What do you guys think? I did my second WOT run today and I found out my MAP sensor dropped the Baro Reading for about two seconds during the WOT run then it went back up to normal reading....

On my first Scanned WOT run my MAP sensor did report a drop of the reading for one full second and it happened twice during the WOT run... it was a very minor drop so I thought it was a scanner problem. On my WOT run today, it reported a drop the Barometric pressure that was more severe. It happened once but lasted 1.5 seconds before it recovered and it was fine till the end fo the Run.


Todays readings were... Per half second readings:

0.. 4.90 volts
1.. 4.90 volts
2.. 4.90 volts
3.. 4.88 volts *
4.. 4.88 volts *
5.. 4.84 volts *
6.. 4.90 volts
7.. 4.90 volts

and it remained at 4.90 till the end of the run.

I havent seen it drop the Baro during normal city driving. Have you guys seen this before? Could it be a loose wire? or a bad MAP?

What do you guys think?


Marvin

 

my vote goes for a loose wire.

Anyone else have any ideas?

 

When you asked about what you thought was a low MAP sensor reading, I suggested that you check the connector, since they "crumble" with age. Just curious.... have you checked out the connector and the wiring yet?

Confusing terms..... when you keep saying "it dropped the Baro reading" do you mean that you were getting an MAP reading that appeared to be roughly the same as the BAR value? You realize of course that the typical BAR reading isn't a whole lot differnet than the +5V bias signal that the PCM provides. Sounds like there is a short between the +5V wire and the signal wire, giving you "0" resistance, and returning the bias volts via the signal wire. This short could be internal to the sensor, or external, in the connector or wiring.

But if you don't follow up on suggestions of things to check, you may never find the problem.

 

They're $35 at O'reilly's. Id replace it instead of wondering. Most likely its the cause, it was in my case.

 

Fred, my scanner reports "both" BAR (Atmospheric (barometric) pressure) and MAP sensor voltage (intake manifold vacum).

1) The BAR (Atmospheric/barometric pressure) Is the reading that I have a problem with. It fluctuates ramdomly during a WOT run and at idle when I gun the engine. "That value should not change unless I am either gaining or loosing altitude." Am I correct?

However If I quickly push the gas pedal to the floor then the reported value will fluctuate and it will stay there even if I turn the engine off and back on again. I would define it as being "Sticky" in my lame terms. For example, The reported reading will change each time I gun the engine. Is like saying one second the sensor tells the PCM Im am at 10 feet above sea level... and "if" I gun the engine again then I am a 100feet above sea level. Keep gunning the engine and it will keep changing between +4.90 volts and +4.82 volts and somewhere in between. (without moving from the parking spot)

2) The MAP voltage reading (intake Manifold pressure) as we all know it, will fluctuate with engine load. At idle my car reads about 20 inches Hg.

I compared my scanner reading with my mechanical vacum gage reading and both agree with each other. Both scanner and mechanical vacum gage readings follow each other perfectly. I dont see any problem there.


3) Yes I did it Fred and you are DAM RIGHT when you say the connector "crumbles" I broke the little tab that secures the connector to the MAP sensor. I tested the conector and wiring with the wiggle test. Nothing changed. It seems the connection to the MAP sensor is solid. Actualy I hoped the wires or connector were loose. No luck there. The reading was rock solid.


4) I know what you mean Fred. My intake manifold vacum reading is just fine. Your explanation was of great help and now I understand.

What I am referring to is the Atmospheric Pressure reading or how about if I explain it like this...

1 atm = One Atmosphere or 101325 pa or 760mm Hg or roughly 15 psi at Sea Level. Thats what I call The Barometric Pressure Reading. My scanner reports it as a max of +4.90 volts at Sea Level.


I Think the confussion here is that I am not longer talking about Intake manifold vacum any more. You explained that to me in a previous post and thanks to you now I know my Vacum readings are fine.


Going back to my Barometric pressure readings (BAR), I know if I park two feet above sea level my BAR reading should be as close to +5v as the Sensor will allow. Whe I did it, my Highest BAR reading was +4.90 volts, and let me tell you here in Hawaii I have parked at what appear to be Below Sea Level. (look like it) Cant get any lower than that and never go higher than +4.90 volts. I am ok with that. Its the reading fluctuations at WOT and when I gun the engine that bother me.

Just yesterday I was at an elevation of about 300 feet Above Sea level My scanner reported +4.90 volts/+4.88 volts/+4.82 volts/+4.90 volts and +4.88 Volts within 6 seconds while I was gunnin the engine. I am not talking about engine vacum ok? (just making sure we are on the same page). I at work gunnin the engine in park. This BAR reading should not have changed at all since I wasnt even moving. It was like reporting different altitudes each time I reved her up.

Also it seems to be sticky. Once a new value is set it stays there till I gun the engine again. I know the engine moves in the engine bay and I am still hoping is just a bad conection. If it turns out to be the Sensor I want to share the info to other members.


Fred forgive the lack of Technical terms in my vocabulary, its hard for me to describe the problem. English is my second language and I know I suck at it so please bear with me if you can. and If you cant, dont worry. I am kinda mentally disturbed anyway so I wouldnt even take it personally. (jest pulling yer chain)

Marvin

 

Meangreen I would love to succesfully diagnose a faulty MAP sensor and what exactly was the problem, how I found out about it... what to look for and then Share it with our fellow Board members...

What symptoms did you get and how did you diagnosed your MAP sensor?


Thanks for your info by the way.


marvin

 

OK, now that you explained it, it makes sense. I can't explain it. From my experience, the MAP sensor reports the "BAR" value before you start the engine, and that value appears to be stored in the PCM. I have seen numerous data logs that people send me, and BAR never changes over the course of maybe 10-15 minutes of operation.

But... what if it does change minutely.... what is it going to affect? The mass-flow fuel management system does not rely on BAr for fueling. It has the actual mass flow.... it doesn't even need the MAP as a speed-density does. Yes, it uses MAP as an ordinate for timing lookup and other corrections. But BAR doesn't seem to be particularly relevant, unless you are comparing BAR to MAP to try and identify pressure losses in you air induction system.

I really can't explain why your BAR is fluctuating all over the place. Perhaps it is an idiosyncrasy of you scanner. Have you tried reading BAR with another device?

 

Not yet. At least not in my car. I did wiggle the conection between my scanner and the car for possible loose wires and that did not affect anything.

I am glad to hear the PCM does not use the BAR reading to calculate air density and fuel delivery.

I have an EZ-scan 6000 that pretty much plugs up to any PCM out there. OBD1, OBD2, CAN. etc. It came with several conection cords. I will find a willing buddy to scan his/her car and see if the BAR readings fluctuate while gunning the Engine.

The reason I bought this type of scanner is because I will be looking to buy a couple of new cars in the next 5 years. One for my daily driving and one for my future Wife's daily driving too. I wanna be able to service all cars with just one scanner.

I am sure the BAR reading fluctuations are of very little effect on my car since It feels good to drive. I have a 14* knock retard when I do WOT runs though and If I get to fix the problem I wanna do it in an intelligent way as opposed to the Auto-dealer style of just throwing ignition and fuel parts at it. If I get the problem fixed I wanna know exactly what caused it and post it here for our members reference.

As soon as my gas tank is empty I will fill her up with unleaded racing fuel and re-test at WOT again. We'll see if I still get the 14* Knock retard again.

Hey I actually like doing this *investigation* type of work. Its fun and its fun to see peoples faces when they see my scanner hooked up to my car. I told one young dude and Im an engineer for GM and was doing a field test on Emissions and Fuel comsuption per delivered Horse Power.

He looked disapointed when I told him I was just kidding



Anyway. Thanks for your help Fred. You are very knowledable and you dont mind helping other people. That is nice of youl.

Marvin

 

Yeah thats the neat part about using a scanner. I noticed how my car begins to develop a small amount of vacum once it hits 2700RPMs... I can tell how bad the air induction restriction is and/or how good a Cold Air Kit will compare.

Even reading Air Mass Flow helps when you ad bolt ons.


Anyway. I will do short posts of my progress from now on.

Marvin