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Idaho KDM King
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Hey,
My 3.8 V6 gencoupe for some reason when I floor it at let's say 2500rpms, it does pull, but it just feels weak. And then it gets to around 4500rpm and it just takes off. It feels like a crazy vtec, the exhaust tone changes and gets louder as well. Only thing done to it is a intake and axleback single exit exhaust. It's been like this for a while and I don't know what to do to get it to run normal again. I've tried putting the stock airfilter system on, helped for a little while. Now I'm at a loss..

Any ideas or suggestions are greatly appreciated, dealership is going to inspect her Wednesday just want to have some info handy for them so they can do it right. Thanks all.
 

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couple things:
If the oil isn't up to operating temp, timing doesn't fully advance and it'll feel slow when you ask for max power
If you floor it and your A/C compressor is engaged, it takes a second or two @ 100% throttle for it to disengage, that might be why it feels like VTEC kicking in

try turning off the air entirely and floor it from 2000 - it should be a gradual increase in torque up to about 5000RPM - it shouldn't feel like VTEC :)

Edit:

Had another thought - if you're worried your mods (intake, exhaust) are doing something funky - you should check:

MAF Sensor + Throttle Body Position Sensor - if those are off, it can screw up your AFR causing weird behavior in closed-loop operation.... when you put your foot down and the car goes into open loop mode, it basically defaults to known-good settings, hence the power jump.

O2 Sensors: If your primary sensors (in the header) is acting up, it can trick the ECU into too-rich or too-lean conditions.... again, going to known-good defaults once the ECU goes into Open Loop operation at 100% throttle

this stuff usually starts throwing CEL codes after a while though
 

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If you floor it and your A/C compressor is engaged, it takes a second or two @ 100% throttle for it to disengage, that might be why it feels like VTEC kicking in

try turning off the air entirely and floor it from 2000 - it should be a gradual increase in torque up to about 5000RPM - it shouldn't feel like VTEC :)
Also a good point, in Texas A/C is on for 80% of the year and I forget about this.
 

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My Genesis Coupe is 100% stock, and what you describe matches exactly how I feel when I floor it. Considering that the 3.8 gets its peak torque at a high RPM, the behavior sounds normal to me.

I am not complaining though. It's fun to get thrown back into the seat at a high RPM with the exhaust being loud and all that. Apparently I have done it so much that I am used to it now - took a colleague around for a cruise the other day and floored it at 4500 RPM. His jaw dropped and said the car felt faster than a roller coaster.
 

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Mature Speeder
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717 Posts
Hey,
My 3.8 V6 gencoupe for some reason when I floor it at let's say 2500rpms, it does pull, but it just feels weak. And then it gets to around 4500rpm and it just takes off. It feels like a crazy vtec, the exhaust tone changes and gets louder as well. Only thing done to it is a intake and axleback single exit exhaust. It's been like this for a while and I don't know what to do to get it to run normal again. I've tried putting the stock airfilter system on, helped for a little while. Now I'm at a loss..

Any ideas or suggestions are greatly appreciated, dealership is going to inspect her Wednesday just want to have some info handy for them so they can do it right. Thanks all.
Can't troubleshoot this without more info.

What is your oil temp reading when this happens (is the car cold)?

How many miles are on the car?

When is the last time your fuel filter was changed?

How old are your spark plugs?

What kind of gas are you using?

The comments about the AC causing it are incorrect since our AC compressor is engaged all the time. The only thing that changes is whether or not it sends coolant through the cooling coils or not. That's how a Hyundai Master tech explained it. They days of cycling the compressor on and off were found to cause more wear and tear on the AC system than just letting it run. I can turn the AC on and off and it doesn't change the load on the engine.
 

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The compressor in our cars has an electronic clutch, if it's always engaged, on and off, high and low RPM, why does it have a clutch? In my experience the only thing that cycling the compressor wears out is the compressor clutch, not disengaging at high RPMs doesn't make a lot of sense, but not disengaging when the A/C is off makes no sense.
 

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Mature Speeder
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The compressor in our cars has an electronic clutch, if it's always engaged, on and off, high and low RPM, why does it have a clutch? In my experience the only thing that cycling the compressor wears out is the compressor clutch, not disengaging at high RPMs doesn't make a lot of sense, but not disengaging when the A/C is off makes no sense.
You answered your own question. It's electronic. The load to the engine is the same in all states. Essentially the clutch is always on to the engine but the actual control of the coolant happens electronically. It's variable so it adjusts itself based on rpm.

https://youtu.be/DPuYF0eNK6M

Variable Displacement A/C Compressor

Posted on May 29, 2013 by macsworldwide

Delphi-5-cylinder-CVC-Compressor

(Delphi Automotive illustration)

by Jacques Gordon

When a belt-driven piston-type A/C compressor is engaged, it produces a noticeable ‘thump’ and a reduction in engine power. This can be eliminated by keeping the compressor engaged all the time and controlling refrigerant flow by varying the displacement of the compressor.

The variable displacement compressor is an axial piston design, with the pistons driven by a wobble plate or a swash plate. Since the angle of that plate determines the length of the piston stroke, changing that angle changes the length of the stroke, therefore changing the amount of refrigerant pumped (displaced) on each stroke. The plate angle is controlled with linkage and springs, and it’s adjusted by changing refrigerant pressure in the compressor housing. So the key to controlling displacement is controlling housing pressure.

When housing pressure is increased, pressure on the back side of the pistons keeps them “higher” in their bores, closer to the cylinder head. This reduces the angle of the swash plate and shortens the stroke, reducing displacement. When housing pressure decreases, a spring pushes the adjusting linkage away from the cylinder head, increasing plate angle and lengthening the piston stroke to increase displacement. Housing pressure is controlled by a valve with ports and passages that connect to the suction (low-side) and discharge (high-side) chambers of the compressor head.

Two different types of control valve are used; mechanical and electronic. The mechanical valve has a precision diaphragm that senses low-side pressure. When the cabin is warm, evaporator temperature increases, which increases low-side pressure and collapses the diaphragm. A port opens to vent housing pressure to the suction side of the compressor head. This decreases housing pressure and increases piston stroke, increasing refrigerant flow through the system.

As evaporator temperature decreases, so does low-side pressure. The diaphragm expands to close the low-side vent port and at the same time open a port that admits high-side pressure into the housing. Higher pressure reduces piston stroke and refrigerant flow volume. Remember, changing flow volume doesn’t change pressure, so a diaphragm-type control valve remains stable.

Picture1

Delphi’s mechanical displacement control valve is actually two valves in the same body. When low-side pressure is high, the bellows collapses (towards the left). The cone valve opens and pressure vents from the wobble plate case (compressor housing) through the by-pass hole and to the suction side of the compressor head, reducing housing pressure. When low-side pressure is low, the bellows expands to close the cone valve and open the ball valve. Pressure vents from the discharge cavity (high-side) to the wobble plate case, increasing housing pressure.

Around 2001, DENSO introduced a pulse-width-modulated solenoid valve to control housing pressure, based on information from temperature and pressure sensors in the refrigerant system. With a computer controlling the valve’s duty cycle, compressor displacement can be used to control evaporator temperature, rather than the other way around.

The mechanical control valve is still used in some systems because it’s inexpensive and reliable, but it’s range of control is limited. Electronic control valves are becoming more common, and on many applications there is no clutch, so the compressor runs continuously. Displacement can be reduced to about 1 percent when cooling is not needed, keeping seals lubricated, minimizing oil pooling and preventing other kinds of damage that result from long periods of inactivity. Ultimately there is less load on the engine when the A/C is in use, reducing the system’s contribution to tailpipe emissions. As mileage and emissions regulations continue to tighten, we can expect to see electronically-controlled variable displacement compressors on more new models.
 

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The comments about the AC causing it are incorrect since our AC compressor is engaged all the time. The only thing that changes is whether or not it sends coolant through the cooling coils or not. That's how a Hyundai Master tech explained it. They days of cycling the compressor on and off were found to cause more wear and tear on the AC system than just letting it run. I can turn the AC on and off and it doesn't change the load on the engine.
IIRC - that was a change for the 2013's. There's a very apparent change in engine load when I turn on A/C or defogger in my 2010. Teeth-rattlingly apparent. Not sure if it's belt-load from the A/C, or if it's alternator load, but it's certainly a significant load on the engine.
 

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Mature Speeder
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IIRC - that was a change for the 2013's. There's a very apparent change in engine load when I turn on A/C or defogger in my 2010. Teeth-rattlingly apparent. Not sure if it's belt-load from the A/C, or if it's alternator load, but it's certainly a significant load on the engine.
If that's the case I apologize, it was my understanding they were the same systems.

It quite possibly could be the alternator load since its doing all the work now. I notice a load on the engine when I turn the steering wheel and I believe the power steering pump is electronic as well.
 

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Yeah, I notice the same load & the ECU compensating when I turn the wheel, or when the HID's flick on.... the A/C is just a different beast, it's ridiculous. I drive with no A/C unless the wife is in the car or it's raining hard and I need the defoggers.

The wife's car (Jetta TDI) has an electric driven A/C compressor and it's weird to me that there's no change in engine load when it's on..... it's magical :)
 

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Hmm, interesting, I knew there was a change in the A/C system between BK1 and BK2 but I didn't realize it was that drastic. Still though, based on what is described above and the video you linked, since there is a clutch in our system it sounds like it's doing some amount of disengagement to help regulate the system, while not as drastic as the older system it should still have some effect on parasitic loss. Maybe it's just me but it certainly seems to be more responsive with the A/C off but not as noticeable as my past cars.
 
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