code 91

stang9150 · Mar 23, 2010

ok iam getting a code 91-ego sensor problem due to fuel pressure or injectors out of balance.

i think this means i have an o2 sensor in trouble i just replaced both sensor with brand new bosch sensor this weekend.I also ran the injector check test and came back with number 7 cylinder in trouble.Does this mean i have an injector going out and the o2 sensor is getting confused or is the o2 just bad out of the box.I notice normal driving i do not have the check engine light on but if i get into it some the light will come on.

The reason i changed the o2 is here http://www.stangnet.com/mustang-forums/809062-strange-misfire.html

I drove with them unplugged with no more breaking up issues for a week plugged them back in and the breaking up came back,thats why i changed them.Thanks for any input guys

SpartaPerformance · Mar 23, 2010

Did you get that code with engine running or is it in continuous memory?

stang9150 · Mar 23, 2010

I got the code 91 today with the koeo test and also with it running

stang9150 · Mar 24, 2010

going to pull the kick panel tonight and check the voltages on the ecu wiring for the o2 sensor.This problem is getting frustrating.

SpartaPerformance · Mar 24, 2010

stang9150 said:
going to pull the kick panel tonight and check the voltages on the ecu wiring for the o2 sensor.This problem is getting frustrating.

According to AllData it can be caused by a few problems, even as simple as contaminated oil or even sludge build up around PCV system or bad/damaged PCV.
Other possible causes are:
• Fuel injectors
• Thermactor
• Vacuum
• MAP sensor
• O2 sensor itself
• Canister purge

liljoe07 · Mar 24, 2010

Swap the O2 sensors around in the H-pipe. Just the sensors not the wiring!

Re run the KOER test and see if code 41 shows.If it does, then the 02 is bad. If not, and code 91 still shows, its a wiring problem, or that bank is actually lean.

stang9150 · Mar 25, 2010

Thanks for the info guys!Moving the o2 sensor was on my list of things to check to see if the code follows it.I also was doing some searching and found that it could be the longtubes causing it.They could be losing heat the o2 sensor are getting out of wack.

Sparta the canister and all other smog equipment is off the car has been for 10 years that i have owned it.The list of mods just for background.

331
afr 185 heads
fti cam
performer upper and lower intake
bbk 70mm throttle body
stock mass air
stock injectors
bbk longtubes
bbk off road h pipe
msd 6al

I also read that if i got the car tuned that they could increase the timing on the o2 sensor due to the long tubes.I have to hold off on this because i have new injectors,mass air and intake to put on.

gstsaver · Aug 9, 2018

Did you ever find out what the issue was - I have code 91 and cylinder 7 as well.

Mustang5L5 · Aug 9, 2018

Swap the O2 sensors. If it's truly a sensor issue the 91 will turn into a 41.

However, likely cause is Cylinder 7 is not firing as well causing one cylinder o be pretty much an air pump and generating the code 91. You need to troubleshoot cylinder 7. Either a bad injector, bad ignition (spark plug or wire), low compression, or fouling of the plug due to oil leaks in the valve seals.

gstsaver · Aug 9, 2018

Thanks. I will check the O2 sensors.
I just changed the plugs and wires and all plugs looked good including 7..there was no fouling etc.
It could be a bad injector ? How does one check for that ?
Additionaly I guess I should do a compression test ?

Mustang5L5 · Aug 9, 2018

Use a multimeter and check resistance of injector coil. Should be 14.4 ohms.

Using a 9v wire and leads, touch to injector prongs and listen for click.

Attach fuel presssuee guage to rail cycle key, touch 9v battery to injector and see if pressure bleeds down.

Swap injectors and see if problem follows injector.

silverlx50 · Apr 26, 2020

gstsaver said:
Thanks. I will check the O2 sensors.
I just changed the plugs and wires and all plugs looked good including 7..there was no fouling etc.
It could be a bad injector ? How does one check for that ?
Additionaly I guess I should do a compression test ?

Old thread but if you are still on here, what was the fix? There are so many good threads but they generally don't get followed up on... even the old ones help some of us...

silverlx50 · Apr 26, 2020

Lol nevermind, looks like they were last here in 2018. Man, it is so frustrating to never know what the fix actually was...

General karthief · Apr 26, 2020

It really sucks when they don't post the cure.

Mstng93SSP · Apr 26, 2020

Here is the cure

View: https://youtu.be/q2aBn-QuPVw

code 91

stang9150

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SpartaPerformance

stang9150

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stang9150

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SpartaPerformance

liljoe07

stang9150

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gstsaver

Member

Mustang5L5

That is…until I whipped out my Bissell

gstsaver

Member

Mustang5L5

That is…until I whipped out my Bissell

silverlx50

silverlx50

General karthief

wonder how much it would cost to ship you a pair

Mstng93SSP

You have a nice rear end there Dave.

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code 91

Member

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That is…until I whipped out my Bissell

Member

That is…until I whipped out my Bissell

wonder how much it would cost to ship you a pair

You have a nice rear end there Dave.

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