1990 5.0 runs fine when cold but terrible when hot

vacuum leaks will generally cause the car to run rough at idle and will smooth out as you put more load on the vehicle.

I would start tackling your codes first. 21 is an ect related code so I would double check that your vehicle is getting up to operating temperature and not overheating. You could have a cooling system issue like a bad thermostat, water pump or radiator or the ect and/or the ect circuit is bad.

As for the code 41 and 91 they're probably just indicating your car is running lean and there's nothing wrong with the sensors themselves

Did the distributor come with a new TFI ignition module ? Even so, I would suspect that it's bad or possibly the heat sink grease is missing or lacking and the heat can't be disapated correctly. Ignition problems after warm up is classic for the TFI.

Mustang5L5 said:

If you run the test cold, you always get a code 21.

Make sure the engine is fully warmed up before running the test, then see if the 21 is still there.

Click to expand...

oh... That's just like on obdII cars doing a koer test. didn't even think of that

whats this heat sink grease? and how can i tell if my distributor is TFI? i replaced my stock unit with a msd distributor about a year ago trying to solve this same problem, but its still running weak sometimes.

All the 86-95 EFI 5.0 Mustangs use a TFI module.
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 155617

See Automotive Tools Specialty | Auto Mechanic & Technician Diagnostic, Testing Equipment | Thexton

See http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif for some more useful TFI Module information.


If your car runs poorly at times, dump the codes.

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.

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.

edit. sorry.

ill dump codes as soon as i can find a helper to start my car for me while i hold the voltmeter. its hard to find good help.

My vote would be for the tfi, if you choose to replace it, only use a real ford unit unless you want to do it again in 6 months.

When the engine warms up it heat soaks the tfi, it's why on 94/95's they moved it.

i didnt get a code 11, just 4, then about a quarter of a sweep, which i didnt count as a "sweep" then, i got 21,91,31. the second time i did it, i got a 4, then 91,31.