Healthy compression numbers?

[325e '87]

I spent the $26 for the harbor freight compression test kit. I got 115psi from both cylinder 1 and 2. I quit after these two out of frustration because as best I can tell those are really bad numbers, right? Is my engine toast or should I just start with a rebuilt head?

[jeffnhiscars]

Yes those are low but I would definitely not make a decision based on 2 cylinders, especially w a HF guage. If its only 2 it could be as simple as a head gasket VS say all 6 being low or the leak down numbers being bad.

I'd check further

[325e '87]

Thanks Jeff. I just went ahead and checked all 6 a few minutes ago. Engine temp is about 80° and I got the same 115psi from all of them. Car has 207,000 miles on it.

There’s a leak down kit too and it’s about $40 so I may go pick it up, or I may see if my oreilly store has a rental kit.

Stuff like this makes me want to rebuild the power train but dang, I love driving it too much. Beats the heck outta my diesel F350 (not faster, just more enjoyable).

[ForcedFirebird]

Return the compression tester and get one from Advanced or equivalent. It's only a couple dollars more and actually accurate. The HF is only accurate enough to see variance between cylinders.

The HF leak down is also junk. If you are handy, you can build one for a few bucks out of plumbing fittings and 2 gauges, and it will be 100% more reliable.

I use tools professionally and usually have to buy quality, but have tried both of those from HF.

[cscracker]

When I last rebuilt mine, I got 180psi on all six. I used the OTC kit off Amazon. The fact that they're all the same makes me think you've either got a faulty tester or a faulty method. If the engine was that worn out, I'd expect variance.

[Nader393]

Did you do it at WOT?

[ForcedFirebird]

When I last rebuilt mine, I got 180psi on all six. I used the OTC kit off Amazon. The fact that they're all the same makes me think you've either got a faulty tester or a faulty method. If the engine was that worn out, I'd expect variance.

180 on a stock good used engine is typical. I see 175 or so after a season of racing on a .5mm over block. Fresh .5mm over all balanced and file fitted rings etc, 185 and fades to about 180 over 100 hours of hard racing. A good used up healthy m20 will be in the mid 160's or so, and power fades significantly after 155. Usually the m20 looses compression through the exhaust valves if maintained properly. I have broken 400k mile m20 cores where the intake valve sealing angles looked mint, but the exhaust on a 100k mile motor look like Swiss cheese.

Did you do it at WOT?

I've been compression testing engines for nearly 2.5 decades and WOT/hot/cold etc is all nonsense. If the needle doesn't jump to 60-80psi on the first stroke and make the gauge jump like mad (stock-ish engines), there's something wrong. WOT or closed, you can turn the engine until the battery is dead and it will reach a delta.



The biggest problem with diagnostics is it's subject to determination. Over the years I can rock a rocker arm on the base circle of an m20 and tell you if it's at least close to spec. It just becomes second nature. So, if in doubt, just count the pulses on the compression tester (all plugs removed) and record results. As long as they are equal, the engine will run smooth and be power-dependent on the ability to keep trapped mass.