No check engine light

[SPSmith]

So here we are again with my 1991 Explorer XLT 4x4. After replacing the ECM and fuel pump relays and checking every ground I could find I, cleaned up the battery cables and replaced both clamps.

I jumped the truck and it fired right up so I disconnected the Jumper cables and let it charge for 20 minutes. I was going out to shut it down thinking "problem solved", got ten feet from it and it died. I tried to start it (turns over normally) but it will not start. After running 15 minutes I had checked the gauges and all were within normal range.

The only thing I notice is no check engine light when the key is moved from off to on. The relays I replaced function properly and the fuel pump cycles as it should.

What are the chances that the computer is flummoxed and if so, why would it have started up and run normally as it did just to die for no apparent reason?

 

[MrShorty]

If the CEL is off KOEO, that is almost always an indication that the PCM is not getting power. The first thing I'd do would probably be to verify that there is power to the PCM. While you're at it, don't forget to check the computer's ground connections.

It's not unheard for the light bulb to blow. I would be tempted to change the bulb to make sure it is good. While you are at it, make sure the wiring between the bulb and the PCM is good.

Pulling codes might be an interesting exercise. If you hook up a test light to the STI OUT pin on the selftest connector (see the test hookup pictures in BrooklynBays test protocol thread in the EEC-IV forum). If the computer can talk to the self-test connector but not the CEL, that could suggest a problem with the check engine light/circuit.

 

[SPSmith]

Thank you very much, MrShorty.

What's puzzling is why it would run just fine for more than 20 minutes and then simply die. So here's a question. When you remove both battery cables for a period of time and then reconnect, is the computer reset to factory defaults or is the reset limited to the trouble codes?

 

[MrShorty]

When you disconnect power, the computer should clear everything in keep alive memory, which includes CM codes and any other parameters, like fuel trims, that the computer "learns." Effectively, as you suggest, going back to the "factory" presets.

 

[Suicide77]

Lol! I actually just replaced the check engine light bulb today! How would you know to replace it if you always want it off anyways?

Before:

After:

The Charred Remains

There is always a possibility the light has gone the way of the Dodo. As far as the not running part there is also a chance that a silly battery fuse has burned out. I had one and couldn't figure out why it would barely run while showing super low amps on the battery. Turns out a cheap 10 cent fuse had burned out in the main fuse box.

Hope this helps!

 

[SPSmith]

After throwing up my hands and walking away from the project for a while I dismembered the dash (what a PITA) and found the CEL to be burned out. I replaced it and now have a CEL. Turn the key to the on position and it stays on until you turn the key off. Is that a good thing?

 

[Bruces expo]

After throwing up my hands and walking away from the project for a while I dismembered the dash (what a PITA) and found the CEL to be burned out. I replaced it and now have a CEL. Turn the key to the on position and it stays on until you turn the key off. Is that a good thing?

Did You Start it And Does It Go Off After Initial Start up ?

 

[SPSmith]

After replacing the CEL it still won't start. I disconnected the battery completely for a week or so, reconnected and the engine turn over normally but will not start. There is no flashing of the CEL to indicate a trouble code. Cranking fuel pressure is normal. There is spark at the plugs.

Now what?

 

[MrShorty]

4 things are needed for an engine to start:

1) Air -- usually not a problem because the air intake would have to be seriously plugged up for the engine to not get any air. Possibly the IAC valve pathways are plugged (still not common, though). Will it start if you hold the throttle open a little bit?
2) Compression -- again this usually isn't the problem because it requires significant mechanical failure to get no compression to the point of not starting.
3) Spark (at the correct time) -- Common, but you say it has spark, and it would be a real trick for the timing to be so far off that it wouldn't start, unless the spark plug wires were not routed correctly.
4) Fuel -- Probably most common cause of no start. You say it has fuel pressure at the rail. If everything else to this point seems right, then my next thought would be that the injectors aren't firing. Can you hear the injectors fire? Have you tried starting fluid or putting a little gas into the air intake?

 

[SPSmith]

The answer to #1 is no. It turns over normally but does not start regardless of throttle position.

#2 It has darn fine compression in spite of having 120K + miles on it.

#3 I went so far as to connect an old school timing light with pickup between the wire and spark plug and used that to check all 6 cylinders. Plenty of spark.

#4 There is still fuel at the rail. Today I went out and tried starting fluid. As one would expect with NO fuel it fired and "ran" until the starting fluid was burned. Honestly, I wouldn't know what a firing injector would sound like. I could build you a 400 HP 1960s era 351, but these computer controlled vehicles are beyond my experience.

Computer or something else?

 

[MrShorty]

As one would expect with NO fuel it fired and "ran" until the starting fluid was burned. Honestly, I wouldn't know what a firing injector would sound like.

It's just a click, not unlike the sound a relay makes. It might help to use a stethoscope or equivalent to pinpoint the injectors when you listen to them. If it runs on starting fluid, it likely is a fuel problem. If it really has pressure, that suggests something wrong with the injectors. See if the injectors "click" while trying to start. If not, then check for power to the injectors. If the injectors have power, then move down to the computer to see if it is doing its part to supply ground to fire the injectors.

 

[SPSmith]

Where would one check to see if there is power to the injectors? (It's a 1991 XLT, by the way)

 

[MrShorty]

It seems like the easiest would be to pick one of the injectors that is easy to get to, pull the plug off, and check the wires for one that has 12 V (KOEO). If one injector on each side has power, then they probably all have power (at least as far as a no start goes).