Saguaro

Hi, so after a few years with my rex i decided to test its compression for fun, and i bought the dct 500. Now my question is how do you do that. In the instructions it says disable ignition and fuel system, can i just fully press the gas pedal to do that, like you do when its flooded or do i actualy have to do other things? Thank you

sonicsdaman

iTrader: (3)

first, welcome
second, read the new owners thread no really read this!
third, try using the search function at the top of the website( you can get results like this https://www.rx8club.com/series-i-tec...n-test-244299/ )

Saguaro

Hi, thanks for the immediate reply. Im not really a new owner so i know all of the stuff in that welcome thread, or atleast i remember much of it. As for the search option, it doesnt really help me. All the threads talk about disconecting the eshaft sensor, but my question is can i get by, by just using the deflood method since it does the same thing... As far as i remember

sonicsdaman

iTrader: (3)

It does not do the same thing. Holding the gas pedal down only stops the fuel pump(doesn't stop spark). Also the sensor takes a whopping 2 seconds to disconnect so why bother risking it
And not trying to sound like a dick but the new owners thread has the official how to from mazda on compression check and a very handy chart once you have all 8 of the numbers from the check.

BigCajun

iTrader: (1)

I think pulling the fuel pump fuse would work.
*edit
Incorrect, ignition needs to be disabled as stated before.

Saguaro

Hi. Thanks, these are the answers i was looking for. All the compresion test instructions i foud talk about disconectimg the eshaft sensor, the fuse or both. So i remembered that the gas pedal does the same thing but wasnt sure about the spark. Would the spark effect the test tho? Can it ignite some residiual fuel wapors or something? As for the why not disconet the sensor, because pressing the gas takes 0 seconds plus i dont really like disconectimg plastic bits or i try to keep it to a minimum, because its an old car and plastic breaks. Last year i changed the glow plugs on my peugeot and the plastic wire protector literally disintegrated. You coud blow on it and it would break into small pieces. I had a similar problem with the lambda sensor, where the plastic was so rigid, instead of the clip releasing it broke, and is now held together with duct tape. Sry for the bad spelling, im tiping on the phone in a car

Red line envy

I would think any type of spark or ignition in the chamber could damage the sensor on the compression tester. IMO

dannobre

iTrader: (3)

I would disconnect the ESS sensor....it ensures no spark or fuel.....

BigCajun

iTrader: (1)

You're right, I was thinking fuel only, and I didn't see sonics reply.

Saguaro

So I finally managed to get down to it and test my compression, but it was a real pain getting to the sensor. I spent about an hour trying to get to it and trying not to brake it. So for the next time, can I do something else? The pedall will cut the fuel and now I only need to kill the spark. Can I do that by pulling some fuse or simply disconnecting the coils/leads?

CaymanRotary

I just disconnect the fuse and disconnect all 4 spark plug wires from the spark plugs. Works for me and no damage to my compression tester.

mazdaverx7

iTrader: (6)

I do the same as above. I remove the plug wires and pull the 20A fuel pump fuse and have never had an issue. I use the same procedure on RX-7's as well, with some minor differences in fuel cut. And that depends on which generation I'm compression testing. Disconnecting the eccentric shaft position sensor is a good idea however.

CaymanRotary

Agreed but if it's really stuck or a pain in the *** for you it wont really hurt anything. You aren't cranking the car long enough for it to matter. I know this mainly because I've never had issues ever doing it this way on multiple cars.

manuRx8

I'm also one of those who disconnect the eccentric axis sensor and I do it without thinking, I recognize that it is uncomfortable and that the first time I was a little scared, afraid to break it, because it was very hard. I had read it in a forum and I had never considered another alternative.
However, in line with the latest comments, I think there are simpler and perfectly valid alternatives.
I have reviewed the electrical diagrams and, in fact, I think that removing a couple of fuses solves the issue without any risk:

1. Removing the 15A fuse from the ignition (Engine - F35) the coils remain without power so there is no spark.
2. Removing the 20A fuse from the pump (Fuel Pump - F27) is no longer pressure, so that the injectors, although activated, can not inject gasoline.

I even think of another more radical possibility, although I do not recommend it to neophytes and it would be to remove the starting relay (Relay 46) and ALWAYS WITH THE CONTACT OFF, bridge the contacts of the relay in the socket. In this way the engine would rotate alone, without activating the PCM, or ignition, or injectors, or sensors, nothing at all, just the starter.
Another small advantage that has this last method is that, not having to give contact, everything is done in the engine area, without having to enter and leave the cabin (and without staining the seats).
My only question is whether the fact of not opening the throttle butterfly will influence the compression measurement.
This method I have never tried, but apart from simple, maybe solve the small inconvenience of the error that appears in the PCM whenever we turn the engine with the eccentric shaft sensor disconnected.
If someone is encouraged to try any of these alternatives, please tell us. For my part, as soon as I have the chance, I will try the alternative of the starting relay and tell you.

CaymanRotary

It's not really complicated:

1. Remove fuel pump fuse and relay.
2. Remove all spark plug wires.
3. Remove trailing plugs and do your compression test by pressing clutch (just leave in park if Automatic) and pressing gas pedal to the floor and crank engine.
4. Put everything back and move on with your life .

Removing the crank sensor not really necessary if you dont want to but I would recommend if you can.

Saguaro

Thank you guys, there is a reason I always come back to this forum if I need anything. On my native forum they always stick to what some ''expert'' says, but you guys aren't afraid to think outside the box and find new solutions One more question, why do you think the spark would actually damage the sensor? I mean it's just a spark, and its not too close to the sensor

CaymanRotary

The spark can cause ignition and the heat from the engine starting can destroy your sensor. It's important that ignition is impossible before cranking the engine to do a compression test.

Red line envy

Also why not be as cautious as possible with such a special tool.

Saguaro

I get that an explosion would damage it, but do you really think there would be enough vapor inside to cause an explosion? As for why not be cautious, honestly, because it's nothing special, and I'm the tipe of guy who is willing to risk and experiment a bit in order to make progress

CaymanRotary

It's mainly because its so easy to do the steps so it's worth it just to be safe. New rotary compression tester like $300 so why risk it when it's so easy to not have to in the first place.

Saguaro

I get what you're saying, but me being me, I'll proboably try it one day Do you know how loud it would be if I were to remove the plug, stop intake and just openly crank the engine to get any of the fumes out? Well I payed around 150€ for mine, and even if I were to break it, it would only damage the sensor which I should be able to get locally for under 50€. So the way I see it, is if I find a really easy and quick way to do the test (skip the 1 hour fiddling with the ess) I would be actually saving money in the long run, because I walue my time

CaymanRotary

Well it's your car and your money so it's not really my business how you do the test. Just suggesting an easy way to prevent ignition from happening so you can test without risk of damaging your tester. Happy testing!

Saguaro

And I thank you for your tips. This forum already saved me about 1hour the next time I do a compression test, so thanks for that

And if anybody is interested, my car has done 80kkm and has a passing compression (just), so I think I'll be one of the few in my country to do 100kkm on the original engine :D

CaymanRotary

What were your numbers out of curiosity?

Saguaro

Sorry I don't remember because I dodn't write them down. They were 2-4 squares above the rebuild line in the graph