Mazda 3 Running Lean

Hi everyone I have a 2005 Mazda 3 with the 2.3l engine that has a major lean problem. It looks to be lean at both idle and at wide open throttle. At idle STFT is about 16ish and LTFT is about 20. Under WOT it still high up there with both trims at about 20 or so.

I have already replaced the fuel pump and injectors and they seem to have made a slight change in terms of the O2 signals and they now look how they're supposed to at idle, but there's still something clearly wrong.

I have done a volumetric efficiency test and plugging my data into an online calculator I'm getting really horrible scores like 50% - 60% range. Looking at my MAP sensor at idle, it stays pretty constant which should mean the engine's okay mechanically but I'm really not sure what I should do next. Judging from this does this scream MAF sensor?

Are there any other tests I can run? I'm just confused on where to go next, any help would be greatly appreciated.

Might have gotten to you too late. :blush: Sorry!

This does scream air measurement issue, OR unmetered air. Being a Mazda 3, I'd have a careful look at the air intake boot between the MAF and the throttle body. They like to split at the accordion and open up when the engine torques under load.

Otherwise, I'd suspect a MAF problem. What's the highest voltage you can get out of it while at WOT through 1st gear?

Hi no problem just thanks for your time, I appreciate it. I'm not sure about actual voltage unfortunately my scan tool doesn't measure that but under those conditions I can get like 100 g/s I think maybe the highest I've seen is 101, 105? As for the intake tube, I actually have a cold air installed by the previous owner so no problem there. I also have performed a smoke test before so I should have seen something if there was a leak. Thanks.

Hi ajherrera,

What's your Maf reading at idle?

That should be near 2.0 g/s.

Hi yes it’s about 2 g/s like 1.9- 2 range give or take.

ajherrera58 wrote: I'm not sure about actual voltage unfortunately my scan tool doesn't measure that but under those conditions I can get like 100 g/s I think maybe the highest I've seen is 101, 105?

Eh, that's not terrible? I threw that into a VE calculator and got a decent result:



Still doesn't explain the lean.

As for the intake tube, I actually have a cold air installed by the previous owner so no problem there.

Oh, really? :huh: I know CAI's are supposed to be an upgrade, but that only works if they allow the MAF to measure correctly. I'm guessing the old intake plumbing is long gone?

So, according to Tyler's VE calculator and Maf reading at idle, i think is not a breading issue.

How about fuel pressure reg? Fuel filter?

I did this a little while ago using this calculator and using the parameters from the scan tool I wasn't getting anywhere close to those numbers. For arguments sake, say it was a VE issue what exactly would I do next?

As for the CAI I haven't had a problem with it for the lifetime of the car like it sorta just came out of the blue so I don't know about it causing a problem now? I have double checked it though, everything's tight, all the airflow should be going through the MAF. But yeah old intake plumbing is long gone

I believe fuel pressure regulator is built into the pump which was replaced not too long ago, and these cars are super weird and don't even have a fuel filter.

ajherrera58 wrote: I did this a little while ago using this calculator and using the parameters from the scan tool I wasn't getting anywhere close to those numbers.

Would you mind doing another WOT run through 1st gear for us? Capture the data if at all possible, or just record the highest values. We mostly need the highest MAF value and the engine speed during peak airflow. It'd also be useful to know what the upstream O2/AF sensor is doing at WOT.

For arguments sake, say it was a VE issue what exactly would I do next?

Then you'd start looking into plugged cats, restricted intake, like that. Still, none of those will account for the lean trims you're seeing. There's a genuine fueling issue at work.

For your fuel trims to be equally lean across all loads, you've got a few options. Ethanol fuel, though I doubt you'd make that mistake at the pump. :silly: A stuck open purge valve, which you could eliminate by disconnecting the vapor line from the purge valve and capping the end. Then there's the MAF, which we'll hopefully address with another VE test.

Hi sorry for the late reply today, had to wait until I finished class. But I put together a table of data from 10 tests that I ran. This was captured from a cheaper autel scan tool, but I went ahead and made a column for the Volumetric Efficiency using that same ATG calculator. Its the last column and its followed by the avg VE of all the tests. Let me know what you think

Love the chart. Definitely low numbers. Not to nitpick, but did you mean to label the O2's as bank two? :huh:

Anyway, it sure looks like your MAF is done. Only other things I'd investigate are debris in the MAF itself, or anything that might cause the MAF to incorrectly measure.

Thanks! But sorry that's my fault O21 is upstream and O22 is the downstream. I already went and cleaned the MAF so cant be that. I don't know what could really be preventing the MAF to not measure properly, I've checked the intake tubing and its all clear. I'll go ahead and replace the sensor hopefully that fixes it.

Also side note, is it weird for my MAP sensor to automatically turn to 99 kPa for every test? And in terms of throttle position do those values at WOT look normal? Thought they might be higher? Thanks

What kind of performance do you have with this condition ? Does it lack power ?

what are the 02's (B1S1 and B2S1) doing at idle and closed loop? graph the 02's, you want to see oscillating 100mv to 900mv.

what are the STFT and LTFT at idle.

Map readings... are you using an OEM BAR1 sensor or have you upgraded.

Just popping in to suggest that you open your oil cap while running. If you begin shooting oil onto the hood, pull the intake and inspect PCV components bolted to the block side.

Just saw this problem still persists after new MAF. yeah there is a lack of power and a decent hesitation sometimes when under 3k rpm.

O2's at idle do oscillate properly, but this was only most recently fixed after I changed my injectors. As for the BARO sensor, yes it is stock.