hey everyone have been a long time reader first time posting as a member,

1994 ford Ranger 4.0 Federal Emissions meaning ( does not have a Cam/sensor ,EGR, or MAP sensor ............... have changed every sensor you can name,,,,,,,,, checked all wiring, problem is on first start up while computer is in open loop engine runs a little rough but idles and does not die while under acceleration bogs down until over 2000 RPMS then takes fuel .......... when engine warms up to normal temp and goes into closed loop could not ask for a better running vehicle? the Ranger does not want to idle or take normal acceleration in closed loop only........... HELP

 

…… title is supposed idles funny in open loop, when truck goes into closed loo everything is fine………

 

All gasoline engines need to be "Choked" when cold, choke means Rich fuel mix and high idle, which was done with the Choke Plate on a carb engine
Fuel injection has no Jets to suck extra fuel from so the computer must set "Choke" mode
Part of Open Loop is Choke mode

With key on computer looks at the ECT(engine coolant temp) sensor, a 2 wire 5volt sensor
If coolant temp is under 140degF then computer sets choke mode based on coolant temp, the colder it is the more choke is required and the higher the idle
Once coolant temp reaches 140+degF choke mode would be off but Open Loop still in use until O2 sensors were fully warmed up
Closed Loop means computer can use the O2 sensors to adjust air/fuel mix on the fly

So on cold start do you get a high idle?
1,100+ RPMs
Can drop to 900rpms pretty fast if its 70degF outside temp, but higher than warmed up idle, 650 manual trans, 750 automatic
If no high idle then computer has not set Choke Mode
Could be ECT sensor is not showing correct temp, or there is air in cooling system


Dash Temp gauge uses a 12volt 1 wire(1994) temp sender, used since the 1930's on engines with dash temp gauges

ECT sensor and Temp Sender look similar but are much different

 

thanks for the reply …you are correct this vehicle was not being chocked correctly due to this find…… I forgot to add that I am a Master Mechanic and a shop owner and operator……this vehicle is in my shop giving me a fit!. I understand all aspirated engine’s also jets …… I have ohmed out all wires and have checked vac leaks with vac gage…… although I did find a bad purge solenoid, Mass airflow sensor, and both O2 sensors the computer only coded out Mass air flow sensor and bank 1 sensor …….. all other sensors found to be bad by ohming out or other testing………. this narrowed my find to a bad computer …… I confirmed this, this morning when I pulled the ECM and popped it open…… found that three capacitors that run the basic program while in open loop we’re burnt,….making it run erratic until operating temp is accomplished then switching to closed loop when this occurs engine runs perfect! I suspected this but was looking for others that have had this happen ….. No one has had a issue like this before I guess …… will update this find after I get the capacitors soldered in ECM …… and then checked out…… I suspected ECM earlier but for some reason could not process in my thoughts that a ECM could operate so many things and still be a bad ECM ……. I ruled out ECM early in the diagnosis because of the vehicle ran ok at idle until closed loop is achieved the vehicle bogged during normal acceleration…… I did learn something useful…… who says you can’t teach a old dog

 

I had to replace the 3 capacitors in my 1994 4.0l a few years back
But had fuel pump running full time with key on, then wouldn't run at all
So after testing FP relay next stop was EEC(ECM), so got lucky there
And 1 cap was obviously leaking
1 cap had a leg break off when I touched it, lol
1 was most likely still "breathing"
But replaced all 3
Only sensors ever replaced on the 1994 have been the O2s, every 10 years whether they need it or not, lol
New set next year

 

I am glad you got it running ….. these trucks are sometimes a pain!

 

FIXED ……. replaced computer/ecm runs starts closed loop runs great ……