I have never been rude to you , you do the job correctly and show professionalism.

I think you have identified the reason this car has a running problem.

Barometric pressure - is the clue.

68 millibar the PCM thinks the car is at 25,000 feet above sea level.

Barometric pressure (BARO) is inferred by the PCM software at part throttle and WOT there is no actual BARO sensor on MAF-equipped vehicles.

At high air flows, a faulty MAF sensor will under-estimate air flow coming into the engine, hence the PCM infers that the vehicle is operating at a higher altitude. The BARO reading is stored in Keep Alive Memory (KAM). What are Long Term Fuel Trims and short term trims like.

I would fit a new MAF and leave the PCM unplugged for 30min's , should clear down the KAM. But I have had some PCM's that would only seem to clear when instructed by WDS / IDS.

switch ignition back on without starting and the barometric PID should be around 998Mb also there may be a frequency PID for Baro that should 158.9hz.

Also one to remember on Ford ECC IV and V disconnect the MAF sensor connector. This puts the vehicle into Failure Mode and Effects Management some people also call it LOS (limited operating stratiges) .

In FMEM mode, air flow is inferred by using rpm and throttle position instead of reading the MAF sensor. In addition, the BARO value is reset to a base/unlearned value. If the lean driveability symptoms go away, the MAF sensor is probably duff.

HTH

Darren

Anyways

Nigel

The HEGO is showing a very lean voltage, during the hesitant period. This changes the instant that it clears. At 4,200 RPM it will pick up, like you have flicked a switch. It maybe something in the mapping, that causes this characteristic. What I mean is that maybe the system is designed to ignore certain parameters (such as Barometric values) at certain engine speeds & throttle openings & this maybe why I feel a sudden increase in performance at that point. (probably talking bolox again)

Darren

FMEM hey! Thats a new one on me . I will remember to throw that into a chat with like minded techs & see if I get any blank looks.

I take on board what you are saying about the inffered value, but I have a MAP style sensor fitted to the nearside inner wing that needs an explanation. It maybe something to do with the auto box I suppose. I found a passage in a tech document about what you said & it offers something slightly different.....

"""BARO must be inferred, usually at high throttle openings or is derived from a MAP/BARO sensor."""

Maybe this one does have a Baro sensor. Either way, you can bet that I will try your suggestions to see what happens.

Nick

I take your point about scoping back at the ECU. I have been caught out recently when I witnessed a good capture taken from the sensor end, only to discover a problem when looking from the ECU end. Its harder work, but ultimately more useful to do it the way you suggest. I will try harder in future .

I may not see this again for a week or two, but I need to sort it, for my own sanity.

Thanks again for the thoughts

Dave (Hill)

As I said before here or on ilexa this car can have a map sensor as I have had several with one. It is not related to gearbox but engine although if its eec5 it controls gearbox anyway.

So what you found looks identical to any other ford map sensor IE like single point one or dohc 2.0.

Infact it is the same and one of those would work fine so beg steal or borrow one to prove your live data.

John

Thanks for the thought. If the pressure sensor is infact the actual barometric sensor, then wouldn’t it update “live” in live data & not like the “snapshot” reading taken from the MAF setup? I unplugged the sensor & there was no shift from 68mb & no fault code logged. Also I did get a digital signal capture from the sensor that looked ok.

I was thinking about what Darren said, regarding Barometric pressure being inferred. I am struggling to see how the PCM can determine this value from the MAF sensor. From my understanding, the MAF calculates airflow by looking at the amount of current required to maintain a temperature of 200ºC in the "Hot Wire" (crudely explained). If a Barometric pressure value is to be calculated at start-up & wide throttle openings, I would be interested to know how, as this is not a pressure transducer.

I have just been looking at Ford TIS & reading about the VI system (variable inlet system). I do recall seeing the letters "vi" on the inlet manifold. TIS also mentions secondary air injection, which I maybe haven’t looked at hard enough

I am looking forward to getting my hands on this car again, so that these questions can be answered

Cheers again.

Dave (Hill)

The Key word you used is "calculated".

The ECC V system determines the air charge / Mass / Density, going into the engine from a measured value of mass airflow. It compares this value with a predicted value of air mass going into the engine,this is the value the PCM infers. Differences between the two values are first attributed to inlet air temperature, which is measured, and then to a change in barometric pressure, which then becomes the inferred barometric pressure.

So basically the PCM as stored Tables and "3D MAPs" which is cross checks the The measured MAF readings with the desired readings.

So if the Air flow readings are lower than predicted at lets say Idle , half or full throttle then it infers the car must be at high altitude and thus a lower Barometric pressure.

The higher you go the the air pressure get less , thinner and also less dense.

2.o litre has an airflow meter and a barometric pressure sensor attached near chassis rail nearside.

John

Just been looking at some old training notes i have and just noticed there is a difference between the manual and auto gearbox vehicles.

The manual gearbox type will have NO baro / Map sensor . just MAF (inferred Baro)

The auto gearbox type will have MAF sensor and a Baro / Map sensor. But the MAP / Baro sensor is used for the gearbox control and not the engine control.(that's how it reads to me )

Both manual and auto gearbox have a exhaust pressures sensor , that could be confused for a MAP sensor.

So I reckon John's car today was a auto gearbox.

Dave if your car is a auto gearbox the pins at the ECU are

pin 91 - sensor ground
pin 90 - 5 volt reference
pin 34 - frequency input ( normal 158.5 Hz is 998Mbar)

So have a play with a signal generator and send a signal into the PCM on pin 34 and check your live data.......

HTH

Darren............

yes mine was auto but when it had a problem a few months ago it caught me by surprise.

It was totally flat and backfired through inlet, extremely week mixture.

Tried maf as it seemed obvious, checked fuel pressure then reading through live data found barometric reading very low I think about 700 mbar.

Thought strange so found sensor I did not know existed and fitted substitute. Read about 1 bar and went like stink. Put the old one back ran terrible.

So the ecu does use it for fuelling obviously reducing fuelling to take affect of lower density of air at lower pressures.

What surprised me is that software allowed it to accept 700mbar as a sensible reading and not default.

Incidentally disconnecting made it default to about 1 bar and ran ok.

This may not be the problem with the current car but just wanted to clarify position incase anyone else has the problem and reads these posts.

Best regards

John

Sounds good to me...........

Finally got my hands on this old Scorpio.

I took the advice given & disconnected the battery & whilst waiting I dropped the PCM (easier said than done) & checked continuity between the pressure sensor on the nearside inner wing/chassis area. The wiring agreed with pin data & continuity was good.

After a couple of hours I reconnected the battery but this time I left the pressure sensor unplugged. The value displayed now is 101Kpa (1010mb) The following road test proved to be very positive, with a good full throttle injection PW of around 12ms.

I shall be happy if it is the same from a true cold start in the morning, but I feel sure that it is sorted now.

Again I have achieved a fix that I otherwise would have struggled with. Thanks very much for the professional input guys!

Cheers

Dave (Hill)

So what will you be replacing the MAF or MAP sensor?

Or are you going to see if it happens again before replacing these items?

Cheers

Sean

Neither mate!

Is that a trick question? As it hasn't got a MAP (manifold absolute pressure) sensor & the MAF sensor seems to have no role to play in this set-up.

I may be being a bit pedantic but I know what you guys are like for the details!

I will be leaving the barometric sensor unplugged for a while to see how it goes. The owner is an old guy who only travels locally. As I don’t think he will be experiencing any significant altitude changes in the near future, I don’t see the need to fit any new parts at this stage.

Cheers for now

Dave (Hill)

Sorry, I meant the "barometric sensor" when I said MAP sensor.....my mistake

I'd do the same, try it for a while and see how it goes.

Cheers

Sean