Hey all, been around RF for a while. Just haven't made an account to comment and what not, that search feature does wonders lol. Anyways, I've got a 4x4 93 ranger xlt with the vin x 4.0 ohv motor and a man trans. 170k on the odo. Fed emissions w/out egr and no cats. I've had this truck since about last August or October, it's been a great truck but has always ran rough with a few little quirks. iac Problems constant po136 and 172 lean codes etc. Otherwise it's always idled (surged) around a clean 750-800. Well lately it's acting like I have a massive vacuum leak, unplugging Iac does nothing. cold or hot it doesn't idle below 1200 except at cold start and only for about 30 seconds before closed loop operation begins? If i tap the throttle or push in the clutch while I'm driving RPMS soar to 1700,-2000 and hang there before slowly dropping off over the course of 20 seconds. Back down to it's "idle" of 1200. Same in neutral, tap the throttle it soars to around 1800 and hands before dropping off. it Only codes I have at the moment KOER are 136, 172, (both banks lean) and 411 (system unable to control rpms during low rpm self test). Previous owner said the top end was rebuilt 6mo prior to me buying it, he did in fact have paperwork on it. Anyhow what I've done in the past year is
replaced
Air filter
ECM
TPS
IAC
MAF
ECT
IAT
FUEL FILTER
COIL PACK
NEW PLUGS AND WIRES
bank 1 O2 sensor
Ive sprayed starting fluid numerous times, last time being today around all vacuum lines the entire throttle body all the way from intake to maf. Around the lower intake fuel rail areas and i've inspected everything a dozen times, I changed the upper intake gasket today to no avail. I have 19 psi of vacuum, fuel pressure was good. I'm stumped, I'm pretty sure it sucking air from somrwhere is the only way it could increase rpms like it is and hanging like it does. Otherwise I'm completely stumped, I've got the gasket to replace everything from the heads on up. I just haven't because if I do that, I wanna get machine work done and rebuild the heads for strength etc and I just don't have the time and money for that right now. Although If it is in fact leaning out I'm gonna crack a head and have to rebuild it anyways. I'm not sure guys I'm a little stumped, ANY advice at all is well appreciated.
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today fiddled with the MAF a bit, normally when unplugged CEL does come on and it would idle like normal for a few minutes before dying. Albeit somewhat rough. Today went out and did the wiggle test on the connector, really not much or no change at all. Unplugged it and the motor stalled IMMEDIATELY. This tells me either A) my MAF is good? Or B) the computers preset values for a faulty MAF won't let the motor run because of some other sensors input. I know when maf is Unplugged computer looks to IAT, ECT, and TPS. TPS has been tested and is reading correctly .8 at idle with a smooth increase throughout throttle movement. IAT is new but never tested, same with ECT. Although I do have a warranty ECT waiting for me to install.

 

Still at a loss for words, truck is driving me crazy but st least she's getting me from point a to b for now

 

Start at the source, sensors are of no value if computer is not interpreting the incoming data correctly

Pull out the 1993 computer(EEC-IV) and have a look at its circuit board and also replace the 3 capacitors, as they WILL fail after 20 years or so, and will cause issues
In 1993 EEC-IV should be in the engine bay, drivers side firewall down low, so under clutch/brake reservoirs and close to the fender, 60-wire connector/computer is mounted vertically
I need to also pull the large round connector for cab wiring to get enough slack in the wires, just a single small bolt in the center and it will pull off

Picture here of a EEC-IV: Ford EEC-IV
You can see the 3 blue barrel capacitors, these will fail and also leak
These are different values and have a "-" side(stripe) so take pictures to get values and orientation
The uF has to match exactly but the "V"volts" can be higher on replacement, so you can replace a 16uF 30V with a 16uF 100V, or 16uF 50V

Capacitors can be black as well as blue and can be in different places on the board than pictured above

Check for corrosion on the board and connector pins and slots, computer has a resin type coating but can still get some corrosion on edges and connections

My 1994 4.0l has all original sensors and controls(IAC Valve), 400k, O2s have been changed a few times since they can only last 100k miles or 12years
But I did have to change the capacitors in the computer a few years back, odd running issues
4.0l eats spark plugs, Motorcraft or Autolite last the longest, NGK is next
Only use regular copper or DOUBLE Coating, never single coating, its a waste of money when using coil packs

Sensors all run on 5volts, O2s under 1 volt
So any corrosion throws them off, so always good to unplug and plug back in ANY connector you find in the engine bay, that cleans the contacts

You can only use Motorcraft or Hitachi brand IAC Valves, other brands are not true solenoid valves, so will always have issues

PCV Valve is a controlled vacuum leak, needs to be changed every 3 to 4 oil changes or at least cleaned to get the oil vapor residue out of it so it can't stick

1993 has Fuel pressure regulator(FPR) on the engine, has a vacuum line attached, not uncommon for an FPR to leak and fuel is sucked into the engine via that vacuum hose, just have a look and a smell of that vacuum hose

It does read like there is a vacuum leak, if when you unplug the IAC Valve on a warm engine the RPMs do not drop
You can check the anti-diesel screw, looks like an idle screw on the throttle linkage, people often adjust this to cure stalling, when an IAC Valve fails
So after unplugging the IAC Valve on idling warm engine, turn this screw counter-clockwise to see if RPMs start to drop, if so keep turning until engine is barely running, 500-600rpms, its now correctly re-adjusted
Plug IAC Valve back in and RPMs should go up, warm idle should be about 625-650 on a manual trans 4.0l, 750 on automatic

If idle still stays high leave IAC Valve unplugged, and with the larger air tube removed from the intake, use a Gloved Hand to close off the air flow into the engine bit by bit, you may be able to hear the vacuum leak as the vacuum in the intake climbs even higher from restricted flow
You can also spray some "fuel" around at that time to see if its sucked in, just be VERY CAREFUL

 

Hey RonD thanks for the reply I really appreciate it. Sorry for the late response I've been pretty busy the last couple weeks. I feel really stupid for replacing as many sensors as I have. I also bought a new computer to no avail, I did look inside at the three capacitors and they were good to go. New copper plugs a few months ago, solved a miss problem I had. I'll have to get back out and recheck all sensors for 5 volts. Last I checked a month and a half ago they all read great. Pcv valve is newer and clean, not the original hose but I have no bends cracks etc. FPR didn't show any signs of leaking and no smell of gas. I was thinking it's gotta be a air leak from somewhere. On startup rpms shoot up to 2k hang at 1700 before dropping to 12-1300 fully warm. Iac unplugged no change stays at 1300, no change with anti deisel screw backed out either. I'll have to do another smoke test and try running it and cutting off it's airflow. This might sound dumb but I did notice while running if I unplug maf the motor dies out completely. It never did that when it was running okay. Just wondering if it could be related. Newerish maf recently cleaned and let dry properly. Thanks for all your help, have a good night
Hunter