I'm lost on this, I apologize this is very long, but I wanted to give all possible information. I've done due diligence with research and trying suggestions, but I'm still stuck


I have a 2004 ford ranger XLT, 4.0 V6 SOHC 5spd manual, 101k miles. In short, the issue is engine RPMs are holding high (around 2500 RPMs) when I depress the clutch fully while at cruising speeds, and they stay high until the clutch re-engages or I'm just about stopped where they return to my typical idle.

Full story:
I don't have a check engine light. Just a few days ago, my engine started going high in RPMs when I fully depress the clutch pedal while at cruising speed either to shift or coast, and they stay high until I come to a near stop or the clutch re-engages. Usually before this, my RPMs would settle in the low 1k range when clutch was pressed, now they're going to around 2500, variable between about 2200-2700 that I've seen. Also, if I'm lower in RPMs in gear, say 1500, when I depress the clutch, the RPMs will climb to that 2500 range. It seems to do this when up to temperature.

RPMs are normal when in gear. Warm idle is constant about 800-900. Sometimes at cold startup, it'll go to 2k+ for a few seconds before settling to lower 1k ish until it gets warm.

I tested the TPS, it tested about .98V closed throttle, 4.8V at WOT, and while rotating the throttle plate the voltage increased consistently so I'm fairly certain TPS is not the issue

I thoroughly cleaned the throttle body and replaced the gasket two days ago, the spring seems good, and I drove ~40 miles with no fixing it.

I tried replacing my IAC and PCV valve, both yesterday, I reset the computer, went through all the gears cold, then again once fully warm, and I've driven ~60 miles running errands since. Still no change.

I'm wondering if I just need to wait a bit for the computer to adjust to the new IAC? HOWEVER, when I unplug my IAC, engine RPMs are barely affected so I was thinking I have a vacuum leak. Unless that's just a symptom of the computer not being adjusted to the new IAC yet?

Seeking a vacuum leak, I have tried spraying TB cleaner throughout the vacuum system and intake manifold with the engine running, and I don't have any change in RPMs that would indicate a leak point anywhere. I have also tried blowing smoke throughout the engine, and it doesn't get sucked in anywhere. Highlighted in red on this picture have been my focus areas when checking for leaks, also back near the firewall where the PCV valve is tucked:
[img]blob:https://www.ranger-forums.com/e04a193b-0cac-4e96-9a8d-b8771d387467[/img]I did my best to determine and check all the components of the intake system and where the vacuum lines are, but admittedly am not the most familiar with the landscape of engine bays even with the diagram on the frame as guidance. I checked basically everywhere with the smoke method recommended to me, was a little bit more cautious with where I put the TB cleaner because of online warnings of fire.

Then I went to my local NAPA, I was looking to borrow a code reader if it were possible to have codes with no dash lights on, they said no, there wouldn't be any codes. Then I talked this issue over with the owner, and he came out to have a good look at my truck but didn't detect anywhere that seemed likely to be giving a vacuum leak.

So basically, I'm stuck. I'm no mechanic, I've had some exposure but I'm still just learning car stuff, this is all very unfamiliar to me

I've done a lot of diving into research on this with a lot of really great insightful threads, but I'm still dealing with the same symptoms of the RPMs going high around 2500 with the clutch fully depressed while at cruising speed, and then returning to idle 800-900 RPMs when nearing a stop. The fact that unplugging the IAC has no effect on my RPMs seems to point to a vacuum leak according to everything I've read. Am I just missing the leak somewhere?

I've read about trying to adjust the anti-diesel screw because it can be used to set the idle and also can tell if there is in fact a vacuum leak, however I'm hesitant to try it because I've also seen other threads that are adamantly against adjusting it. I bought this truck at 97k miles, I have no clue if it may have been tampered with previously.

Another thought from what I've been reading is maybe this has something to do with the computer side of things? I tried calling a few Ford locations to see if there were any software updates for my truck, and there weren't, but what if it doesn't need an update it just needs a complete reprogram? Or does the computer reset when unplugging the battery take care of that?

Any input would be greatly appreciated! I planned a 3000 mile (mostly interstates) road trip for the end of the month, and I don't know how bad it may be for the engine and other internals to drive this way, really tryna keep the lil lady in good shape. Sorry for this being so long, wanted to give absolutely all information I have, thank you!

 

Random thoughts here:
1) Did you buy a Motorcraft or Hitachi brand IAC valve? I've read that others do not work well as the controlling signals put out by the 'computer' don't jibe.

2) The PCV valve is a bit fiddly to replace as it's right up against the firewall. Did you seat it correctly?
Also, the "L" shaped plastic pipe which connects the PCV valve to the intake system commonly cracks and is hard to spot.
It goes from the PCV valve across the rear of the engine then turns forward to reach the intake; a 90-degree L shape.
I read they crack on the bottom thus creating a huge vacuum leak.
Also, you may have cracked it when you replaced the PCV valve. They get brittle with age, and you may have moved it too much.

3) It's amazing to me how crowded the right side of the top of that engine is. There's a plastic vacuum tree in there, I believe.

4) Could your CTS (Coolant Temperature Sensor) be bad? It might be telling the computer that it's ice cold, thus demanding lots of fuel.
A broken one reading "COLD" can result in the equivalent of manually choking the engine at all times. The additional fuel can cause the engine to run faster.
Look at the top of the thermostat housing. There'll be one or two wired "sensors". If there's only one, it's a sensor to the computer operating on 5 volts. The computer sends a signal to the dashboard gauge, if so equipped.
If there are two, the other one is a 12 volt 'sender' to the dashboard gauge directly. Be careful to accurately determine which is which if you have two.

4a.) Could your thermostat be stuck wide open, thus overcooling the coolant?

5) Did you inadvertently break/disconnect the vacuum source to the HVAC reservoir (a vacuum reservoir which closes-off the hot coolant to the heater core when "MAX AC" is selected)?
You'll see a ~5" sphere mounted low on the right front fender well. It has a vacuum line in and one out to the heater hardware. Ford used smaller, hard lines on this, not familiar rubber hoses.

Also, save up and get a fairly decent code reader.
The readers at parts stores are always the most basic as customers often steal them. None of the cheapest ones read 'pending codes'.
Select one which serves the newest of vehicles in your life. Being it's 2024, you want one with SRS reset and fluid temperature capabilities, at the very least.

 

if you unplug the IAC and the RPM stays high or no change theres a good chance you have a vacuum leak. I'd start around the PCV valve, diver side rear valve cover.

 

Update:

I really appreciate your feedback on this! And thank you for taking the time to make it make sense to a non-mechanic

I got a USB ELM 327 v1.5 that I had never used before up and running with FORScan, not sure if that is a sufficient scanner? I couldn't really find any way to tell if the one I have is one of the "good" ones

Scanned my truck and came up with P0511 on the PCM (additionally B1317, B1318, and P1804 on the 4x4 which I believe are separate/unrelated but I will look into them). Some searching got me that P0511 is related to IAC/PCM/wiring issues.

re 1: The replacement IAC was standard brand from oreilly, so before buying a new OEM part, I deep cleaned the original IAC and put that back on. I discharged the battery, ran through all the gears both cold and warm, and then drove it about 30 miles. When I re-scanned, the P0511 was gone.

While driving, with the clutch depressed I didn't see a rise RPMs at all. It did seem pretty high on startup still, hitting ~2.5k initially before calming down, but my idle RPMs when warm were 750 which is the lowest I've seen. Not sure if it could still be relearning, I'll see as I run it more.

When I get a chance tomorrow, I'm going to see what happens when I unplug the IAC to see if I still suspect a vacuum leak, I didn't do that tonight.

re: 2. I am fairly certain the PCV is seated right, I dislike how it doesn't seem to lock into place in any way, you just twist it in all the way and the hoses seem to keep it in place, (assuming I did that right). I'm going to check on that L pipe tomorrow, if it's pretty inexpensive I might just replace it for good measure.

re: 3. agreed, I think myself and the owner at NAPA both did a pretty thorough look of everything in there, but I'll do a thorough re-check

re: 4. I'll check on those voltages just for good measure.

re: 5. I'll check on that HVAC area too.

Seriously, thank you for taking the time to explain each of those things so in-depth. Not fully understanding what everything is, you really helped me know exactly what and where to look, I appreciate it!

 

You're welcome.
Re the PCV valve. It seats into a grommet at the rear of the right valve cover, as you know. It "quarter turns" (perhaps a tad less than a true 90 degrees) into place.
There's "got to" be a small notch for it to come to rest. I didn't concentrate on how definitely or positively it settled into place.
I can tell you this; go easy on the grommet as a replacement left side valve cover or even that grommet itself, is becoming impossible to find.

 

I had this exact problem and posted on this forum a 6-month process I went through to solve it if you check my threads. What it was in my situation was a bad throttle position sensor. I discovered that because I went through everything you've gone through and I tested every part and sensor that has to do with gas, air, idle, you name it. I replaced everything and had all the given up when I noticed in an OBD2 scanner that when I would come to a stop my TPS would never fall on the same Base number which is how I realized it was faulty and as soon as I replaced it the next stop sign I came to my baby dropped right down in part like a kitten

 

As far as the high RPM's at highway speed. These trucks will idle high until they get to under 5 mph. It has to do with the emissions. Seems that there's less pollution being created with a higher RPM than idling at lower RPM's.

While other IAC's will work to a degree the only ones that work the best are Hitachi and Motorcraft.