I've been reading for probably the past week or two on everything I could pull up on google. I have tried a ton of things and done a bunch of testing and haven't been able to pinpoint the problem here, time for the description!

Just picked up a 91 ranger and was told it needed a new head gasket and water pump. I pumped up the cooling system and a heater hose was leaking as well as the weep hole on the pump. Changed the pump and heater hoses. When I got the truck it ran and shifted good, just ran a bit warm. I pulled the heads and had 2 cylinders on the drivers side that were cracked between valves and getting water in the cylinders. I got a new set of heads from clearwater cylinder head and popped those on with all new gaskets/o-rings/head bolts on the way back out of the motor.

Before I even changed the heads my problem started. The truck would drop rpms once shifted into any gear (R, OD, D, 2, 1) and sometimes it would rev back up and run but most of the time it stalls out. Sometimes it stumbles and dies, mostly it just cuts off almost immediately.

-I have cleaned the MAF, and tested the voltage/ground/signal voltage from the MAF and it seems in range. *When I unplug the MAF while running there's no difference in the idle though if that means anything.*

-Tested the voltage/ground/signal from the TPS and it seems in range.
-Tried a new IAC and it made no difference.
-Cleaned the throttle body.
-Checked ohms on the ECT sender to the computer and it was like 27 ohms cold and i believe 8 ohms warm.
-Thought it was transmission solenoids, unplugged those and no difference. Ended up putting a new TCC solenoid and 3-4 solenoid in anyways to rule it out and figured it would be a good time to do a trans fluid/filter/gasket service anyways.

The thing runs good, maybe a tiny bit rough in P and N but it doesn't stall.

I have read the ICM could be getting warm but that seemed to be more on the trucks with the ICM on the distributor, but the ICM on this is behind the passenger side headlight on this truck. Took the ICM to autozone and it tested good 3 times in a row.

ALSO it will shift into gear and drive fine when the motor is cold. Once up to temp I have my issue, but if I blip the throttle and shift while the rpms are just a little bit higher than idle it shifts into gear and doesn't die.

I think that's pretty much the majority of it so far, but if I remember anything else I will add it to this. Could it be a bad ICM? bad ECU? I didn't find how to test those so I haven't been able to check them yet but any advice on testing will be appreciated. I don't want to throw parts at it.

Thanks for any help!

Also I'm an HVAC tech here in Los Angeles so I will gladly help anyone with heating/ac/refrigeration questions and troubleshooting that you may have!

 

Welcome to the Forum


1991 ranger 4.0l automatic
2WD?
4WD?

Yes the 4.0l OHV engine had heads that would crack between valve seats when over heated, never just crack out of the blue, had to overheat them.

The heater hose leaks and weep hole were probably caused by system being over pressurized by the cylinder leaks, after first over heating happened.

When you start a fuel injected engine cold, idle RPMs should be 1,100 or higher
As engine coolant warms up idle RPM will start to drop, that's the ECT sensor working and the computer seeing the temp go up so it starts to close the IAC valve to lower the idle RPMs.
Target idle in computer memory for a warmed up 4.0l with automatic should be about 750rpm in Park/Neutral
When you shift into gear computer will bump up idle by 50rpm or so, so open IAC valve a bit, same thing if you have AC, idle RPMs are bumped up so engine won't stall when AC compressor comes on.

After engine is warmed up and idling, unplug the wires on the IAC Valve.
RPMs should drop to 500 and engine may even stall, either is good it means no vacuum leaks.
If idle stays above 700 then you have a leak.
OR someone has been messing with the anti-diesel screw, thinking its an "idle screw", can't use an idle screw with fuel injection, no jets.

next would be spark plug gap, the 4.0l needs .054, that's one big gap but it won't idle well with a smaller gap

 

Thank you for the reply. It's a 2wd.
I replaced the ect sensor and it made no difference. Ohmed the new and old sensor out and seemed about the same. I fully agree the truck was overheated to crack the heads. I put an actual temp gauge on just so I could make sure it was running good temps before just using the dash gauge, since I don't want to warm up the new heads. Gets maybe 200-210 then drops down to like 180-190.

I did notice when I start the truck after it sits over night, it doesn't kick up to high idle when started. I've tried a new set of plugs gapped at .054 and even put the old plugs back in considering it didn't have any of these issues when those plugs were in.

Also I put a new harmonic balancer on because the rubber on the old one was cracking like crazy and ford gave me the new style balancer. But I don't see how that would make this happen.

Maybe someone messed with a screw but the truck ran absolutely fine and had no issues other than cracked heads and the two water leaks until I changed some parts.

I'll list the parts I changed, again no shifting or stalling issues before changing these:
Thermostat
Upper/lower radiator hoses
Water pump
Timing chain cover gasket and seal
Harmonic balancer
Spark plugs (gapped to .054)
Fan blade
Fan clutch

Now the other issue I have is the crank position sensor seems to be closer to the harmonic balancer than it was on the old balancer. I can't tell but that pointer on the sensor might even be riding on the balancer.

 

I'm gonna check tomorrow. I think the pile of trash at Galpin ford gave me the wrong harmonic balancer

 

CKP(crank position) sensor is the Big Kahuna of sensors, engine will run with any other sensor disconnected except the CKP sensor, since engine starts it should be OK

CKP sensor connects to ICM(ignition control module), ICM runs the Coil Pack spark timing based on CKP pulses, it can run the spark without computer connection.
ICM passes the CKP pulses on to the computer(PCM, ECU), the PCM uses that pulse to time the fuel injectors.
PCM also sends back driver input data to ICM, to advance or retard spark based on throttle position, this is like the vacuum advance used on old distributors that reacted to driver pressing down on gas pedal, opening throttle plate or closing it.

New style balancer may need different CKP sensor, don't know, newer CKP sensor may be made to allow more clearance for larger balancer, worth a look at the new style, remove current one and take it to parts store and compare with newer model.

Yes, coolant operating temp should be 190degF, thermostat is 192-195degF, depending on driving conditions when thermostat opens 210degF is normal, my 4.0l will climb up to 220degF on longer uphill grades with warmer outside temps.
It's the 260+ degF that can cause problems.

Remove IAC Valve and plug in its wires, turn key on and IAC Valve should open all the way, unplug the wires and it should close all the way, plug wires back in, test it a few times to make sure it isn't sticking.
Computer controls IAC valve by pulsing 12volts to it, with key on IAC gets full 12volts to open all the way for starting.
After engine starts computer will cut 12volts in pulses, actually it cuts Ground but same difference, this pulsing closes IAC valve partially, computer also has RPM reading from ICM/CKP, so it can pulse the 12volts to set a specific RPM, and it will be within 15rpm of target so very accurate.
Target RPMs are based on engine temp when ECT is below about 150degF, then target RPM switches to preset RPM for warmed up engine.


These systems are all monitored by the computer and any oddball stuff will cause CEL(check engine light) to come on.
Does your CEL work?
You should see CEL when key is turned on, than means computer got power and is starting up, CEL will then go off after engine starts.

If CEL is not working then you may have burned out bulb, and you could have some codes in memory that could ID a problem.

 

I fixed the stalling in gear issue. It was the old style sensor with the new style balancer that was causing my issue. So when I have the old balancer/sensor or the new balancer/sensor it runs without stalling now. And the CEL does work it just isn't illuminated. I have tested KOEO and KOER and both get an 11/pass.

BUT now I have a rough idle!

The previous owner had a 'tornado' deal in the intake tube which I removed. I have cleaned the MAF sensor. I have checked the air intake temp sensor and I believe it is okay. New coil, plugs, wires. Sprayed brake cleaner all over the motor and can't find any vacuum leaks. Replaced any dry looking vacuum hoses.

I have about 170 psi compression on all cylinders. I have 17 inches of vacuum at idle. It drives pretty smooth but idle is really rough.

I have pulled the injectors and took them in and the guy didn't test them at low pulses/voltage like it would be at idle so I don't know how they are working at idle but he claimed they flowed good at what I would assume is comparable to WOT. BUT when the test was over one injector did continue to drip. I have a new set of injectors coming in on Friday since that one dripped and I am hoping they are having a hard time at idle which would explain my rough idle.

Also when I start the truck in the morning after a cold night the truck doesn't rev up very high for a fast idle to warm up. Maybe 900rpm then starts dropping down to normal idle speeds like the motor is already warm within 20 seconds of starting. Any reason for this? I assume whatever may be preventing my fast/warm up idle might also be causing my rough idle but I'm not sure.

Anything else I should be checking or anything you know that may be the culprit?

 

I haven't read your entire thread, just this single post, but about your rough idle, have you changed the spark plugs or at least looked at those? If your plugs are shot then therefor no amount of cleaning is going to bring that idle up to spec.

 

I've tried reusing the plugs that came in the truck when we got it and it ran smooth, no difference. Tried a set of motorcraft copper plugs at two different gaps, i think .034 and .054, no difference. Threw in a brand new motorcraft Coil pack, motorcraft wireset, and motorcraft platinum plugs all from the Ford dealership which were all parts for the truck based on the vin number and no difference.

BUT!!!!

If the key is on I have 11.81 volts to the plug for the IAC but when it's plugged in the valve doesn't move. When I put 12 volts directly to the terminals on the IAC it moves.

So I had the IAC on the intake plenum and left the plug off and put 12 volts directly to the IAC terminals and it revved up higher than it will if the IAC is just plugged into the harness.

What controls the voltage to the IAC??

Thanks for the help so far guys.

 

I just ran the motor with the wires to the IAC probed, and they are only getting 4.9-5.0 volts to the IAC while running while the motor is still cold.

Also just took readings from the signal wire off the MAF and it varies a bit as it idles and the motor bogs down a bit at the signal wire voltage drops to about 0.87. Then it jumps up a bit to somewhere around 0.92-0.95 and it idles smoother at those voltages. I don't touch anything on the motor while the voltage fluctuates like that either. Is that normal? Are those acceptable voltages at idle?

 

The IAC valve should get battery voltage with key ON
The computer pulses the Ground wire on the IAC to set its position, not the 12volts.
Most digital volt meters can't "see" pulsing Ground, but your approx. 5 volts should be about right, if engine is warmed up.................

The computer sets IAC Valves position based on engine/coolant temp.
The fact that your idle goes high on startup and then drops down means IAC valve is working, computer is controlling it, but computer may be operating with incorrect info.
ECT(engine coolant temp) sensor is where computer gets the engine temp info.
ECT sensor is located above thermostat housing on lower intake, it has TWO wires connected, there is also a ONE wire temp sender located next to it, that is used only for the dash board temp gauge.

The ECT sensor runs about $6, so you may want to check its wiring for corrosion, but replace it if you haven't already

One wire on the ECT sensor will have 5 volts with key on, this 5 volts is shared with air temp sensor, throttle position sensor and MAF sensor, it comes from the computer.
The other wire is the Temp wire, 3volts going back to computer means Cold engine, under 1 volt means warmed up engine, so corrosion on the Temp wire or 5volt wire, at the ECT sensor, will Lower the voltage, so................computer sees that as warmed up engine

 

I replaced the ECT sensor already. The motor really only revved up when i hit the IAC with 12 volts directly. Otherwise it only hits like 900 rpm for a sec. I have a 1990 bronco with a 302 that will idle over 1000 for a bit until it warms up a bit, this ranger doesn't do that at all.

 

I would get a sewing pin and check voltage on ECT wires while they are connected, just to take that off the table.
Key ON, engine OFF, should have 5volts on one and above 2volts on the other, assuming cold engine.

Even hitting 900rpm for a second means the IAC valve is moving, but I would expect 1,500rpm at first then a quick drop to 1,100rpm
So could be computer issue

This page shows pin numbers and voltages a 1994 and earlier Ford computer uses and expects: Ford EEC-IV

If you want to test the computer wiring.
There can be corrosion on the computer connector that can effect voltages, since you have checked everything else..............

 

Try clearing the PCM. This is done by removing both battery cables from the battery, then connecting the battery cables together with a small jumper wire, leaving it for about 15 minutes.

This forces the computer to forget anything it had learned prior.

 

Cold engine, probed the ect wires WHILE HARNESS PLUGGED INTO SENSOR and the green wire has 3.367 volts and the gray with either a red or pink stripe wire has 0.026 volts with the key on engine off.

Checked the +5 volts for sensor reference on pin #26 off the ECM and as soon as the key is on I get 5 volts to ground off the wire going through the firewall.

Pulled the PLUGS OFF the air temp sensor, throttle position sensor, engine coolant temperature sensor, and mass air flow sensor and here are the results (KOEO TO GROUND):

ATS: 4.658
TPS: 5.037
ECT: 4.657
MAF (listed by terminals):
A: 12.12
B: 0.020
C: 5.862
D: 0.027

What if I bypass the ECT sensor, run the 5 volts through a dimmer switch, and manually adjusted the voltage going back to the ECM and see if it runs better??

I ohmed out two different sensors both read about 22ohms which was pretty on point for the temperature, but the voltage coming out of the sensor isn't right.

 

Green wire on ECT is return voltage and 3.3v would be cold engine, about 55-60degF
Not sure why the gray wire didn't show 5volt when connected but did when disconnected?

MAF sensor gets 12volts and Ground on the two outside wires, A and D, this is the heater
Inside wires are the sensor, B voltage would get higher as engine was REV-ed up, .4-.6 at idle, up close to 4volts at 5,000+ RPMs

Diagram here: http://www.therangerstation.com/tech...0_3_0_3of3.JPG

 

A word on the dimmer switch: If you plan on using a dimmer switch like you'd see on a wall in someone's house, that won't work. That is designed for 120VAC and won't work for the 12VDC in our trucks.

What you want to do is use a potentiometer, a variable resistor. It's identical to what is on the engine already, except instead of coolant temp changing the resistance, it's a ****.

You can if you want, although I personally see no reason to do so. They're cheap enough to where it's (in my eyes) a good idea to swap it out.

 

I've tried two different sensors in there already

 

My green wire is hot. The gray showed nothing to ground, the green wire shows the 4.6 to ground when unplugged

 

Well if i am getting the right voltage from the ect wouldn't that point towards a bad ECU since it's getting a good voltage reading but still not kicking into high idle when cold?

 

In the diagram the 5volts, computer pin 26, is distributed to all the sensors on the grey/red stripe wire.

Light Green wire goes to computer pin 7, ECT input voltage

EEC module = Computer/ECM/PCM

 

With the computer plugged in or unplugged my gray/red wire has continuity to ground on any metal on the motor. 0.2-1.2 ohms from gray/red to ground. The green wire on the ECT doesn't.

I'm assuming that means my gray/red wire is broken somewhere and touching the motor??

Did some more digging. I unplugged the wiring harness from the ECM, unplugged the wiring harness plug that plugs in near the passenger side head, then rechecked. My gray/red shows 1.6 ohms to ground on the section of the wiring harness that plugs into the computer on one end and the motor harness on the other and OL (no continuity) on the gray/red on the motor half of the wiring harness.

The gray/red should have more than 1.6 ohms of resistance to ground correct?? (unplugged from computer and motor half of the wiring harness)

 

Yes, it reads like to have a short to ground on the grey wire.

Unhook computer connector
Hook ohm meter up to ECT grey wire and a ground, yes 0 to 100 ohms is bad
Now unplug each sensor that uses the 5 volts, and watch meter to see if ohms jump up high.
Move wiring around to see if a wire is shorted.

 

With the computer disconnected, and the wire unplugged from the ECT, I got 0.8 ohms to ground on the grey/red wire. It ended up settling a little lower at 0.6 ohms and unplugging/plugging in the sensors made no difference.

Now the only thing I do notice is there are 2 sections of the wiring harness. There is a plug that connects the section of the wiring harness that comes through the firewall and the harness that's on top of the motor. With that plugged in it's 0.6-0.8 ohms from the grey/red to ground. When I unplug it, the wiring harness on top of the motor shows OL on the gray/red to ground meaning no ground. The gray/red on the section of wiring harness that goes through the firewall (with the computer and other half of the harness unplugged) shows 0.6 ohms to ground making it seem like that section of the harness is where the short is. Wiggling wires didn't seem to make a difference though.

Am I on the right path here?