Intermittently high idle speed, 1992 4.0 V6.

[Fast Doc]

Good morning friends. I’ve been a member this forum for a very long time but have not posted in a while. I need some help with my beautiful 1992 4.0 V6 four-wheel-drive automatic transmission Ranger. It has 112,000 miles, and does not get used very much. I have other vehicles that I use most of the time, the truck mostly gets used for those times when I need a truck. Bad weather, pulling motorcycles, things like that. Maybe 2000 miles a year.

I’ve dealt with this problem intermittently for years, but it is becoming increasingly frustrating.

Sometimes, it will idle too fast, and there is nothing that I could do to bring it back down. It’ll usually start normally, idle a little bit faster than I think necessary (about 1000 RPMs when cold) then drop down to maybe 750 rpm’s, in gear. As the truck heats up, it’ll usually drop down to 650 rpm’s in gear, which I consider maybe a little fast but reasonable.

Occasionally however as it warms up, it idles faster and faster, sometimes much faster, which makes it a little bit uncomfortable to drive in stop and go traffic.

I have done a lot to try to sort this out.

Originally I thought it was a vacuum leak, but all the usual tests (spraying carb cleaner, starter fluid, around intake manifold and vacuum lines etc.) did not reveal any change in idle speed.

I checked the throttle linkage is which are clean and lubricated and definitely not hanging up.

I fastidiously cleans the throttlebody and the mass airflow sensor with the proper cleaners which didn’t have any effect.

New spark plugs, coil packs, fuel pump, air filter, fuel filter, etc. did not help. It can still idle as fast as 1500+ RPMs if in park or neutral.

I have not had the injectors formally cleaned, but I typically run Seafoam, which does not seem to help.

When I brought it to my mechanic, naturally it worked normally, so no diagnosis. It has never thrown a check engine light, and typically runs perfectly well except for this. You can also go days or weeks or a month at a time, and behave normally.

When it is hot, if I unplug the idle air controller (also new and replaced) it idles at about 500 in gear, which I consider to be perfect, however then it will not idle when cold.

Sorry for the long post, but this is been frustrating me, and I thought people would have an easier time figuring out what’s going on if I laid out the details. Thank you in advance for your help gentlemen.

Bill

 

[RonD]

Your expectations of idle RPMs is too low

1992 Manual trans 4.0l after full warm up would be 625, automatic 725, auto in gear 775-800, usually about a 50rpm bump in gear or AC on

Cold start should REV to 1,500(IAC Valve is open all the way for startup), then drop to about 1,100(choke mode)
Then as engine warms up it would slowly drop down to warm idle, seen above
This is based on ECT sensor's temp reading, so if ECT is acting up computer may hold cold idle rpms too long, or raise RPMs
Its a cheap sensor to replace, it has TWO wires, there will be an ECT sender for the dash board temp gauge next to it, it has 1 wire
ECT sensor uses 5volts, ECT sender 12volts, so NOT interchangeable

Idle RPMs are based on Emissions requirements for that year, later years will have higher idles to pass stricter emissions
Most new vehicles HAVE TO shut off the engine when stopped to pass idle emissions, no its not done to save gas, lol

Check the mechanical first, make sure return spring on throttle plate is OK and that throttle plate can open and close easily, worn out TPS can cause it to stick open a bit
Also check Throttle cable and Cruise cable for binding, they can hold throttle open a bit as well

Change PCV Valve, it can stick open a bit causing higher idle intermittently

Then pull and clean IAC Valve, as shown here: https://www.explorerforum.com/forums/index.php?threads/iac-valve-cleaning-thread-w-pics.84220/

3rd party IAC Valve's often don't work too well, and cause alot of wasted time because we just "assume" a new part will work, old school thinking, lol, new just means NEVER tested now, so grain of salt

Engine shouldn't idle when cold with IAC Valve unplugged, cold engine needs to be choked, which means rich fuel mix and high idle, also advanced spark

 

[Fast Doc]

Thank you for the thoughtful detailed and helpful reply! This brings up another thing I forgot to mention, at some point my mechanic replaced my thermostat with one that runs the engine at a cooler temperature. If I recall though, I had the idle issue before that change. Could that be part of the problem? Would you recommend that I replace the controller with a Ford unit? I think I bought a run-of-the-mill one off of eBay if I remember correctly.

 

[Fast Doc]

Even adjusting my expectations for how low the idle should be, it still occasionally stays too fast, 1000 RPM in gear. 1500 or so in neutral. But again, unpredictable.

 

[RonD]

I added some stuff to my first port

No on the t-stat, Choke mode high idle ends when coolant is at 150degF or so
While you should run a 192degF t-stat for better MPG and cleaner oil, a 180degF won't change idle RPMs

SAE did a study back in the late 1970s that showed minimum coolant temps of 190degF were best for longer engine life, cleaner oil and better engine efficiency(MPG)
So most automakers switched to 190-195degF t-stats in the 1980's

 

[Fast Doc]

Again Ron, I am grateful for your help. I think you will help me get to the bottom of this. I did change the PCV valve sometime ago, and it makes the normal rattling sound when I shake it but it is cheap maybe I could try another one.

I didn’t ask the mechanic to change the thermostat, I agree, I think that it was unnecessary. Maybe I’ll put a stock one in next time I change the coolant.

Also, I tried the experiment of disconnecting the idle air controller, and yes, it will not idle when cold.

As far as the controller itself is concerned, I did see that link before, and cleaNed the original one. It did not affect its function, so I replaced it, and the new replacement does the same thing. So I think that’s probably not the problem.

So I can be clear, should I be thinking of replacing the ECT sensor, or sender? Is there a link on this that you are aware of? Also, the temperature gauge on the instrument panel seems to be working normally. Wouldn’t this mean that the sensor is working?

 

[RonD]

Yes, I would change the ECT sensor, its cheap, just to take that off the table

No, not the ECT sender, if temp gauge seems to be working OK

The IAC valve gets 12volts(system voltage) on the red wire with key on, direct connection to battery voltage
The other wire is the ground and connected to the computer, computer "pulses" the ground to control the valve, wider pulse = lower voltage
You can use sewing pins inserted into the 2 wires to measure voltage with engine running to see if computer is raising the voltage to IAC valve during higher idle incidents, so a computer issue, might be sensor issue as read by computer
Voltage with alternator on is 13.5-13.8volts after warm up, thats what Red wire should show with meter grounded to engine metal
Using the computer ground should show 7-11volts, lower volts = lower RPM, there is no exact voltage to compare to RPMs, the system/computer adapts/learns what voltage = what RPM, target RPMs are programmed into computers memory

If problem continues you could be at the point where you need to pull out the computer and have a look at the circuit board inside
There are 3 capacitors that can leak after 20+ years and that can cause any number of issues, including undriveable vehicle

Picture here: http://www.auto-diagnostics.info/ford_eec_iv

The 3 Blue capacitors are the ones that can leak, under $5 to replace but need soldering ability
Also general look at the board can show other issues

 

[Fast Doc]

OK Ron, here’s an update.

Your information has been very helpful.

I went out to the truck, and started looking around for the sensor. Before I found the sensor, I found a disconnected vacuum hose!

I don’t know if this was there all the time, or if it’s something also that just happened. In any event, I took some pictures, I don’t know what this thing is supposed to do. As it turns out, the hose was deteriorated, so I replaced it temporarily with a piece of fuel line for a motorcycle.

I took the truck out. Once it is warm, it idles in park at 1000 rpm’s; in drive at about 700 rpm’s. Drivability seems fine. If I remove the blue hose that I attached, the idle jumps up about 500 rpm’s. What is this thing? I am hoping that I have it fixed. If I spray the area of the blue hose that I just attached with starting fluid, the idle remains the same.

I also attached a couple pictures of the truck, in case you or anybody is interested in pictures of old Ford Rangers. This one is a survivor. It is original, but I get compliments on my “restoration“ all the time. It’s a great truck and I appreciate it. Thank you all for helping me keep it on the road ?

 

[Fast Doc]

Great, now I have to figure out how to attach pictures LOL

 

[Fast Doc]

William shared 1 photo with you

www.amazon.com

There’s probably a better way to share pictures, but this is what I figured out so far. I thought you may enjoy seeing the old truck.

 

[Fast Doc]

William shared 1 photo with you

www.amazon.com

The pickup is called SLOWDOC. I have a 500 horse power sports sedan that’s called FASTDOC. ?

 

[Fast Doc]

William shared 1 photo with you

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Original interior. Pretty nice for 1992.

 

[Fast Doc]

William shared 1 photo with you

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Idle speed while warm in neutral.

 

[Fast Doc]

William shared 1 photo with you

www.amazon.com

Idle speed while warm in drive.

 

[Fast Doc]

William shared 1 photo with you

www.amazon.com

This is the mystery part that was disconnected.