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Losing power after 20% throttle

JCorbo25

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Oct 10, 2011
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533
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Sacramento, CA
92 Galant VR4, all stock, mostly original from what I can tell.

I was hoping on the freeway last week and pushing the throttle past 15-20% would just get an engine growl but no power. I figured the 14B finally froze (been burning oil) and swapped it out over the weekend with a small 16G, but the problem persists.

I'm thinking it may be a dead TPS sensor. I inspected the throttle body and everything was working fine mechanically. I'm going to test the TPS sensor tonight, but wanted any other ideas of what I should look into.

I bought a pocketlogger, but haven't got it yet /ubbthreads/images/graemlins/frown.gif
 

grocery_getter

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Jun 20, 2004
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Kent - industrial suburbs of Seattle, WA
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.
 
Last edited:

JCorbo25

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Sacramento, CA
Thanks for the ideas
Plug wires look relatively new.
No wideband or datalogger yet, but it could be a clog in the fuel system (fuel filter, stock-FPR) and the ECU is trying to compensate.

Some more info:
Will free rev to redline no problem, only happens while driving. It doesn't stutter or jerk, will pull smoothly but past 20% throttle the engine just roars and doesn't increase power.
It just passed smog 2 weeks ago.
 

JCorbo25

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Clutch is fine, it's not slipping.
 

grocery_getter

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/ubbthreads/images/graemlins/dunno.gif

Will free rev to redline no problem, only happens while driving. It doesn't stutter or jerk, will pull smoothly but past 20% throttle the engine just roars and doesn't increase power.

Seems you are only losing power under load.
 

JCorbo25

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"Seems you are only losing power under load."

I agree, so inspect ECU for leaky caps?
 

MuffinMan7580

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Aug 6, 2011
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Fort Bragg, NC
Boost leak. Watch the stock boost gauge and see if it maxes out while doing this. If it stumbles and you're gauge is showing max bust, this is most likely your problem (only at low throttle should you be able to tell). Visually check all connections. The stock intercooler piping is probably old and leaking. Do you have a real boost gauge? You can also check your plus to see if you've got boost leaks, if they're bad enough (black instead of white/tan).
 

JCorbo25

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Sacramento, CA
The stock boost gauge (digital) would only build 2-4 bars (4 bars is at the center). It would build up to 4 bars in first gear and only 2 bars on the street in 2nd/3rd. Anything past 1st gear, it would not go past 2 bars (half way to 0 boost).

I hooked up a boost gauge yesterday and it wouldn't even attempt to make it to positive boost, it would always be in vacuum.

Also, during the small 16G turbo swap, I linked up a MBC... could the BCS still being plugged in cause an issue like this? Since I was short on vacuum nipples, I just routed the UICP and MAS into the BCS to cap them...

hmm... great ideas, keep them coming. I'm eager to start testing ideas this evening.
 

MuffinMan7580

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Fort Bragg, NC
Did you unplug the stock boost solenoid? Do so and see if that makes a difference. or better yet, put the bcs back in thr loop and see if it changes anything.
 

gramkrakr89

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Bunker Hill, WV
Quoting grocery_getter:
/ubbthreads/images/graemlins/dunno.gif

Will free rev to redline no problem, only happens while driving. It doesn't stutter or jerk, will pull smoothly but past 20% throttle the engine just roars and doesn't increase power.

Seems you are only losing power under load.



no power past 20% throttle? Chugging under high-load? smoke?

are you running big injectors on the stock ECU? Check to be 100% sure. It sounds like it is running in an unusually rich condition.

Air, fuel, or timing... it's one of those 3.(IMHO)

Definitely check the ECU for leaky caps, though. Good luck!
 

JCorbo25

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Sacramento, CA
Thanks for all the suggestions. It's on stock injectors. I'll unplug the stock BCS and/or try tossing it in the loop, pull my ECU, and check the TPS.

I was thinking could be air/fuel/timing but it's very weird that it will free-rev fine but suffocate under load (non boost).

Could also be a failing MAS... damn I wish I had my pocketlogger already.
 

luis84one

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lynwood, ca
wow im having the exact same problems.. and im waiting on my pocket logger too.. i hope you figure this out quick /ubbthreads/images/graemlins/uhh.gif
 

JNR

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ca
Doesn't there need to be an additional ground placed on the throttle body to the firewall? Not sure if this will help or not though...
 

JCorbo25

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Oct 10, 2011
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Sacramento, CA
Quoting luis84one:
wow im having the exact same problems.. and im waiting on my pocket logger too.. i hope you figure this out quick /ubbthreads/images/graemlins/uhh.gif



Well fix it and let me know!

Had to swap a battery into my bmw e30 to get it going tonight. Ill see if I have time in the morning.

Things to check:
ECU
TPS
BCS
TB grounding cable
 

luis84one

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Aug 19, 2009
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127
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lynwood, ca
im trying /ubbthreads/images/graemlins/bawling.gif i checked my ecu.. it looks like it still has the original CAPS, but no leaks what so ever.. my guess is the tps.. i need to buy a meter with the needle my digital one reads OL.. hopefully my pocket logger gets here tomorrow.. i know for a fact that there's no ground from the firewall to the TB could that be a problem? there's one to the IM though..
 

beaner

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b'ham, mi
Check your bov, specifically the vacuum line going from the bov nipple to the intake manifold.
 

mitsuturbo

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Near Seattle, Washington
If it were a boost leak, the factory "boost gauge" would likely light up all six bars. So that's probably not it.

If it's not making any power/boost under load, with NO CEL coming on, I'd recommend trying a known good ECU and see what happens. Sometimes when caps go, you can't see it, but the car will run like you describe. If you ARE getting a CEL when this happens, it could be a knock sensor issue. However, if this were the case it would pull timing like mad and again, the factory boost gauge would likely indicate more airflow.

I once had a power transistor unit go out and the car behaved much like you describe as well. It was dropping spark on 2 cylinders under a load. Replaced the ignitor/power tr, and issue was resolved.
 

JCorbo25

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Oct 10, 2011
Messages
533
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Sacramento, CA
My pocketlogger is scheduled for delivery Saturday. I'll see if I can source a local ECU and test that theory.

Thanks for all the responses, just wish I had more time to troubleshoot during the week.
 

656of1000

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Jan 18, 2005
Messages
292
Location
Phoenixville, PA
It sounds like a clogged exhaust to me... either a clogged cat, muffler, or collapsed flex section.
 
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