This one almost seems kind of silly but it has got me scratching my head. Two RX-7s, I changed the oil on recently. One is a S5 GXL rebuilt (Thanks RR ) with about 6,000 miles on it and the other is an S4 base with a Mazda remanufactured engine pulling about 21,000 miles.

Here is the thing, I changed the oil at the same time with the same Castrol oil 10W-30 and the same oil filters for each. My GXL's oil is almost pitch black, while the base's is still middle amber colored. Should I be concerned or is this normal? I am glad the oil is removing the particulates and such, but what I am more worried about is it should be getting that dirty, that quickly or should the base's oil be as dirty.

 

Most oil contamination comes from fuel residue.

My first guess - The GXL is running rich compared to the S4 base.
The fuel mileage may confirm that..

 

Are you premixing on one and not the other?

 

No premixing is done on either. Is the AFR guage the only reliable way to check whether or not the GXL is running too rich? If there is some ghetto way that I can tell other than just by smell and have the results be somewhat reliable, please tell. The fuel mileage also seems to be about equal for both cars. I fill them up about once every week and a half normally.

 

I've heard about some "more than normal" blowby problems with the Viton oil control rings compared to the OEM when used during rebuilds. Maybe one of your engines uses these rings, and the other doesn't. I used them in mine, and I get premix/gas blowby. I just change the oil a little more frequently, no big deal...

 

Well, as long as it is not something major and maybe just running rich I can deal with that. These cars have taught me to keep an eye on even insignificant things. The small issues usually tend to turn into things that gives you many headaches.

 

i'm getting something very similiar, it seems my oil gets very dirty almost pitch black within a few days or a week after doing an oil change. my car is running extremely rich. it was 4mpg, been working on it and now it is up to 12mpg (still rich but better than 4mpg) in any case any pointers or advice would be helpful.

 

4 MPG! I thought my 14MPG was bad.

I used to tune cars with a $2 screwdriver, now it takes a $200 S-AFC.

 

IS this after the first changes btw? cause there could have just been dirty/contamination left in the remaining oil in the oil cooler etc.?
(one of my 12A's scared the **** out of me when after an oli change afew hundy later the oil was balck...by the 3rd changeand FLUSH it ran clear and clean)
Just a thought.