Compression test

[md21722]

I was curious about the compression of my '88 Accord LX so I did a compression test yesterday. This was my first ever test on any vehicle. I am not sure I followed procedure.

I drove the engine about 50 miles and then let it cool off for 20 minutes or so.

I removed one spark plug, inserted hose for guage and cranked till it stopped reading higher... 180 psi on 1. Put spark plug in, and repeat on next cyl. 1, 2, and 4 were all 180 psi. I had trouble with cyl 3 because of my compression guage kit hose and threading it correctly.

I removed the coil wire so the engine wouldn't fire, and made it turn over by manually engaging the starter (did not use the ignition key). However, I did NOT have the pedal at WOT or wire open the carb - does this matter??? (Can't retest right now, its raining).

Thanks,

[racerx]

yeah, i'd like to know this too. the last time I did a test, the engine was cold, and i didn't have the throttle open at all.

170-160-130-130

bad, I know. way too much variance. that's why I'm getting the other engine rebuilt in the meantime.

[Jareds 89 LX-i]

Not having the throttle open all the way will affect the compression readings you get. In order to get an accurate reading, you do need to have it open to allow enough air to get in. 180 across the board is excellent though! But when you do it again don't just crank until it doesn't go any higher... you have to do each cylinder the same number of cranks to get an accurate reading, and take all the spark plugs out when you do this so it cranks easier. Usually takes about 6-8 cranks to get a good reading.

When I did mine awile back, I don't remember which cylinder got which reading, but the lowest was 199 and highest was 203. Pretty damn good for 364k miles on the motor! :rockon:

[Immeraufdemhund]

dang the best i ever got out of a civic was 167. My new accord gets 170 with new everything.

[md21722]

When I had ALL the plugs out it seemed like it spun "too fast" and I got 210 on one of the cyls before I figured I would have only one plug out at a time.

I guess this goes to show its hard to compare compression #'s because people use different procedures...

When the weather gets better I'll try at WOT... does that mean with a brick on the pedal or do I have to otherwise keep the throttle plates open?

[md21722]

When you say the "new accord gets 170 with new everything" its important to realize that compression increases as the rings seat and the engine "breaks in".

[BlueBead]

According to my Haynes you should
1) Have the engine at normal operating temp.(so choke is open) and the battery fully charged
2) Clean the area around the plugs (compressed air)
3) Remove all spark plugs from engine
4) Block throttle wide open
5) Detach coil wire and ground it to engine block
6) Insert compression guage in plughole (starting with #1)
7) crank engine at least 7 compression strokes while watching guage
Repeat for each cylinder
8) Add some engine oil to each cylinder and repeat test

Quickly building pressure = Healthy engine
Gradual increase in pressure indicates worn rings
Low compression that doesn't build up pressure indicates leaking valves, blown head gasket, or cracked head. Deposits on the undersides of the valve heads can also cause low compression.
IF the compression increases noticeably after the oil is added the piston rings are definitely worn. Otherwise the leakage is occuring at the head gasket or valves (burned valve seats/faces and/or warped, cracked, or bent valves
Two adjacent cylinders with equally low compression may mean head gasket blown between them.
Unusually high compression may mean carbon deposits on the inside of the combustion chamber (remove cyl.head and decarbonize)
One cyl 20% lower than the others(along with rough idle) may mean a worn camshaft lobe
Have a leak down test performed for very low compression or very uneven compression(at shop).

[Jareds 89 LX-i]

Remove all the plugs like it says in the manual... they list the compression spec for a certain RPM, which in our case is 300 RPM. Compression will change based on the engine speed, so it's important that it's spinning as close to the speed they call for as possible. If you ever do a running compression test you'll see what I mean... engine efficiency drops as speed increases. Just follow the procedure in the manual and it will work fine :)

[md21722]

Remove all the plugs like it says in the manual... they list the compression spec for a certain RPM, which in our case is 300 RPM. Compression will change based on the engine speed, so it's important that it's spinning as close to the speed they call for as possible. If you ever do a running compression test you'll see what I mean... engine efficiency drops as speed increases. Just follow the procedure in the manual and it will work fine :)


What manual are you referring to? I did not see a test procedure in my '88 Accord manual, just the spec that says nominal 171 psi WOT @ 250 RPM ?

Does the FSM procedure, if available, agree with the Haynes manual?