loss of power during steady state driving

[mransr]

loss of power during steady state driving UPDATE!

hello guys,
I know internet troubleshooting is tough at best but here goes...

I'm starting to see a loss of power during steady state cruising...I'll be driving down the freeway at 70/75 mph and the power will just slowly drop off, if I let off the gas, or push in the clutch and blip the throttle, power returns to normal. Occasionally power will just return to normal on it's own.

the car idles fine, I replaced the fuel filter and cleaned the MAF last weekend, checked the plugs two weeks ago and they looked brand new (and very even)...

I have no mods to speak of, just a K&N filter and Magnaflow weld-ins (I'm a Stangnet slacker) and a ton of miles, just under 174,000...

any suggestion on what to check out?

EDIT:
it's no longer intermittent, and now I have a SES...went to Autozone and read the code: P1131 "Lack of HO2S switch - sensor indicates lean"

 

[Dragstr05]

I understand what you're saying, but that issue just does not make sense. I just cant think of anything that would cause that.

 

[Superhereaux]

first i'd start looking for a vacuum leak (vacuum is highest at cruise), then make sure the egr is working properly (egr operates at cruise)



any other problems? the more symptoms we know the better

 

[MarcoCapone]

Maybe its only in your head!!! Does anyone notice it????

 

[mransr]

EGR is a good thought...I couldn't find any obvious vacuum leaks (cracked hoses loose lines etc), but I'll check that again

as far as other symptoms...gas mileage hasn't changed significantly one way or the other, it doesn't seem to matter if the car is warm or cold (relatively cold anyway)...

when the power loss is most noticeable you can hear a "stumble" or "wet-ish sound" in the exhaust note...

<shrug>

 

[mransr]

I added some info the original message, but I figured I copy it here too

it's no longer intermittent, and now I have a SES...went to Autozone and read the code: P1131 "Lack of HO2S switch - sensor indicates lean"

 

[Dragstr05]

I added some info the original message, but I figured I copy it here too

it's no longer intermittent, and now I have a SES...went to Autozone and read the code: P1131 "Lack of HO2S switch - sensor indicates lean"

Did it specify bank 1 or bank 2? Now you have a very good reason to check for vacuum leaks.

 

[mransr]

more research turned up that that was bank 1..and "sensor indicates lean" meant that it trying to correct for a rich condition on the right bank

that makes sense as the car felt and sounded really rich

anyway, I went ahead and replaced the O2 sensors and the car sounds a lot more "crisp" than it did that last few weeks...I guess I'll see if that fixed it or if something else was causing it

 

[Mike97gt]

more research turned up that that was bank 1..and "sensor indicates lean" meant that it trying to correct for a rich condition on the right bank

that makes sense as the car felt and sounded really rich

anyway, I went ahead and replaced the O2 sensors and the car sounds a lot more "crisp" than it did that last few weeks...I guess I'll see if that fixed it or if something else was causing it

You have that backwards, it was trying to correct for a lean condition.. This code is set when the sensor is "stuck" at around 0 volts..

Now one thing you have to keep in mind that this can be set due to misfires. When a cylinder misfires you will have a lean reading on that bank because the 02 sensor will only pick up the excess oxygen ( it can not sense the unburned fuel)

 

[mransr]

...this is what I found:
P1131 - Lack of HO2S-11 Switch, Sensor Indicates Lean A HEGO sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when the fuel control system no longer detects switching for a calibrated amount of time. See Possible Causes for DTC P1130
that's it's wrong? or I'm interpreting wrong???

anyway, the right bank O2 sensor had much much, more black carbon (?) build up on it than the left sensor

 

[Mike97gt]

...this is what I found:
P1131 - Lack of HO2S-11 Switch, Sensor Indicates Lean A HEGO sensor indicating lean at the end of a test is trying to correct for an over-rich condition. The test fails when the fuel control system no longer detects switching for a calibrated amount of time. See Possible Causes for DTC P1130
that's it's wrong? or I'm interpreting wrong???

anyway, the right bank O2 sensor had much much, more black carbon (?) build up on it than the left sensor

Your problem may just be a bad 02 sensor.. but I will try to explain what I mean about that description.

it's not really wrong just kind of confusing.

this code is set when the 02 sensor is stuck below .45 volts.. the range of the sensor is 0-.99 volts the higher the number the richer the mixture. so if the sensor is reading let say .17 volts then that is leaner than stoch, .67 would be richer than stoch.

If the mixture is so lean that the mixture is out of range then the PCM can not find stoch and the sensor will stop switching.. the reading then will be near 0 or 0 volts. You will see this a lot with HUGE vacuum leaks.



The descriptions is referring to how the OBD2 HO2S monitor tests the 02 sensors

If the sensor is stuck lean ( less than .45 volts) for 30 seconds or longer then the PCM will force a overly rich condition to "test" and see if the 02 sensor responds.

So what they are talking about is the sensor fails the test trying to correct for a overly rich condition..

NOW this is where it gets interesting.. and I don't think this is what happend with your car.


IF something drastic happens instantly that will radically change how much air is entering the intake stream compared to what is sensed by the MAF ( you can do this by pulling a major vac line off then start the car) if the leak is big enough then the 02 readings will go out of range so quick that the pcm is not able to adjust fuel trim and bring it back around stoch ( 02 sensors switch around stoch) then the pcm might not make any fuel trim adjustments at all because it has lost control of where stoch is and it will set codes for the sensors being stuck lean.

I had this happen before a few times when I was doing a intake then started the car with a major vac line off..

This does not happen often at all outside of forgeting to put a vac line back on in the shop. 99.9999999 percent of the time the vac leak is small and the pcm adjusts the fuel trim and may set a p0171,174 code when the fuel trims max out..

but the reason I said that is kind of backwards is because the mixture might have never been overly rich EXCEPT when the pcm forced rich to test the bad 02 sensor that is not switching.

my earlier explanation did not make much sense now that I think about it, unless you had multiple misfires on one bank and it would have to be a steady misfire so you would have had codes for the misfires. Only way you would not have had codes for misfires in that case is if the misfire monitor was not ready.

so as long as your KAM was not reset or the battery disconnected recently then your misfire monitor should be active, so ignore what I said in my first post.

 

[mransr]

OK thanks, I follow you...

I disconeted the battery when I changed the O2 sensors, was that a bad idea?

 

[Superhereaux]

no, you'll be fine

 

[Mike97gt]

OK thanks, I follow you...

I disconeted the battery when I changed the O2 sensors, was that a bad idea?

changing at least the 02 sensor with the code was a good idea. might be your problem..

 

[mransr]

thanks Mike,

even if it wasn't the whole problem, it was probably worth changing them just due to the mileage on 'em...

I've gone about 150 miles since I replaced them and the car feels a lot better, and gas mileage seems to have improved greatly too

 

[Mike97gt]

thanks Mike,

even if it wasn't the whole problem, it was probably worth changing them just due to the mileage on 'em...

I've gone about 150 miles since I replaced them and the car feels a lot better, and gas mileage seems to have improved greatly too

I think you found your problem.. now I will remember this if i ever run into a car loosing power that way at work, I would have not guessed at a 02 sensor causing that..

02 sensors going bad used to cause problems like this often in the OBD 1 cars, 1996 and newer cars ( obd2) are a lot better at weeding out bad readings and most of the time can operate pretty much normal with a bad 02 sensor.