Compression test interpretation?

[Brezzi]

My car just died on me in the middle of the street I'm thinking it had something to do with these:



While I was in there I ran a compression test and I'm a little worried with what came back. I haven't done a compression tests on an m20 yet so I was wondering if anyone could help assess?

After four cranks they came back:
170/181/185/170/160/179

[dnguyen1963]

Your numbers look fine. You have the wrong plugs for this engine. Use NGK ZGR5A.

[M-technik-3]

simple single electrode is what M20's run the best. Save yourself some money next purchase and buy copper vs multi's

[packratbimmer]

Agree - No Platinums

[Brezzi]

Your numbers look fine. You have the wrong plugs for this engine. Use NGK ZGR5A.

simple single electrode is what M20's run the best. Save yourself some money next purchase and buy copper vs multi's

Thanks guys I knew the plugs looked wrong. I'll get the right ones in there. How concerned should I be about cylinder 5? 160 seems like a pretty big dip?

[ForcedFirebird]

simple single electrode is what M20's run the best. Save yourself some money next purchase and buy copper vs multi's

This x100000000000^100000. With a stock ignition system, there is no need to go to multi-electrode plugs, even with boost we just change the heat range on them. Typically the multi-electrodes are for COP or wasted spark coil packs with extremely high output (~50-70k volts+ per cylinder) that have ample time to recharge during the engine roation since it's only firing twice per revolution (once on compression, once on waste), whereas the single m20 coil has a lower rating (closer to 40k volts) that has to be split into 3x per revolution - less time to recharge. No need to split the already weak(er) spark into multiple grounds.

We use the wr8lc Bosch Copper, even in the high strung N/A builds.

[Brezzi]

This x100000000000^100000. With a stock ignition system, there is no need to go to multi-electrode plugs, even with boost we just change the heat range on them. Typically the multi-electrodes are for COP or wasted spark coil packs with extremely high output (~50-70k volts+ per cylinder) that have ample time to recharge during the engine roation since it's only firing twice per revolution (once on compression, once on waste), whereas the single m20 coil has a lower rating (closer to 40k volts) that has to be split into 3x per revolution - less time to recharge. No need to split the already weak(er) spark into multiple grounds.

We use the wr8lc Bosch Copper, even in the high strung N/A builds.

Thanks for the detail! Do you think this would cause the engine to die on me? It ran on what felt like 2 cylinders and had around 10% of the power I usually do before it died on me.

I'm thinking the massive amounts gunk as well as unnecessary extra electrodes would have caused this?

Sorry to keep bringing this up but I really am a little worried about the compression in cylinder 5, any thoughts on that?

[M-technik-3]

You didn't mention specifics. Miles and so forth. Best way to check engine is honestly a leak down test vs just a compression test. I'd do new plugs them pull em in a month just for giggles and see where compression results are. Bet it comes up.

[ForcedFirebird]

I see that often. The rings are worn, oil builds up on the spark plugs, the multi just compounds the situation. Go get some Bosch and throw them in. Just be aware that a lot of parts stores will sell you plugs that don't have threaded tops, so check them before you leave. I have even had them give me 4 that were threaded and two that weren't, just order them in bulk anymore lol. Usually bring 2 spare sets when we go to the dyno, got pinched once when we fouled some plugs tuning an m20 after hours, had to stop since we couldn't get more as stores were closed :/

As far as compression, the numbers look good. I don't fall for the whole "pump 7 times" or "throttle has to be open", I have done tests all different ways and the results are the same. Just crank the car the the gauge will stop at it's compression, but take notice if one takes much longer to pump one up. Best to have the engine warm as the that will actually change the final result, though. Also, test the cyls more then once, sometimes human error is in the equation.

[Brezzi]

You didn't mention specifics. Miles and so forth. Best way to check engine is honestly a leak down test vs just a compression test. I'd do new plugs them pull em in a month just for giggles and see where compression results are. Bet it comes up.

Fair. OD doesn't work (shocker) It says it has around 160,xxx on it but that was when I bought it. I'm assuming that it prob has upwards of 200k. Def need to buy a leak down tester, and a compressor for that matter.

I'll put the new plugs in and check it out after a month.

I see that often. The rings are worn, oil builds up on the spark plugs, the multi just compounds the situation. Go get some Bosch and throw them in. Just be aware that a lot of parts stores will sell you plugs that don't have threaded tops, so check them before you leave. I have even had them give me 4 that were threaded and two that weren't, just order them in bulk anymore lol. Usually bring 2 spare sets when we go to the dyno, got pinched once when we fouled some plugs tuning an m20 after hours, had to stop since we couldn't get more as stores were closed :/

As far as compression, the numbers look good. I don't fall for the whole "pump 7 times" or "throttle has to be open", I have done tests all different ways and the results are the same. Just crank the car the the gauge will stop at it's compression, but take notice if one takes much longer to pump one up. Best to have the engine warm as the that will actually change the final result, though. Also, test the cyls more then once, sometimes human error is in the equation.

Ouch that can't be fun. Good advice I'll make sure I check that before I buy.

Tested cylinder 5 three times. I'll get the new plugs in and see how she does on a warm test. She was dead cold for this one.

[Brezzi]

spark plugs and a few other maintenance items ordered :hitler:

[digger]

are those plugs oil fouled to the hilt or are my eyes playing tricks? ive used those plugs on a stock m20b25 with 160k on it and it didnt do that....does the engine burn oil?

its worth noting that compression and leakdown often don't mean a hell of a lot for oil control.

old m20b23 i had carbon'd the combustion chamber and plugs with oil because of bad stem seals

[Brezzi]

are those plugs oil fouled to the hilt or are my eyes playing tricks? ive used those plugs on a stock m20b25 with 160k on it and it didnt do that....does the engine burn oil?

its worth noting that compression and leakdown often don't mean a hell of a lot for oil control.

old m20b23 i had carbon'd the combustion chamber and plugs with oil because of bad stem seals

I do believe it burns oil a little. It doesn't smoke out of the tail pipe though. I just notice after multiple longer (1hr+) high way trips that I need to throw a half a quart or so in. Might just be leaky as well.

The oil doesn't go all the way to the hilt. It was on a couple of the first threads though.

[squidmaster]

What oil are you running? Have you tried running some snake oil through it? "Restore" helped with oil burning on some old cars I've had, as well as with compression numbers. 160 isn't bad but it's not ideal.

Did your compression probe get oil on it while you were cranking?

[Brezzi]

What oil are you running? Have you tried running some snake oil through it? "Restore" helped with oil burning on some old cars I've had, as well as with compression numbers. 160 isn't bad but it's not ideal.

Did your compression probe get oil on it while you were cranking?

I run 10w-40 Castrol Edge non-synthetic. I was going to "sea foam" it will that have the same effect? Thoughts on going up to a heavier oil like a 10w-50?


I think it had some on it now that I'm thinking about it but there was a little bit of oil pooled in the indent in the head where the plug goes in. I had a leaky valve cover gasket too so I'm thinking it came from there. Hopefully.