confused about my compression readings

[xlMcMlx DsM]

So I did a compression test on my car to see if the rings were the cause of my hard starts, and I got the following: 100, 50, 50, 60. The car will start up everytime, idles perfect, does not burn a spot of oil, and pulls extremely hard with a little bogging at around 2k when cruising. Does this sound like piston rings to you? It just seems weird that the car runs fine considering the compression numbers.

 

[jrohner]

Low compression like that will definitely make the car harder to start. Low compression would probably make your car spool the turbo slower, but once the boost hits it may not hurt you much.

It could be that your valves aren't sealing. You could try clean the valves by sucking something in through a vacuum line -- engine fully warmed up and rpms up to a couple grand. Seafoam, or i actually just use water or water and ethanol.

 

[BoredDSM]

Low compression like that will definitely make the car harder to start. Low compression would probably make your car spool the turbo slower, but once the boost hits it may not hurt you much.

It could be that your valves aren't sealing. You could try clean the valves by sucking something in through a vacuum line -- engine fully warmed up and rpms up to a couple grand. Seafoam, or i actually just use water or water and ethanol.

Generally seafoam doesn't really do anything. If your valve isn't closing because theres carbon buildup on them then i would worry more about the springs and retainers and the valvetrain.

How did you perform this test? Did you make sure your throttle plate was open or someone was holding the gas pedal down all the way? Those compression readings do not sound correct and should be in the 100s. If its in the 50's and 60's you would be burning massive ammounts of oil or your valves would be bent. Your car shouldn't even start.

 

[bluegs03]

Did you hold the gas petal all the way down? Did you disengauge your mpi relay or unplug your fuel injectors? these can all cause a slightly lower compression reading

 

[Cs4g63]

I once had a 90 gst that did the same thing yours is doing. It had really lousy compression but started fine, didnt burn any oil, and still pulled hard enough to break the crappy tires loose when booost hit in first. I honestly cant tell you what caused the bad readings or how you can fix it, but I can tell you your not the only one that has experienced this phenomenon and mine worked great the way it was as a daily driver for about two years before i sold it.

 

[delta448]

Wow, those are some low numbers. I'm surprised you're not having more issues with it. I think a leakdown test may be in order, when all cylinders are reading low, I'm inclined to blame the headgasket or the timing. Have you performed a wet vs dry compression test? -Which way were the results above arrived at?

 

[KelTsi90]

Check you head gasket. Also you can try to put a cap of oil in each cylinder and start it up for minutes and then check compression again. Then put spark plugs back and start the car and let it idle because there is going to be a cloud of smoke for a while!!

 

[xlMcMlx DsM]

sorry guys i've been a little busy. The car had the head rebuilt back in 2005, so I'm fairly certain it's not the valvetrain, although the motor does have 170,XXX miles on it, I'm not sure if the valvetrain components were replaced when the head was rebuilt. Yes I had the throttle at WOT, disconnected MPI and coil pack and had let the car idle up to operating temp. I am going to get a compression tester at Sears tomorrow, one that comes with an adapter so I can do a leakdown test. I know how, I read up on them, I was so surprised by the readings myself, I completely blanked on putting some oil in each cylinder to see if the readings would go up. I'll definitely do all this wednesday after school.

 

[xlMcMlx DsM]

Oh I forgot, I did check the timing belt and the marks all lined up, i used this tech article http://www.dsmtuners.com/forums/art...-1g-2g-quick-timing-belt-alignment-check.html I also checked the base timing at Idle, and it was 8* BTDC at operating temp. I'm gonna take it out for a hard drive next time and then recheck it. thanks for the input guys.

 

[Freerevving]

Does your oil wreak of gasoline?

those numbers are too low if this was done by turning over the engine... I believe the test was done wrong if your oil smells fine

 

[xlMcMlx DsM]

my oil smells a little like gas, not a ton, but it's noticeable.

 

[xlMcMlx DsM]

I did the compression test last night, the stupid tester adapter for the deeper spark plug holes got stuck down in the plug well a bunch, but I got the same readings. I added oil and the numbers went up about 20 psi per cylinder. So I'm pretty positive it's the rings.

 

[KelTsi90]

Yes, it can be the rings. Does the upper radiator hose inflates when running the car for a while? It can be the head gasket.

 

[z24frank]

Well I just learned 2 things today.I didnt know your radiator hose will swell,and i never keep my throttle wide open either,now I know.

 

[delta448]

I did the compression test last night, the stupid tester adapter for the deeper spark plug holes got stuck down in the plug well a bunch, but I got the same readings. I added oil and the numbers went up about 20 psi per cylinder. So I'm pretty positive it's the rings.

If the readings only went up 20psi, you may still have another problem. Yes, your rings are certainly worn, but an additional 20psi per cylinder is still going to put you well below the service limit. Going off of 120, 70, 70, 80; your lowest reading is ~58% of your highest, the service limit I believe is something like 65-70% max difference. I still think you need to investigate further. Maybe you only have bad rings, but I think you should check the headgasket and valves before you call it done, especially when the high readings increased the same as the low readings.

 

[xlMcMlx DsM]

I was planning on doing a leakdown test to see what I come up with, but I just gotta find some time. And no the headgasket is fine, I cranked the car with the radiator cap off and no bubbling at all. All this was done at operating temp. The thing is I don't have any money at all to rebuild this thing, so It's just a boat anchor for me right now. Also how could I tell exactly if my valves are bent or not seating properly? Or will that show up in the leakdown test?

 

[Boost92awd]

Dont forget to do a running compression test. Remember the compression pressure of a running engine is much lower than a cranking compression test. The engine is revolving faster, and therefore, there is less time for air to enter the combustion chamber. With less air to compress, the compression pressure is lower. Typically, the higher the engine RPM, the lower the running compression.

Your looking for 60-90 PSI. The running compression of all cylinder should be equal. Broken valve springs, worn valve guides, and worn cam lobes are some items that would be indicated by a low running compression test reading on one or more cylinders.