8is>enuff

Thanks Go48.

I wasn't too happy with the low readings myself, however, I just received a call from my service center (My 8's in the shop due to a lack of power). They informed me I have a clogged cat and need a new one.

Would the increased backpressure affect the #s like that? Also, would driving around with the cat clogged and the compression so low the have any long-term effect on the engine?

ZoomZoomH

iTrader: (1)

well you shall find out once your car's back with a new cat....

Go48

I doubt that there will be any long-term effects from a clogged cat assuming the car was not operated for a very long time with that condition. The clogged cat would certainly have a negative effect on engine performance, though, as you have experienced.

Not sure about the effect of a clogged cat on engine compression readings. My gut feeling is that a clogged cat would not have a significant effect, but that's just a guess.

8is>enuff

...and I will share the information with the world.

sferrett

Another thing I'm slightly curious about is - that to get proper compression readings the throttle has to be wide open. With the electronically operated throttle on the rx8 how do we know for sure that floored pedal during cranking actually translates to fully open throttle plates, since the ecu has ultimate control over them?

Brice-RX8

I would think that they are pressing the pedal to the floor just to keep fuel from being injected into the motor, sort of like the deflooding procedure. I could be wrong though and it actually be a factor in the compression check.

sferrett

Having the throttle not wide open definately would affect compression numbers due to the signifiant intake restriction a closed throttle would impose on the system. That's the main reason why you're supposed to hold the gas to the floor for the test. As far as preventing fuel, you really should have the egi fuse removed to do the test to prevent the injectors from firing at all otherwise you've got a pretty dangerous situation.

Brice-RX8

Ok, makes sense.

8is>enuff

They just told me the test was performed on a cold engine because it wouldn't start. Sigh. Though I don't believe anything they say, I'll bite...

Any ideas on how a cold engine would affect the numbers?

mlx8

I think you also have to make allowances for altitude, a somewhat major factor in our area, and ambient temperatures, probably not as big a factor, but a factor. At @ 5,280 feet I think one may be looking at as much as an 18% drop off in maximum numbers, so depending a bit on Jaguar's battery condition, oil viscosity etc., i.e. rpm's turned by the starter, a 7.2 could be reported as low as 5.9 (7.2 less 18% or 1.296) As I recall, on piston engines, compression tests and leaks downs are supposed to be done on warm motors, and the tech needs to know what he's doing ~ i.e. throttle wide open etc.

8is>enuff

I got another compression check done, this time on a warm engine. According to my understanding, the numbers should be much higher than they are, due to the higher engine RPM of ~306.

Rotor 1
713kPa - Chamber 1
730kPa - Chamber 2
737kPa - Chamber 3
305RPM - Engine RPM

Rotor 2
746kPa - Chamber 1
739kPa - Chamber 2
732kPa - Chamber 3
307RPM - Engine RPM

24.2kPa max. rotor 1 - min. rotor 1
13.8kPa max. rotor 2 - min. rotor 2
8.65kPa max. rotor 1 - max. rotor 2


Thoughts anyone?

I confronted the technician and mentioned that my numbers should be much higher due to the higher RPMs, and suggested that if he read the graph correctly I was still under minimum spec. He countered by telling me the new engines only read about 800kPa, implying I should be content with my numbers. When I pointed out that those #'s were for a cranking speed of 250RPM, and therefore not comparable to mine (with cranking speed of ~305RPMs), he didn't seem to understand.

I'm planning on taking this to another dealer. How concerned would you guys be if this was your car?
Thanks in advance.

Brice-RX8

I would be really concerned unless there really is a logical explination to why the numbers are low, like ALL new engines have less compression than older ones. But who know, you are right you might just want to get a 2nd opinion on this one.

Also, how long has the tech been a Mazda tech? I know that the one at my dealership he has been a Mazda Master tech since 1970 or something like that, so I repect and value his opinion on my car, maybe a good thing or maybe not, I guess time will tell with dealing with him.