I got a 2005 rx8 a few days ago and it won’t rev above 4-5k rpms. It can go through all of the gears but stops accelerating past 4-5k rpms. It doesn’t seem like a “rev limiter bounce” more of like a loss of power at 4-5k rpms. 100% on the throttle pedal and it’s like I’ve hit a wall of dough.

First Attempt
The first day I got it, it showed up with codes P0172 (System too rich) and P0850 (Neutral switch circuit). I managed to fix the P0850 code by connecting two unplugged wires under the car. I also cleaned out the MAF sensor using MAF in an attempt to clear out the P0172 code.

I did the 20 brake pumps and cleared out the codes while it’s on the ON (engine not started). When I started the car, it didn’t return the P0850 code anymore, so connecting those plugs near the transmission might’ve fixed it. But the P0172 code popped up again after around 1-2 mins of idling. I tried to do a neutral rev and it only got up to around 4.5k rpms. Also, I tried to turn the AC on after driving and while it’s idling: it stalled the engine.

Second Attempt
I cleaned the MAF sensor again with MAF cleaner and let it dry out. Did the 20 brake pumps and cleared out the codes. It wasn’t giving out the P0172 code, right up until I started driving it. Around a minute into driving it, it gave out the P0172 code and I wasn’t able to go above 4-5k rpms on any gear (even the neutral).

Third Attempt
It might be helpful to mention that the previous owner installed a pretty janky intake system. They installed an aftermarket MAF sensor housing and a cone filter intake, but kept everything else stock on the intake system side (Accordion tube and everything else after it). Also, the air filter was dirty and dusty. I cleaned the air filter but after the amount of particles (sand and dust) inside the air filter I decided to just call it quits and buy a new one. I bought a cheap Spectre cone filter from O'Reilly's. Also, I should mention that the previous owner installed an unbranded silver 2.875in interior diameter MAF housing (this info may be important and I’ll expand on it later). Then, I checked the SSV: I can move it freely with my hand and it snaps back.

Fourth Attempt
This attempt goes along with the previous attempt because I haven’t started the car yet with the new cone filter. I wanted to rule out the catalytic converter as a suspect. I saw on some threads and videos saying it might be the cause of the “rpm wall” (hint: it wasn’t). So then I dropped the catalytic converter and unplugged the downstream o2 sensor because it was dangling and putting tension on the wires. I cleared the codes and did the 20 brake pumps. With the new cone filter and the cat dropped, I started the car. I got the codes P0113 (IAT sensor), P0102 (MAF sensor), and P0172 (System too rich). It was idling pretty rough; it would spike and then drop and then it would stabilize and then spike and then drop over and over again. Until it was stabilized. I did a neutral rev and only got to around 5k rpms until it hit that “rpm wall”.

That’s about how far I got into it. I installed the new filter, hoping that it wouldn’t make the MAF sensor dirty (I theorized that the old filter was making it dirty and reading bad numbers). This attempt confirmed that the problem is not on the exhaust side because it still hit the rpm wall with the catalytic converter flange disconnected from the header.

Fifth Attempt (pending)
What do you guys think? I previously mentioned that the owner before me installed an aftermarket MAF sensor housing that’s 2.875” and I think the stock interior diameter is around 3.375”, it also doesn’t have a turbulence screen. Is this causing the bad readings? Should I get a new MAF sensor, get the stock airbox back to the previous diameter, or both? I’m pretty stuck on what to do. Is it an entirely different problem I’m not aware of? My goal is to just be able to access the full rpm range.

Extra notes:
- On the second attempt, I actually started it twice. First time I just idled it and 100% throttled it slowly until it hit that 4-5k rpm wall. Then the second time I started the car, I revved it and it was able to achieve all the way up to 9k rpms. Does that confirm the fact that the engine is able to achieve the full rpm range, but the ECU is stopping it?
- Do you guys think it’s the MAF sensor housing is causing the wrong readings and causing the STFT and LTFT to be all over the place (because of the pipe continuity principle)? Does the decreased diameter of the housing increase the airflow speed? The housing may be causing a double problem, it increases the perceived airflow speed and it’s also not tuned to that certain diameter. Is it thinking that more air is coming through a 3.375” housing and it multiplies the problem because of the 2.875” housing?

I’ll post my MAF, STFT, and LTFT readings in the comments.

Please let me know how to fix this "rpm wall" problem. Thank you!

 

Most recent reading (idle):
-MAF: 1.03 lb/min
-STFT: Spiked to +70% then settle to -7%
-LTFT: -22.7%

Other MAF readings (done at different times):
-Initial reading (first time): 0.97 lb/min
-Highest recorded value: 1.21 lb/min
-Avg. range throughout: 0.88-0.99 lb/min


Yes I know it’s supposed to be g/s but I have no idea how to change the it to g/s. I have just been converting it manually on Google.

 

Your MAF is reading too much air for some reason. 7.5g/sec when it should be 5.6 at 800 rpm idle. Clean the MAF? Replace?

 

Okay thanks for your reply. I cleaned out the MAF sensor around 3 times now and I replaced the dirty filter on it. It seems to be throwing out the proper IAT, throwing out ambient temp then going up as it idles (I don’t have the stock airbox). I’m not sure if it’s the MAF sensor or the MAF sensor tube housing, because it’s a different diameter from oem and it’s not calibrated for the smaller diameter I have on it rn.

 

Oh well yes if the MAF housing is a different diameter none of this will work. Its very hard to imprve on the stock intake, so start by going back to that. Else show what you're working with and well see wha's possible.

 

Yeah the MAF housing I have right now is smaller than the oem MAF housing. I would like to install the oem airbox system but I haven’t found a reliable private seller or a retailer that doesn’t charge $600+ (I’m a broke college student). What do you guys think about buying the AEM 21-458C Cold Air Intake System? I’ve read that they’ve designed their system around oem dimensions. I’m not really looking for hp gains yet, I just want my car to reach full rpm range. Also, do you guys have any tips where I can get the stock airbox intake without having to spend a hefty dime?

 

Stock airbox - ebay, junkyard. AEM is a great intake, one of the few. But usually not exactly cheap and long since out of productio .

 

I have the same issue, except it will only hit the 5k rpm wall under load, and it’s been driving me NUTS for months. I have the stock airbox though. New MAF, plugs, wires and I tested the resistance of the coils through the PCM connector. Tested the backpressure on the cat, new O2 sensors, all three solenoids on the back of the intake manifold have been tested, SSV not stuck and holds vacuum, swapped out OMP. Anyone knows what else it could be? Data logging shows the short term fuel trims maxing out when it hits the rpm wall and it has new fuel pump too

 

Dang. Yeah its annoying. My car hits the "rpm wall" under load AND in neutral. Could it be that your previous owner had a tune on it, and before selling it to you, he installed the stock airbox without loading its stock tune back? My car maxes out the STFT and LTFT during idle lol. I am thinking that the downsized aftermarket MAF housing the previous owner installed on my car was never tuned/calibrated for it (making it run rich), so now I am looking at buying an OEM airbox or an intake system that was designed around the stock housing dimensions.

 

Not sure on the tune, but it did have an aftermarket airbox and maf which I replaced with stock parts and it ran fine, it actually ran a lot better than with the aftermarket stuff!

 

Interesting. Hopefully I don't get stuck with this problem we are having rn after I replace my intake with the AEM intake system. Do you mind sharing what kind of intake you had on it before you installed the stock airbox? Do you have codes on it rn and what are your MAF readings?

 

I don’t remember the intake, that was quite a few years ago, sorry! It has no codes at all and MAF readings are 102 g/s when it hits the wall

 

I dont remember the intake, this was quite a few years ago, sorry! No codes at all and the reading is 102 g/s when it hits the wall with STFT at +25%

 

I've read that the turbulence screen before the MAF sensor is really important for airflow conditioning. Without it, it can cause unstable air signal and air swirls inside the housing. I don't have that screen on my current setup right now and it's probably amplifying my issue. Could you check inside your airbox and see if you have the turbulence screen, it looks like a door/window mesh screen (and there should be two in the oem airbox)?

 

Yes, I do have that screen but there’s only one. Not my picture but looks just like this

 

Oh okay. Yeah, I'm not sure it would be a turbulence issue if you have at least one screen and it's the one before the MAF. Sorry, I'm pretty new to this platform. Hopefully someone can reply to this thread with the solution to our problems.

Reference:
https://www.rx8club.com/series-i-tro...creens-237596/

 

I'm morbidly curious about this reduced MAF housing and why anyone would do that.
Its going to be very confusing to troubleshoot both your problems in the same thread, you might want to start your own Z14. Your issue sounds more like injector miswiring

 

I'm not sure either. The previous owner installed a housing that resembles Performance MRP's MAF housing (the one without the screen). They have different options for the inside diameter. If I can recall, they have options for 2.25" up to 3.75" inside diameters. The one I currently have installed has the same finishing and also has that MAF shroud welded into the tube, it just lacks that arrow signage (maybe I have a knockoff?). But either way, it doesn't have the same diameter as oem's. This Friday, I am planning on ordering an AEM intake to see if that fixes my problem. Do you think it'll fix it entirely or just kind of move that "rpm wall" to a higher rpm range?

 

I dont know if it will fix all your issues, it could, but its entirely possible the root cause is some other previous owner project or side effect :/
If your MAF reads too high across the range, its likely maximg out fuel trim and the computer is no longer able to correctly meter fuel at high rpm