Hello,

I am getting horrible gas mileage from my 2004 RX8.

I'm running AEM smart coils, original NGK plugs, stock ECU, stock air filter and MA6025T CAT that I bought new in order to pass emissions.

Oil is Castrol 10W40 A3/B4 semi-synthethic as it is has a max deviation of 5% in properties from Miller's Trident because I can't get Miller's Trident in my country so I took the next best thing.

My compression is not great. 6.14 / 5.8 BAR but still it shouldn't be this bad
*minimum is 6.8BAR on both rotors, brand new engine has 8.6BAR


Not that we got that out of the way, my mileage is... 35L/100Km or 6.7MPG

And this is on highway driving. 3,200~4,000RPM . It actually gets better during city driving, which makes no sense...

I'm suspecting it might be the CAT. I was expecting restrictions but nothing this huge. I'll drop it and go back to being CATless

Power on the low end is virtually zero. If I have below 3,000RPM in any gear any I press the accelerator half way, I have to wait a couple of seconds for any type of response.

I know an RX8 usually gets between 15 and 20MPG but almost 7 is more than half its tank for ever 60 miles of highway driving...which basically means I can't go very far

 

There is no magic answer for fuel mileage, you have to be systematic in troubleshooting it. The ECU injects fuel based on airflow, so can you read and log OBD sensor data for rpm, intake air temp, airflow, airflow voltage, commanded afr, actual afr, calculated loadand injector pulse width/dury cycle?

Both during cruise and at warm idle. Save the data out and post the files somewhere we can read.

Obviously the low compression is not helping, but that's still pretty extreme.

 

I'm using Torque so I have access to most if not all of those.

Forgot to mention : I cleaned the air-filter (even though it was new but I was already working on the MAF) and I cleaned the MAF also with a can of brake de-greaser. Was pretty clogged with oil and other dirt but cleaning the MAF didn't do much of anything in terms of throttle response or mileage.

Car has brand new OEM DOX-1449 and DOX-0330 O2 sensors also.

Any time frame for the logging? 1min or more? I imagine it is going to generate a lot of data.

 

Nooooooooooooooooooooooooooooo... you're supposed to use MAF cleaner. Brake cleaner changes the conductive properties of the MAF sensor, ruining it. I don't know how much brake cleaner is too much.... but zero is best.

A minute of idle and a minute of cruise is a good place to start.

 

Ah....too late now. And I used like half a can of brake cleaner on it...

I have a spare MAF but I'm guessing my MAF is ok if nothing changed?....but then that would explain why power down low is...low.

I don goofed!

I'll get the times for that next week. De-CATting and other mechanical work comes first.

Thank you for your help!

 

A bad MAF can seriously affect your performance. Be systematic in your detailed diagnosis as Loki suggested.

 

Bothare correct - you can only use MAF cleaner on ANY MAF. The solvents in brake and carb cleaner will "melt" the MAF sensor. Even putting your "clean" fingers on the MAF sensor can alter the thermal conductivity properties of the surface and throw off the readings.

I would also make sure that the oil in the area of the MAF is cleaned well and that oil is not continuing to be aspirated into the area fouling the plug. When you fill the oil, use an oil fill cone/spout to ensure you're not sending oil down the breather ports along side the wall of the oil fill tube.

 

" breather ports along side the wall of the oil fill tube ". Who designed this ****? Why didn't they also put a gasket that dissolves when in contact with oil while they were at it?

Of course I spill oil on the engine every time I fill her up. But yeah, an oil cone would fix that. Great advice, thank you!

 

all things EPA...must not have any unburnt hydrocarbons in the atmosphere but CO2 is just fine thank you. Most of the complexity of modern engines to eke out another 10% boost in mileage and reduce engine reliability and increase service costs is traced to EPA mandates on emissions systems and control. I hope that changes such that CAFE standards are lessened and the RX-9 is allowed to be created.

 

Yeah it's not like LA was blanketed in smog before CAFE rules were put in.

The breather setup was revised and you can get an improved filler neck/pcv/breather thing from Mazda. Or.. you know use a 50-cent funnel and the correct amount of oil.

 

On another Bad mileage report, at 20k miles on car I was only getting 5 mpg while driving hard on sports car track.

When I put the BHR ign kit on at 23k miles it went back up to 10 mpg average on track.

Last month I got 26 mpg on a spring day with crap winter gas, but no A/C.

 

Good news and bad news.

The good news is that I managed to get logs for about 1min of idle running.

The bad news is that when I tried to start the car and go on the highway to do the other set of logs, it didn't start anymore.

It cranked for about 3 revolutions and then made a horrible metallic clanking sound and it just stopped.

Whining sound coming from the starter motor, lights dimmed when trying to crank and a bit of smoke coming from the battery terminals. Conclusion: starter motor is pulling juice like crazy because it is trying to turn a seized engine.

Either bearing failure or a seal popped out, either way the car doesn't crank, not even 1mm

This explains the pool of oil and fuel in the airbox and the intake manifold, not to mention the horrible gas mileage. Fuel and oil were gushing out of the engine in every which way possible.

Does anyone know what other parts I need besides the stationary gear bearings? I already ordered those and I already have 2 other low compression engines from which I can get the irons:

• 6x Apex seal Set
• 6x Apex seal spring long
• 6x Apex seal spring short
• 4x Rotor Oil seal set
• 4x Outer water seal
• 4x Inner water seal
• Eccentric shaft oil seal front
• Eccentric shaft oil seal rear
• Set of through bolt seals
• Set of dowel "O" rings
• Mazda RX-8 Engine rebuild workshop manual

 

Yikes. Sorry to hear that.
Also these logs look like GPS logs. Not engine data.

 

I trusted you, Torque! But oh well, can't do anything about it now. I thought it would log all of the readings the ECU gave not to mention I even added all of the dials to make sure it did that but apparently, it only logged the GPS. It's not a bug, it's a feature I guess.

I was lucky she didn't die on me during the trip, in the middle of nowhere. I have another starter motor with which I will try, maybe something happened to the starter motor (mechanical issue, not electrical) since the engine didn't get a chance to fire.

Or maybe the apex seal popped out just when it detonated but then why would it stop abruptly and not turn? Sounds fishy to me. It's not like the inside of the engine is made of glue or something. But I'll check the starter and try with the other one, see what gives.

Thank you for your help. Sorry the log data was not relevant

 

A free apex seal can jam between the rotor and the housing, ruining both. It's possible the starter or flywheel have become jammed. See if you can turn the engine by hand with the trans in neutral.

You might need to pick the best parts from both of your engines, or buying known good big parts. I'm not sure if this has been stated sufficiently, but cutting corners in a rebuild only shortens the time before you need another rebuild.

 

Might also be a failed OMP. I have been running 10W40 semi-synthetic oil so maybe it seized due to heat expansion and welded the rotor to the rotor housing.

I tried moving it by hand and I tried pushing the car in 6th gear and in reverse. Wheels lock up= engine seized.

I don't want to cut any corners in the rebuild, I just want to do it as cheaply as possible but I'll see how that goes.

I might get my hands on a 13B-MSP for free. I have a guy who wants it out of his RX8 so he can put something bigger for drifting and if I help him, he said he'd give it for free. It's still a low compression engine but it runs.

Until then, back to my Smart ForTwo

 

Popped the old engine back in, the one with 3,6bar compression. It lasted for about 100Km (60miles) then the seals on the 2nd rotor popped out. Managed to get some readings if this helps.

I'm going to try and salvage a working engine from the two dead ones.

I know popped seals mean a lot of internal damage but actually, this may be my saving grace.

Engine 1 has blown bearings which seized during crank, so BEFORE the engine could run = minimal internal damage = seals and rotors are fine.

Engine 2 has blown seals but working bearings.


I really hope there is a way of replacing the bearings without completely taking the engine apart but given my limited knowledge of mechanics, I could be dead wrong

 

I generally achieve a constant 20 mpg (8,5 km per 1 ltr)......during 58.000 km's (36.500 mls) already......
My last fill up after 20% - 55mph / country road and 80% - 80 mph / highway turned out to be an average consumption of 22 mpg.....(exception for me)
I drive my car in a smooth/moderate manner.......
BTW: 2-3 times a year at night a 135 mph on a familiar straight road with very few traffic and no cobs......it creates adrealine in a natural way.........


A month ago I did a first compression test wit my own "RCT-V5.2 Rotary Compression Tester".......

The result:
Front rotor:
kg/cm2 at adjusted 250 rpm.........8,53 / 8,31 / 8,02

Rear rotor:
kg/cm2 at adjusted 250 rpm.........8,83 / 8,39 / 8,09


I started premixing from new at 1 oz/gallon plus active OMP (increased the flow rate of the OMP at 10.000 km with 50%)........on which the engine runs very smooth; especially at idle you can clearly hear the difference
I run 5 / 10W40 engine oil in the moderate Dutch sea-climate.........oil change every 5.000 mls.........spark plugs + leads every 20.000 mls........OEM Mazda coils every 30-35.000 mls ..........recently I changed the motor mounts......again OEM Mazda; the old ones still had free space at the passenger side.


I protect my coils against the engine heat by a double layer of ceramic heat insulator tape (2x3mm.) plus alu sheet plus air space between alu sheet and the bottom of the coils.......furthermore I arranged the coils fingerspreaded versus each other.








I have a digital temp. meter enabling me to measure the oil sump temp. or the temp. at the oilfilter (2 probe elements and a switch)
The difference between both spots is between 50-57 degr F.
99% of the time I pick up the sump temp.
In summer (84 F.) the sump temp. varies from 194 to 203 degr. F.....at steady 80 mph highway.......sometimes reaching shortly 212 to 220 degr. F. in a traffic jam.
With cold weather 40 degr. F generally 185 to 194 degr. F.





.

 

update: it wasn't the seals that popped on Rotor2. The spark plugs just exploded. see: https://www.rx8club.com/series-i-tro...-issue-266425/