Poor Full Throttle Acceleration

[NemesisEJ22t]

Hey everyone, this has been a rather recent problem for me (last 2 months). I have installed a Saab 900 IC and a MBC. The MBC is set up to about 11-12 psi. The problem i've noticed is that once the turbo spools up at part throttle, i can push the pedal down the entire way and it feels like nothing happens, or even like a little power is being lost. The engine gets louder of course, but no increase in acceleration occurs. I think it may be some sort of fueling problem, but i don't know for sure. Has anyone else experienced this, or have any ideas that may point me in the right direction?
Thanks

[vrg3]

Is your MBC connected between the compressor outlet and the wastegate actuator?

It kind of sounds like you have it connected between the intake manifold and the wastegate actuator. That overspeeds the turbo at part throttle and makes boost levels kind of weird, and heat soaks the intercooler.

[BAC5.2]

What are your gauges doing?

Any boost spikes or drops along the throttle path?

[NemesisEJ22t]

Nope, the MBC is directly between the compressor and the wastegate act. The factory boost control is disconnected like it should be when running a MBC. The gauge shows that the boost will spike 1-2 psi before settling down to the proper pressure (in this case 12 psi) when mashing the gas pedal down. The boost spike is expected with the ball and spring boost controller i am using, pushing the pedal down slowly can pretty much eliminate the spike. I realized that i haven't reset the ECU in a while, i am going to try that tomorrow to see if anything changes. The weird thing is that sometimes the car will really "pull" when i floor the pedal, and sometimes it will just sit there and not accelerate. This appears to be random, although once it starts to not accelerate, it doesn't go away until i let the car cool off completely. That isn't to say that the car only works right when cold, it can work fine even when the engine is up to operating temperature. One more guess, do you think a slipping clutch could have anything to do with it? Sometimes the clutch will slip and I see the revs climb rapidly, but other times the car is still having acceleration problems and the tach isn't moving quickly.

[vrg3]

That's weird... So at part throttle you're making like 7 psi, and at full throttle you're making like 12 psi, but they feel the same?

Unless the poor acceleration is always accompanied by a slipping clutch, I don't think the clutch is responsible for the problem.

Could this just be heat soak of the intercooler?

One thing you should understand is that the stock engine management can become a bit unpredictable if you routinely push the car outside its normal parameters.

To oversimplify it a bit, the ECU keeps a grid of numbers for the parameters it learns. The grid is scaled so that it has good resolution for the stock configuration. Loads above those seen by a stock engine end up all having to fit in the last column of the grid, so to speak. That means you can have two very different situations that the ECU's learning processes can't properly distinguish from one another.

Several people have felt that the car is most zippy right after a long period of low-load cruising.

[NemesisEJ22t]

About the boost. At part throttle, it will go up to 12 psi (at about 25-30% pedal), but it won't spike. I get the most power at about 40-50% pedal, at full boost. Then when i push any further the power level and boost pressure level do not increase at all in fact it sometimes seems that the car accelerates more slowly, although that may just be in my head. The boost only really spikes when i mash the gas down quickly.

[vrg3]

Where are you measuring boost?

[legacy92ej22t]

The factory boost control is disconnected like it should be when running a MBC.

I know the answer already I think but you mean that the OEM EBC is unhooked from the turbo but you still have the electrical connection hooked up to the ECU, right?

[LegacyT]

you are getting slower, the stock turbo is too small to continue pumping out 12 psi. I have the same problem. With boost set to 12 psi, it peaks there untill full throttle is reached then falls off.

Mark,

[NemesisEJ22t]

Ok, i took it for another drive and this is what i found.

The sluggishness seems to only occur under about 4000 rpms, once i get over that it pulls like normal. That pretty much eliminates the too small of a turbo problem.

The EBC solenoid must remain connected to the engine or else i'll get a check engine light i believe, am I wrong about this?

I am measuring boost on a tee from the bypass valve actuation line, so unless its leaking i hope i'm getting fairly accurate readings, at least within the limits of my gauge.

Thanks everyone, I'll beat this problem sooner or later with all your help.

[legacy92ej22t]

The EBC solenoid must remain connected to the engine or else i'll get a check engine light i believe, am I wrong about this?

Yes, it needs to remain hooked up and I assumed it still was in your situation, I was just making sure. Unhooking it, you'll probably get a CEL in the least but it would cause the car to go into limp mode.

I am measuring boost on a tee from the bypass valve actuation line, so unless its leaking i hope i'm getting fairly accurate readings, at least within the limits of my gauge.

.

Hmmm...I don't know. I and everyone else that I know of, taps the line to the pressure sensor off the #1 intake runner. IMO this is the best place to measure boost. You're gonna want to know your manifold pressure.

[NemesisEJ22t]

I have been thinking that the BOV line might not be the best place to measure, i will move that around as soon as i get a chance after finals. Still doesn't explain the problem, but maybe it will give me some better info about how things are working in the engine. I really need to get that scan tool working first to see if i can find any odd things.

[vrg3]

The BOV vacuum line isn't a bad place to measure boost. It's just often easier to tap at the #1 intake runner.

It's not possible for an MBC plumbed between the compressor outlet and the wastegate actuator to give you full manifold boost at part throttle. Can you double-check the installation of the MBC and the boost gauge?

If you disconnect the stock boost control solenoid's electrical connector the ECU throws a trouble code for it and cuts fuel to prevent you from going on boost at all.

[BAC5.2]

Run the VRG3-Scan program and see what the ECU sees as manifold pressure. It could be that your gauge either 1) sucks, or 2) has a leak somewhere.

Never trust the buttdyno.

[aspect]

well, I have EXACTLY the same problem, with no boost controller and no intercooler.

[totech]

Clutch slipping?

[aspect]

clutch is fine. I'd notice if the rpm increased but the car didnt move...this is a definate power drain...

maybe the knock sensor is giving false signals cos it's a POS, and the ecu is pulling timing way back? does that make any sense?

I havn't got any codes but I'm still using the original sensor...

[NemesisEJ22t]

Ok, i'm gonna build the scan tool right after my last final tomorrow so i can use it for the 2 hr ride home. Maybe i'll catch something by doing that. I'll let you guys know.

[aspect]

Yeah, please post up any info you find out. This is driving me nuts...

[NemesisEJ22t]

Well, i think i inadvertantly fixed the problem today. It all started with no boost and a cloud of black smoke behind the car right before i got into a tunnel leaving Pittsburgh. Yup, my intercooler pipe blew off again, and after limping it about a mile, i pulled off and re attached it. I also noticed that the other 4 or so clamps were loose as well, so i tightened them all and now it pulls like it should. I think that the loose pipe was causing pressure leaks in the low rpms until the engine was using enough air (high rpms) to make the problem go away. It does seem to be working fine now, i'll let you guys know if that changes. Thanks for all your help everyone.