Anybody ever try checking compression with engine running?
 

I just got my engine back from the shop where they fixed a bent valve on cylinder#4. However after my 3.2l engine warms up it starts to miss on cylinder #2.

Spark is good, fuel from the webers seems ok, compression when cranking is 150lbs, and a leak down test shows only a 2% loss. So we tried starting the engine with the compression gauge attached and it would only build up to 40-60lbs.

Why the difference when running ...and what does this tell me is wrong?

Thanks
S.Chapman

Maybe a sticking valve on that cylinder...?

My mechanic thinks it's another (slightly) bent valve... Does a bent valve turn into a sticking valve when warm?

Thanks
S.Chapman

Pure speculation, but it seems possible as it "grows" with heat.

Lacking a compression spec with a running engine (it's not an industry test), you'll have to interpret the regular compression and leakdown tests results.

With the engine at a fast idle, try a cylinder balance test to confirm power is down on cyl. #2 or wherever. With a pair of insulating pliers, remove a spark plug wire from each spark plug, then record the decreased idle speed. Repeat for each cylinder. The cylinder(s) that do not demonstrate any idle drop are the weak cylinders (not contributing). If compression and leakage tests are okay, inspect the ignition system. If a cylinder has low compression and not contributing much to overall power, it won't necessarily misfire. A spark is a spark, with or without compression pressure.

If you think compression has changed due to engine temperature, retest when it's up to temp. It's suggested that compression and leakage tests be performed on a hot engine anyway.

Sherwood

Thanks for the replys!

If you let the car idle for 15-20min to get it up to temp everything is ok ...until you rev it up then it starts popping in the exhaust and running slightly rough.

One of the first things we did is pull the plug wires and nothing changed on #2. We changed the sparkplug, cap, rotor, and plug wire and it still didn't help. We also held the plug against the block and got good spark.

I have never seen a compression test done while an engine was running though and I was wondering if the number was supposed to be that low? I tried to duplicate the test we ran at the shop with my cheap compression guage just now and got 120lbs (when the engine was hot) turning over and 60lbs when running. I also checked the cylinder that just had the exhaust valve replaced for a comparison and it was 120lbs/70lbs.

Whatever the problem is I don't have time for it ...so back to the shop it goes.

Thanks
S.Chapman

I hope you don't make a habit out of idling your car for 15-20 minutes.

I guess I forgot to mention that I was trying to reproduce the hot engine temps it would see out on the track...

S.Chapman

Holly crap!! the difference is the poor gage can't react that fast or was it meant to!! This is a dangerous practice as the hose or the gage might come apart. On a 4 stroke engine, like your engine, only one out of four is a compression stroke, the other time the valves are open. So I don't think that is a viable test result. Use the compression test and the leak down test. On aircraft engines if I don't get a good leak down test cold, I run the engine and check it hot.

Whatever bent your 1st valve may very well have bent the 2nd one. The question is why didn't the mechanic anticipate this?

Is it time for a 2nd opinion?

Unless you are running the engine at WOT (unlikely) you will have a low compression reading because the throttle valve only lets in a little air. Seems most likely a carburation problem. Lots of things to look at there. You can try driving it to see when the miss occurs. If it misses at low power but fires when wide open the the problem is probably in the idle circuit.

-Andy