Map sensor failure symptoms.

[AnotherPerson]

Trying to judge a couple things on my car and this is kinda a 2 part question. When a map sensor is failing has anyone ever had it report higher boost than it's getting?

Second part, if the map sensor is saying to the ecu I'm getting 20lb of boost does the ecu fuel for 20lb of boost? I know newer cars can adjust but didn't know if the alh based fueling on sensors or if it was just set throttle position and rpm maps for fuel.


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[oilhammer]

Never seen a MAP sensor fail on an ALH. Ever. So I could not tell you the symptoms.

Obviously if it is plugged in and the circuit is complete, you'll have no DTCs, so that much is OK. The question is, is it reading the correct value? The only way you could verify that would be with a manual boost gauge.

Depending on the software, the ECU which is controlling the boost, will fuel accordingly but a tuner would be better to answer that question.

Perhaps some context to your question is in order. Are you trying to fix a problem that doesn't exist, or are you just curious how the system works in general?

 

[AnotherPerson]

Still trying to figure out my bad mpg. Vcds shows requested and actual almost spot on. Before my tune, lots of smoke 30mpg. After tune. Much less smoke 30mpg. Just cleaned the turbo because vanes were sticky from before I replaced the injectors and a pretty old and dirty air filter,30mpg. We have gone through almost everything and the only thing left is the map sensor. At upto like 3k rpm the car seems to be fine. Above 3k it blows a lot of smoke. Which is why I'm thinking maybe it isn't actually getting the boost it should for the fuel. Vanes this time we're at the point where sometimes it would do that little chirp when it would break loose.

Wish someone would have made a steering wheel pod for my car and I would have a boost gauge. But the only ones I come across out there are so expensive it's a joke for a piece of plastic.


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[oilhammer]

Did you go through and check off everything in my sticky thread in the fuel economy section?

 

[eddieleephd]

Haven't read checked timing.
I find that advanced is necessary for efficiency and EGT's.

Timing needs to be played with to get right.

 

[Shoveltrev]

had one fail on the 2002 bug at 200000 miles . it was easy to diagnose it showed boost before you started the engine. when i changed it out it was full of oil. was kinda slutty to change on the bug as its mounted in the intercooler

 

[turbocharged798]

Turn the key on and don't start it, MAP should very close to the baro sensor.

 

[AnotherPerson]

Have had the map off previously so I know it's a pain. What vcds groups to compare with engine off for baro and map?. Timing is good. Even my brake drag issue is 100% fixed now. I know people will always look back on that one lol. I feel like maybe the map is showing High at boost and more normal at idle.


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[Enabled]

No, the ECU does not add extra fueling if there is more air. It's only negative feedback.

Does your intake manifold have carbon deposits/buildup?

 

[AnotherPerson]

Intake is clean. So it wouldn't fuel for 18lb of boost even if it's actually getting 14? I would have figured if it was calling for 18lb of boost it would fuel for it if the car said it was getting it. The n75 doesn't have set levels for boost correct? Like it will adjust pressure for what psi is requested based on map?


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[turbocharged798]

It fuels based on MAF, not MAP. Yes it will adjust the N75 based on MAP but to a point. It compares it to a set value and adjusts it as needed.

Just scroll though the VCDS groups, one of them will be the baro and MAP readings.

 

[AnotherPerson]

Alright, do you know what the variance has to be to throw limp mode?


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[Enabled]

In static air to fuel ratio cars (gasoline cars), the ECU will add more fuel when there is more air measured because it has to keep the engine between 12:1 - 14.7:1 afr. Any leaner and it will get damaged.

In diesels, you can go as lean as you want without damage. Diesels do not have a programmed positive feedback. It will fuel the same.

 

[eddieleephd]

Alright, do you know what the variance has to be to throw limp mode?
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I can't say exactly, however, it's probably in excess of 225 to 300 more than requested.
I have mine set for 150ish over and had a sticky actuator for a while causing a regular overboost that didn't throw it into limp mode.
Something makes me think it's actually more than a set limit of boost.
Over 18psi for a stock car and maybe more than 26psi for my tuned car set to around 26psi.

 

[AnotherPerson]

Malone emailed me back even though there on vacation. Said its a mix of the map and maf that do fueling. May give it a couple weeks and just let malone send me some new chips for a 3bar than change it to a 3bar.

 

[eddieleephd]

3 bar is better, then you know it's good.
Order a maf and 3bar now so you're ready.

 

[AnotherPerson]

Got a new Maf already. Did have it reading low at higher rpm prior. Gonna get some chips from Malone than do the 3 bar. They are on vacation so I got a couple weeks


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[AnotherPerson]

Here's a question. If the tune was setup for a 3bar instead of the 2.5. Would running the 2.5 cause it to run very rich?


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[eddieleephd]

This is a question for Malone.

They told me that the sensor reads different all together and isn't compatible with stock sensor tunes.

You'll have to keep the original and reinstall to go back.

 

[Enabled]

Putting a 3bar sensor on a 2.5bar designed tune will artificially raise the amount of boost. (But no changes in fueling so it will do nothing to power, keep in mind).

These sensors work on 0-5V.
A 2.5bar sensor will be around 2.5Bar at 5V, while a 3Bar will have 3bar at around 5V.

The tune just redefines the pressure definition at 5V.
If the scaling is wrong, the ECU will not know the correct boost level and try to correct (upwards in your scenario), giving you a risk of overrunning the turbo.

 

[AnotherPerson]

I don't know much on bars. How many psi would that be under where it should be if it was the other way around?


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[vanbcguy]

1 bar = 1 "barometer" or atmosphere = 14.7 PSI at sea level(ish)

These are absolute pressue sensors, with no boost it should read about 1 bar as it's exposed to the atmosphere. So a 2.5 bar sensor is good for 1.5 bar, or about 22 PSI of boost. A 3 bar sensor is good for about 29 PSI. You actually don't want a tune that takes you right to the edge of the sensor limit though, as once it hits the limit it can't tell if it is 0.001 bar over or 10 bar over. The ECU can no longer act in a proportional manner.

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[Nevada_TDI]

Another Person, to answer your last question; yes it is possible running a 2.5 bar MAP sensor on a 3 tune could very likely cause your engine to run smokey.
And historically speaking, MAP sensors don't usually die. But if they never died, why are replacement units sold to fix the bad ones? And they are cheap enough to replace if all other option have been depleted.