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Losing power after 20% throttle

JCorbo25

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Joined
Oct 10, 2011
Messages
533
Location
Sacramento, CA
Updated
Things to check (in order of probable causes):
ECU - leaky caps
Power transistor - maybe coil pack, spark plugs, or spark plug cables
clogged exhaust/cat - creating backpressure
TPS - not going past % throttle open
TB grounding cable - playing hell with sensors
BCS - failed(?) - maybe less likely unless the digital boost gauge was reading full boost

I posted on my local DSM group for a loaner ECU to test out that theory.
 
Last edited:

fuel

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Feb 23, 2009
Messages
2,165
Location
Toronto, ON, Canada
I think you should check for boost leaks.
 

beaner

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Jun 22, 2005
Messages
1,562
Location
b'ham, mi
Quoting mitsuturbo:
If it were a boost leak, the factory "boost gauge" would likely light up all six bars. So that's probably not it.


Explain.

I still think the bov is not staying closed. That or a clamp is loose or rip in a coupler.

/ubbthreads/images/graemlins/dunno.gif
 

SouthCaliVR4

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Jul 31, 2010
Messages
984
Location
North county San Diego
Quoting 656of1000:
It sounds like a clogged exhaust to me... either a clogged cat, muffler, or collapsed flex section.



This!!!
 

fuel

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Feb 23, 2009
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Toronto, ON, Canada
Quoting Beaner:
Quoting mitsuturbo:
If it were a boost leak, the factory "boost gauge" would likely light up all six bars. So that's probably not it.


Explain.

I still think the bov is not staying closed. That or a clamp is loose or rip in a coupler.

/ubbthreads/images/graemlins/dunno.gif


I guess because the factory boost gauge isn't an actual manifold pressure gauge, it 'estimates' the boost by the amount of air through the AFM which would mean any leak after the AFM would be consuming as much air as it would be when boosting.
 

mitsuturbo

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Jun 2, 2008
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3,551
Location
Near Seattle, Washington
Quoting 656of1000:
It sounds like a clogged exhaust to me... either a clogged cat, muffler, or collapsed flex section.



This is super easy to check. Drop the 2 bolts for the downpipe and test drive. Although i have serious doubts about this being the issue. It wouldn't be a "collapsed" flex section, but the possibility remains that a catalytic converter could be clogged.

Fuel is exactly right about the factory boost gauge. It simply runs off the airflow the MAF sensor is reading.
 
Last edited:

JCorbo25

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Joined
Oct 10, 2011
Messages
533
Location
Sacramento, CA
I picked up a known working ECU out of a 1991 Galant VR4 and will test that out tonight.

If that doesn't work, I'll go in this order:
1)unplug the BCS
2)drop the cat out
3)test the TPS

Of course, depending on how much time I have.
 

JCorbo25

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Joined
Oct 10, 2011
Messages
533
Location
Sacramento, CA
Swapping in a known good ECU didn't do anything. But my 'rebuilt' ECU looks like it's leaking caps or someone did a piss poor job of soldering...



Also, disconnecting the BCS did nothing.

Next up:
1) drop cat, check for clogged exhaust
2) test TPS
3) check spark plugs, wires, and coil pack?

Any other ideas?
 

grocery_getter

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Joined
Jun 20, 2004
Messages
1,225
Location
Kent - industrial suburbs of Seattle, WA
Quote:
A TPS failure will often accompanied by hesitation. If you are just losing power, but not hesitation, or the engine chugging back and forth then most likely your tps sensor is fine. The best way to troubleshoot a TPS sensor is with an old school multitester with a neddle display, not digital display. Black to ground and red to tps feedback wire. Open the throttle with hand very slowly in a linear fashion from all the way close to all the way open position and observe if needle display also moving in the same linear fashion or if it is jumping around or hesitating.

Other possibilities:
If you still have a cat, maybe a clogged cat.
If you still have old plug wires, they could be bad.
If you have a wideband, observe afr may be getting lean which might indicates not enough fuel or poor spark problem.


 

JCorbo25

Well-known member
Joined
Oct 10, 2011
Messages
533
Location
Sacramento, CA
pulled the CAT off and the car boosted fine /ubbthreads/images/graemlins/applause.gif

But when pulling the spark plugs, i found a TON of oil in there. Must have a leak in the valve cover seals for the spark plug wells. Guess I know what I'm doing next weekend!
 
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