90 Lx Convertible

Tom Musso 90 lx

New Member
Aug 9, 2013
Hello, fellow 5.0 ers, I am new to this forum, I just purchased a 90 lx I bought it knowing it was not running to its full potential. I can see that it has a lot of new parts maf sensor, new injectors,tps,fuel regulator, egr, water pump, etc. The first problem is it won't start when cold, a little spray of either fires right up,let it run for a minute or so and shut off it will fire right up. Sensor? When running it runs extremely rich, burn your eyes rich. Vacuum leak? I have 14 lbs of vacuum warmed up. Enough? I cannot find any visible leaks. I have tried to run self test but it does nothing, engine light just stays lit and does not flash. Ecu defective? If you have not figured it out yet I'm not used to computer controlled engines last car I built was a 67 FAIRLANE with a 289, twenty five years ago. Any help would be much appreciated. Thanks Tom
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SN Certified Technician
Aug 27, 2012
In the garage
first thing to do is to run the codes stored in the computer to determine why the CEL is on. You can do this both with the key on engine off and key on engine running. Then come back with the codes and people can have a better understanding what is wrong. Jrichker has an excellent writeup on how to pull the codes. All you need is a paper clip to jump the OBD test plugs together.

You need to tell us more about the setup of the engine- stock. modified, N/A or power adder, etc. Your vacuum reading is normal for a stockish engine with no radical cam or a blower.


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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-$36.

Tom Musso 90 lx

New Member
Aug 9, 2013
I tried the paper clip method before last post I could not get it to do anything I made sure contacts were clean, and the check engine light did nothing but stay on, it would not flash. I tried it several times. I will check pressure this week and report what I find out. As far as I know engine is stock, it has the 19lb injectors ( yellow). All the receipts that came with car don't show any performance parts. The exhaust is the only thing altered, the o2 sensors are in place. Thanks for the help I will post pressure reading as soon as I get it.