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Check engine light is coming on randomly and then goes off


black_demon69

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94 ranger 4.0l 5 speed 4x4




CHECK ENGINE LIGHT comes on usually during hard acceleration getting onto the hiway then will go off either driving @ slower speeds for a little while or shutting truck off.



It is ODB and I am not getting any codes because CEL is off after truck gets shut off



Recently had transmission out so y pipe was pulled down along with the o2 sensors but I can't think of anything else that would possibly cause CEL as a result of pulling the transmission. BTW o2 sensors have been replaced in the last year.





Only other thing that has been done is to replace factory Amp with a 4 channel alpine Amp but I am using all the factory Amp wiring with the exception of the ground and the battery feed. I really don't think that this would be the problem.



Any thoughts as where to begin with this are welcome :icon_confused::D:icon_idea:
 


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Usually when they do that it indicates a missfire.
 

black_demon69

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Usually when they do that it indicates a missfire.
PPossible but this started as soon as transmission was put back in and engine runs smooth as glass even with light on. Aside from the fact plugs and wires have less than 10000 miles on them and fuel injectors were cleaned @ the same time. However I will pull plugs and inspect and report back if they are the cause.



Engine has always pinged a little on hard acceleration but when heads were off and redone (valves were bad) I cleaned all carbon from the pistons, replaced plugs and wires, and cleaned injectors while I was @it.


Could Maf be dirty? Or maybe just a bad connection on the o2 sensors?
 

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Have you checked for codes?

Might be one in memory.

Fuel Pressure?
Could be that under load the fuel pressure is dropping, and what that does is to cause the computer to adjust the open time of the injectors because O2s start to show lean, if computer has to push open time above 15-20% of its calculated time it will turn on the CEL, and then it would go off once load was lessened(fuel demand dropped and pressure came back up.)
Computer might not set a Lean code for that because it doesn't last long enough, but it might be in memory.
Memory is there so computer has a History of occurrences, computer has KAM(keep alive memory) just like Radio does for clock and presets, long term fuel trims and IAC valve settings are stored there along with temp codes so computer can "check" if a code is reoccurring.
Unhooking the battery clears this memory, same as it clears radios clock and presets, lol.


CEL will usually Flash rapidly during misfires then go off once misfires stop, thats for OBD2 for sure, not sure if its the same for OBD1

Yes, clean the MAF sensor, with the 4.0l OHV engine always clean the MAF sensor, even if a tail light bulb burns out, clean the MAF then put in a new bulb, lol :)
 
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black_demon69

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Have you checked for codes?

Might be one in memory.

Fuel Pressure?
Could be that under load the fuel pressure is dropping, and what that does is to cause the computer to adjust the open time of the injectors because O2s start to show lean, if computer has to push open time above 15-20% of its calculated time it will turn on the CEL, and then it would go off once load was lessened(fuel demand dropped and pressure came back up.)
Computer might not set a Lean code for that because it doesn't last long enough, but it might be in memory.
Memory is there so computer has a History of occurrences, computer has KAM(keep alive memory) just like Radio does for clock and presets, long term fuel trims and IAC valve settings are stored there along with temp codes so computer can "check" if a code is reoccurring.
Unhooking the battery clears this memory, same as it clears radios clock and presets, lol.


CEL will usually Flash rapidly during misfires then go off once misfires stop, thats for OBD2 for sure, not sure if its the same for OBD1

Yes, clean the MAF sensor, with the 4.0l OHV engine always clean the MAF sensor, even if a tail light bulb burns out, clean the MAF then put in a new bulb, lol :)
Codes
41,67,79
 

RonD

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1993/1994 computer uses 3 digit codes, for sure.

And there is no 416 or 779?

If it was 1991 or earlier(1992 can be either, but not both)
41 (R) System lean – Fuel control
67 Park/Neutral circuit fault – PNP
79 A/C is on or pin 10 is shorted to power

System Lean could be fuel pressure as mentioned before
 

black_demon69

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1993/1994 computer uses 3 digit codes, for sure.

And there is no 416 or 779?

If it was 1991 or earlier(1992 can be either, but not both)
41 (R) System lean – Fuel control
67 Park/Neutral circuit fault – PNP
79 A/C is on or pin 10 is shorted to power

System Lean could be fuel pressure as mentioned before

Ran codes 3x to be sure there was a 4second pause in between each code


I will check fuel pressure and report also smoke test


Also found where code 41 was supposed to be referring to the right side o2 sensor
 

Denisefwd93

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I have the same year engine transmission combinations it's OBD2 because I've been getting a code 172 rich condition Bank 1
 

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1995 and up Rangers used the 104 wire EEC-V computer which were OBD2

1994 and earlier used the 60wire EEC-IV computer with OBD, OBD wasn't standardized, different manufacturers used "whats best", for them

black_demon69, just out of curiously, if you have access to the computer, get the date code from it
It's the first 2 digits of the part number, like F17F-12A650-..., F1 = 1991, F4 = 1994, E8 = 1988
Just never ran into a '93 or '94 that used 2 digit codes, anything is possible, and doesn't really matter except, the other 2 codes don't really make sense, so Lean code would be suspect as well

Yes, the 41 code is older so was used at a time when there was just one O2 sensor, on the "Y" pipe of a "V" engine, not one on each bank of the engine.
So there is just 41 Lean, and 42 Rich no designation for Banks

The 3 digit codes have the Right/Left designations for two O2 sensors on a "V" engine, as does OBD2
 
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black_demon69

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1995 and up Rangers used the 104 wire EEC-V computer which were OBD2

1994 and earlier used the 60wire EEC-IV computer with OBD, OBD wasn't standardized, different manufacturers used "whats best", for them

black_demon69, just out of curiously, if you have access to the computer, get the date code from it
It's the first 2 digits of the part number, like F17F-12A650-..., F1 = 1991, F4 = 1994, E8 = 1988
Just never ran into a '93 or '94 that used 2 digit codes, anything is possible, and doesn't really matter except, the other 2 codes don't really make sense, so Lean code would be suspect as well

Yes, the 41 code is older so was used at a time when there was just one O2 sensor, on the "Y" pipe of a "V" engine, not one on each bank of the engine.
So there is just 41 Lean, and 42 Rich no designation for Banks

The 3 digit codes have the Right/Left designations for two O2 sensors on a "V" engine, as does OBD2

Computer is from a 91 Mazda Navajo 4.0l been there since I bought the truck also have the original computer which is still good just never wanted to do the work to swap it out. Previous owner put it in trying to fix something that I don't remember what previous owner said :icon_confused:. Never had a CEL before.



Might be interested to know that I can shut engine off while going down the road and start engine again code is off for a little while...



Maybe I should swap original computer back in. :D See what happens
 

black_demon69

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Found original to the truck

F47F-12A650-CMA
maybe I should put it in and see what happens
 

RonD

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That would explain the 2 digit codes, 1991 computer

Yes, rebooting computer, power off/on, will reset computer so CEL would stay off until it "saw" a/the problem again, but code set before key off would be in memory unless battery power was disconnected.

If engine has 1994 wiring harness and two O2 sensors then putting the 1994 computer back in might give you better diagnostics for the problem of why the CEL is coming on now with 1991 computer.
Wiring is basically the same but............1994 wiring harness would have more wires to computer than 1991 harness, i.e. 2nd O2 sensor

You could also pull off the top off both/either computer and have a look at the circuit board, the Blue Capacitors are known to leak after 20+ years or so, and that will cause CEL and/or odd codes and running issues.

Clean circuit board seen here: http://www.auto-diagnostics.info/ford_eec_iv
3 Blue Caps can be seen, look at their bases for discoloration, which is leakage
Cheap and not that hard to replace these Caps

In the link above there is a list of what each connection/pin does, "unused" is not true for most, it should say "unknown at this time", most have internal connections to circuit board and were used for different vehicles running different software.
Which is why 1994 wiring harness to 1991 computer might be one of the reasons you are getting a CEL now, a sensor or voltage on a 1991 "unsued" pin is now out of range, which it wasn't before, so you are getting codes but 1991 software doesn't match 1994 wiring pins
 
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black_demon69

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That would explain the 2 digit codes, 1991 computer

Yes, rebooting computer, power off/on, will reset computer so CEL would stay off until it "saw" a/the problem again, but code set before key off would be in memory unless battery power was disconnected.

If engine has 1994 wiring harness and two O2 sensors then putting the 1994 computer back in might give you better diagnostics for the problem of why the CEL is coming on now with 1991 computer.
Wiring is basically the same but............1994 wiring harness would have more wires to computer than 1991 harness, i.e. 2nd O2 sensor

You could also pull off the top off both/either computer and have a look at the circuit board, the Blue Capacitors are known to leak after 20+ years or so, and that will cause CEL and/or odd codes and running issues.

Clean circuit board seen here: http://www.auto-diagnostics.info/ford_eec_iv
3 Blue Caps can be seen, look at their bases for discoloration, which is leakage
Cheap and not that hard to replace these Caps

In the link above there is a list of what each connection/pin does, "unused" is not true for most, it should say "unknown at this time", most have internal connections to circuit board and were used for different vehicles running different software.
Which is why 1994 wiring harness to 1991 computer might be one of the reasons you are getting a CEL now, a sensor or voltage on a 1991 "unsued" pin is now out of range, which it wasn't before, so you are getting codes but 1991 software doesn't match 1994 wiring pins



91 computer has been there for about 5 years now
 

Denisefwd93

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You should not have kept it secret in the beginning LOL

anyhow this may not help anybody but me I found what I think may be the cause of my code 172! (I'll start another thread for this one);

Bank One connecting wire (all four of them) are pinched between the engine and the top of the bell housing near the one of the top bolts!!!

I can't get to it, will have to cut it and reroute

Techs if you're reading this where does it connect at the other end?

Just when I found it, and of course it started raining and it's really dark out there anyhow!

This probably also shorted out and caused overheating alternator.
 

RonD

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91 computer has been there for about 5 years now
Yes, no problems usually, 1994 wiring would have more wires in the harness than 1991 needs, but something has changed, it happens with age, lol
 

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