compression?? odd readings

[Jeff4764]

ok, so i have an 87 n/a, ive explained this problem before... It starts fine when its cold, but once it warms up it has a hard time starting and sometimes wont even start. I was told to check compression so i bought a tester, (didnt remove schrader valve, just held in relief valve) the pulse is are very even steady pulses and all read the same. Heres the problem, its reading 50 psi every time.. But i was told if there steady and even pulses the compression is fine.. can someone give me some insight.. maybe a different problem the compression

[Phoenix7]

50 is too low. Are you sure you tested it right. That's crappy compression. Normally anything around 80psi means your motor is dying.

[Jeff4764]

im pretty sure i did, i tested both trailing, and pulled the injector and egi fuse and held pedal to floor while trying to turn it over. but if it was bad would the pulses be even

[djmtsu]

If it was tested correctly than 50psi is toast. Rebuild time.

I suggest Rotary Ressurection.

[87turbo_turtle]

sounds like its time to rebuild

[Jeff4764]

I do believe you guys, but does it sound like i did it right?? and would the pulses be even if compression was bad? could it be in the intake? I have no idea.

[Jeff4764]

and if it was 50 psi would it even start cold.

[Phoenix7]

Take a video of both rotors being tested. 50psi is too low, whether or not the pulses are even (your apex seals intact).

[djmtsu]

Just because the apex seals are there, does not mean they are efficient at sealing anymore.

Heat causes expansion, expansion lowers compression.

[Jeff4764]

ok I can do that and post it tomorrow. But does it sound like i did it right...
I tested both rotors on trailing side. the compression tester fitting screwed into the spark plug hole and i pulled both the inj. fuse and the egi. fuse. I had my girlfriend get in the car and push the pedal to the floor, and cranked it for about 10 seconds while i held the pressure relief valve in and got a steady reading of about 50 psi. I will take video, and if you guys can check it out tomorrow and give me some input that would be awesome.

[classicauto]

I do believe you guys, but does it sound like i did it right??

Was the engine warmed up when you did the test? Otherwise everything else sounds pretty good as far the tests execution is concerned.

Although those are low numbers even for a stone cold engien that is in decent shape....

[Phoenix7]

Plus, you can drive it until it can't hold compression. Just don't expect it to go anywhere, fast.

Some people add a bit of premix to raise compression too. If you can't get it rebuilt soon then I'm sure others will chime in with ways to help the motor last a little bit longer.

[vex]

:banghead: You're doing it correctly. 50PSI is fine for checking the apex seal condition, now you need true compression value of the rotor. Do not hold the relief valve and you should get the max number on the compression. I bet you're going to be getting above 100PSI, just a guess though.


http://www.aaroncake.net/RX-7/blown.htm

[Jeff4764]

If i hold the relief valve it will just keep bouncing to 50 psi. If i let go of the relief valve it will bounce a fewtimes and stop at about 95 psi. so if what your saying is right then i am a happy camper.

[djmtsu]

Happy that you have compression, but sad that you still have hot start issues.

[vex]

95psi on an NA isn't all that great. You may want to do a few things to try to increase your compression numbers. (You're only 5psi off from my rebuild limit--where I would tear it apart and rebuild.) Do the water trick or sea foam treatment. This "should" remove excess carbon. Do an oil change and replace the filter and oil with quality products. Make sure that your oil is the right thickness and has the right visc. for the temperatures you're seeing.

[Jeff4764]

sorry to ask, but what is the water trick? and also ive heard you can put about a teaspoon of oil in each cpark plug hole and that will increase compression

[Whizbang]

if you are using a normal compression tester for piston engines, the the value that you get can be skewed heavily. take it to a dealership and use a rotary engine compression tester and get numbers.

its cheaper than a rebuild!

[WE3RX7]

My two cents on this....

The piston tester is ok for "general" assumptions. What it proves is that you haven't lossed a seal or one isnt stuck.

50psi is low, even for a cold engine so vex's suggestion of letting it pump up to the overall psi is a better assumption. for the s4 NA 100psi would be better, but 95 isn't the end of the world (but I would start saving for that rebuild).

I would not do the ATF/MMO/Water trick. In certain cases, this is acceptable such as a motor that has been sitting in storage or got completely locked. Its a last ditch effort for freeing an engine, but not something you should do on a running one primarily because what it can "free up" can also cause MORE damage to the seals/springs in the engine than good.

Start running premix 100:1 at least on the ratio, do regular oil changes and start saving.

This is all speaking from experience on two separate NA engines - so I'm sure some people have had different experiences...

[NoDOHC]

Is your battery fully charged and was the clutch down? The engine has to be cranking at about 200 rpm to get good compression readings.

[vex]

My two cents on this....

The piston tester is ok for "general" assumptions. What it proves is that you haven't lossed a seal or one isnt stuck.

50psi is low, even for a cold engine so vex's suggestion of letting it pump up to the overall psi is a better assumption. for the s4 NA 100psi would be better, but 95 isn't the end of the world (but I would start saving for that rebuild).

I would not do the ATF/MMO/Water trick. In certain cases, this is acceptable such as a motor that has been sitting in storage or got completely locked. Its a last ditch effort for freeing an engine, but not something you should do on a running one primarily because what it can "free up" can also cause MORE damage to the seals/springs in the engine than good.

Start running premix 100:1 at least on the ratio, do regular oil changes and start saving.

This is all speaking from experience on two separate NA engines - so I'm sure some people have had different experiences...

I'd love to hear those experiences some time (not calling you out just curious). I used to get really low compression on my engine--around 90-95psi but that was probably from the 6 months to the year it wasn't run. I ended up doing the seafoam treatment and I gained back alot of compression. I don't do it as much as I used to anymore because it fouls the plugs, but the positive benifits have happened more than not to me. Then again I only have 50k on my engine so it might just be night and day.

OP, i'd listen to WE3RX7. He has much more experience with engines on the way out--where I haven't experienced my first one yet. You don't necessarily have to take it to a dealership. There are units out there that can tell you the full compression readings and conditions of the seals I believe full units sell for around 200-300 bucks while you can pick them up for 150 at the cheapest end.