Hey guys... just ran a compression check on the engine in the street car...(stock) and compression is 60psi in the front and 55psi in the rear... I know these engines are not high compression engines to begin with...however...thats too low...right???

Rebuild??? What should I do? Oil pressure is great...No smoke...Just short on power...Suggestions???


Compresion was done cold. Battery might have been a LITTLE weak...

 

60psi! That's WAY too low.

Is that done from the trailing plug with the engine already warmed up? Is that a piston engine compression tester? with or without the valve that holds it back in?

IIRC, FB engines (12a and 13b) should be 85psi min.

Jon

Edit: If you think your battery is weak, hook a set of jumper cables to the battery and have another car running, to make sure you've got good juice. That can definitely make a difference

 

Trailing plugs. Engine Cold. Piston Engine tester. Valve in.

Maybe someone needs to educate me on how to do a compression test on one of these things.... Admiting stupidity!!

 

Doesn't sound right. These motors become Hard to start with compressions below 70-75. At 60 they are a pain and you almost ALWAYS have to use tranny fluid to start them. Take vipernicus42's lead.

Go back and take the reading the right way so that you have a good idea of the actual compression. I would hate to hear that you tore apart a good motor due to an unrealistic compression reading. I've seen people do it, though.

The short on power could be lots of other things. With only 105-115 HP stock, a little loss is a lot!!

 

I have never had to run a compression check on these engines before... Can someone tell me how... Thanks guys....

The engine will start...I have to pump the gas.....but...thats normal... Does not smoke or require tranny fluid.

 

what about the injectors?

 

It is not fuelinfected!

 

The RiGHT way?
With this:

Mazdatrix Price Check

Part Name Description Part Number Price
TOOL COMPRESSION TST DIGITAL COMP TESTER 49-F018-9A0B $1,640.0

but none of us have it so we do it like this:

Rather waiting for my slow typing, I found you a good link:

in "how-to" www.banzairacing.net

Don

 

this is how i did mine, with a regular piston compression checker.

1. Warmed up motor
2. Pulled both the fuel pump and the ignition fuse.
3. Pulled rear leading plug. put into comp checker and turned key. (look for 3 even pulses, and at least 85 psi.
4. Put rear leading plug back in and repeted process on front leading. Put a rag around the base of the carb as any fuel that was in the carb will come out.

I got 115 front and 110 rear, which was great, but i have a nasty dowel pin leak now so im currently building a half BP 12A do drop in when the car comes out of storage in the spring.
If you really have 65psi, your in the "blow anytime" category.

 

your engine could be very carbon fouled, causing leakage past the seals..

search for "carbon lock" and youll find everything about removing carbon (not that your motor is locked... but it still applies)

but thats only an if... shouldnt hurt anyways.

 

There is a guy local that has the Mazda tester.

 

All good information except one thing:

Testing should actually be done on the *trailing* plugs, amazingly enough. I didn't believe it myself, but someone drew my attention to it in the FSM, it's the trailing plugs.

You can remove the valve from the end of your compression tester's hose, so that you get the 'bounce' effect. Otherwise it will hold the highest value, which doesn't tell you anything. (if you have one blown apex seal, you still have two good ones that can give you a good compresion reading. So your engine could be blown and a regular pistion-engine compression tester with the valve still installed will read good compression!).

However, after you've verified that you have three even "bounces" of the needle, you can put your valve back in to find out what the highest one is. If all bounces are even and your highest reading is 103psi (for example), then your avg compression is *somewhere around* 100psi.

So
1. Remove the valve from your compression tester
2. Warm up the engine
3. Make sure that your battery has good charge (get boosted if necessary)
4a. Remove T1 plug, install tester, test it. Look for 3 even bounces
4b. Do the same thing with T2
5. Re-install the valve and repeat 4a and 4b

That should tell you what's going on with your engine.

Jon

 

You're right, trailing. The site I sent him to did say either. Should have caught that. Thanks Vipernicus.

 

Incorrect.

One seal takes out two chambers

 

Thanks guys... I will get back into it tommrow.