A little background on my car. It's a 2004 M6 with 96k miles on it. Has a BHR ignition kit and new plugs both have about 10k or less miles on them. Fuel pump replaced about 8-10k ago. Car has a gutted cat and the computer has had all the recall/updates done to it. The radiator is new (about 1k on it) and I have used sea-foam as per the Mazda instructions. I've owned the car for 3 years and take very good care of it.

The problem started about a year and a half ago. If I drive on the highway the car will start to loose power after about 15-20 minutes of steady highway driving. It will bog down, get much louder (sounds like a WRX) and will sometimes loose so much power that I can't get the car going faster than about 75-80. The power loss seems to be throughout the rev range but worse at higher RPMs (above about 5000 - 6000). No amount of city driving has ever produced these symptoms and they only seem to go away if I either park the car for a while or do about 20ish minutes of city driving it sometimes returns to normal. It seems to happen about 50% of the time I drive on the highway.

Outside temperature doesn't seem to effect it much. I've had it happen both in 100+ degree summer weather and in single digit winter weather tho i think it happens more often when warmer but if so not by too much. It happens both with a low tank of fuel and a fresh full tank. The car has never stalled or even threatened to and always starts up quickly both from cold and hot starts and has never flooded even if accidentally shut off cold. The only code in the computer is for the cat being gutted.

The problem has been happening since before all the above work was done on the car. Some things made it get better for a short time (BHR ignition) but it never went away completely and always got progressively worse until it's back to the way it was before. Around town the car runs just as strong as ever and drives perfectly. I've done a ton of searching, read many similar threads but I'm at a total loss as to what's causing the problem.

 

Hmm. Interesting problem. Everything I'd point to seems to be new or a non issue. Do you have any obd2 logging ability? I'd be interested to see what your fuel trims, both AFRs, cat temp (yes, it's still reported), and MAF data is from before it starts to when it's fully present.

 

No I don't have the ability to log. It's something I need to invest in tho I think. Any recommendations on a good tool to use?

 

ur fuel pump is going.

 

The cheapest option is a bluetooth OBD2 adapter (lots of options from ebay and amazon) paired with an OBD2 app. If you have an android based smartphone/tablet, Torque is by far the best app. There is a free version, but I don't think it can log. The paid version is WELL worth the $5 price, and a fractional increase in the cost of the adapter if you think of it that way. The logging rates tend to be a bit slow over bluetooth sources, but it should be fast enough to see what is going on.

Alternatively, a pricer cabled OBD2 option will probably give better logging rates. The fastest logging I've seen is from the AccessPort, but getting one is a bit overkill for just this issue (there are other benefits though)

He mentioned that the fuel pump is relatively new. Fuel pump failure usually is an abrupt shut-off though, not a slow power loss.

 

what he describes is pointing at some sort of fuel related issue, its most likely the pump, new doesn't mean anything as new one can fail prematurely. something is overheating, and the only thing on the car that can overheat during highway cruise is fuel pump, I know the fuel inside the tank help to cool things down, but it does not change anything if the pump itself was defective in the first place.

it can also be coil, but coil usually fails when its heat soaked (slow traffic), I fixed a Rx8 before with failed coils, it only fails when the car is slow, highway it drives perfectly fine. and I am checking another 2006 Rx8 right now with dead fuel pump.

 

Agreed, just far less likely. I'm more inclined to think something like one or both O2 sensors is drifting way out of target and sending the AFRs very lean or very rich. This would occur faster on highway driving than city driving, since city driving spends more time in open loop.

 

I got me a bluetooth adapter and borrowed an android phone for the day. I took about a 10-15 mile city drive, mostly around 40-50 mph.

MAF: 13.9g/s Cat:1471 F RPM:2464 LTF:4.7% Coolant:183 F Timing Adv: 38.0 AFR: 14.65

MAF: 13.0g/s Cat:1469F RPM:2342 LTF:4.7% Coolant:187 F Timing Adv: 36.5 AFR: 15.03

MAF: 20.2g/s Cat:1597F RPM:2950 LTF:6.2% Coolant:187 F Timing Adv: 42.5 AFR: 14.36

At this point I put 3 gallons of gas in the car and got on the highway. It started loosing power almost immediately so the next 3 data points are all as the power loss is getting steadily worse.

MAF: 35.5g/s Cat:1627 F RPM:3312 LTF:6.2% Coolant:187 F Timing Adv: 34.5 AFR: 14.50

MAF: 31.7g/s Cat:1625 F RPM:3311 LTF:6.2% Coolant:187 F Timing Adv: 36.0 AFR: 14.81

MAF: 19.3g/s Cat:1655 F RPM:4024 LTF:3.9% Coolant:192 F Timing Adv: 43.5 AFR: 14.63

I got off the highway and the car was down on power. When I accelerated the AFR would drop to 12.5-13.5 then come back to the mid 14s at cruising. The cat temp around town averaged in the high 1400s and on the highway around mid 1600s. Accelerating to pass people would spike the cat temp up to around 1750 for just a few seconds then it would drop back down. The car has a hallowed out stock cat/exhaust on it for the record.

This was the data at idle when I parked it:


MAF: 4.0g/s Cat:1377 F RPM:840 LTF:6.2% Coolant:190 F Timing Adv: -3.5 AFR: 14.87

Ambient temp was 44, the stft was typically + or - 2 from 0 and the car reported being in closed loop almost all of the time.

 

Well, this are my thoughts on it, which may or may not be right.

The cat temp spiking like that on the highway would suggest that you are either going very lean or getting combustion in the exhaust instead of in the engine. Since the AFR remains close to 14.7, I don't think it's going lean, or that there is a significant O2 sensor problem, or that it is a fuel delivery problem.

It suggests to me that you aren't igniting the air/fuel mixture inside the engine very well, and that mixture is being transfered to the exhaust stream, where it's ignited there. This would result in power loss and spiking cat temps without really changing the AFRs much.

I'd revisit your ignition. I know you have the BHR kit and the plugs are relatively new, but if I was in your shoes I'd first confirm the solid installation of the BHR coils AND the provided grounding point, then replace the plugs anyway, then if still nothing, swap back to OEM ignition to see if the problem persists. Defective coils have happened (Ray says about 1%, which is still >0%), and so have defective plugs where the plug starts coming apart internally. I'd also check the engine block grounding points to be sure they are clean and secure, since that electrical charge needs a path to return on.

I'd also check the e-shaft sensor and see if there is any debris on it, and clean it, then reset the NVRAM (20-pedal-stomp), just in case it's acting odd. Your symptoms sound load based however, and the e-shaft doesn't care about the engine load.

If still no change, I'd take the car to a shop and have the intake track smoke tested for vacuum leaks. Your idle MAF is lower than it should be, suggesting a weakening engine or a vacuum leak. So if no vacuum leaks, get a compression test to confirm that, or rule it out.

 

Thank you for the help. After thinking about it I tend to agree about the fuel igniting in the exhaust. When it's doing the power loss thing really bad the exhaust pops like when it pops flames at redline but at ~6.5k to 7k rpm. It's only done this a few times when the power loss was really bad.

I have one more question. My car needs to pass smog before I can register it in the state I moved to (Utah). If I were to buy a new cat and install it long enough to pass the sniffer test would I be at risk of destroying it before I got it to pass the test?

 

I've been having similar issues for the past year. I have an 05 with 83k miles about 2k on coils and wires and 10-12k on plugs. Previously i had bhr ignition with about 8k on the coils. In december developed a misfire in rotor 2 that i think led to my cat melting. Installed my midpipe reversed the bhr kit and got misfire in rotor 1. So now i'm running oem coils.

To get to the point i was having the high cat temps with my midpipe on and recently got a used cat and want to make sure i don't melt this one. A few things i've seen watching data on my tablet with torque app is as soon as i go into open loop my afrs will drop from around 14.7 to 13.5ish. Also if i drive for around 20 min then park for 5-10 min and get back into my car my iat will rise 40-50f degrees, my fuel status will read open loop due to system failure and my ltft will rise around 5%. My maf reading is 4.6 at idle at 875 rpm which is a little down from before the cat metled. It was normally 5.2-5.3 at the same rpm at idle.

I'll try new plugs for the cat temp and planning on compression test soon since my engine warrenty expires in may. I've been thinking maf sensor, front o2 sensor for the other problems just trying to get input from someone with more experience on these cars than me. I don't post much just wondering if i should have started my own thread for this or is this an ok spot to post all this

 

Sorry, I didn't see this.

As long as you don't disconnect the battery or clear codes, etc... let the system readiness monitors go to "ready", then install the cat and drive straight to the sniffer test and you will have your best chance at passing. Unfortunately, even with a cat, you still may not pass a sniffer test with that problem.

Your open loop numbers look accurate.

The low idle MAF is likely a vacuum leak, and likely from something missed when you reinstalled the intake after reverting to stock coils.

The open loop due to system failure means that there is some other problem at stake, like an O2 sensor failure. You SHOULD have a CEL for whatever that problem is (since you know the ECU sees the failure, it should be reporting on it), unless you have a tune that blocks whatever CEL would be thrown. That could also be the source of your misfire, as could the vacuum leak you introduced.

 

I'll look for a vac leak. It only stays in the open loop due to system failure for a short period of time, probably less than a minute, maybe that's why it's not throwing a code but i'll try to find someone with a good o2 sensor to test with. My tune only has the idle rpm raised and fan temps lowered a little on the cobb stock tune. Still reading a lot before i start trying to tune.

Is it normal for the intake air temp to get that heat soaked to rise 40-50 degrees? That's with the stock intake.

 

Yes, normal intake temp increase. The components under the hood get upwards of 160F-200F, and that heat soak will get to the intake easily if you come to a stop.

 

Changed plugs and the power loss came back after a run on the highway and low rpms i sounded like a subaru. Double checked wires and plugs and did the same thing. Maybe it's closer to time for a compression test. I'll try to get a couple logs to see if there's something that can be seen that's going wrong.

 

Problem seems to only happen once the car gets hot. I get a rattling sound from the driver side and an oily smell in the higher rpms when the problem is there. I'll try to get the logs up later. Other things i've tried for this problem was clean the ssv. I have looked for a vac leak a few times by spraying around the areas it could leak and haven't found one. Maybe still need to take it in to get it smoke tested.

 

a couple logs from monday maybe someone can see what is going wrong

The morning run i had no problems with. The afternoon had the rattle in high rpms power loss and missing at idle.