I am totally lost - LTFT and MAF problem

[rao]

I will try to keep this short.

402 with stage 2 ported heads 228/228 cam and equal length headers, lucas 42 lb injectors. Mail order tune.

The motor has around 800 miles on it and runs pretty well, but developed a stumbling problem almost immediately. I have changed the o2 sensors, plugs and wires; triple checked for ANY exhaust or intake leaks and looked at anything else I can think of. I just put on 8 new coils. I checked the fuel pressure and it is rick solid and I installed another new fuel pump. Every part on the motor is 800 miles old or less.

It runs perfectly when it first starts up but then develops some break up when you open it up at lower RPMs and tit is not making all of the power it should be.

I got HP tuners and started taking a look around. What I found was that the LTFTs at Idle are going positive around 10-15%.

I decided to see if the VE table was off so I followed the standard procedure to check them out and found that they needed some fairly large adjustments, which I made and the LTFT when operating in SD mode are pretty good (not as good as with a wideband, but before I go and get 2 widebands I want to make sure that I am headed in the right direction) and the car drives somewhat better.

Interestingly I noticed that the behavior of the OS sensors change the SECOND I disconnected the MAF - before the sides were differed by 5% or so. In SD they acted like you would expect.

After I finished tinkering with the VE table I reset the PCM to re-enable PE and plugged i the MAF and the os sensors and Fuel trims IMMEDIATELY went back to their old ways. leading me to suspect that it is the MAF sensor that is bad.

Any ideas. I am stumped.

I will attach some log files and the tune file.

[rao]

Here is the mail order tune with original VE tables and a log with a new MAF sensor that I just made.

[TiredGXP]

After I finished tinkering with the VE table I reset the PCM to re-enable PE and plugged i the MAF and the os sensors and Fuel trims IMMEDIATELY went back to their old ways. leading me to suspect that it is the MAF sensor that is bad.

Any ideas. I am stumped.

I will attach some log files and the tune file.

These may be dumb comments, but here goes:

Did you recalibrate the MAF after adjusting your VE? Your post didn't indicate that you had. If not, you may want to spend some time doing so.

Did you reset your fuel trims after adjusting your VE table - Fuel trims take some time to relearn after VE or MAF calibration changes - the rule of thumb I've read in the threads seems to be to drive for an hour or so after resetting the trims b4 relogging.

[rao]

I did not recalibrate the MAF. I did reset the fuel trims.

What I don't understand is how the idle fuel trims can be 0 to slightly negative and then I plug the MAF in and they go +15 LT and a few more percent positive ST.

The car seems to run the same no matter what tune I have in it, so I put the mailorder tune back in with the new MAF, which was probably a waste of money.

[TiredGXP]

We probably need to have one of the experts weigh in here, but my understanding is that, when enabled, the MAF is the primary sensor used to measure airmass. If you MAF tables do not properly reflect the actual mass of air flowing, your fueling will be out.

Based on the differences you are seeing between SD and MAF fuel trims, it appears to me as if your MAF tables need adjustment.

I believe that there are some histrograms that will allow you to log STFT's against MAF Hz - try disabling LTFT's and logging the STFT's then adjust your MAF tables accordingly. This should bring your MAF airflow measurements in line with the SD airflow measurements.

[rao]

I get it. I will try to get the VE tables figured out tomorrow and leave it in SD until I get the MAF dialed in. If I make some progress then I will get some widebands and do it right.

Thanks.

[TiredGXP]

Just one more thing, you won't be able to tune your MAF tables in SD when tuning with fuel trims. You need to log trims and make adjustments based on the fuel trims that exist when the MAF is "driving".

Good luck

[rao]

I got that part. Thanks again.

[rao]

Well this morning I cleaned up the VE table and the MAF table as best I could with the narrow band O2 sensors and the difference is impressive

Off to get a wideband and do the entire tune correctly.