ecu goes to open loop at hot idle?

[nates6969]

Hey guys I'm having an issue with my idle when the car is hot. I posted a bit about it in a knock/timing thread but is better suited here. On the logger the o2 graph fluctuates nicely while cruising around 2000 rpm and then when I drop down to an idle when the car isnt fully warmed up it idles around 1100 in closed loops and the o2s fluctuate again nice and smoothly and the idle is smooth. Then all of a sudden after sitting for that extra minute at a red light, the car warms up to full temp and the idle slows to a nice 750. however right when it drops so does the o2s down to about a constant 0.05-0.07. my AF gauge has one red bar and then if I look at the short term o2 fuel trim its reading like 160%! my low fuel trims are maxed at 139.8! then all of a sudden the o2 fuel trim reads 100% and the o2 voltage drops to 0.0-0.01v. Now if I'm not mistaken that means its in open loop mode. it idles like crap like that aswell. my mas had been hacked by the previous owner and my theory is its pulling air the ecu is not accounting for and causing it to run lean. is this possible to cause the computer to flip back to open loop. and yes i have leak tested is and only have some minor leaks in the TB seals. also the coolant temp appears to be reading right on the logger (like 205 degrees). So what do you guys think. has anyone else seen this. Thanks for your help guys!

[steviek]

Is your engine light on? I had error 505 (system idle malfunction).
I had something similar but not quite the same. Mine was only a problem when my car had been run hard for a fair amount of time. And my revs would refuse to drop at idle. I tinkered with the air fuel ratio a bit gave it a little less fuel if I recall correctly and found a vacuum line that was unhooked and capped it. You can always try swapping your ISC or check your spark plugs and their gapping but I doubt they are your problem. Idle surge as previously stated by crunchy is a common problem with our cars... If its any consolation its not particularly harmful to your engine.
I wish I could help you more.

[nates6969]

ya its not really idle surge thats my problem though, its just for some reason my computer is running so lean at idle its throwing it into open loop. and it doesnt throw any code, its really messed up. My car doesnt really idle surge. Any other ideas?

[corning_d3]

Checked the fuel pressure?

[nates6969]

hmm no actually I haven't, I never thought of that one. I guess that could probably be the cause to all my other lean fuel trims! thanks, I'll have a go at that, right now I have my fps bypassed and have the regulator just hooked up to the intake, is this ok? I've heard of people doing this and that it really has no adverse effect.

[kjewer1]

Yes, the FPS is worthless.

I'm at a loss at this point as to the main problem, but I'll continue to think it over. I'm leaning towards something like the hacked MAS leaning out the idle so much that the sensor either has nothing to read or is too cool to read (due to lack of fuel). The former is more likely than the latter. I would unhack or replace the MAS first to see what effect that has.

[gthompson97]

Originally Posted by kjewer1

I'm at a loss at this point

Did I read that right? Typo maybe?

[nates6969]

Thanks for the quick replies everyone, ya one thing i was wondering about the Mas was on an average 750-800 rpm idle what should the air flow frequency be at? mine is at 25.2Hz with 8 degrees of timing. is that number low or just as low as it reads?

[kjewer1]

You should be around 60 hz at idle IIRC. The hacked 2g mas would lower my airflow by something like 40% at 1k, 35% at 2k, 20% at 3k, and 4% from 4k up. This is off the top of my head, but you get the point. It's a ridiculous curve. I got these numbers by compensating for the hack with the AFC, and these were the percentages it took to get things back in line. There are much more reliable nubmers out there in the DSMlink world, but they only apply to 2g so I won't look them up. But it's pretty clear now that airflow signal from the hacked MAS is the root cause of this problem.

[kjewer1]

Originally Posted by gthompson97

Did I read that right? Typo maybe?

Hey, I'm only human, it happens

[nates6969]

these damn hacked mas's! would there be a good way of testing this theory before throwing in a new one? maybe running some tape around the inputs on the unmetered passage of the mas to block it of, huh i wonder if this would cause too high of a reading at idle

[kjewer1]

You'll never get it to be right. The factory units are finely calibrated instruments.