I can’t seem to figure it out, and figured I’d try and figure it out here. Before I redo the entire front end suspension I’d like to tackle something out. When I am driving at highway speeds my RPM gage will drop, and won’t go back up unless I gas it or let off. Believe it might be a throttle body issue with the linkage but I’m not sure. I’ve replaced the TPS sensor, IAC, full tune up, fuel filter, air filter, clean the throttle body, don’t everything I could imagine would be the case... the whole 9 yards. Trucks been doing this for a while and I’d like to try and figure out what it is?

any idea?

 

Welcome to the forum

So 2002 Ranger with 3.0l Vulcan engine?

Manual or automatic trans?

 

Yes it’s the 3.0 Vulcan 4x4 automatic trans.

 

So the actual engine RPMs drop not just the gauge(tach)?

And does the tachometer drop to 0 or just drop down to engine RPMs, like 1,500?

If tach drops to 0 or below actual RPMs then I might replace the Crank sensor on speculation, not cam sensor although that might be next, lol

The TPS was a good choice as long as you used a Motorcraft(Ford) replacement, 3rd party parts have been giving people a hard time recently, bad out of the box or short life

Have you checked for any Codes in computer memory, check engine light(CEL) doesn't come on for intermittent issues, but code is still set and saved in memory

 

It drips down to 1500, 2000, 2500 it differs at different speeds. TPS sensor I haven’t had an issue with, it’s Duralast and I’ve had a lot luck with them. I’ve hooked up my 2500 dollar snap on code reader and even then it’s not bringing anything up. No lights or anything.

 

Then it may be a computer(PCM) issue

If crank sensor or cam sensor lose sync then a code would be set

If fuel pressure should drop then Lean code would be set

If TPS and MAF sensors don't match then a code would be set

If spark should be lost then misfire code would be set


I would unplug MAF sensor or ECT sensor and start engine, see if PCM sets correct codes, basically a "sanity check" for PCM

 

It throws the codes like it should. It’s just weird, I’ve tried everything and can’t seem to figure it out.

 

Then its a PCM issue

Occam's Razor, the simplest answer is usually the right answer

 

Ill get a video and see if you can’t diagnose it.

 

Best I could get of it. Was trying to drive and record at the same time. Does it sometimes but not all the time. Still slowly gains speed which is what gets me.

 

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Not sure how a video would help

If any computer controlled engine is running out of spec, you check the hardware first, i.e. air filter, fuel filter, spark plugs
Then the sensors, if they all test OK...............
Then the controls, i.e. coil pack, IAC Valve, if they all test out OK..................
Computer problem