Ford Focus - High BARO Hz reading

Was hoping someone could enlighten me about something I witnessed on a Focus in the shop recently. FYI, this vehicle has come and gone, so I won't be able to do any follow-up testing I'll answer any questions as well as I can.

'01 Focus comes in with a stalling complaint, and a check engine light. P0171 stored. Freeze frame has vacuum leak written all over it. A test drive doesn't reproduce the stalling, but I can easily see how it could be:

www.scanshare.io/share/YOfrRs-7eEehJIA2LlOlJQ#0,1,7,8,12



Oh yeah, 70% total correction at idle! I find a big ol' vacuum leak at the EGR valve that's been loosely installed with the wrong bolts, and no gasket. Yep, someone hacked this one up before I got to it. I noticed the abnormally high BARO reading, but decided not to read much into it given the other problems.

Fixed the EGR valve, clear the KAM and I go for another drive.

www.scanshare.io/share/tHRtAg7SJUKoQcKzn5n93g#0,2,6,13,14,19



Fuel trims are much better overall, but the BARO ends up high again. A BARO of 155 to 157 Hz is normal for my elevation, so a reading of 162 Hz is very strange. For reference, that reading would be normal if I were below sea level, instead of above.

This is the first time I've ever seen an abnormally high BARO reading. Normally, a breathing issue or a under-reporting MAF will produce lower BARO readings, so I'm not quite sure what to make of this. The engine is breathing TOO well?

I'd be happy to hear any thoughts you have as to why this Focus thinks it's in Death Valley instead of Kansas!

At the risk of being laughed at, how is Baro calculated? I do notice the pid at work but I'm used to seeing around 100kpa as we're near the coast. From memory our old ford gear recorded it on the map just before crank signal was sent. I'm not familiar with the Hz signal. How does the maf translate airflow to atmospheric pressure? Is it based on full throttle airflow? If so, could that come back to an inaccurate tps?

matt.white wrote: At the risk of being laughed at, how is Baro calculated? I do notice the pid at work but I'm used to seeing around 100kpa as we're near the coast. From memory our old ford gear recorded it on the map just before crank signal was sent. I'm not familiar with the Hz signal. How does the maf translate airflow to atmospheric pressure? Is it based on full throttle airflow? If so, could that come back to an inaccurate tps?

No worries, I probably could have explained some of this in the first post. On most Ford vehicles with a MAF and no MAP, the BARO is calculated while the engine is under load during a drive. Not WOT, necessarily, just some kind of steady load. I can't find any documentation that explains the math exactly, but I'm pretty sure the PCM compares throttle opening to the MAF and 'calculates' the barometric pressure.

The BARO PID itself is kinda weird. As you mentioned, older setups measured barometric pressure with the MAP sensor KOEO. Those sensors created a square wave frequency signal (instead of an analog signal like we're used to), and sea level barometric pressure produced a signal of 159 Hz. Ford carried this frequency measurement of barometric pressure over to their MAF systems. Why not use a conventional unit of pressure? 'Cause Ford, that's why

I hadn't thought about a TPS problem, good thinking! Looking at the data, it seems like the PCM is recognizing closed and wide open throttle positions correctly, but it could still be out of adjustment. After seeing what these other guys did to this engine, I consider anything possible.

It starts out OK after the repair and the KAM is cleared. At the time of the "correct" reading, STFT is high, but is soon countered for by LTFT and everything seems to settle into an acceptable range. MAF looks to be reporting within range at WOT (what I assume is WOT by the TPS anyway) maybe a smidge low....
I've been trying to draw a correlation to the Baro PID update to MAF and TPS values, but they aren't consistently changing together....
Very curious...

in reference to the end of your first post Tyler, if the egr was blocked with carbon or similar, would that make the MAF read higher than expected and skew the readings. Low egr flow should set a code though?!
So the higher the hertz the higher the pressure, which is lower altitude?

Noah wrote: It starts out OK after the repair and the KAM is cleared. At the time of the "correct" reading, STFT is high, but is soon countered for by LTFT and everything seems to settle into an acceptable range. MAF looks to be reporting within range at WOT (what I assume is WOT by the TPS anyway) maybe a smidge low....
I've been trying to draw a correlation to the Baro PID update to MAF and TPS values, but they aren't consistently changing together....
Very curious...

Yeah, I noticed how the trims started out too high, then worked out once the Baro and long term started learning. I've never seen a Ford with a weighted LTFT value, so I think the Baro is indeed playing with trims. It just so happens to be in the right direction, lol.

Matt, fair question, I actually don't remember what the DPFE was reading during the test drive. I can't view ScanShare.io on my phone, but I think that would answer a lot of questions about the EGR system. I'll check back later.

Exactly right about the Baro frequency. If I'm thinking about it correctly, then a higher barometric pressure would require more fuel to match the denser air.

The Baro PID updates (going higher), computer adds more fuel, trims go closer to zero. So then, the engine would actually be lean if the Baro would update correctly...

Update outa nowhere!

Just thought I'd share some other data I gathered. Had a different '01 Focus in the shop recently, oil change and A/C concern, no driveability problems. Just HAD to take a peek at the Baro reading:



Also on the high side... Makes me wonder if there's something about this engine application that causes that?

Then yesterday, diagnosing a misfire on an '04 F-150. The Baro didn't have much to do with the problem, but I took a glance anyway:



Little bit low. Again, no breathing problems that I'm aware of.

Honestly, I'm not very confident in the Baro calculation any more. Not that I placed a ton of faith in that PID in the first place, but it seems like there's too many variables at work to get a consistent Baro calculation. Any thoughts/comments you have would be welcome!