Compression Test
 

I found this video on You Tube on how to perform a compression test on an RE using a conventional compression tester.

Is this the next best way to perform a compression test on an RE if an RE compression tester is not available?


I will be performing this test Tuesday, because my engine is idling low, is low on power, and stalls sometimes when I'm coming to a stop. Sometimes the engine does not restart after the engine is shut off after driving around. It will start after it sit for an hour though.

from what I've read ... yes. that's the way to perform the test.

Great, thank you for confirming :icon_tup:.

I'll post the results, praying for the best, I hope all readings are at, or above 90psi.

such a calm and soothing voice....almost put me to sleep.

Comression Tests
 

Here are the results from my compression tests I performed today.

I'm not too sure about the readings even after watching them countless times :scratch:.

What are your thoughts?

For the rear rotor video, it looks like the cranking speed might have dropped after the first ~10 seconds of cranking, which may explain why the pulses drop to around 60 psi.

Otherwise, both videos appear to show relatively even pulses around 75-80 psi. Was the throttle open, and was the engine warm? Having a fully charged battery is also important. Minimum spec in the manual is something like 85 psi, but there will be some variation between gauges. Length of the hose connected also plays a role.

Ah! I knew I was forgetting a step, I forgot to hold the throttle wide open. The battery is good, the engine was warm, but cooled down a little since I had to prep for the compression test. The connection/plug that was inserted in the spark plug hole was short like the one in the top You Tube video. It did not have the same length of threads as the spark plug threads, the hose is about a 1' to 1' 1/2" long.

Also I did not disconnect the CAS. Should I have done that as well?

I am about to do this test for my 90 vert and would like to know if you need to disconnect the CAS while performing the test as well? The manual says to, but some confirmation would be helpful. Thanks for this thread because I was using that first vid as a starting point also.

So I just did the test and yes you do have to disconnect the CAS. On a side note (this is my first time doing the test ever) I think my rx7 passed pretty good, even bounces at around 100-105 i think. Hope for your rx7 to be running great.

Awesome
 

You are welcome :icon_tup:.

Ok, thank you for the confirmation, because I was still wondering if it was necessary. Your vert is in good condition, I hope my retest has results in your range. You are out of the in need of a rebuild compression test results.

I will follow up again.

This is what my test looked like, you can tell me what you think and maybe it will help. First one was front rotor, sorry for the negative sound in my voice but i did not want bad results..still do not lol.

Your good man :icon_tup:. The two lower pulses followed by an above 90psi pulse is good :nod:.

I hope my retest with the throttle wide open and CAS disconnected will get me similar results :fingersx::fingersx::fingersx::fingersx::fingersx: :fingersx:.

I'd just pull the EGI fuse instead of disconnecting the CAS.
That connector is a pain in the ass.
You just don't want Spark ,or fuel,(haha!),when testing for compression...Ftt,fft,fftt..Woosh..(holy crap!)

maybe it's my hearing/vision getting old but to me the needle from both 'S4 Vert''s and 'Lonely Driver''s is not bouncing evenly on every pulse.

to me there is skeeping going on. I like to back off the schrader valve a bit to bleed off some pressure which minimizes the needle's tarvel. Makes it easier for me to see what the needle does on every pulse. I also suggest hooking up vacumm gauge to see where the #s are @ idle.

p.s. 'S4 Vert', notice 'Lonely Driver''s RPM is higher. That will give higher #s.

I recently had hot start issues on my rebuild that turned out to be a shit Autozone rebuilt starter that cranked slow. I got a Mazda rebuilt and now cranks on steroids (like 'Lonely Driver''s) and hot start is a thing of the past.

THANK YOU :icon_tup:!! I tried disconnecting the CAS and could not get my hands in there to pull the clips apart, DAMN MY MAN HANDS :ugh2:!! ha ha.

You are seeing correct, there should be two low pulses and one high pulse in a three pulse compression test. And LD's compression readings are better then my readings, so his compression is better then mine.

I will retest my compression, because I did not have WOT, and the ignition was not disconnected. Maybe then I will have either a better, or worse compression reading then my first tests.

I think the hot start issues might be from recent repairs I had done to my engine, because before the repairs my engine started on hot starts and did not stall when coming to a stop.

Thank you for the advice and tips, I will follow up this week when I have the time retest.

That top vid, the guy is like the Bob Ross of compression testing

[QUOTE=

I recently had hot start issues on my rebuild that turned out to be a shit Autozone rebuilt starter that cranked slow. I got a Mazda rebuilt and now cranks on steroids (like 'Lonely Driver''s) and hot start is a thing of the past.[/QUOTE]

I thought about what you said about your hot starting issues you had. Is it really possible that my starter could be causing my engine not to start on hot starts? My brother said he had the same issue with his dually, and a new starter had fixed his hot start issues too. My starter is the original unit, so it's possible a new starter may be needed.

The problem I am having on hot starts is the engine will not fire up, the starter cranks the engine and I smell gas instantly. However the engine will not fire up, so I figured it is a compression issue. Because when the engine has time to cool down the metal retracts and creates a better tolerance for the apex seals to create compression (imo).

The longer I drive my car, the idle gets lower and the power falls off and the engine dies when coming to a stop unless I blip the throttle once, or twice. I figured the hotter the irons get they expand causing a gap between the apex seals and irons. And that is where I am loosing compression.

Correct me if I am wrong, I am still trouble shooting and need any and all correct advise I can get.

Thanks

^^^^
make sure your battery connections are clean. Hook up a charger set to start assist next time you do the test. this will rule out insufficient voltage as the cause of low RPM.

let's say your hot start issue is low compression. low compression is a function of both mechanical (i.e., poor sealing) and RPM. cranking RPM should be 240-250 RPM. let's say compression shows >95 psi @ 240-250 then at 140-150 RPM compression might come in @ <80 psi. At 95 psi hot start should not be an issue (specially if less fuel than factory is being injected). At 80 psi it's a challenge.

If it turns out compression is pretty even but borderline w/proper RPM and hot start is still an issue, you should have luck by reducing the amount of fuel during cranking. You can do this by introducing a switch on the 12V start signal wire to the ECU (it's pin 3B on the S4 TII, might be the same for the n/a). By cutting the 12V during cranking, the pulse width is ~1/3.

You need to hook up a vacuum gauge. Cheap way to monitor the engine's health. The high compression n/a motor should be north of 18 inHg at idle.

I do have an AEM 320 Fuel Pump, I am not using an fpr to regulate the fuel pressure, and I am still using the stock fuel injectors and S4 TII ecu.

So now I am leaning towards too much fuel is being introduced. Which could be the cause of the hot start issues, stalling when coming to a stop, poor fuel mileage and performance??

I will try the measures you have advised me to do during my next compression test.

Thanks :icon_tup:

Thanks for the info OP

You're welcome.

Thanks for the info!

You're welcome.

Ah!!
 

Well after being tired of my car stalling almost every time I come to a stop, or let off the gas, I finally decided to adjust the idle. After let the screw out a couple of compete turns, my idle is steady at 900rpm and my engine no longer idles super low, my gas mileage is better and the engine no longer stalls.

Thus is such a relief, because I was worried that the problem was worse.

Glad you have your running how you would like! Did you ever redo the compression test to see how it looks? Even though I think mine was good, while trying to troubleshoot my own separate problem, I think I have a blown coolant seal (which can happen even with good compression as I understand it) and am looking at a rebuild. But Ill do what I must to make my baby run nice again, just might be a while lol

Thanks!

No I have not performed another compression test since, but I really need to just to be sure my compression is within good standings. But now I have a new problem as of this morning, I can't get my car to go into gear when the engine I running. If it isn't one thing it's another ha ha : /.

Is your engine using a lot of coolant, or smoking? I hope that your water seals are good and that your engine does not need a rebuild.

I tried min today and need some input from you guys. 1. car was not warm up 2. my tb is off due to a lot of changing stuff but i figure would be the same as holding the gas down.
rear
http://i209.photobucket.com/albums/b...psf19261fd.jpg
front
http://i209.photobucket.com/albums/b...ps0098eb26.jpg

It hard to tell where your readings are in the pictures, however they do look very low from the sweeping motions.

It is best to perform your compression tests when your engine is warmed up, and be sure to remove the valve stem from the compression plug. Also be sure to look back on the previous post in this thread to make sure you are following the proper steps as described.

Is there any way you can video record your tests? That would help, or if you can tell us your three pulse psi results, ie 70psi, or 80psi.

my bad I thought it would take you to the video. Only reason not sarted it becuase I just put big injectors aem coils and a single setup and have no map yet and was to the point wanted to make sure it was good before kept going. Mistake of taking the guys word for it claiming comp was 120. note this is my first rotary car.
rear
F52B00CB-F155-44A4-B262-09586A1F1A7F-1045-0000013250329C1F_zpsf19261fd.mp4 Video by lanet420 | Photobucket
front
F6C8A5F9-31C7-45C0-A00F-A1BBF03EE7D6-1045-00000131E2C97878_zps0098eb26.mp4 Video by lanet420 | Photobucket

First RE car? Welcome to the world of Rotary!

Your rear rotor reading look very low, the front rotor looks a little better though. As I was advised in this thread, be sure your battery is fully charged. Maybe the throttle body might need to be installed to get a better reading.

Lets see what some of the more experienced members have to say, they can give you better advise than me.

battery was fully charged to do this. this was done with the valve removed. I didn't eve put it back in to see how high comp was cuz it looks like the rear has blown apexi seal from the reading I've done. If its blown I'll probably part it all out and do a ls swap :( Thanks for the help and info guys!

You are welcome.

LS swap sounds like a plan, a turbo LS swap would even be better ;).

Rear rotor looks like one apex seal is gone (1 high followed by 2 low) but the front is fine.

In general I think the piston testers will test low.

New FC Project Compression Tests
 

Well I just purchased three more fc's that were running but have been sitting for years. I have tried starting the s5 vert, but it did not turn over, so I performed compression tests on the front and rear rotors.

Here are the video:



What do you think?

Your rear is crappy..
I mean, it is rebuild time..
your engine..not your ass..lol!

Just to let you see for yourself,watch the video of the front..3 nice even bounces.
The rear does not have the 3 bounces..it is like one good one,and a pooooff..

Ha! Ha! :lol: Crappy rear, that is funny! Thank you for clarifying what you meant ;).

Ok, I guess it's time to role up my sleeves and get busy.

Compression Test on FC Number 2
 

Ok, so I performed compression tests on my GXL. This time around I got much better results, but I am still not sure if the reading are satisfactory. The engine would not turn over, so I am thinking maybe fuel is not reaching the fuel injectors.

Front rotor compression test:


Rear rotor compression test:

front rotor on the GXL is toast, that needs a rebuild also, rear rotor is good, which is surprising since usually the rear rotor goes bad before the front

but, anything can happen

the last test on the GXL seemed to give strange results since it wasn't always 2 low peaks but alternated from 1 to 2 low peaks. it could be an issue with the gauge but the starter follows the sound, so i'm guessing they're both toast.

IMO a digital system isn't much more to put together than buying a new mechanical gauge. i just put together a dual transducer unit to test both rotors at the same time for under $100. you can also use the data to calculate cranking speed during the test as well for more definitive results. after building it i regret not doing it sooner.

credit given here:
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/$150-pc-based-compression-tester-1021454/

credit to me: the unit listed is out of date though and the DI-145 is capable for only $35 shipped. the resolution on the DI-145 has much to be desired though at only 240 samples per second if only one channel is enabled where the DI-149 has 500 times that resolution for $59, both would work fine but if i want to test both rotors in the same sample or a piston engine's cylinders all in the same test i would choose the DI-149.

even a chinese transducer will work but the scaling will suffer somewhat since it is a 5v 150psi sensor so the PSI readings will have a 3% margin of error(read as 3psi, the logger has a .02v interference factor) based off my tests at 100PSI, a 10v transducer will read more accurately but they also cost over $100 each. a generic 100psi sensor will have less error margin but the peak would be at 100 and no further info given for those with healthy engines.

http://www.ebay.com/itm/Pressure-transducer-or-sender-150-psi-stainless-steel-for-oil-fuel-air-water-/251362960044?pt=Motors_Car_Truck_Parts_Accessories&hash=item3a866662ac&vxp=mtr
disregard the 5v input reference, it will work with 12v as well as a 5v input, you could even run the system off a 9v generic battery. so digital sampling can be done as cheaply as $70 with a single sensor system and your computer.

:( man... I was hoping the GXL had a good engine. So now I have two rebuilds I need to do.

Would low compression cause an RE not to start?

Thank you for your input fellas.

in those cases yes, it will make an engine very difficult to start.

in most cases a single failed seal makes these engines more difficult to get started than if a rotor was completely blown, because the backfires in the intake(although discrete) disrupt both rotors.

Ok. Thank you again for the confirmation.

Rebuild time it is.