Hello, Curtis,
I don’t recommend aiming a parts cannon to “fix” stuff unless you know the part you're buying is the fix. But if you already have a known-good part available (free), then it's reasonable to swap the part.
Based on the symptoms you provided… and understanding I’m only guessing based on my experience.
And also understanding for most of stuff I’m suggesting here, you’ll need a scope, a multimeter, and some experience or someone who knows what to look for and/or how to interpret waveform data. If you’re a gung-ho? DIY’er, then jump on the internet. Lots of excellent video tutorials available by some amazing auto techs who willingly share their expertise with everyone.
1. Vacuum leak (unmetered air) - as rpm's and run-time increase, the ride smooths out. The engine gets hotter, and heat tends to minimize or nearly "fix" small vacuum leaks at cylinder heads and valve cover and other places so the ECU doesn’t go bonkers with unmetered air and flood the fuel injectors. So check for leaks at egr, pcv (sticking pcv can cause idle and other performance issues), air flow meter, brake booster (although the booster can be trickier to diagnose), any place there’s a grommet, hose, gasket or mating surface on engine. Really easy and cheap to diy test for many vacuum leaks with a spray bottle of water. Lots of excellent videos online. Go deeper using a vacuum gauge and/or smoke machine. Easy to setup.
2. Fuel pressure regulator is getting “stuck” but gets happy at higher rpm. Many regulators have a rubber o-ring seal. Sometimes those get smooshy-brittle and leak. You’ll need to test fuel pump pressure as other posters suggested and check before/after regulator and filter. A related issue is intermittent voltage drop to the fuel pump from the fuel pump relay. If power is being restricted to the fuel pump, the pump won’t pump the correct pressure or volume of fuel. In your case, most of the performance issue is at lower rpm, not highway driving…so inclined to think may not be the pump pressure or volume per se unless…
3. Fuel pump is ready to go – you’ll need pump specs, a scope and a multimeter to really test this, maybe a little experience as well to understand/interpret waveforms and because you’ll be checking grounds and power from the engine bay to the fuel pump.
4. Fuel injectors – really need a scope on these and check against ignition spark and crank. Are they the original 140,000 mile injectors? Ever been replaced/removed/cleaned?
5. MAF sensor – needs cleaning or needs replacing. Even with a scope this is a difficult sensor to troubleshoot. You’ll need a scope, multimeter, experience. And you’ll need to perform drive cycles. Even then, it can be a crapshoot, especially if no codes.?
6. IAC – Idle Air Control valve needs cleaning or is faulty. Not sure if your vehicle has one.
7. PCV sticking sometimes but related to air leak. Easy to check. Make sure to check grommet if it has one. If it’s rubber, it likes to leak. So check for vacuum leak. Don’t buy a new pcv if the installed pcv checks okay. That’s money you can save for a cheap burger. If you have to buy new, then make sure to get the grommet as well. ?
8. ECT. Faulty Engine Coolant Temp sensor. If temp sensor misreports, then ECU adjusts fuel because it thinks the engine isn’t running at proper temp (adds fuel if it thinks it’s “cold” which is part of reason why rpm higher on startup). Easy to test…but a real pain to get out sometimes. This is a rare issue.
If I were the tech, I’d be asking questions and looking at stuff:
How long have these symptoms been present (when did you notice and why)?
What time of day?
Are symptoms present all the time or intermittent?
What color is the exhaust at idle? After sitting all night or day? At running temp? After driving for 15 minutes on the highway?
Any fuel smell in the engine compartment?
Any wetness near the fuel injectors or fuel pressure regulator?
How’s gas mileage?
Any strange noises from the engine compartment?
What’s your air filter look like?
Battery at full charge and getting charged properly?
Were any other parts replaced before or besides the computer?
What happens when you stomp on the accelerator (wot – wide open throttle)?
What happens to rpm when you turn on a/c (if you have a/c)?
Surprising the ECU replacement “fixed” anything. It's more typical for something to get "fixed" unknowingly when a tech is banging around in the engine bay -?if the stalling is still fixed as you say – excellent!. Replacing an ECU is a big deal. It’s more typical for techs not to perform a proper drive-cycle relearn pattern when an ECU is replaced. So what you get is the default performance protocol from the ECU until the ECU adjusts itself for the signals from input/output sensors and then sends its corrections to some sensors…so from air flow to fuel to spark, the big three of any combustion engine and from evap system. Performance can change after the default system is adjusted real-time from sensors.?
