Weird Misfire

[krazedstang]

I have an 88LX 5 speed with BBK equal lenght shorties, bassani xpipe with hi flo cats, and american thunder mufflers, i just finished putting a clutch in it and about a week after driving it around, ive had this little problem, when i start it up she's fine for about 5min of driving, then she gets what feels like a slight-medium misfire until about 2500rpm, and this continues for another couple min then suddenly she runs great. not sure if the O2 sensors have anything to do with this, i had lost one of the sensors and just used a 18mm drain plug in there until i buy a new one. ideas??
Thanks

 

[jrichker]

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 19-May-2009 to update drawing for dumping the codes on 86-88 Mustangs with no check engine light.

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

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 http://www.troublecodes.net/Ford/

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.

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.

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.

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 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.

 

[Kdubslugga]

I would think that no oxygen sensor would cause all kinds of problems. It makes sense not to have any problems when its cold because your car is in open loop, meaning your o2s arent working. When car warms up a bit, the car enters closed loop, meaning 02s start operating and control all kinds of EEC functions!

 

[HISSIN50]

I agree that you need to get an O2 back in that pipe. It might not fix your issue but you need it anyhow.

 

[krazedstang]

i dont have to take emissions in this state so if its not going to fix the current problem, im not gonna worry about it yet, and i understand how it would run fine in open loop until the engine warms up, but once the engine is warm i have a misfire for only about 5min or so then it goes away, thats the part that seems odd to me

 

[HISSIN50]

Closed loop is a function of time, not engine temperature. The heated O2s require up to a couple minutes to light off and then the car is in closed loop, regardless of engine temps.

The car simply needs both O2's - they are not for emissions but for driveability.

Good luck.

 

[krazedstang]

i understand that, but why would it run rough, then all of the sudden run fine. once it got out of open loop wouldn't it run rough the whole time instead of just for a couple min?

 

[HISSIN50]

i understand that, but why would it run rough, then all of the sudden run fine. once it got out of open loop wouldn't it run rough the whole time instead of just for a couple min?

I agree that you need to get an O2 back in that pipe. It might not fix your issue but you need it anyhow.

No one really knows what a particular car will do with only one O2. For it to run decently, it will need both O2's, Thus one might as well install a new one and see how/if the issue changes. It very well might not fix the issue but you probably will not know when the issue is fixed because the lack of an O2 is causing feedback and driveability issues.

What codes did you retrieve?

 

[krazedstang]

I've had a hectic weekend and havent gotten a chance to scan it yet, im going to try and scan it tonight after work, and i get what your saying now, we cant really eliminate that as being the problem if i only have one sensor

 

[stang9150]

http://www.stangnet.com/mustang-forums/809062-strange-misfire.html

Turned out to be my O2 sensors.

 

[krazedstang]

http://www.stangnet.com/mustang-forums/809062-strange-misfire.html

Turned out to be my O2 sensors.

well settles that, looks like im getting an O2 sensor on my way home from work today