91 ford ranger 2.3


0uptain

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So i just got a 91 ranger with the 2.3L when i got it it already had plugs,wires and coils replaced. I put a new mass air flow sensor on it and also a ignition control module. When the mass air flow sensor was replaced it ran good for about 20 miles then lost all power and would not idle. You unplug MAF sensor and it starts idling good and runs good but the fuel mileage is not good and still lacking a little power. Truck cools down and you can plug it back up and will run about 20 miles and does the same thing. Anyone have any problems with this or can give me a direction to go in?
 


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RonD

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Welcome to TRS :)

The engine computer(PCM) has two software routines it runs
Open Loop
Closed Loop

Open loop is what boots up when you turn on the key
Open loop uses preset air/fuel mix tables for the engine size(2.3l) programmed into it at the factory
PCM uses engine RPM and throttle position to "look up" correct air/fuel mix to use in the tables
Part of Open loop is Choke mode, PCM uses an engine temp sensor(ECT) to determine how Rich to run the engine based on Coolant temp
This offsets the tables air/fuel mix on the Rich side
MAF sensor is used but doesn't effect air/fuel that much in Open loop

BUT..........if there is no MAF sensor signal(unplugged) PCM will stay in Open Loop.........forever, which runs a Rich mix to avoid engine damage from a too Lean Mix, so poor MPG


Closed loop is used for "on the fly" air/fuel mix calculations, the point of using a computer to begin with, lol.
Closed Loop runs engine more efficiently, best MPG and power.

Use of the O2 sensor(s), is the big difference between Open and Closed Loop, Open Loop ignores O2 sensor(s)
To do "on the fly" air/fuel calculations the computer needs Feedback on those calculations, is the mix too Rich or too Lean

An O2 sensor sees Oxygen in the exhaust, it uses a chemical reaction to to this.
Too much Oxygen is Lean exhaust, too little is Rich exhaust, this is the Feedback the computer needs
Problem with O2 sensors is that they don't working until they get above 650degF, the chemical reaction doesn't work unless its VERY HOT, so all O2 sensors are heated, but still need exhaust heat to get that HOT

So Open Loop runs until engine AND O2 sensor gets hot, this can take 5 to 10 minutes depending on outside temp


Your issue as described reads like the problem comes with Closed Loop operation
Unplugging the MAF sensor just prevents the switch to Closed Loop

Since you don't know the age of the O2 sensor, it may need to be changed, old and out of chemicals or ??, they rarely fail but do need to be changed every 150k miles
But you should have a Check engine light(CEL) if O2 is not working well

And this could also be a computer issue, did you buy the Ranger "as is", not running well?

Does the CEL come on with the key, it should, and then go off after engine starts?
If it doesn't come on then bulb may be burned out or..................previous owner disconnected it

Is the temp gauge on the dash working?
Does it get above 1/3 after a few minutes driving?
 
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0uptain

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I did get the truck as is but he told me with the MAF sensor unplugged the truck runs good but i replaced it thinking it was the problem. I found the ground bolt broken off on the ignition control module so i just replaced it and put another bolt in it. As far as the temp gauge it moves very little after the truck has been running.
 

RonD

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What about the CEL?

Someone may have put the wrong thermostat in, it uses a 192-195deg thermostat people often replace it with 180deg thinking "Cooler is better", like cooking a steak on an ice cube tray or a BBQ, "cooler is not better" for all things, lol.

Although steak tartare does have an appeal :), its not so for engines, 190-210deg coolant is best operating temp best MPG and keeps oil cleaner

Ford temp gauge uses 215deg as 1/2 way
 

0uptain

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I dont know what thermostat it has in it but it still does not explain why it runs good after you unplug the MAF sensor Unless they work together some how.
 

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Runs better because its stuck in Open Loop(MAF unplugged) and MPG is down even more because computer is running Choke as well because engine is running cold.

Runs poorly in Closed loop(MAF plugged in), you need to find out why computer on the fly calculations are not working
 

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What and where would i need to start checking why the calculations are not working?
 

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Does CEL(check engine light) work..............................important?


O2 sensor is the main sensor used in Closed Loop
 

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Yes it does. I have used a jumper wire and the flash code was 35. I have tryed to fined a code chart but each one i found was different.
 

RonD

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Ford 2-digit codes here: https://therangerstation.com/tech_library/2digitcodes.shtml

35 has a few options all have to do with the EGR(exhaust gas re-circulation) system

EVR is Vacuum solenoid that is used to open EGR Valve

EVP is EGR valve position sensor problem
PFE is a DPFE sensor problem, not sure if the 1991 2.3l had one of these or used EVP

Ford used the EVP OR PFE(DPFE)

Find your EGR valve it will have a vacuum hose attached, if it also has Wires attached then it is using EVP sensor bolted to it
If no wires then it is using DPFE sensor

When I looked up the 1991 2.3l EGR valve it shows a place to add the EVP sensor


AND...............this could cause your issue
EGR system is not used in OPEN loop only in Closed Loop
If the EGR valve is opening too much in Closed Loop then it will cause running issues
To take this "off the table" remove the EGR valves vacuum hose and plug the hose, put a screw or bolt in it.

Hook up MAF sensor and go for a drive
Its a long shot but not a "no shot"
 

0uptain

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It does have wires going to it do i still need to plug off the vacuum line. The wires go to a white plastic peace that bolts on to the EGR with the vacuum line.
 

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Yes, unhook and plug the vacuum hose, to see if this is causing your issues.

You can deal with the reason for the code later
 

scotts90ranger

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vacuum leaks will play heck with a metered air system

DPFE didn't start until '93 or '94, a '91 should just have a EGR position sensor

an oxygen sensor problem could be involved as well, but I would look for vacuum leaks before throwing more parts at it.
 


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