RPM Rising to 3K after Installing New O2 Sensors!!! Please Help

MaRu_GT

Member
Sep 11, 2006
66
0
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The O2 sensors were removed when I bought the car. So, I got 2 new motorcraft O2 sensors and new O2 sensor harness (which plugs into the main harness). After installing these, I started the car for first time and it was normal. I stopped the engine and tried to start it again but this time the RPM went all the way to 3K, hanging there and fuel smell was getting more and more all over the place.
Now I disconnected the O2 harness and it is back to normal! Any idea why did that happen? I thought my A9L will recognize the sensors and run normally.
These are some of my car’s details:
•Engine:
302 CID BOSS BLOCK with GT-40 HEADS AND B CAM
FRPP Cobra 93 intake
Accufab 70MM Throttle Body
FRPP 24 LB/HR injectors
BBK 190 LPH fuel pump
BBK adjust fuel pressure regulator (40 psi).
PRO-M 75mm Mass Air Meter
BBK Cold Air Fender Well intake

•Exhaust:
HEDMAN equal length headers
2 ½ MAGNAFLOW X-Pipes
MAGNAFLOW Cat-Back

•Ignition System:
MSD ignition system (coil, distributor, wires), timing set on 12 Deg
Optima battery (RED)
Motorcraft spark Plugs gapped to 0.040

•ACCs:
March Underdrive pulley set
Air pump is deleted (No EGR )
 
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See the "Surging Idle Checklist for help with all your idle/stall problems.

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.
 
It is possble that the previous owner(s) adjusted the idle so the car would idle under control for the situation they created. So clean the items as suggested in the answer to your question and reset the computor and idle.
Good Luck,
olbob
 
It is possble that the previous owner(s) adjusted the idle so the car would idle under control for the situation they created. So clean the items as suggested in the answer to your question and reset the computor and idle.
Good Luck,
olbob

All sensors in the car were replaced to new ones from motorcraft including o2 sensors.

I disconnected the battery for 30 mins to clear the KAM... and run the engine.. same problem exists. When i disconnect the O2 harness , the problem is gone!

I posted this on others forums.. someone told me that you might be connecting the wrong harness for o2... I bought the one which is applicable for 87-93 5.0 manual.. they said you will need to match wire colors on pins from main harness.. and swap the colors between the 2 connectors (main and o2) but really i did not find the way to match since all colors on the sensors connector are different!! i compared this to my bro's 88 saleen .. which is stock.. and his main harness 8 wires colors are totally different from the o2 harness wires colors and he has no issues. So I started using some Ford's wiring diagrams for my car model and matching ground/igintion/signal wires from the sensors all the way to main harness, and it looks just fine !!
 
The engine fuel injecton harness has an orange wire in it that is the ground for the O2 sensor heaters. make sure it s bolted down to a clean shiny spot on the engine or body.

The correct O2 sensors are 3 wire units. The 4 wire ones are for the 4 cylinder engine and will not work on a V8 without modifying the O2 sensor harness.

Dump the codes to make sure you don't have other problems.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 18-Mar-2011 to advise differences in code information for engine running and engine off codes.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical

Underhoodpictures007-01.jpg


Underhoodpictures010.jpg


If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

attachment.php


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

attachment.php


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
Or for a nicer scanner see http://www.midwayautosupply.com/p-7208-equus-digital-ford-code-reader-3145.aspx– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
 
The engine fuel injecton harness has an orange wire in it that is the ground for the O2 sensor heaters. make sure it s bolted down to a clean shiny spot on the engine or body.

The correct O2 sensors are 3 wire units. The 4 wire ones are for the 4 cylinder engine and will not work on a V8 without modifying the O2 sensor harness.

Dump the codes to make sure you don't have other problems.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 18-Mar-2011 to advise differences in code information for engine running and engine off codes.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical

Underhoodpictures007-01.jpg


Underhoodpictures010.jpg


If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

attachment.php


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

attachment.php


The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
Or for a nicer scanner see http://www.midwayautosupply.com/p-7208-equus-digital-ford-code-reader-3145.aspx– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

I checked that orange wire! it is not bolted to anywhere!!!!! Could that be a reason why my RPM is behaving so when I connect the o2 harness to the main harness????
Can I bolt it to any part of the body like other grounds?
 
I checked that orange wire! it is not bolted to anywhere!!!!! Could that be a reason why my RPM is behaving so when I connect the o2 harness to the main harness????
Can I bolt it to any part of the body like other grounds?

Any clean and shiny metal on the body or engine.
 
That's the first part of the two parts of my post. What codes did you get?

I do have Actron for codes pulling- the one you are talking about. Is it similar to the first method?
I am asking so because I remember I connected it to my self-test connector and it didn't pull any codes, i have no idea why? However , I used the same Actron on my brother's 88 and it pulled the codes with no problems. This was like 6 months ago, I never tried to use it again thinking that there might be an issue with my self-test unit.
 
The A low cost cton scanner does the same thing as jumping the test connector. It's just a lot more expensive. You still have to count the flashes of the indicator.

You may have some computer/ wiring problems. I need to know what year the car is to post the correct tech note to troubleshoot & fix the problem.
 
The A low cost cton scanner does the same thing as jumping the test connector. It's just a lot more expensive. You still have to count the flashes of the indicator.

You may have some computer/ wiring problems. I need to know what year the car is to post the correct tech note to troubleshoot & fix the problem.

It is 1990 5.0 manual. It is with A9L.
Maybe it's worth mentioning that few days ago I was curious and replaced my A9L with A3M, I was thinking maybe the problem will disappear if I am using another computer.

So I connected the A3M to my car, and when started the engine , it would die after 5 seconds. I tried to start again but it didn’t run at all , only cranking. I used this A3M into another car 2 months ago and it just run perfectly with no issues. I am not sure if that car was originally A3M or not, but it was manual.
When I put back my A9L into my car , the engine run again with no issues..

Now I am really confused! I thought that A9L and A3M are the same and they will both run on any 87-93 5.0 Manual.

I really dont understand what's wrong with this car:shrug:

Thank you very much for your help and following up.
 
The A9L, A9P and A3M are interchangeable in a 89-93 5.0 Mustang. Cars 86-88 will need a modified harness or a harness out of 89, 91-93 5.0 Mustang. The 90 harness will not work without re-pinning several connectors.

Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.


How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned will work
properly. That will severely affect the car's performance. You will have hard starting, low power
and drivability problems.
Since it is a dedicated ground, it passes through the computer on its
way to the computer main power ground that terminates at the battery pigtail ground. It should
read less than 1.5 ohms when measured from anyplace on the engine harness with the battery
pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector black/white wire gets jumpered to power
which either burns up the wiring or burns the trace off the pc board inside the computer.
That trace connects pins 46 to pins 40 & 60. Only an experienced electronics technician can open
the computer up & repair the trace if it burns up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering
it to power can produce unknown results, including damage to the computer. The ohm test
simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than
1.5 ohms.

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If that check fails, remove the passenger side kick panel and disconnect the computer
connector. There is a 10 MM bolt that holds it in place. Measure the resistance between
the black/white wire and pin 46 on the computer wiring connector: it should be less than
1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the
computer is suspect. On the computer, measure the resistance between pin 46 and pins
40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal
ground has failed, and the computer needs to be replaced.


See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.


If the first ground check was good, there are other wires to check. Measure the
resistance between the STI computer self test connector (red/white wire) and pin 48 on
the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer wiring connector.
eec04.gif


Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every
sensor on the engine except the MAF is connected to it.
88-91_5.0_EEC_Wiring_Diagram.gif


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info

harness02.gif
 
You may have gotten the wrong 02 harness. They are not inter-changable between all years. Having the wrong year 02 harness will toast an ECM, you wont be able to pull codes, and more than likely explains why it goes away when the 02 harness is unplug. Run the alternate code pullling method JR listed above. If you get codes for TPS,ECT,ACT,EGR and MAP/BP sensors being out of range, the ECM is toast, and you need to re-pin the 02 harness.

A quick look at the 02 harness will tell the truth. There is a looped wire on the back of the 02 harness where it plugs into the main EFI harness. If that looped wire connects a blue/yellow wire to a white/pink wire, then its setup wrong for a manual transmission setup.
 
Liljoe07: I ordered the harness for manual 87-93 , I saw the other one also for Auto, anyway I attached here a picture of what I have noticed in my car for main harness wires colors that connect to o2 harness - I picked the colors as I saw , don't mind if I missed the exact color due to aging of wires.



How to re-pin the O2 harness ? is there any logic behind matching the colors from main harness to O2 harness?

jrichker: I will check out the grounds and resistance as you suggested and I may come back with questions.
 

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OK!!! so the loop from the picture i attached is the black wire (1st top right and 2nd bottom right)--- between NDS and AUTO, am I RIGHT?
So the harness they sold to me was for Auto not Manual -- ??

I am also comparing that to my bro's 88 manual, and the loop is swapped to the last line of 2 pins -- he has the 2nd bottom pin (from right) empty while I have the 1st bottom pin (from right) empty, he has no issues with O2 sensors connected.
 
It depends... Like I said, even though you bought one for a manual, not all year manual O2 harness are compatible for all year cars, even though they are manuals. You need that loop to connect the blue/yellow to the purple/yellow on the main efi harness side.
 
It depends... Like I said, even though you bought one for a manual, not all year manual O2 harness are compatible for all year cars, even though they are manuals. You need that loop to connect the blue/yellow to the purple/yellow on the main efi harness side.

Why I was saying that because if you look at the picture below from fordfuelinjection, BLACK CONNECTOR#40 looks like the O2 harness connector, and it says that the pin no1 at bottom from left is for MANUAL and no2 at bottom from left is for AUTO, I thought that makes sense with what you have explained earlier, and that I got the Auto O2 harness. If you look at this picture, my harness loop is between AUTO and NDS pin 30 , while MANUAL is empty.
 

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Yes but not all O2 harness's are Pinout the same as far as those circuits go. That picture won't jive will all years. The wiring on the main efi harness may have those circuits in a different location, but the loop wire hooking to something it shouldn't. I'm driving right now... LOL.... Let me get home and ill post up what I'm saying a little better.
 
What I was trying to say was that even though your loop wire indicates its setup up for a Auto Trans, that the Circuit on the opposite side may not be the NDS and Auto circuits, It may be connecting Manual and Auto which is probably what it is doing.

There is a few other things it could be, but this has to be fixed first regardless.