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Guest Message by DevFuse
 

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Why more power at part throttle than floored?


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33 replies to this topic

#1 Mantonite

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Posted 18 August 2005 - 05:20 PM

Done so far:
Fuel Filter
O2 Sensor
Plugs (Platinum)
Plug Wires
Air Filter

So why do I have more power at part throttle than full? Is it too rich/lean when floored? Any ideas on what to check? I'm a little stumped here :-\

Thanks in advance for all the replies. You guys have helped a ton so far!

#2 TurboSPFI

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Posted 18 August 2005 - 05:21 PM

When I changed out an old ECT my car ran like new..

Also, I changed my PCV valve out and that helped as well.

#3 Sweet82

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Posted 18 August 2005 - 05:40 PM

Have you checked your timing? :confused:

#4 Numbchux

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Posted 18 August 2005 - 05:58 PM

Have you checked your timing? :confused:


agreed....it sounds like you're running too rich

try running it with your air cleaner out, maybe it's clogged and isn't letting enough air in...

#5 WoodsWagon

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Posted 18 August 2005 - 06:27 PM

I'd agree with the timing. I know the power only when you lead the rpms up with the throttle feeling, and it was because the timing was set 12 degrees too retarded. Check your timing with the green connectors together and see what you come up with.

#6 Mantonite

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Posted 18 August 2005 - 09:23 PM

When I changed out an old ECT my car ran like new..

Also, I changed my PCV valve out and that helped as well.


ECT?

Forgot to mention that I checked the timing and it was spot on. That was with the green connectors together... but if I remember correctly there were two weird things: I remember only one wire to the green connector (looked like space for two). Also, the disty looked like it was turned all the way one way (not sure which). Any of those point to anything?

I may try the air cleaner thing, but I live on a dirt road, so may just swap it out.

Maybe the PCV is the issue. We shall see...

#7 TurboSPFI

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Posted 18 August 2005 - 10:56 PM

ECT = engine coolant temperature sensor.. sorry, emissions lingo.

As far as the timing, how can that make it run rich?

Also, you could have a clogged exhaust somewhere thats only letting a certain amount of CFM through the system before it starts hesistating.. Just an idea.

#8 Numbchux

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Posted 19 August 2005 - 12:06 AM

timing and running rich are seperate, sorry, that wasn't clear...I meant either thing could be possible

as far as running it with the air cleaner off, just try it in parked, and rev it a couple times, see if it stumbles or not, if it doesn't, and does with the filter on, that certainly isn't helping...

I agree on the exhaust, though not sure of a good way to check it, maybe you're cat is clogged...I think they make some stuff you can run through it (not sure how, just remember reading about it in jcwhitney)

hope this helps...good luck!


here's that cleaner stuff....maybe you can find it locally http://www.jcwhitney...3 10201/c-10101

#9 Mantonite

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Posted 19 August 2005 - 09:53 AM

Haven't had any problems with it stumbling when not under load, but I'll pay a bit more attention to it to see.

As far as the cat, if anything the rear one would have an "out of body" experience. Can the front be replaced with a y-pipe and an o2 sensor bung (for off road use only of course ;))?

The more I think about it, it does feel like clogged exhaust. I'll look into it...

#10 Numbchux

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Posted 19 August 2005 - 10:11 AM

yea, you could just remove the rear one, but the one in the y is a pain, short of disconnecting it from the heads (hmm maybe just one side?) I don't know of a way to troubleshoot it.

I don't know about your area, but here in MN it's almost impossible to find y pipe that's not completely rusted through, and I'm far to cheap to pay anyone to make me one...luckily, my parts car had one :grin:

more good luck!

#11 Mantonite

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Posted 09 September 2005 - 09:32 PM

UPDATE:

Tonight, the EGR Valve left, but still had the problem. Rechecked (and adjusted) the timing and still have the problem. Put a gage on the fuel line just in from of the (replaced) fuel filter and had 50 psi! Seems like too much if anything. Held pressure well too. PCV replaced, to no fix. Sprayed carb cleaner around under the hood and didn't notice any jumps.

I'm left with the air cleaner (not replaced but looks good) or the cats. Don't even know which one to lean towards. Air cleaners are cheap, so I'll probably pick one up tomorrow. Rear cat may leave tomorrow too. Can you tell the wife and kiddos are out of town? All this time, but lots of projects too.

Okay Subaru gurus... help me out!!!

#12 Snowman

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Posted 09 September 2005 - 09:39 PM

That fuel pressure reading seems excessive. Maybe the regulator isn't working properly and the massive pressure is making it run rich. Check the little vacuum connection on the regulator to make sure it's hooked up.

#13 jeffast

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Posted 09 September 2005 - 11:22 PM

i had the same problem until i ghanged my fuel filter i didn't check my pressure before hand so i don't know what the real prob was

#14 Mantonite

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Posted 09 September 2005 - 11:43 PM

Where would one find

the little vacuum connection on the regulator


Additional info: This was done by cycling the key, not a "T" while running... does that change your opinion, or is this really a high reading?

#15 ShawnW

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Posted 10 September 2005 - 02:50 PM

You can put a vac gauge on the engine to verify clogged exhaust.

1. Put the gauge on an intake manifold vac source.
2. Allow engine to reach full operating temp. (fans cycle on/off)
3. From idle (700-850 RPM) rev up engine to 2000 RPM.
(Vac level on gauge should remain the same)
4. Quickly release throttle and watch gauge. Reading should slowly rise then smoothly return to normal reading you were getting at idle. If it rises or does not quickly return to normal the exhaust is likely restricted. This can be the cat or the muffler even if the muffler is clogged with cat material from a previous cat failure.
5. Disconnect exhaust and test if car fails test.

#16 Mantonite

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Posted 10 September 2005 - 04:00 PM

I like that test... I'll hopefully give it a try tonight.

Swapped out the air filter... small improvement, but not gangbusters by any means.

I am beginning to think it is actually the muffler, not the cat. The pipe just in front of the muffler seems to be failing at an alarming rate. Time for some flex pipe and a glass pack (cherry bomb).

I'll try to update as I have more!

#17 nassor

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Posted 10 September 2005 - 04:15 PM

Have you checked the TPS (Throttle position sensor)? You should be able to test it with any cheap multimeter.

afterthought added---> the restricted exhaust does sound likely, but if the prob. remains with the cat disconnected (don't run for too long this way or you may damage valves) then check the TPS. Actually it is an easier test, you might check it before diagnosing the exhaust system.

#18 Mantonite

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Posted 10 September 2005 - 06:11 PM

Have you checked the TPS (Throttle position sensor)? You should be able to test it with any cheap multimeter.


Cheap multimeter I have... clue as to how to test, I don't have. Can you clue me in?

#19 Mantonite

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Posted 10 September 2005 - 06:44 PM

Did the vac gage test:

At Idle: 20" Hg (inches of mercury)
When Accelerating: Can go down to 0
At 2000(ish, no tach outside): 20
Release: 25
Returns to 20 with RPM (When it reaches idle, the vac gage gets to 20)

Is this good or bad???

#20 ShawnW

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Posted 11 September 2005 - 02:26 PM

Looks good but depends on how fast it returns to 20.

#21 Mantonite

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Posted 11 September 2005 - 03:02 PM

Looks good but depends on how fast it returns to 20.


It gets to 20 when the engine gets back to idle... is that good or bad?

Remember the rapidly increasing in size hole in front of the muffler??? Well, I am now mufflerless. Fell off today, and I know what you are thinking... "sure it fell off, but it really did! Still have the issue. Time for a cherry bomb!

How do I test the TPS?

#22 nassor

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Posted 11 September 2005 - 04:44 PM

The TPS is located on the throttle body and should have a three or four wire connector. It consists of a potentiometer (variable resistor) connected between the ECU refrence voltage and ground with the wiper feeding the signal back to the ECU. The four wire version also incorporates an idle switch.

3Wire:
Pin1: GND
Pin2: REF voltage (5volts from ECU)
Pin3: TPS Signal
4Wire:
Pin1: Idle switch (opens with throttle)
Pin2: GND
Pin3: REF voltage (5volts from ECU)
Pin4: TPS Signal

Key on, engine not running:
Without removing the connector, read the voltage between the TPS signal pin and GND. As the throttle is opened the voltage should go from about 1 volt with the throttle closed to 5 volts with throttle wide open. Check for a smooth, continuous change as the throttle is opened.

Key off, TPS disconnected:
If the pins are not accesible then remove the connector and read the resistance (Ohms) between the signal pin and GND. Observe the reading as you slowly open the throttle to full. The the reading should increase steadily over the entire range.

I do not recall the numbers for the resistance test. I am pretty sure the ECU automaticly calibrates itself for this.

The 'key on, engine not running' test mode is preferred because it will also indicate problems with the connector or with the refrence voltage.
****

A few other thoughts and questions:

This problem sounds like it being caused by exhaust restriction more than anything else.

Is there a specific point in the throttle position where you lose power or do you lose power only at WOT (wide open trottle)? Many systems have a WOT switch that allows it to respond to maximum power demand with actions such as turning off the AC compressor, alternator or other systems that draw horsepower. Some systems will also implement a 'passing gear mode' that will advance the timing for more power at higher RPM. If you are losing power only while at WOT, you might investigate what changes your car makes for WOT conditions.

If the problem is the TPS, it is probably causing a lean condition. This can also be caused by fuel restriction at the injector, fuel filter, fuel pump.

If the knock sensor it not working, you might be losing power due to detonition. Does the engine knock when you are missing power?

-Nassor
'89 XT-6 (retired at about 200k mi.)
'98 LOB 2.5L DOHC 4EAT AWD (has a spun a rod bearing, looking for short block)

#23 Mantonite

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Posted 12 September 2005 - 05:26 AM

Holy Crap! That is one GREAT post. Hope others see it! I'll check the TPS when I have time, but tonight will likely be spent putting a muffler on.

Regarding the power loss, it seems that I feel the change at about half throttle, but the magic point will change time to time. If I pump the throttle, sometimes I can get it to give me a bit more power, but still feels like she has more.

Engine doesn't knock when losing power (at least not that I can hear), but occasionally I do get what sounds like an intake backfire, which would indicate lean. I have heard (somewhere) that an exhaust restriction can also cause a lean backfire... anyone else concur?

Looks like I have some more work to do. Is the TPS an adjustable unit, or is it just good or bad?

Thanks for all the help so far. Hopefully we can get this subie running like a champ!

#24 nassor

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Posted 12 September 2005 - 11:12 PM

quote: Looks like I have some more work to do. Is the TPS an adjustable unit, or is it just good or bad?

Short answer: The performance of the TPS can degrade slowly or it can fail completely. It is adjustable for the throttle open threshold point (a critical adjustment for the 4 pin type with the throttle switch).


Long Answer:
The TPS is a potentiometer (sp?) similar to ones used for volume controls etc.. A metal contact sweeps the length of a band of carbon to vary the resistance (Ohms). They can develop problems and get 'noisy' when the carbon wears down or if any crud gets in there. If the carbon band actually breaks it will only function for a portion of the range.

Thanks for the kudos on my post ;)
I hope the info is all accurate. It came from personal experience and a Haynes Manual.

-Nassor

#25 Mantonite

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Posted 13 September 2005 - 05:47 PM

Well I got under there after putting on a muffler and did a feel check on the two cats, intending to remove the rear one. The front one was freaking HOT! That prolly means that it is clogged.

So... any suggestions on that one? I'm not going to pay for a new one, so how does one make this one better? Seems like it would be a bear to gut... any idears??? Any option for a (cheap) replacement?

BTW... thanks for all the help folks.




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