Hi. I have a 97 Expy with 5.4L 189,xxx miles.
My daughter has reported two isolated incidents over the last month that I can not duplicate. No CEL, and no codes when scanned by OBD-II.
She said she was stopped at a stoplight (last month) and while waiting the engine accelerated and lurched forward. She applied harder brake pressure to keep from hitting the car in front of her. Then it dropped RPM back to idle. She got on the hiway and a mile later at 65 mph, going up hill, the vehicle shuttered and decelerated to 35mph. She pulled off to shoulder and called me. I had her shut off and restart the engine and she came home with no other issues.

When I got home I drove it for several miles and could not get it to do anything unusual, nor get a code.

Yesterday she was on the same hiway, this time level ground and had the shutter and loss of power again. This time she checked the parking brake (off) and RPMs. She said the RPM stayed way down (1000 or so she said) even under throttle, She limped home...when underload it shuttered. She backed off throttle and it would smooth out.

I've had coil packs go bad in the past, but never had a severe drop off to where you could not keep speed/rpms up. I again test drove (ten minutes after she got home) and under heavy load (up to 90 mph) and at idle..all is normal, in fact great. No stumble, nothing.

I did find the hose from throttle body to Vapor Management Valve was bad, so I am changing that. I also have the IAC off and am cleaning that and made a new gasket. I replaced the fuel filter 10,000 miles ago, and never let the fuel get below 1/2 full. I will put injector cleaner in while I am addressing the concern. I have not checked fuel pressure yet. Any other things I should look at? I guess I can pull the TPS and clean that?

I was hopng for a code, but none yet. If the only symptom was the idle surge, I would be confident the IAC may be the culprit. But the loss of power is confusing me. Perhaps one or more cops are about to go and have not thrown a code? Maybe the vacuum was affected by the Evap hose? I will also check the PVC hose which I had repaired a few years ago.

Sorry for long post, but wanted to eliminate some of the obvious questions.

 

Well the PCV elbow was "repaired" in the past, not replaced! I now recall making my own elbow with vacuum lines, a coupler and RTV to "temporarily" seal it until I got home, I was far from any parts store at the time. That worked for 30,000 miles! What an idiot...I completely forgot I had done that. It was starting to come undone, so perhaps that was the intermittent power loss problem.

New PCV elbow is on. Clean IAC is back on. Resetting the computer now so hopefully that ends the issues. I'll post back later if not.