Engine New Issue Now That Tach Works...lol!! :(

45,000 RPM!

The TFI module mounted on the distributor is usually the culprit for a high speed miss on a warm engine. If the problem does not occur when the engine is cold, the TFI module is definitely suspect. You may need a special socket to remove the TFI module, but most auto parts stores will have one for $5-$7.

Be sure to use plenty of the heat sink grease on the new TFI and clean the old grease off the distributor.

View attachment 142806

See Automotive Tools Specialty | Auto Mechanic & Technician Diagnostic, Testing Equipment | Thexton
diagram courtesy of Tmoss & Stang&2Birds




Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

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.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

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





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. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity 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.

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 clutch (if present) is pressed to the floor, 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.

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

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




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.

When your tach is reading 4500rpm (72mph in 2nd gear according to you), the engine's actually turning at 6000rpm. This assumes that you really WERE doing 72mph at the time.

I take it this is an automatic?

What's done to your car?
Factory Fox gears are 2.73, 3.08, and 3.27.
3.27 was an auto gear set only, and 5 speeds had the option of 2.73 and 3.08.

Even if the OP's car did have 2.73 axle gears, the engine would be spinning at 5300rpm at 72mph in 2nd gear so the tach's still reading way too low anyway.

Jace78 said:

5 speed



All I was told is the car has flowmasters, everything else stock. When I asked about gears the seller said he thought it had 3.23's... calculations and the rpm vs speed chart jrichkter posted def points to me having at least the 3.08 if not in the 3.20 range.

Click to expand...

If the rear end hasn't ever been touched, the highest ratio you could get for a 5 speed fox was the 3.08 so it's more than likely that you have that gear set.

Jace78 said:

I am not sure what you are saying, or asking. I know I am doing 72 mph...verified speedo with gps up to 100 mph and speedo is dead on. According to RPM calculators if I have 2.73's I should be about 5000 rpm at 70rpm. That being said This morning the tach went up to 5k in first gear... and doing other calcs last night it points to me having a 3.23 or so rear end. I think I am going to just:
1: Clean MAF
2: Replace fuel filter
3: Test/Replace TFI
4: Rebuild fuel injectors
5: Plugs rotor cap wires

Also check other voltages, cats checked out so either fuel or ignition at this point.

Click to expand...

And its a 5spd. So you could go any rpm in any gear. This whole post makes no sense to me. I've read it a few times and I have no idea what we are even trying to fix. I guess me dumb.

The rear end gear question is very simple to solve.
1.) Jack the car, up so that the rear wheels are off the ground. Be sure to use jackstands under the body mount for the lower rear control arms for safety's sake. With the jackstands supporting the body, the rear axle will drop so that it will be easier to see and count the number of turns the driveshaft makes.
2.) Make a chalk or paint mark on the driveshaft so that it is easy to see.
3.) Make a paint or chalk mark on the tire.
4.) Turn the tire one full turn while counting the turns make by the driveshaft.
2 /3/4 turns of the driveshaft = 2.73 gears
3 turns of the driveshaft = 3.08 gears
3. 1/4 turns of the driveshaft = 3.27 gears
3 1/2 turns of the driveshaft = 3.55 gears
3 3/4 turns of the driveshaft = 3.73 gears.

Jace78 said:

^^ in 4th gear for 1:1 ratio right?

Click to expand...

No. Do it in neutral. What gear the trans in doesn't matter at all in this.