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What is wrong with my 3g MAF?

jepherz

Staff member
Joined
Aug 8, 2004
Messages
7,877
Location
KC, Missouri
For starters, I got a brand new 3g MAF from a member here a long while back, probably 5-6 years ago. To be honest, besides burning a chip with the 3g MAF tables in it, I didn't do much tuning OR driving of the car for years. The last few years I've tracked it and had a conservative tune. It was hard not to notice, though, that there was some weird behavior on partial throttle around 2-3000 RPM. AFR's would first jump up to around 15-16 AFR and the car felt SLOW. Then, as soon as AFRs rose, they dropped off again and the car took off like a bat out of hell.

I've now installed an Evo ECU with the intent on being able to log and tune easier. To my surprise, with a few Evo maps with only injector corrections (PTE 780s) the car behaves the EXACT same. I tried to take some logs although I'm no tuner so I'm not totally sure what to look for. I logged MAF hz vs. AFR vs. RPM and it seems like the only similarities are partial throttle acceleration. Somewhere between 150--200 hz I am having issues. But the strange thing is, if I am just cruising and not under load, the AFRs seem fine at that same frequency from the MAF.

In order to narrow it down a bit, I locked the ECU into open loop and set the fuel tables to 11:1 AFR to see what would happen. Here is a log segment to show what I'm talking about:

Code:

Code:
TPS     RPM     AirFlow ECULoad	W02
20      2187    150     58.75   11.68566
20.3    2187    150     59.375  11.86143
20.3    2218    150     59.375  11.74425
20      2250    150     59.375  11.68566
20.3    2250    150     59.375  11.74425
20.3    2250    150     58.75   11.74425
20.3    2250    156.25  59.375  11.80284
20.3    2281    156.25  59.375  11.80284
20.3    2281    156.25  59.375  11.80284
20.3    2312    156.25  59.375  11.86143
20.3    2281    156.25  59.375  11.86143
20.3    2312    156.25  59.375  11.97861
20.3    2312    156.25  58.75   11.97861
20.3    2343    156.25  60      11.97861
20.3    2343    162.5   60      12.0372
20.3    2375    162.5   58.75   12.0372
20.3    2375    162.5   59.375  12.15438
20.3    2375    162.5   59.375  12.15438
20.3    2406    162.5   60      12.27156
20.3    2437    162.5   59.375  12.15438
20.3    2437    162.5   59.375  12.21297
20.3    2437    168.75  59.375  12.21297
20.3    2437    168.75  59.375  12.27156
20.3    2468    168.75  60      12.21297
20.3    2468    168.75  59.375  12.21297
20.3    2500    168.75  59.375  12.21297
20.3    2468    168.75  59.375  12.38874
20.3    2531    168.75  59.375  12.44733
20.7    2500    175     60.625  12.38874
20.7    2562    175     59.375  12.27156
20.7    2531    168.75  59.375  12.38874
20.3    2562    175     59.375  12.6231
20.3    2562    175     60      12.6231
20.7    2593    175     59.375  12.6231
20.7    2562    175     59.375  12.79887
20.7    2593    175     59.375  12.79887
20.7    2593    181.25  60      13.209
20.7    2625    181.25  60      13.32618
20.7    2625    181.25  60      13.38477
21.1    2625    181.25  60      13.44336
20.7    2687    181.25  60      13.32618
21.1    2656    181.25  58.75   13.32618
21.1    2687    187.5   60.625  13.44336
21.1    2687    187.5   60      13.67772
21.1    2718    187.5   60      13.61913
21.1    2718    193.75  60.625  13.67772
21.1    2718    187.5   60      13.61913
21.1    2718    187.5   59.375  13.61913
21.1    2750    187.5   60      13.209
21.1    2781    187.5   59.375  13.09182
21.1    2750    187.5   59.375  12.91605
21.1    2781    187.5   58.75   12.79887
21.1    2812    187.5   58.75   12.68169
21.1    2812    193.75  58.75   12.6231
21.1    2812    193.75  59.375  12.50592
21.1    2843    193.75  59.375  12.21297


I haven't messed with the MAF tuning at all because frankly, I'm not sure I should have to. Yes, my ECU rom is set up for an Evo MAF but they flow so closely that it shouldn't be a problem. I had the actual 3g tables in my ROM on my 1g ECU and had the same problem anyways.

Anyone have any ideas? I'm tearing my hair out trying to think of what the problem is. I will buy a new 3g or Evo MAF if there is something wrong with mine, I just don't want to throw parts at the car without having a good feeling about what is wrong. Could my intake piping have anything to do with it? My BOV is recirc'd fairly close to the MAF coupler, could that matter?

Help!
 
Last edited:

jepherz

Staff member
Joined
Aug 8, 2004
Messages
7,877
Location
KC, Missouri
Injector scaling was taken from EvoM in this thread:

Quote:

PTE 780cc

Scaling: 680

Latency:
4.6875 3.624
7.03125 1.992
9.375 1.344
11.7188 0.984
14.0625 0.744
16.4062 0.576
18.6768 0.456





I will double check fuel pressure, but my intent is 36 psi (stock DSM). Not sure if that should be different since I am running an Evo tune, anybody know what base pressure is on an Evo 8?

In the end, this results in LOW and MID fuel trims within 4%.

I've read a little bit about Accel enrichment, and although that immediately sounded like it may solve my problem, my car runs lean for A WHILE before the fuel catches up. Also, I had never heard of anyone adjusting accel enrichment on DSM ECUs. I don't even remember if the tables had been found for it.
 
Last edited:

Andy_S

Well-known member
Joined
Jan 30, 2007
Messages
982
Location
Shithole Wisconsin
I think an evo is around 38-39psi. The trims are almost spot on though. You don't have a lean spot if you WOT, only at partial throttle? I'm thinking this has more to do with the physical setup instead of tuning. At partial throttle, your BOV is probably cracking open. Maybe some weird flow stuff is going on with that?
 

toybreaker

iconoclast
Joined
Apr 30, 2006
Messages
3,581
This car still have the egr system installed?
 

jepherz

Staff member
Joined
Aug 8, 2004
Messages
7,877
Location
KC, Missouri
Quoting Andy_S:
I think an evo is around 38-39psi. The trims are almost spot on though. You don't have a lean spot if you WOT, only at partial throttle? I'm thinking this has more to do with the physical setup instead of tuning. At partial throttle, your BOV is probably cracking open. Maybe some weird flow stuff is going on with that?



That's exactly what I was thinking. It's a factory BOV, but all custom IC piping. I can take some pictures of the actual setup and entry into the intake to see if you guys think there are any issues there. At idle, the factory valve should be open, right? Maybe at that instant when I open the throttle plate it causes some disturbance in the maf reading or something? How can I test this? Disconnect the BOV for a quick test perhaps?

Oh, and WOT will demonstrate the same problem. It's coming back from decel or idle around 20% throttle OR above at that same RPM range. Basically any time I exit a corner and start in on the gas, it has about 100 hp /ubbthreads/images/graemlins/rofl.gif
 
Last edited:

jepherz

Staff member
Joined
Aug 8, 2004
Messages
7,877
Location
KC, Missouri
Quoting toybreaker:
This car still have the egr system installed?



EGR is removed and a block off plate is installed. What are your thoughts there, John?
 

beaner

Well-known member
Joined
Jun 22, 2005
Messages
1,562
Location
b'ham, mi
I'd set the base fuel pressure to what the ecu is expecting.

The BOV not staying shut seems plausible. I remember I could get my car in a weird situation, especially on the highway, where the bov wouldn't close all the way. If I blipped the throttle real fast, the bov would shut and car would take off.

/ubbthreads/images/graemlins/dunno.gif
 
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