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Does the ECU really limit boost when cold? at what temp?

Discussion in 'A3/S3 Forum (8L Chassis)' started by Prawn, Nov 30, 2009.

  1. Prawn
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    Prawn My other car is a MINI!!!!

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    Hey guys, this is purely for my own interest, as it's something I've been wondering for a while.

    It's commonly said on here that the ECU limits boost when the engine is cold, I've even said it to others myself before, but I do not know the exact details of how it does this.

    Does the ECU definately limit boost (obviously via the N75) when the temp sensor senses cold water in the system, and if so, how much does it limit it by, and what is the cut off temperature at which it allows full boost? it it a stepped limit of Low boost then full boost above a given temp, or is it incremental?

    The reason I ask is this:

    my stat is stuck open, I know this, and it takes AGES to warm up.

    Today, I left work, and the car heated up to 90 whilst stood in traffic, I floored it comming onto the A31, and it flew, boosting 19psi

    After 5 mins cruising along the A31, due to the VERY cold winter air and an open stat, the car had cooled to 61 degrees (climate49 viewing). obviously the block and oil were warm after 25 mins driving, but the coolant not so.

    Comming off a roundabout onto the A331 I floored it again, and it felt a little sluggish, and only boosted to around 14psi, instead of the typical 19-20.

    then, at the end of my journey, in slow moving traffic in town, it warmed back up to around 85, and the boost was back in full.

    Sorry for the randomness of this post, I know I need to change my stat, I'm just intereted to know the exact details of how much it's limited, and at what temperature this limit is lifted.

    Thanks for reading my epic long post

    Prawny
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  2. jojo
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    jojo S3 Drift King! Staff Member Moderator

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    I always thought it was the other way round Prawn, the turbo increases boost in warm weather to make the same power with warmer air for standard cars, but once the car is remapped, then it doesn't do it anymore?
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  3. Prawn
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    Prawn My other car is a MINI!!!!

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    I'm talking about the coolant being cold jojo, like when you've just started up in the morning.

    my thinking is that although the car has warmed up, it's thinking it's not yet warm when the coolant temp drops so far due to the weather and stuck open stat.

    if my stat worked and it stayed at 90 degrees I don't think I'd have the problem
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  4. god_thats_quick
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    god_thats_quick Numptie of the highest order

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    Na, don't believe any of the 1.8t engines prevent you boosting before the water / oil are up to temp! Would probably be sensible if they did mind.
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  5. Westy
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    Westy Double Dark Side! Diesel & 8P

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    Prawn, I've always thought that there might be some sort of a warm up cycle on my S3 but its never been confirmed. The only reason I thought the boost was restricted on warm up was because I get a fluttering noise from the under the bonnet if I boost up and the car has just been turned on. After a few miles the fluttering goes away!

    I've had the dump valve checked and I've also had the whole boost system pressure tested with no faults. I could only imagine that the N75 was releasing some of the boost!
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  6. 1animal1
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    1animal1 The Clar!! it mouves!!!

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    never noticed a difference myself..... know the reasons why you should warm a car up but i run my S3 like i did my A3..... boot as and when i feel, hot or cold.... that car lasted 6 years, i got her at 30k miles and sold her with 120k on the clock...man that car took some hammer :)
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  7. ianysm
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    ianysm Active Member

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    Remind me never to buy a car from you then!!

    You should always let the oil temp reach operating temp before 'booting' it. The M3 had lights around the rev counter which lit as the engine warmed, it also limited revs to this point but it was based on coolant temp, not oil temp which is useless IMO

    I agree with Prawny though, mine seems to do the same thing. In fact, it did it this morningwhile nipping into a gap in traffic, felt like it had nothing, when i looked at the temp needle it was around 60 ish. Think i'm due a change of sender or thermostat too!
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  8. 1animal1
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    1animal1 The Clar!! it mouves!!!

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    haha i tend to run my cars into the ground though..... they are upkept properly and i will normally try and give it a couple of miles to let the oil circulate....
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  9. ianysm
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    ianysm Active Member

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    Only a couple of miles? It must take about 5-10 before its fully bootable! Frustrating as about 2 miles from my house is a nice slip road onto a dual carriageway...But i still don't boot it until its warmed, even though the Focus ST the other day thought he'd thrashed me...I'll see him again one day and show him!!
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  10. Westy
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    Westy Double Dark Side! Diesel & 8P

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    I've been caught on warm up by plenty of motors because its only a 4 mile journey from home to work and work to home :keule:
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  11. ianysm
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    ianysm Active Member

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    Its soooo frustrating when you know you will be the faster car (but they assume you're running standard!)

    Still, my face must have been a picture to the bloke in the 480bhp TT i met! He pulled up behind me and i wrongly assumed his was standard or at best stage one, then he blew me away!

    Anyway...
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  12. Westy
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    Westy Double Dark Side! Diesel & 8P

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    I find its best to prey on NA engined cars, that way you know that they will be rarely tuned. Anyone running a turbo these days has a stage 1 map as a minimum!
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  13. ianysm
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    ianysm Active Member

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    I seem to find alot of Turbo cars non mapped. Seems people are too scared to do it or can't justify the cost. Only enthusiasts tend to bother with mods.

    Now though, it seems everyone is too scared to even 'drive' their performance cars, so whats the point in them owning one!
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  14. MintyS3
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    MintyS3 Works 60% the time, Everytime.

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    Im not sure if my boost is lacking when car is cool but certaonly my dv makes a slightly different noise almost like the boost has been reduced.
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  15. Westy
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    Westy Double Dark Side! Diesel & 8P

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    This time of year is the best time for us quattro cars, we can muller most FWD cars in the cold and damp :thumbsup:

    What sort of noise do you get Minty? I get a flutter as the turbo is spooling not when I let of! This noise goes after a few mins of warming up :uhm:
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  16. 1animal1
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    1animal1 The Clar!! it mouves!!!

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    mine makes a nice wurring noise in the mornings.... you can hear it spin off boost when travelling the 15mph off my estate :)
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  17. HTC
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    HTC Active Member

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    I've never heard mention of a direct correlation boost and coolant temperature. Email Bill Brockbank and I'm sure he'll know.

    For later engines the MAP records spec and refers it to the N75 and/or N249 (to open the DV)

    On early AGUs it's via the knock sensor.

    As far as I'm aware you need less boost in cold situations to force the same amount of air due to the raised density.
    Last edited: Dec 4, 2009
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  18. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator

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    I know my A4 certainly doesn't. The ECU is too stupid to know if it's warm or hot TBH.
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  19. S3KO4
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    S3KO4 Member

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    I have noticed my car was under boosting when coolant temp was cold, slow and slugish took forever to warm up and consumed more petrol, replaced the thermostat that helped alot.
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  20. ianysm
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    ianysm Active Member

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    Just took mine out this lunch and it felt pretty kick-in-the -back ish so obviously isn't related. More likely to do with atmospheric conditions i assume?

    I guess if MAF/DV/Thermostat were to be slowly failing you would be so used to it you wouldn't notice. Might be worth changing some bits to see if it helps

    How much are MAF's?
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