help; acceleration problems

esinger · Post by **esinger** » Wed Sep 17, 2003 5:27 pm

Hi,

I have a 94 legacy turbo that just hit 100k miles, and I've recently been having problems when I accelerate. Often, if I am driving at a steady speed and then accelerate, the car really hesitates...kind of jerks and doesn't pick up any speed - if I back off the gas and then hit it again, it zips away just fine. If I start to accelerate slowly and then hit it, there are no problems - it just when I try to accelerate from a steady speed. The car certainly drives fine otherwise. Any ideas? Could this be a boost problem?

boostjunkie · Post by **boostjunkie** » Wed Sep 17, 2003 5:39 pm

Actually, this sounds like a similar problem that I'm having, although I've been checking my a/f ratios lately and have noticed that if I slam on the gas I'll get the a/f ratios to drop OFF THE gauge (as in VERY, VERY, lean - fuel cut-like). If I roll on the gas it's a little better, but sometimes it'll still hesitate.

Post by **Legacy777** » Thu Sep 18, 2003 1:52 pm

have you done any recent maintenance? Changed plugs, wires, fuel filter, etc?

If not....may want to start there.

LegacyPunk · Post by **LegacyPunk** » Thu Sep 18, 2003 2:25 pm

Might be your fuel pump. I had a problem a while back when i would accelerate normaly and the car would kind of bog down like the fuel was cutting out, then one day my fuel pump died. But i got a new one and it fixed everything, even got better gas mileage

Brat4by4 · Post by **Brat4by4** » Thu Sep 18, 2003 4:29 pm

Get both fuel filters changed right away. Do a basic tune up - plugs, wires. Then reset the ECU.

The ECU will go lean for an instant before it switches to closed loop static fuel maps. Something might be exaggerating that switch...

NuwanD · Post by **NuwanD** » Thu Sep 18, 2003 5:07 pm

sounds like a possible O2 sensor issue from past experience. Any chance this happened after it rained? i've noticed this occur when the car's been in a wet environment for a while.

good luck

FreddexTurbo2Go · Post by **FreddexTurbo2Go** » Thu Sep 18, 2003 9:02 pm

Re. hesitation I found this from Sport Compact Car very interesting. This was about the new WRX but I wouldn't be surprised if the same applies to our Legacy Turbos.

The magic box
We've been driving Project WRX with an early version of Vishnu's Stage Zero upgrade package (turbo up-pipe, underdrive pulley, colder spark plugs and a re-tune with various piggyback computers) for more than six months, and while we've enjoyed the huge improvement in power and responsiveness, there was one driveability issue that started bothering us shortly after our last update. Accelerating on boost at part throttle, the powerband felt lumpy. Instead of a smooth surge of power, it seemed to come in little hits and surges. It turns out this is an artifact of Subaru's engine management strategy not being entirely compatible with aftermarket boost controllers.
The stock ECU normally controls boost, and in what must be an effort to dodge an emissions bullet, the ECU won't allow full boost unless you give it more than half throttle (55-percent throttle to be exact). The stock ECU also runs closed loop at these light throttle settings, meaning it constantly looks at the O2 sensor to keep the air/fuel ratio at 14.7:1. With the manual boost controller we have, or just about any electronic boost controller, you can get full boost at part throttle. This greatly improves throttle response and makes the car much more fun to drive, but if you don't mash the pedal, the ECU will still stay closed loop and the air/fuel mixture will be too lean (under these conditions, the engine would be safer and more powerful between 10:1 and 11:1). The result of this lean condition was the strange, surging powerband.
Over the last several months, we've tried various prototype versions of the EMI Adaptor, a magic black box Vishnu has been developing to solve this problem. The patented box watches for the high-boost, light throttle conditions that cause this lean condition, and sends a fake throttle positions sensor (TPS) signal to the ECU when necessary to fool it into switching to open-loop mode. In open loop, fuel delivery will be based on pre-programmed values in the stock ECU, all of which are nice and rich. Most versions of this box worked brilliantly, though just before the final production version appeared, we changed to an Exede piggyback ECU that was able to handle the box's functions itself, rendering the EMI Adaptor useless. It's still worthwhile on any WRX with a boost controller and either a stock ECU or any piggyback controller that doesn't specifically address the closed-loop part throttle problem. In fact, we'd go so far as to call it a critically important safety measure.

-K- · Post by **-K-** » Sun Sep 21, 2003 4:32 am

I don't know if we have that bad of an ECU. I've heard it is kinda easy to blow up a WRX with a MBC and a long hill. I've never heard that about a Legacy. I just replaced my fuel pump and it made a big difference, mine was on the way out though. (I raised the boost from 10 to 12 psi and it was slower)

vrg3 · Post by **vrg3** » Tue Sep 23, 2003 5:26 am

-K-, can you tell us more about the WRX-blowing-up stories?

The problem in the article FreddexTurbo2Go mentioned is part-throttle-full-boost; part of the cause of that problem is discussed in this thread:
viewtopic.php?t=2116

Basically, if you hook up your MBC to the intake manifold, you can blow up any engine if you drive it a certain way.

-K- · Post by **-K-** » Tue Sep 23, 2003 6:53 am

Ok here is what I get. The WRX keeps the boost down unless you floor it. It does this so it can run the A/F mix to the lean. If you put on a MBC it will not raise the mix because of the extra boost, Only if you floor it. 14.7:1 is way to lean for 15-16 psi that you would set on the MBC, EGT's will go way up. So you pull a long hill at part throttle full boost, #3 piston melts..... I don't know any other car that you can't raise the boost a few psi and the ECU doesn't just give it more fuel.
I DON'T BELIEVE THE, PART THROTTLE FULL BOOST = BAD. I think that Subaru engine managment is bad for it. If the ECU gave the engine enough fuel for the boost it was running to keep the A/F nice and rich it would be fine.
And if you hook up the wastegate line right it won't over run the turbo.

vrg3 · Post by **vrg3** » Tue Sep 23, 2003 7:40 am

Ah, okay...

-K- wrote:I DON'T BELIEVE THE, PART THROTTLE FULL BOOST = BAD. I think that Subaru engine managment is bad for it. If the ECU gave the engine enough fuel for the boost it was running to keep the A/F nice and rich it would be fine. And if you hook up the wastegate line right it won't over run the turbo.

The engine management is supposed to go into open-loop when on boost. It just doesn't overfuel like you'd have to under PTFB.

Think about it -- if you have full boost with a partly closed throttle, you must be overrunning the turbo! The compressor has to produce enough pressure to give you your desired boost level in spite of the pressure drop of the throttle butterfly. The only way to deal with this situation is to dump huge amounts of fuel in to try to keep combustion temperatures sane.

The stock engine management can't really deal with this somewhat nonsensical situation (where the compressor is making boost only for it to be strangled by the throttle plate) mainly because it doesn't have an intake air temperature sensor. It doesn't know that the boost is being inefficiently produced.

-K- · Post by **-K-** » Wed Sep 24, 2003 1:01 am

If you hook the wastegate line up off the turbo outlet it will open the wastegate at the same pressure whether the throttle plate is fully open or just part open. If there was pressure building up behind the throttle plate and you run the wastegate line off the intake manifold I see that what you say will happen.

vrg3 · Post by **vrg3** » Wed Sep 24, 2003 1:31 am

Right, you got it... PTFB causes high intake temperatures because compressor outlet pressure (before the throttle) is extremely high.

-K- · Post by **-K-** » Wed Sep 24, 2003 1:36 am

It can't go to high if your wastegate is run off of a line before the throttle.

vrg3 · Post by **vrg3** » Wed Sep 24, 2003 1:44 am

Right, but if your wastegate is controlled by a line before the throttle, you can't have part-throttle-full-boost either.

-K- · Post by **-K-** » Wed Sep 24, 2003 3:14 am

Well, close if I'm pulling a steep hill.

esinger · Post by **esinger** » Sun Sep 28, 2003 3:48 pm

Legacy777 wrote:have you done any recent maintenance? Changed plugs, wires, fuel filter, etc?

If not....may want to start there.

Changing the wires took care of it, thanks!

Eric

vrg3 · Post by **vrg3** » Sun Sep 28, 2003 5:26 pm

Cool!

So I guess your ignition wires' insulation had probably wasted away to being marginal... The problem probably was only happening above a certain level of boost, when the mixture is hard to ignite.

Glad you're back onto boost. :)

EJ20TMAN · Post by **EJ20TMAN** » Sun Oct 12, 2003 1:42 am

I DON'T BELIEVE THE, PART THROTTLE FULL BOOST = BAD. I think that Subaru engine managment is bad for it. If the ECU gave the engine enough fuel for the boost it was running to keep the A/F nice and rich it would be fine.
And if you hook up the wastegate line right it won't over run the turbo.

[/quote]
But dont u get it, a MBC tricks the ECU into thinking that there is less boost than there really is. So if u run under 55% throttle with a MBC letting it run full boost there is no way the ECU can tell its making full boost because the MBC masks it therefor the ECU cant just add more fuel!!!!!!

vrg3 · Post by **vrg3** » Sun Oct 12, 2003 4:35 am

EJ20TMAN wrote:I DON'T BELIEVE THE, PART THROTTLE FULL BOOST = BAD. I think that Subaru engine managment is bad for it. If the ECU gave the engine enough fuel for the boost it was running to keep the A/F nice and rich it would be fine.

Sure, it's possible that it wouldn't cause problems, but there full boost at part throttle is still hard on the engine, with high intake air temperatures. At PTFB that the compressor is producing more boost than the manifold's full boost level, because the throttle plate is a pressure drop.

If temperatures are high, dumping in lots of extra fuel will help cool the intake charge and prevent detonation. And it does appear that the stock Subaru engine management doesn't dump in quite enough extra fuel. Because it doesn't have an intake air temperature sensor, it doesn't know how inefficiently the boost is being produced. It has to be calibrated for something, and it makes perfect sense to calibrate it for what it's expected to be when it leaves the factory.

But dont u get it, a MBC tricks the ECU into thinking that there is less boost than there really is.

No it doesn't.

An MBC "tricks" the wastegate actuator.

The ECU still measures boost through the manifold pressure sensor.

So if u run under 55% throttle with a MBC letting it run full boost there is no way the ECU can tell its making full boost because the MBC masks it therefor the ECU cant just add more fuel!!!!!!

That's not true at all.

The ECU is aware of the boost level and the actual airflow, and fuels accordingly. It is not, however, aware of the increased temperature of that airflow.

Brat4by4 · Post by **Brat4by4** » Mon Oct 13, 2003 3:24 am

It's a stupid condition to have and there is a reason Subaru doesn't allow it on their engines. Why would you need full boost if you are applying 25% throttle? Think about it... why?? Full boost makes sense under lots of throttle, especially full throttle. High boost kills your gas mileage and as has been pointed out puts lots of pressure and thermal strain on the engine. These engines don't run for 200,000+ miles for no reason...

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help; acceleration problems

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