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2.5L ('98-'01) Stuck engine RPMs until dead stop - gone fishing


Aretaic

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The truck is a '98 XLT with the 2.5l 4cyl, manual. When driving (hot or cold, highway or in town, 25ft or 200 miles) the engine speed stays high, or goes higher, when depressing the clutch. It will stay there, no matter what, never come down, until the truck comes to a dead stop. I've coasted for miles this way, hoping it would drop down to idle like any normal vehicle would. Like so many others I have looked and searched high and low for this same problem, found many solutions, and no luck with my particular truck. I inspected everything, vacuum lines, electrical connectors, particularly the TPS connector, the entire intake manifold, and started testing and replacing likely culprits. The truck has almost zero issues now but for this problem.

So far I have replaced the VSS with three different units (all good working sensors), the IAC valve, the EGR valve, the DPFE, the purge valve, both O2 sensors, the MAF sensor, the TPS sensor just today, and a plethora of other parts in resolving other issues such as: vacuum lines, EGR pipe, timing belt, intake gaskets, head cover gasket, thermostat housing and sensors (rest assured the problem persisted before and after). I opt for Motorcraft when available. Some other facts, the throttle body has been cleaned (I've heard this is an issue, I have an older spare TB from a Maxda 2300 but unsure of its condition - although that truck ran) and the truck has a perfectly functioning Sportrac cluster as the original speedo had stopped working (stuck indicating 50mph until stopped, possibly related, no luck researching that issue until I replaced the entire cluster). I have no trouble codes, and I do own a OBD2 reader that can do some live data. The ABS light is always on, but will flash if I brake and change gears at the same time. The only thing I haven't tried is charging the A/C, although it blows cold I wonder if it could have an effect if pressure is low.

What a mess! At any rate the truck is great, but this stuck RPM issue has me going crazy. I'm going camping this weekend to get away from it, if anyone bites on this hook - I'm definitely curious what I am missing or ignoring. I'm open to any and all suggestions, even if I've done them before. Hope I catch lots of trout up there, maybe something here too!
 


Rock Auto 5% Discount Code: 70D65189E6D8FF: January 5th, 2022

scotts90ranger

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When you are stopped, does it matter if you have the brake pedal down?
 

RonD

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OK, well it is normal for the manual trans engine to hold RPMs high until speed is under 5MPH

Computer does this using the IAC Valve..............so lets see if its the computer that's holding RPMs high
Warm up the engine, at least 5min, so Choke mode is done
Shut engine off and unplug IAC Valve
Restart engine, it may not idle(no IAC Valve) so you will need to open the throttle a bit to keep it running

CEL will come on because IAC Valve is unplugged

Test drive, lets see if RPMs stay high when shifting or coasting, don't let it stall<<<reminder

If RPMs are dropping between shifts and when coasting then you now KNOW that the computer was doing this high RPM thing, not a vacuum leak or ??

If it is the computer you can check with Ford to see if there was a software update for this issue, and it was an issue in late 1990's and early 2000's Rangers, high idles up to 2,500rpms

If RPMs are still staying high then its not a computer issue so thats off the table
We can move on to mechanical issues like vacuum leaks
 

PetroleumJunkie412

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OK, well it is normal for the manual trans engine to hold RPMs high until speed is under 5MPH

Computer does this using the IAC Valve..............so lets see if its the computer that's holding RPMs high
Warm up the engine, at least 5min, so Choke mode is done
Shut engine off and unplug IAC Valve
Restart engine, it may not idle(no IAC Valve) so you will need to open the throttle a bit to keep it running

CEL will come on because IAC Valve is unplugged

Test drive, lets see if RPMs stay high when shifting or coasting, don't let it stall<<<reminder

If RPMs are dropping between shifts and when coasting then you now KNOW that the computer was doing this high RPM thing, not a vacuum leak or ??

If it is the computer you can check with Ford to see if there was a software update for this issue, and it was an issue in late 1990's and early 2000's Rangers, high idles up to 2,500rpms

If RPMs are still staying high then its not a computer issue so thats off the table
We can move on to mechanical issues like vacuum leaks
Bookmarked. Thank you.
 

Waldoduhh

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I don't know why everyone keeps saying its normal for the idle to stay high when shifting. Not a single manual including my old 96 ranger stayed at high idle when shifting. When you push the clutch in, the idles are supposed to drop, not stay at 4000 rpm like this 2.5 ranger I have was doing when you change gears. Does it immediately dead drop, no, but within about 6 seconds it should have dropped to atleast 1k rpm. Its your abs wheel speed sensor. It's on top of your differential above the driveshaft. I had same problem. 2.3l and 2.5l use same part number sensor.
 

Waldoduhh

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Also, if you can't get the sensor right away and need to drive, just unplug it.
 

RonD

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I don't know why everyone keeps saying its normal for the idle to stay high when shifting. Not a single manual including my old 96 ranger stayed at high idle when shifting. When you push the clutch in, the idles are supposed to drop, not stay at 4000 rpm like this 2.5 ranger I have was doing when you change gears. Does it immediately dead drop, no, but within about 6 seconds it should have dropped to atleast 1k rpm. Its your abs wheel speed sensor. It's on top of your differential above the driveshaft. I had same problem. 2.3l and 2.5l use same part number sensor.
Its was done to lower emissions on manual transmission vehicles, yes should only last 5 to 8 seconds with clutch in, after that time the RPMs should drop to just above 1,000rpms, then under 5MPH it should drop again to 650-750rpms
Idling engines are a big polluter, so RPMs stay at or above 1,000 until stopped(under 5MPH)

This is why vehicles now have to have Auto stop/start so they can meet their year's Idle emissions standards, only way car makers can do that is to shut off the engine when stopped
 

Aretaic

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Thanks for the replies, everyone. Had a great time catching lots of trout.

Scotts90ranger - no, whether the the brake is applied or not it behaves the same.

RonD - I get that rpms will linger a little bit, but this is excessive and beyond anything appropriate, I guarantee it. If this were normal, I would sell the truck immediately (or put a 4BT in it and say to hell with the computer). I have checked for vacuum leaks by unplugging the IAC previously and the truck does indeed stumble and stall, though I have not tried to drive it this way - I'll give that a shot as you described. Not a big fan of the dealers in this town but I'll see what I can dig up insofar as potential software updates.

Waldoduhh - Thanks, I have already replaced the speed sensor three times with new units. No change, I did it three times because I was convinced it would resolve the issue as well, and I thought I might've gotten defective sensors. I've tried it unplugged as well.

I will report back on the truck.
 

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If RPMs drop while driving with clutch pedal down and IAC Valve unplugged, then computer is the reason for the high RPMs, so no need to look further, but unfortunately it doesn't solve the problem
 

Aretaic

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I've been driving the truck with the IAC unplugged, CEL came on for IAC circuit, and... lo and behold, no stuck RPM behavior. It drives like it should, albeit with a wavering idle. RPMs come down. Guess I'll see if I can get the ECU updated locally - I'll also check for a J2534 pass-through tool, I'd rather have this capability myself...
 

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Thanks for the update :)

Yes, unfortunately reads like a computer software issue

There is a small small chance that the IAC Valve's internal spring is not strong enough to push it closed with high vacuum in intake(foot off the gas pedal)
Not sure how you would even test for that, lol

Not sure if OBD has IAC Voltage reading PID, if it does you could watch Live Data to see if Computer is dropping IAC Valve voltage but RPMs are staying high
But I would expect Computer to set a code for that
It has these 3 basic codes
P0505 Idle Control System Malfunction
P0506 Idle Control System RPM Lower Than Expected
P0507 Idle Control System RPM Higher Than Expected

So not setting a code means computer knows RPMs are high because it wants them high, lol
 
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Aretaic

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That makes sense... Thanks for the context and information RonD. And thanks for nothing, Ford PCM!! For now at least, the truck is more tolerable with the IAC unplugged.
I may go the route of a VCM2 and IDS license in order to resolve this myself. My current scan tool doesn't appear to support J2534 pass-through 🙃
 

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Not sure if OBD has IAC Voltage reading PID, if it does you could watch Live Data to see if Computer is dropping IAC Valve voltage but RPMs are staying high
My current scan tool doesn't appear to support J2534 pass-through
In case it helps and your scantool/software is capable of monitoring custom PIDs, my 2004 Ford Ranger reports IAC valve position (in %) via the PCM, Mode $22, PID $1153, formula is "AB/327.68". It might help determine if your replacement IAC valve is faulty before you start re-programming the PCM(!).
 

Aretaic

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2WD / 4WD
2WD
Total Lift
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In case it helps and your scantool/software is capable of monitoring custom PIDs, my 2004 Ford Ranger reports IAC valve position (in %) via the PCM, Mode $22, PID $1153, formula is "AB/327.68". It might help determine if your replacement IAC valve is faulty before you start re-programming the PCM(!).
I will check into this, I'm using a Launch 90 Millennium and I think it can monitor some PIDs. Thanks Orca!
 


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