550s and 2g MAF Fuel Trim Question

[habitatguy187]

Alright I'm going to be a post whore tonight, my first question is this;

Since I got my logger my long term fuel trims have always been 81.2% (or somethin like that, the lowest they will go) I knew I had a bad o2 sensor, so I changed that, reset my battery a couple times, and still the same thing.

Last week I installed 550s with a 2g MAF, no other tuning, and my fuel trims are still 81.2%??? I know that with this setup I should actually be running a little lean, so I'm wondering what else could be up?

I have a rewired 190lph and that's it for fuel stuff. I suspect I have a boost leak but I thought that would actually lean it out more since it would suck more uncounted air in. I also swapped my fpr with a buddies and still same thing.

Sorry for the long post but I don't really know what to check next.

 

[mhuffman]

Strange. Your best bet is to get an AFC and lean things out to see if you can get your trims where they should be. Have you checked your base ignition timing? Were the 550's new, or are they actually maybe 650's causing it to run rich?

 

[habitatguy187]

The base timing should be dead on, (mitsu mastertech changed my timing belt) but I guess it doesn't hurt to check it. And the injectors were used, but I looked up the serial #s on google and they are indeed 550s. I've seen afc 1's go for $100, would that work or should I get a safc 2? Thanks

 

[mhuffman]

it is definitely strange that you are having your LTFT's in the 80's. Were they in the 80's back when you had a 1g MAS?

You can find SAFC's for 150-200 on the for sale boards. I decided to get a new Apex'i safc neo because i didnt want to run the risk of buying a faulty SAFC unit.

However, if price is a concern i'd look around for an SAFC-II. the II has more rpm correction points than the old knob style unit allowing for a better tune.

 

[habitatguy187]

it is definitely strange that you are having your LTFT's in the 80's. Were they in the 80's back when you had a 1g MAS?

The LFT was still the same, but the mid and high were a little over 100%, then I got rid of my bad o2 sensor, and they all went down to low, changed to a 2gmaf and 550s, same thing. I'm wondering what else could be causing me to run so rich?

 

[jking29]

Is the new o2 sensor cycling properly? Were all your trims reading 100% when you reset the battery?

 

[habitatguy187]

Is the new o2 sensor cycling properly? Were all your trims reading 100% when you reset the battery?

Yeah the new o2 is cycling just fine, and yes they were all at 100%, for a very short time. My o2 fuel trims usually hang around 60-70%. I could post a log up later today if anyone thinks it will help, just tell me if it should be idle or cruise.

 

[spyderturbo007]

If the O2 sensor was the problem, your trims would be > 100%, not below 100%. When the O2 dies, it reports a false lean condition to the ECU and in an attempt to correct the problem, the ECU increases fuel delivery. Anyway, I would look at your tune if your fuel trims are maxed out rich. I would also do a thorough boost leak test, as having a boost leak, will lead to a rich condition.

A 2g MAS will get you close to the proper correction, but it's not exact.

 

[habitatguy187]

If the O2 sensor was the problem, your trims would be > 100%, not below 100%. When the O2 dies, it reports a false lean condition to the ECU and in an attempt to correct the problem, the ECU increases fuel delivery. Anyway, I would look at your tune if your fuel trims are maxed out rich. I would also do a thorough boost leak test, as having a boost leak, will lead to a rich condition.

A 2g MAS will get you close to the proper correction, but it's not exact.

The ohm resistance on my o2 was dead 0, so I replaced it. I don't have anything to tune with yet, still deciding whether to buy bigger injectors and a chip or get a safc.

And for it being a boost leak, my understanding is that a boost leak is a vacuum leak at idle and under light throttle, so wouldn't that suck in unmetered air and cause a lean condition? Thanks for your help.

 

[habitatguy187]

And for it being a boost leak, my understanding is that a boost leak is a vacuum leak at idle and under light throttle, so wouldn't that suck in unmetered air and cause a lean condition? Thanks for your help.

Is this correct or am I missing something here? Someone help me out please.

 

[spyderturbo007]

That's what I used to think, but every time I've had a boost leak, my WBO2 has read rich. This also confirms what I've seem with other users' experiences with boost leaks at idle.

I'm not sure why, but the logic doesn't seem to correlate with my real world experiences. The only thing I can think of, is that it is determined by the size of the leak. Let's go to the extreme for a second. Imagine if you were to cut a big hole in one of your intercooler pipes. At that point, all of the metered air would never make it to the combustion chamber and you would be running rich.

Anyone else care to chime in with their experiences or thoughts?

 

[mhuffman]

That's what I used to think, but every time I've had a boost leak, my WBO2 has read rich. This also confirms what I've seem with other users' experiences with boost leaks at idle.

I'm not sure why, but the logic doesn't seem to correlate with my real world experiences. The only thing I can think of, is that it is determined by the size of the leak. Let's go to the extreme for a second. Imagine if you were to cut a big hole in one of your intercooler pipes. At that point, all of the metered air would never make it to the combustion chamber and you would be running rich.

Anyone else care to chime in with their experiences or thoughts?

Boost leaks ahead of the throttle plate will almost always cause rich conditions, though not really noticeable during low throttle.

Boost leaks after the throttle plate will make you run lean under 0psi and rich over 0psi because unmetered air comes in when the intake manifold is vacuum and metered air goes out under manifold pressure.

 

[steve]

Boost leaks ahead of the throttle plate will almost always cause rich conditions, though not really noticeable during low throttle.

Boost leaks after the throttle plate will make you run lean under 0psi and rich over 0psi because unmetered air comes in when the intake manifold is vacuum and metered air goes out under manifold pressure.

I assume your though is that prior to the throttle plate the intake piping doesn't see vacuum only boost so any leaks can only cause metered air to escape. Post the throttle plate both positive and negative pressure happen so leaks can cause both unmetered air to enter under vacuum (lean) and metered air to escape under boost (rich).

There is also the possibility that the leak is self sealing, under vacuum the opening closes on itself and seals but under boost its forced open.

I wonder if somebody didn't put a auto FPR on the car.

 

[habitatguy187]

Well Steve I think I found my leak, I'm almost positive that my egr valve is leaking. I've never been able to hold pressure on a boost leak test and I think it's the last place left besides blow-by. I just learned from this thread - http://www.dsmtuners.com/forums/tun...694-boost-leak-test-question.html#post1803609 - about the egr valve running into the #4 runner. I've always heard a sound around here when doing a BLT but could never find anything.

So I ordered a block off plate on monday, might be here today. But I'm wondering if the egr is open all the time if it will pull too much exhaust vacuum in and make me run rich? Sounds logical I'll just have to wait and see if this is the problem.

Thanks so much for you info though, right when you think you've got it down...you realize there's always more to learn.

 

[steve]

Wow, all sorts of interesting posts from me in the past that I've forgotten about.

 

[habitatguy187]

Well the egr block off didn't seem to do anything, so now I'm on to this.....

Not necessarily. There's a good chance you'll be far too rich and some point in the curve.

I ran a 2G MAS on my FWD with the stock 450's and a Big 16G at 12psi for a little while and aside from being a little lean at idle, the air/fuel was just about perfect under hard acceleration. If I threw a set of 550's in there with no additional compensation, it would've been pig rich.

I'm going to try it and see what happens, I know this shouldn't be so but since my FTo2's are around 60% i'm going to give it a shot and see what happens, I'll keep you guys posted.