91 gt throwing codes 41, 63, 91

I thought 1991-93 mustangs had 3 digit trouble codes? but anyways how old is your exhaust system? The reason I ask is if it's getting old and rusty it is becoming a poor return ground path back to the ECM via the orange wire that is bolted to the bell housing, I.E. poor electrical connection from O2 sensors metal shell, the metal tubing of the exhaust system, the header, the engine block you get the point, I know this to be and actually happends.
Also go thru the routine of cleaning the Mass Air senser, if it has crap on it it throws it's calibration off.

41 and 91 are o2 codes
41 means o2s reading dead rich and 91 is lean
could be bad o2s or a ground issue. i have just recently cured an o2 problem. not good enough grounding can cause problems. make sure your orange HEGO ground wire is good and make sure your main ground and exhaust are grounding good.

i ended up running extra grounds, from my motor mount bolt to the frame and from the back of my H pipe to the floorpan. seemed to fix my problem. you can never overdo your grounds

and the 63 is a TPS code voltage too low code. could be a bad sensor.

FOXFAN, thanks for backing me on this one, I didn't think many people knew about this problem. 1987-93 (3) wire O2 sensor, 1994-present (4) wire O2 sensor, ford finally figure the out the problem.

i was getting some o2 codes yes.

i have a tweecer rt so i can datalog my engine running so i can actually watch my o2 sensor voltages move around and they werent acting right, i knew something was amiss before i ever scanned the codes.

one of my o2s was reading ok, the other was reading dead lean when the car was actually running rich due to a baseline tune i was trying to work with. but since i wasnt getting good o2 feedback and never getting into CL (o2s fault) and never could do any useful tuning.

a good working grounded o2 should flucuate above and below .45 volts so the o2 actually reads "rich/lean/rich/lean/rich...." and will never settle on .45 volts (stoichiometric AFR of 14.7) its not supposed to settle they are supposed to switch from hgh to low.

one of my o2s wouldnt switch, since the sensor was bad it just read lean all the time and threw a code.

i went ahead and forked over the dough for 2 new o2s and threw them in, logged the car again and all seemed better. but then i started having problems again.
turned out i had some ground issues. the orange HEGO ground is just for the heating element of the o2 not the actually o2 itself. the metal case of the HEGO sensor itself must ground to the exhaust.

if there isnt a good ground then the voltage travels up the header and tries to go through the header bolts and then has to manage to try to jump past all the gaskets and etc, just not the best way to have it ground.

i mess with it and went to ACE and bought like 4 feet of 4 gauge wire (only used 3 feet) and 4 1/2" copper eyelets, cost me 15 bucks for it all.

starting out i bolted my small braided ground strap (behind drivers side cyl head) to the back of my drivers sied head. i bolted my orange HEGO ground down under an upper intake bolt.

i then relocated my main power ground to b/w a drivers side header bolt and header flange. it was originally on the timing cover is where i had it and there is has to jump gaskets or run through the water which can corrode and cause pitting on engine parts.

i then took out a motor mount bolt on the drivers side, then made a small ground wire, bolted it up b/w the motor mount bolt and the mount ( travels through bolt threads to get a GOOD connection to the block. i ran that wire to the old swaybar mount location (dont use swaybar) so i just bolted it down there.

then i unbolted one of the 4 bolts on the read of the H pipe i have, ran a small ground wire from there to a bolt sticking through the floor that fastens my DS safety loop on the car.

i also cleaned some other small grounds around the engine, like the EEC ground and etc, just sanded the eyes to clean shiny metal etc etc.

be sure to get soem sandpaper or something that you can scuff the metal and get good bare metal contact. its also not a bad idea to throw a small layer of primer on the connections to keep moisture out, but if you like in a relativly dry climate you should have any problems.

after i did this it seemed to cure my problems and my o2 seem to work good so far.


as for the TPS, you mentioned you replaced the sensor, if you just bolted it down that may be the problem.

take a good digital volt meter and then loosen the small screws holding the TPS down, snug just one up, but not tight enough to where you cant move the sensor around.

take the DVM and probe the green and black wires and then turn the key on, take the sensor and rotate it and watch the DVM, with the throttle 100% shut you should try and adjust it to where it read about .99V. snug it down when you get it there. then push the throttle open by hand, voltage should rise smoothly as you give it more throttle by hand, and at WOT it should read over 4V

if for some reason you cant adjust it to .99v or its not smooth reading with the moving throttle. then the sensor needs replaced.

a TPS out of whack can cause some small drivablilty issues, but a flat out bad sensor will cause alot more problems. my old 4cyl car had a bad sensor and tried to adjust it to .99 volt and it wouldnt do it the voltages went all over and the car ran funny, throttle response sucked etc. i replaced the sensor and cured the problem. the TPS is the main feedback to the EEC of what the driver is trying to do. so the EEC knows if the engine is trying to idle, accelerate or go WOT. without good feedback the EEC pretty much has to guess.

85cleangtvert said:

ok im definitely gonna check it out tomorrow. were ur cars throwing the same codes?

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Yes, for 9 months it would throw codes (41,91) the engine light would come on after it would warm up and the idle surged up and down, had no power. I bought a SNEEC-4 to help find my problem, I wish I had known about the tweeker at the time, I'd have bought it.

honestly at the power level your at now i will throw stock injectors back on, they will handle that fine IMO. you can always put the bigger ones on later if you get more power.

24# injectors are blue tops and if you have them with a stock MAF the car is going to run horridly rich, which would contribute to the o2 codes

Foxfan88 said:

honestly at the power level your at now i will throw stock injectors back on, they will handle that fine IMO. you can always put the bigger ones on later if you get more power.

24# injectors are blue tops and if you have them with a stock MAF the car is going to run horridly rich, which would contribute to the o2 codes

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LOL. I think foxfan's got ya covered, He's saving me a lot of typing.
Oh if you have a K&N filter, I can almost guarantee your mass air sensor is dirty.

you can tune it with a tweecer or have pro tune it for you on a dyno? both of which wil cost around 500 bucks if not more. not worth it IMO for your combo. judging from your mod list in sig i woulkd guess your still under the 300BHP mark

i still say throw the stock injectors on. it will run ALOT better and IMO 19s will feed your engine fine.
if 19s will handle your combo just fine there is no reason to run 24s unless your running a dry shot of nitrous but even then you still have to have it tuned for them.

my friend had the same problem in his mustang, he had GT40P heads, cam, etc etc all the bolt ons, about like your combo. he threw 24s on thinking he needed more fuel. the car ran like crap even with the "calibrated MAF" and he even tried to cobra maf and it didnt help. the car would idle and drive good but he lost power. he thew the stock injectors on and got a bigger MAF meter with the sensor for 19s and the car ran tons better, drove better, better WOT performance.

bigger injector are not just a bolt on and go deal. sure some people throw 24s on a stock untuned eec and it may run and drive fine. but the car probably will run richer and normal and lose a little power. try it with 42s and you really lose some power. i have expierenced tht first hand. some of the aftermarket setups are supposed to fool the EEC but thats a hack way and never does run perfect like that... the only good and correct way to put bigger injectors on is to have the EEC tuned for the, match the MAF transfer to the MAFS you run with the bigger injectors and have the injector slopes changed.

just give the 19s a try, if it doesnt work out, you can always put the 24s back on and have it tuned.

good luck.

Summit Racing is where I got mine and was the cheapest I could find at the time( Ford Motorsport injectors), look around online you might find some good prices.

I had 41,91 and my CE light would go off and on while driving quite often. About two weeks ago I read this thread and then grounded my H-pipe from a rear bolt to my seat bolt. Haven't seen a CE light since. I'm probably going to reset the computer today and then see if I get 41,91 with a KOER test. I'll let you guys know.

Also, i think I may have gained about 2-3 MPG on this last tank because my 02s are now reading properly. Thanks guys.

85cleangtvert said:

i put stock injectors and maf back on and it runs but still like before. the plug wires look good and the plugs could stand to be changed so i am going to put those in.

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Hows your car running? running better or still like Poop?

loveblueoval said:

I thought 1991-93 mustangs had 3 digit trouble codes? but anyways how old is your exhaust system? The reason I ask is if it's getting old and rusty it is becoming a poor return ground path back to the ECM via the orange wire that is bolted to the bell housing, I.E. poor electrical connection from O2 sensors metal shell, the metal tubing of the exhaust system, the header, the engine block you get the point, I know this to be and actually happends.
Also go thru the routine of cleaning the Mass Air senser, if it has crap on it it throws it's calibration off.

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What's up with this? I ran into the same problem when reading the codes off my 93 cobra. The code reader manual indicated a 3 digit code as well for 1993, but the reader displayed two.

Foxbody93 said:

What's up with this? I ran into the same problem when reading the codes off my 93 cobra. The code reader manual indicated a 3 digit code as well for 1993, but the reader displayed two.

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It's a "5.0L Mustang" thing....... most of post '91 Ford s followed the 3-digit format, one of the exceptions was....... 5.0L Mustangs. The 3-digit format was implemented just for the SN95 5.0's.