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Discussion Starter · #42 ·
SRB - That open loop piqued my curiosity, so I took my bone stock 95 LX 4.6 around the block, ran good. Then swapped out my "good" MAF sensor to the one that gave me double digit fuel trims. Still double digit fuel trims but never came out of closed loop. So I still don't have any suggestions for you why your car comes out of closed loop, other than a loose electrical connection or loose sensor.

Like stated above, in open loop the computer should default to a set of preprogrammed settings for air/fuel mixture rather than the MAF and O2 sensors. It shouldn't run bad just because its in open loop. I don't know if there could be a problem with the resident fuel delivery tables but If it was me, I would disconnect the battery for a few hours, reconnect then drive it hard but not rough. Try to teach it how to run again. Sorry, I'm all out of ideas. Give it another hard look for vacuum leaks and loose electrical connections. Good luck with it. - Jim
Thank you very much for your efforts. This is good info!


I am a Motorcraft guy thru and thru but for MAF's, I am NOT impressed with the Motorcraft. They do not last, and/or get out of range enough to cause WOT pinging. I've replaced them on every MN-12 I've owned and also chased WOT pinging issues on every single one.

On my latest T-Bird, I wasted at least $100 for a year or so of 89 octane use until I finally replaced it with a Delphi unit. Now I am back to good 'ol 87 octane, and the MAF will pay for itself after about a year, then it's gravy after that.

I even had great luck with a Cardone unit on my old '95, plus gas mileage went up 2 MPG.

Al
Thank you for your experiences, I will end up with Delphi more than likely.



Sorry I haven't been posting here, working 10 hour days 6 days a week leaves me not very enthusiastic about working on the car, lol. It will slow down next week though so I'll actually have some time to look for vacuum leaks. I easily could have cracked a plastic line when I removed the intake. For now it does well enough with the MAF unplugged, gets me to work and back.


Brandon, I still plan on doing the data log as you suggested. Idle then 2nd gear up to 3500 rpm. What are the specific sensors, measurments, etc that I'm looking for? Is O2 sensor voltage relevant? Do I just need to look at everything and look for spikes on the graphs? I appreciate you babysitting me, lol. What I'll end up doing I think is going through it all and if I see anything that doesn't follow the smooth uptrend, I'll screenshot and post it here for further scrutiny. I'm still re-reading the stuff you posted man, great info!
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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The goal is to keep the car in closed loop so the O2 feedback allows the PCM keep the lambda at a perfect 1.0. We examine the STFTs and can see how far off the MAF sensor measurements are throughout the normal operating range. This is actually how tuners fine tune the MAF transfer function after dialing in the rough curves using a wideband O2 sensor on a dyno - but that's a different chapter. :)

The car will switch to open loop if you exceed about 70% throttle, so keep your foot out of the pedal. Using manual 2 is probably the easiest/safest way to accomplish getting this data.

The MAF counts (or airflow values if you don't have AD counts) should be a nice, smooth line (like you saw in my graph on the other page). If the MAF is behaving correctly, the STFTs should stay within a couple percent of 1.0 throughout the whole run as well. Obviously the MAF needs to be connected, but also leave the battery disconnected for a couple minutes to reset any LTFTs and be sure the car is up to operating temp. before the data run. 180 degrees is sufficient (use your scan tool to verify if you aren't sure). I would examine the STFTs for both bank 0 and 1, MAF counts (or airflow), TP relative, RPM, load and fuel source.

Look for both banks of STFT to stay within a percent or two of each other (regardless of their values - larger variances could mean faulty O2 sensor(s) or exhaust leaks), and within a couple percent of 1.00, with the normal switching I described earlier. If it goes down suddenly (with a change in fuel source value) it could mean the PCM switched to open loop - which throws off the data run. The MAF counts should increase steadily with RPM. As the RPM increases, the rate at which the MAF counts (or airflow) increases will decrease as long as TP stays constant.

If the STFTs are consistently more than a few percent under 1.00 at idle but after opening the throttle it goes back up to about 1, that is a classic sign of a vacuum leak. STFTs that are significantly off across the board would mean something else - a bad set of O2s, FPR, MAF, IAT or ECT as examples. A vacuum leak lets in a given amount of air that doesn't really change. That particular amount has the biggest impact on fuel delivery calculations at low RPM and load, since it's a larger percentage of the total air entering the engine. As you open the throttle for normal driving and let more air in, that fixed leak becomes a much smaller total percentage of the total air ingested and fueling gets more accurate. Remember the PCM only expects to deliver fuel based on the air measurement reported by the MAF sensor. Vacuum leak (aka unmetered air) is extra air the PCM doesn't know about.

Remember, if the STFT is below 1, the PCM is commanding a richer mixture. Over 1.00, and it is commanding a leaner mixture.

The fact that the car runs better consistently with the MAF disconnected, forcing it to use the load_with_failed_MAF table (delivering fuel and spark based on RPM and TP, without any MAF sensor's data) leads me to believe the most likely culprit is the MAF, or the MAF sensor circuit having issues.

The only thing I'm having to adapt to is how your scan tool presents data. My ScanGaugeII and LiveLink give me two different numbers for engine load, for example...
 

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Discussion Starter · #44 ·
You’ve really given me something to chew on here! Thanks. Are there any books on the subject that you would recommend for a beginner like myself?
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Even if you're not planning on getting into EFI tuning, Don LaSota's book is a great read to help you understand how the process works. It's not that pricey, IIRC. Matt or Greg might know of a few other books. Greg Banish has a few good ones out there too but I have not read them...
 

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1994 Cougar XR7 DOHC/5-Speed
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Yeah I read the Greg banish book engine management advanced tuning for the hard knowledge, the rest I learned from notes I made talking to Oscar and others who are better at numbers than me. You included Brandon, thanks for the free refresher course 🍻🙂
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Oh believe me, I'm refreshing myself as I go too, lol. It's been almost 2 years since I've had Advantage open. :)

General Zod, where art thou!? Reign your terrible fire upon us and teach us thy ways once more!!
 
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