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Old 09-14-2001, 11:16 PM   #1
amelnikov
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Alex Performance

Angry Overboost?

Hi guys. I have a qestion for the gurus. I'm new to the MAF equiped turbo cars. I know how to deal with MAP's. I upped the boost a few lbs on my WRX and it was fine for almost a week. The highest it went was around 15-16 in higher gears. Well today it was raining and was cool and I got what flet like overboost shutdown. Engine check lite went on and the car was doing it every time I tried to step on it. I checked the codes and it gave me MAF signal too high. I cleared it and later in the day when it stopped raining and got a little wormer it did not do it again. Is this the overboost shutdown? Even though the boost never got above 16-17 or is this something that people that put cold air on experience and I need to reset my computer to let it learn the colder weather?

Thanks,

Alex...
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Old 09-15-2001, 12:24 AM   #2
JenisonWRX
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Well...I'll just share an experience with you...

Last night was the first night here in Michigan that was 'cold'. It idled like a dream. Very smooth...no sputter...first gear...oh boy...this thing feeling good. Then second gear...if I eased on the gas...it would lurch forward a few times...then go. But if I mashed on the gas...holy frick! it shot like a rocket!

I'm guessing...with the colder air at partial throttle...it jsut wasnt getting enough gas. I didn't get any CEL's...but man...it freaked me out. But always..when I mashed on the gas. Watch out...shes runnin like a bat out of hell.

Eventually your ecu should learn...but it wouldn't be a bad idea...

I just have a cold air intake right now...but I did acutally think about resetting the ecu because of the new colder air...
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Old 09-15-2001, 08:18 AM   #3
Jan Shim
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Alex,

Change in ambient temps isn't likely to cause the ECU to throw a fit like that. I have run my WRX in both MAF (hot wire) and MAP and have never experience cut outs like you described. I feel i am qualified to say this as the temp fluctuations between a perfectly hot day (91.4 F) and rainy day (69.8 F) is rather significant. The ECU makes automatic fuel compensation to maintain correct A/F ratios, as seen on my LambdaLink.

There is a chance the AFM wire element came in contact with water moisture momentarily throwing it off balance until it achieved stabilisation over a period of time. I run an Apexi Power Intake air cleaner and last month, i drove into a pool of water, water splashed and wet the entire engine compartment including the air cleaner ... shortly after that, the car began to buck everytime I attempted WOT. Since it was raining heavily, I continued driving slowly until after half an hour later when the engine is hot and dry again, the problem disappeared completely.


Jan
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Old 09-15-2001, 01:47 PM   #4
YeeHa
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Quote:
Originally posted by Jan Shim
i drove into a pool of water, water splashed and wet the entire engine compartment including the air cleaner ... shortly after that, the car began to buck everytime I attempted WOT. Since it was raining heavily, I continued driving slowly until after half an hour later when the engine is hot and dry again, the problem disappeared completely.


Jan
Had the same problem after going through a touchless carwash. Engine warmed up and it went away. I fixed the problem by washing my car at home.
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Old 09-15-2001, 02:00 PM   #5
IMPREZIV
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Default Like this?!

Hey guys, check out my post. I dont mean to be cross posting, but I wanted to make sure this is the same problem I was having.

http://www.i-club.com/forums/showthr...threadid=92052

Thanks,
Pete

It was raining yesterday, so I am assuming that something got a little wet, and it was cold out. It was wierd.
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Old 09-16-2001, 12:14 PM   #6
dragonwagon
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I did some calculations on air temp and boost for a given air flow. I used extremes. 20 degrees F. and 130 degrees F.
20 pounds of boost at 20 degrees gave the same airflow as 42 pounds of boost at 130 degrees. Temerature does make a difference. During the hot months extra boost wasn't a problem however at colder temps the turbo will run out of steam. The good part is that lower boost can be run to give the same power in the winter. Be careful about higher boost in colder air. the engine will lean out a whole lot quicker now.
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Old 09-16-2001, 03:52 PM   #7
procrustes
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Dragon - you are correct - temp does make a difference, however you may want to check your math (use Kelvin & absolute pressure). I think equiv. boost at 130 will be 28psi.

Also I have experienced this problem on other (overboosted) turbo cars - including ones w/o a wire MAF sensor - it feels similar to detonation or a lack of fuel/ignition on full boost & occurs particulary on wet/cold mornings but goes away once the car is warmed up a bit. I don't think it should be due to lack of fuel on the WRX, which we all know can run pretty rich - but I don't know what it is due to. I would be very interested to know the cause of this behavior.

Bob
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Old 09-16-2001, 04:00 PM   #8
Sophocles
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How do you deal with the temperature differences? I hope your solution was in no way related to your namesake's.
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Old 09-16-2001, 04:15 PM   #9
hkwan
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What to in winter if we have MBC installed? Let's say we have the MBC set up @ 16psi, 4th, 5th gear (other gears = ~14psi), do we actually have to tune that lower during winter?
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Old 09-16-2001, 05:26 PM   #10
dragonwagon
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I have been locking at graphs on some turbochargers and it looks like there is always a point with any turbo that you run outside the rating of the turbo. It might be 20 below zero that there is a limit. The car will reduce boost to prevent high MAF cutouts. An external boost controller is were the problem can come into play. It will always look for the same pressure, regardless of temperature. The high MAF reading is probably the same as a fuel cutout on overboost because they are the same as far as air into the engine. MAF being the air density, which creates HP.

My calculations could be off but I wanted to show the effect of temperature.
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Old 09-16-2001, 08:06 PM   #11
Jan Shim
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Temps do make a difference in the context that you guys are now discussing-opposite extremes. The problem Alex had was clearly had to do with the momentary 'contaminated' air flow signal from water moisture which is not uncommon when it's "raining and cool".

Quote:
Engine check lite went on and the car was doing it every time I tried to step on it. I checked the codes and it gave me MAF signal too high. I cleared it and later in the day when it stopped raining and got a little wormer it did not do it again.
The limits of the stock ecu to deal with such wide temp flucuations is not known except for cold start enrichment that goes way below 0'C. The tendency to overboost is greater with temp drops. Those of you who have a completely stock boost setup and living in very cold climates, does it overboost to point of hitting the MAP limit (whatever the value may be)? The Subaru has both MAF and MAP and as far as i'm aware, MAF is not responsible for boost cuts, only MAP.
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