'94 5.0L reluctant to idle below 1500 RPM
 

Scenario: You're driving along, need to slow down, so you lift your foot from the accelerator. Whatever the RPM happens to be at that point, it stays there for a moment, then drops to 1500 RPM... and stays there. Keep in mind I rarely drive it faster than 60 MPH (about 2000 RPM). Anyway, you apply brakes, the truck does slow down, but it's clear the brakes are fighting the engine. It's only when you get below about 3 MPH that the engine abruptly returns to a normal idle.

It behaves like this only when the trans is engaged. In Neutral or Park, it's fine. But the weird part is, sometimes, the problem goes away while I'm driving it.... but then if I stop somewhere (store, gas station, etc.) the problem comes back when I restart the engine.

Recent Service History:

1. Feb. 2015: New plugs (Autolite), cap, rotor, PCV valve.

2. April 2015: Replaced brake line to rear wheels (OE line had corroded-through and was leaking brake fluid beneath the truck below the spare tire). Also replaced fuel filter.

3. May 2015: Engine was stalling when shifting from Park to Reverse (or Drive). Mechanic checked the TPS (he had replaced the OE TPS in April 2013) and found nothing wrong. So he removed and cleaned the throttle body. This cured the stalling but I noted the engine would surge a little (to around 1500 RPM) when accelerating from a stop. And I believe he used RTV instead of a regular gasket when he reattached the throttle body.

A few weeks later, the idle problem began happening. Last week, he swapped-out the TPS and IAC. Had no affect.

There are no codes.

Mechanic kept the truck 3 days but has run out of ideas at this point, short of replacing the ECM.

I've found that if I put the trans in Neutral (while the engine is "stuck" at 1500 RPM), the engine revs briefly to about 2200 RPM, then drops to normal idle. Is there anything in the dash cluster or transmission linkage that can cause this?

So there you have it... any suggestions greatly appreciated.

What brand is your PCV valve? This is very important as your PCV should be Motorcraft only and will likely affect your idle if it isn't Motorcraft. Also, was the voltage adjusted on your TPS? Also, I've found that a bad EGR position sensor or EGR DPFE will cause a high idle. Along with bad vacuum hoses.

Make sure you disconnect your battery to clear out your ECM.

Sorry, QDEEZIE; I don't know what brand of PCV he installed. Why would that matter if the problem only happens when the trans is engaged?

The TPS was swapped-out but didn't solve the problem, so he reinstalled the old part. I don't know if the voltage was adjusted. I did disconnect the battery for about 4 hours before I posted my first message.

Since my last post, I've noticed a few other things.

About 20% of the time, the idle returns to normal when I take my foot off of the gas.

Every once in awhile, the PSOM goes dark (no mileage display, E4OD hard-shifts, etc.) While it's "out", the fast idle problem goes away. Once I reset it (or restart the engine), problem returns.

Could a flaky PSOM be the root cause of the idle problem?

Follow-up on idle issue
 

Apologies for the time it's taken me to follow-up on the problem I described. Ultimately, it turned-out to be a bad IAC. But getting it fixed was an ordeal. I had taken the truck to a long-established shop that someone recommended to me. When I realized that the mechanic at the place ("Shop #1") drove a truck similar to mine, I had confidence it was in good hands. Instead, he gave me a truck back that wouldn't idle below 3,000 RPM in Park/Neutral... then tried to say that I'd dropped it off that way. Couldn't believe my ears... well, I refused to drive it home in that condition, so he agreed to look at it again the next day. I end-up getting a call from the place tellin' me to pick it up and take it to a dealer... he had done all he was going to.


I waited for traffic to clear out and drove the truck home in second gear, where it sat in my driveway for a few months.


Then I took it to a different independent shop... A few hours after I dropped it off, he called to ask me who had last worked on it, then told me: 1. The IAC was electrically nonfunctional, and 2. the mechanic at Shop #1 had bolted the IAC to the plenum =upside down=. That's what it was. Replaced it with a new part, problem solved. Shop #1 is allegedly ASE certified and been in business for decades, but he sure as hell screwed up on this!