1993 ford ranger 4.0 M5speed. 2wd - Ranger-Forums - The Ultimate Ford Ranger Resource


4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 01-13-2015
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1993 ford ranger 4.0 M5speed. 2wd

1993 ford ranger 4.0l extended cab xlt 2wd 5speed manual.
Im having trouble i just put a new computer in my truck and 1) when i start it in the morning and go down the road it lags and wont get up to speed very quick 2) after i am going down the road for bout 15 min the truck starts to backcfire really hard and the engine light will go off and on but if i disconnect the o2 sensors the truck runs like a champ and both the o2 sensors r brand new.
So i ran a code reader on it and it said there was a curcut in the ignition a curcuit in the cooling fan and a bad ground in the MAF sensor so i checked all my grounds and the truck still has the same problems and all the sensors r new within the last 10,000 miles plse help i need to registed it come february
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Old 01-13-2015
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Computers are different from year to year but not that much.

What was the old number and the new
F37F-12A650-XX
F37F = 1993 Ranger
12A650 = generic serial number for all Ford PCMs(computers)
XX or XXX = software revision

There should also be a Large 4 digit code, with numbers and letters, that one is important.

O2 sensors are feedback sensors so if they are disconnected the computer will "best guess" air/fuel mix.
Same if MAF sensor is disconnected, computer "guesses" air flow based on RPM.

Certainly reads like new computer doesn't like your wiring or computer is bad.
Not all wiring to computer is the same on all models

Last edited by RonD; 01-13-2015 at 08:09 PM.
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Old 01-14-2015
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1993 ford ranger

Hey thanks for that ill get back to u on the #s but the last two letters on thay serial number were "LA". But i still dont under stand why the truck is backfireing if everything is plugged in and the truck is recieving the correct air to fuel mixer it shouldnt do that correct? And it only backfires aftrr its at normal operating temp and im either holding it at 2000 rpm or when im in 5th gear going bout 50mph
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Old 01-14-2015
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Your '93 4.0l will use an EDIS-6 module for spark timing, usually located on the rad support, can be the front side like my '94 or back side usually by battery tray.
These are pretty bullet proof and can run the spark system without computer input, but they are hooked to the computer so computer can set correct injector timing from spark timing.
Computer also gives EDIS module spark advance info, i.e. you step on the gas, TPS(throttle position sensor) voltage goes up, computer tells EDIS to advance spark so there is less lag, like an old vacuum spark advance but less delay.
Spark timing can of course cause backfires.

But since Unplugging O2 sensors would not change anything on this I can't see spark system being part of the problem.

BUT.........Ford EDIS is a "waste spark" system, meaning each cylinder's spark plug sparks twice on each 4 stroke cycle, once on power stroke and one on exhaust stroke, this can come into play for backfires.

There are two kinds of backfires
From intake and from exhaust

Usually intake backfire is a sign of Lean mix, lean mix causes cylinder to heat up, then when lean mix comes in via open intake valve the it self-ignites in overheated cylinder.
Backfire is heard.

Exhaust back fire is usually a Rich mix, on power stroke not all the fuel is burned but all the oxygen is, when exhaust valve opens unburn fuel gets oxygen and can self ignite that's in the old days, Ford's waste spark helps that along with the second spark on exhaust stroke.

Since disconnecting O2 sensors seems to lessen the backfires I would assume this is an air/fuel mix issue, not spark timing.


So, with O2s hooked up, does the engine ping/knock while under load, thats a sign of Lean mix?

Does the engine exhaust smell rich or are the Cats getting Very Hot, thats a sign of rich mix?

Leave O2 sensors connected and unplug MAF sensor, MAF sensor is the main sensor the computer uses to set the 14:1 air:fuel ratio needed.


Also on the computer side, Calif. computer needed Cam Position Sensor and EGR Valve, if your Truck is from Calif. then you need a computer for those, if it is not then you don't.
And they can't be swapped around
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