1993 Ford Ranger V6 4.0 bucking and stumbling on acceleration


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After the truck warms up with a few miles driven with normal acceleration the truck starts to lose all acceleration and bucks or stumbles. There are no check engine lights or codes recorded. I have completed the truck running test and it passes all tests even when warm and the problem is present. I have attached a fuel pressure gauge and the pressure reads about 30 PSI when driving or at idle except when I disconnect the fuel pressure regulator vacuum line which then jumps up to 35 PSI. I have cleaned the MAF sensor and tested the TPS which reads a nice even voltage increase as the throttle is moved. I have checked all vacuum lines for leaks using smoke and repaired a few without it helping. I have replaced the spark plugs, spark plug wires, coil pack and the throttle cable. Possible computer? On only one occasion the truck was sputtering along and I was trying to nurse it up to 35 MPH and it stalled. I pulled it over to the side and started it right back up and for about four miles it ran perfect with plenty of power even squealed the tires and then right back to the same problem again.
 


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RonD

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Welcome to TRS :)

Reads like you have covered all the normal reasons for the bogging issues, so....................

Time to pull out the computer and open it up

Should look like this inside: http://www.auto-diagnostics.info/ford_eec_iv

You can see the 3 Blue Capacitors in the picture
These can leak after 20+ years and cause any number of issues
$5 to change them and clean up circuit board

Repair Info here: http://support.moates.net/capacitor-repair-ford-a9l-ecm/

Anyway, have a look to either see that this is the problem or to take it off the table as the bogging problem
 

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Thank you for the feedback. I have ordered caps from Amazon as all the Radio Shack stores in the Phoenix area have closed. Can someone tell me the most likely place to find the computer on the 1993 Ford Ranger V6 4.0 ? I have looked but found.
 

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It is in the engine bay, drivers side between firewall and inner fender
Look down between Power booster and wheelwell

Video here:
 

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It turns out that the truck is a 1991 Ford Ranger, where is the computer on this truck?
 

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In the cab, behind Passenger side kick panel
 

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Yes thank you. I found a utube video that showed where it is. Has different caps then the 1993 3.3 micro farad and 47 micro farad. I ordered an entire kit of caps so I will still get the ones that it has. The capacitors look good but I will try it anyway just is case.
 

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There was some dried mud in the bottom corner of the ECM case so water must have been above the floorboards at some point. The connectors of the ECM 60 pin must have been underwater and don't look great.
 

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And because I thought it was a 1993 I put in the coil pack, spark plugs and spark plug wires all as 1993 parts from Autozone. I hope they are all compatible with the 1991.
 

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I cleaned the ECM 60 pin connector and replaced the two electrolytic caps on the ECM. I still have the problem with the ranger but I did find out something new. After driving the truck for a few miles I start to loose all throttle control and the truck does not respond to the pedal just bucks and slows down. If I continue to nurse it along the problem gets worse until there is no longer any acceleration at all. I then pull it over and turn it off then restart it and full acceleration is back until I drive a couple more miles then it bogs down again. So what part can be reset when it is powered off briefly and then build up an issue until there is no response at all to pushing on the gas pedal? Before I turn the truck off I can push the pedal to the floor and nothing happens just stays at 1000 RPM. After I restart the truck it responds normally to the pedal in park with the RPM revving right up.
 

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Well there is no "gas" pedal, its an "air" pedal with fuel injection, no accelerator pump :)

So it reads like you are losing fuel pressure, maybe dirty fuel filter in the frame rail or in the gas tank(the sock on the fuel pump)

So as you drive the the fuel available is less and less, after it sits, with engine OFF, enough fuel can pass thru the "blockage" so you can drive it again

I would test for that by NOT turning off the key, just let it sit stalled, then try to restart

It could be computer is turning off fuel pump, or fuel pump power is intermittent, or low voltage, corroded connector, would be same symptoms.
Corroded connector heats up and lowers voltage to fuel pump, shutting power off cools it down instantly, so you get full voltage back on restart
 

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Sounds good I will try that. When it starts to get really bogged down I will pull over and let it idle for awhile without turning it off and see if it has power again after sitting. I am going elk hunting for a week so I will try it when I get back.
 

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I retested the truck today and here is what I found. All acceleration is normal until driven about 3 to 4 miles then the truck loses all power and pushing on the gas pedal does very little or even causes it to slow down. I pulled over to the side put it in park and let it ideal for about 5 minutes. After sitting for a few minutes I put the truck in gear gave it gas and still no acceleration. Pulled it over again and turned it off and restarted it immediately and full power comes back with full acceleration to highway speeds. After 3 or 4 miles the problem comes back and can be cleared by turning the truck off and restarting it. Any ideas?
 

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Really reads like a clogged fuel filter or "sock"(inside the tank filter)

It's possible the computer is at fault and powering on and off resets it.
If you have a manual trans, stall the engine instead of turning key off, then wait as before and restart to see if power returns WITHOUT rebooting the computer
If you have an automatic then you would need to pull a fuse in engine bay, to stall the engine, 20amp fuel pump fuse, then put it back in

This would tell you if PCM(engine computer) is the problem
 


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