92 mustang lx 5.0 tach surges to 3500 but engine idle

1992foxbody50

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Jan 25, 2011
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Looking for suggestions! The tachometer on my 92 mustang LX 5.0 raises to 3500rpm when engine starts however, the engine is idle. When the engine is revved past 3500rpm the tach appears to work appropriatelyy and when the engine is shut off the tachometer falls to zero, turn the key on and the tach appears to jump to 250rpm, start the car and it is at 3500 again. I just purchased the car a month ago from the guy I sold it to 11 years ago, pretty neat story. The car has been maintained stock except for; cold ram induction, pulleys, 160 degree thermostat, aftermarket rims(have original and will put back on immediately), aftermarket stereo, and catalytic converters cut off (straight pipes) to the mufflers, sounds great but not street legal in any state. Also, When accelerating through 1-2-3 the check engine light comes on when car is at 3500rpm and above however goes off when rpm's fall. Thinking this is an emission thing since cats are cut off, what do you think? Also, I am looking to put cats back on but would like to go aftermarket to reduce it down to 2 converters instead of the 4 that came stock. Again, any input appreciated.
 
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No idea just yet on the tach behavior...I would however be curious about which electrical lines were tapped into for the aftermarket stereo.

My first step would be to identify the malfuction codes which are resulting in the check engine light. I would start there since the computer is identifying known issue(s).

I don't believe the cat removal would be associated with any MIL lights as the 92's did not have any sensors which could determine if a cat was present or not. In fact, the only sensors I recall being on the exhaust are the O2 sensors...nothing more.

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

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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.

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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.

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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 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.