, 08:08 PM
#11
Three Wheelin'
Thread Starter
mtnman, i wouldn't be surprised.. seems like just about everything ends up being produced in China, Taiwan etc.. mine looks a little different. Odd though you got almost the same exact readings. what is the yr. & mileage of your car when done? I'm curious now to find another tester.. Dan, I'm also curious about this taper? do you mean that there would be different measurments for compression from top to bottom in the cylinder? how can you test for this? thanks for all the info!! Dennis
03-05-2007, 08:22 PM
#13
Originally Posted by dand86951
Caution make sure the dowel rod is at least 12" long you don't want to watch it dissappear in the cylinder. Yes I have done this much more than once.
-Darwin
03-05-2007, 09:21 PM
#15
Rennlist Member
I've got ~120k miles on my '89, and I'm at ~2500 ft. elevation. When I did the test it was a nice day (80ish temp-wise) and I held the throttle wide open during the test. Car was warm, but not hot (probably 20 minutes after driving around town for a while).
I just got an air compressor for Christmas, and since my gauge has a quick release I decided to hook it up to my compressor for comparison. The Actron gauge reads 5-7 psi LOW compared to the gauges on the compressor (consistently from ~80 psi up through ~150 psi). The compressor has 2 gauges (air tank and outlet pressures). When I max out the regulator on the air compressor(outlet = tank pressure), those two gauges agree. For what it's worth .....
I just got an air compressor for Christmas, and since my gauge has a quick release I decided to hook it up to my compressor for comparison. The Actron gauge reads 5-7 psi LOW compared to the gauges on the compressor (consistently from ~80 psi up through ~150 psi). The compressor has 2 gauges (air tank and outlet pressures). When I max out the regulator on the air compressor(outlet = tank pressure), those two gauges agree. For what it's worth .....