Compression Test

#1

At the start of the last VIR race, I was gridded behind Chris Cobetto. I actually got a bit of a jump on him and got close to his rear bumper as we shifted into second gear. Then, he pulled about three car lengths on me before we got to turn one…and Chris doesn’t have a strong car. And during the race, I would be just off Mike Davidson’s rear bumper coming out of Oaktree turn and I couldn’t stay in his draft going down the backstraight. He would pull about 8 car lengths away.

I got away for a couple of hours last night and did a compression test. The directions say to have the engine at operating temp but I did it cold. I removed the fuel pump relay, wired the throttle open, and disconnected the positive side of the coil. All the plugs were out, all per the directions.

Cyl 1 - 160
Cyl 2 - 50
Cyl 3 - 95
Cyl 4 - 80
Cyl 5 - 60
Cyl 6 - 85

At the end, I redid Cyl 1 to make the sure the brand-new compression tester (bought at Auto Zone last night) wasn’t failing, and got 155.

Would the numbers be better if the engine had been warm? Should I get the valves adjusted by a pro and do it again?

I also bought a Harbor Freight leakdown tester and am planning to try that.

Any suggestions from the group?

Carter

#2

I’d certainly try it warm. You could also try a cold "wet" test–put oil in each cylinder to see if it seals the rings or if you are leaking elsewhere. If your leak down tester works, that will show you better than anything else.

#3

ough, that doesn’t look good. Did you try adding table spoon of oil and repeating the test? That would potentially tell you if it is rings (if comp. goes up) or valves (if comp. stays the same). If you did the test correctly, it sure looks more then just rings or valves (but hopefully not).
Leak down test will tell you more, but do it with the engine warm. Most likely your leak down tester will come with some instruction but here are some pointers.
For leak down leave the spark plugs in except for the cyl. you are testing. Make sure that cyl is at TDC - use your favorite method to find TDC but I also like to have marks on the crank pulley (one at every 120 deg and use it in conjunction with firing order) . Thread the air hose into the spark plug hole (don’t strip treads), put a socket with prybar on the crank nut and apply air. Then gently rock piston around TDC (you are seating rings). The air in the cyl. might want to push the piston down so be careful. Record readings from the gauge. Listen for hissing sound where air is escaping. There are only few paths - rings, intake valves, exhaust valves, head gasket… Remove the oil dipstick tube and listen - if you can hear hissing then air is getting past the rings. Open the the throttle body (remove rubber elbow first)- hissing may indicate air past intake valves. You might be able to hear air escaping at the exahust pipe tip if it is getting past exh. valves. You also may want to listen for air past valve seals. It’s self explanatory if you hear hissing sound past head gasket.
repeat for other cylinders.

Respectable numbers would indicate about 8-12 percent leak down with about 5 % variation between cylinders. Also, for tired engine 15-20% leakdown is normal.

Good luck and let us know how it turns out.

HTH,
Igor

#4

Carter wrote:

[quote]I got away for a couple of hours last night and did a compression test. The directions say to have the engine at operating temp but I did it cold. I removed the fuel pump relay, wired the throttle open, and disconnected the positive side of the coil. All the plugs were out, all per the directions.

Cyl 1 - 160
Cyl 2 - 50
Cyl 3 - 95
Cyl 4 - 80
Cyl 5 - 60
Cyl 6 - 85

At the end, I redid Cyl 1 to make the sure the brand-new compression tester (bought at Auto Zone last night) wasn’t failing, and got 155.

Would the numbers be better if the engine had been warm? Should I get the valves adjusted by a pro and do it again?

I also bought a Harbor Freight leakdown tester and am planning to try that.

Any suggestions from the group?

Carter[/quote]

I suppose different testers maybe have different instructions but we always remove just the plug needed at the time. I think the only thing we do to prevent actual firing is remove the fuel pump relay. Course none of this should have anything to do with your numbers.

Anyway, the 155-160 is right but obviously the others are not. I’d definitely do it warm. As for a valve adjusment, you certainly don’t need a pro to do it, these motors are super duper easy to adjust valves on, refer to the Bentley or any other decent repair manual for specs and instructions. Requires the simplest of tools…feeler gauge and a small allen key.

Let us know the outcome of the redo…and good luck!

#5

Even if you do it warm you won’t see your number come up that much. Certainly add some oil to each cylinder and run the test. However when you add the oil make sure the car is at normal operating temp.
Also if you can run a leakdown test.

#6

I am not sure if a valve adjustment will fix those numbers. A little oil in the cylinder will help pinpoint the compression loss but the leakdown test is better. With numbers that low it doesn’t really matter if one or all the plugs are out of the engine. When we get a used motor we generally will perform a leakdown test on the truck before unloading it, we have sent quite a few back…

#7

double post