my 1991 4.9l 5spd has issues at idle and while under a load and i cant figure out what it is

symptoms- rough idle, it feels like its missfiring and smells like its running rich. when your under a load the truck basicly falls on its face and doesnt want to run unless you feather the throttle or hammer it. i have white smoke coming out the tailpipe. the truck doesnt over heat nor does the motor make any noises other then the "leaky manifold" sound coming from the air valve on the left front of the motor?

ive replaced the plugs and set the proper gaps, replaced the plug wires and last yr i replaced the cap and rotor along with the fuel pump. i just recently replaced the fuel filter the last time i changed the oil about 3500 miles ago.


the truck ran fine up untill now minus a little sputter at cruising rpms every now and again.

im at a loss for once here, any opinons or ideas would be greatly appreciated

 

pull codes, also sounds like you have a vacum leak which will cause the motor to run rich.

 

not sure how to pull codes, waiting on my buddy to show me how to do it. and yeah im pretty sure its a vaccum leak, ive done alot of trial and error just not sure if im getting it right.

 

Welcome to FTE and the Big Bronco Forum. We will do our best to get you the most accurate info we can.

Check out THIS SITE for answers to all your EEC-IV fault code questions.

 

sounds like the problem i had, it ended up being the fuel injectors

 

well my brother just pulled codes for me, the main ones were concerned about is, and i dont remember code numbers right now but they were, map sensor circuit signal low?, intake air temp sensor circuit ground low, coolant temp sensor, ect sensor fault(egr valve sensor), my injectors are working fine according to live feeds just running at a high pulse width due to all the other codes or so he says, i think there were other codes but i cant remember em, when i get the paper tonight ill post up all the codes.


i think im going to start with the coolant temp sensor like he said and then go from there to the map sensor.


Welcome to FTE and the Big Bronco Forum. We will do our best to get you the most accurate info we can.

Check out THIS SITE for answers to all your EEC-IV fault code questions.
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thanks for the site greystreak92, its helping out alot!!

 

was the engine up to temp before you pulled codes. if not some may be false codes

 

yeah i drove it from my house to my brother shop which is about 6 miles from my house so she was nice a toasty upson arival lol.

 

ok so my list of codes are, 126 map sensor, 122 tps sensor(signal low), 327 evp sensor(egr) signal too low, 558 egr vac. regulator circuit, 565 canister purge circuit fault, 112 intake air temp sensor signal low cicuit ground.

someone also told me check my ecu from internal probs as there common on older fords? not really sure about that.

ive replaced my coolant temp sensor which made it run better afterwards that was another code i had so i figured to start there. when i unplug the tps sensor or the evp sensor it doesnt make a difference in the performance, if i unhook the vac. line from the map sensor the truck runs smoother but when i unplug the sensor it doesnt really make a difference (with the vac. line hooked up). im at a total loss here, im about ready to just start replacing sensors one by one till the problems solved.


any help or ideas would be greatly appreciated!!!

 

ECM problems are about 1 in 10,000 or more... anyone who assumes the ECM is the problem BEFORE ruling out all other potential causes either knows the ECM was shorted out somehow or just doesn't know the EEC-IV system's reliability at the ECM level.

Before going ANY further though, you need to check something. Voltage between PIN 26 and PIN 60 of the ECM should be approximately +5VDC with +4.75 VDC being bare minimum. Make absolutely certain the GROUND for the ECM is intact. Poor grounding will cause voltage drop especially in such low voltage systems.

I say this because ALL of those Codes indicate that the voltage the computer is "seeing" coming back to it from EACH of these sensors is LOWER than it should be. While its possible that you have weak connections at some of these sensors, it is highly unlikely that ALL of them have weak connections. So the most plausible reason is a problem with the +5VDC (VREF) power supply INSIDE the ECM. However, as I said, these failures are extremely rare and a failed internal power supply is typically the result of a short circuit caused by either damaged wiring or OE (operator error).

If the voltage between PINS 26 and 60 is good, the problem lies in the wiring or the sensors themselves.

 

thanks man ill try that this week. my brother said try the same thing because he said that theres no reason all of these sensors went bad all at once. the problem literaly happened over night with no warnings to boot.

 

That honestly sounds like you had a ground work loose.

 

not to bump my thread but diagnosed my problem over the weekend. i traced the problem to my ecu and when i opened it up i had to burnt bleed resistors. i replaced it and now my truck runs better then it has in yrs.

my only question is do i need to get it flashed or no??? the truck runs smoother then ever and isnt throwing any codes or anything

 

nope, run it as is. question is did it get wet or was it an electrical spike of some sort. Make sure all of your grounds are clean and tight. and check the area around where the main harness connector comes into the firewall and seal it up.

 

ok, good i was hoping i didnt have too and it didnt get wet. i check my entire wiring thru the whole truck and didnt find and lose or back connection nor bad grounds. the only thing i can think of is like a yr or so a go i went with my brother to look at another bronco for him and it was dead and when the guy jumped it i think he crossed the jumper cables because i had to replace my terminals that weekend. Im just glad to have my girl back on the road again.

thanks all for the help and ideas made it soooo much easier for me