My '04, 4.0 SOHC, only 108,000 well-maintained miles, runs perfectly, pulls hard, is smooth and quiet. BUT . . . at highway speeds, anything steeper than the mildest of hills will mean flooring the gas just to keep up with traffic. Like 65 mph, hill coming on, all the rest of the Hyundais and Corollas are zooming past me and if I mash the gas to the floor, the truck downshifts and just increases the revs but won't go any faster. Is this normal for the 4.0 engine? I used to have a '91 3.0 that did not have this problem!

 

No, not normal at all.

I would test the vacuum on the motor.
18"-20" is normal
Lower vacuum could mean valve timing has slipped, while not common it can happen on these engines because the timing gears are not "keyed" to the cams, just a bolt in the center holds it in place.
But acceleration would also be effected.

Then with engine idling blip the throttle full open and let it snap closed
Vacuum should drop to 0-2" then snap right back up to 18".
If it slowly comes back up to 18" then you have a partially blocked exhaust.
This would effect power at higher maintained speed as back pressure builds up less and less new air and fuel is being pulled in.


4.0l SOHC has a Knock sensor, this engine runs 9.7:1 compression ratio so can not run on regular gas(87 octane) without knocking/pinging.
When knock is detected(you won't hear it) the computer retards the spark so spark plug ignites the fuel mix before it can self-ignite/knock.
This causes a loss of power, advanced spark = more power.

You could try running a few tanks of 91 octane and see if that might be the problem.

EGR system is adding too much exhaust, Unplug the vacuum hose on the EGR valve, plug the hose.
Go for a drive, Check Engine Light(CEL), will come on because EGR valve is not opening, this is good it means CEL is working and computer is seeing faults.
I assume CEL is off and and so no codes to read.
With no working EGR see if it gets up to and above past speed limit, carefully, no tickets

Speed limiter, is speedometer working OK, the computer does have a REV limiter and Speed limiter.
REV limiter is only used when transmission is in Park or Neutral, it kicks in just above 3,000RPM, it also has run away engine limiter at 7,000rpm

Speed limiter is usually around 90MPH, but if Speed sensor is wrong...........

Fuel filter is a long shot from description, it does limit power when trying to maintain speed for any distance, but most would describe as "it felt like it was running out of gas", because it was running out of gas, lol.
Fuel filter should be changed every 4 or 5 years in any case

 

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Thank you for the detailed response!

I've been using 91 octane since I first bought the truck at 86K miles and noticed audible pinging pulling up hills. I don't rev the engine, have probably exceeded 3000 RPM maybe three times so far only when passing on the highway. Passing, or any other higher speed is just fine on level ground; it's just those hills. 90% of my driving is on the highway, and my hotrod days are, sadly, over!

I disconnected and plugged the EGR valve hose and went for a drive up to highway speeds. The truck did nothing unusual, and set no codes. When I got back, I checked for vacuum at the hose end and there was none I could feel. I quickly revved the engine and still no detected vacuum or pressure at the hose end. What does all that mean? No difference with or without the EGR? Also, no increase in RPM with the hose disconnected.

Another test: I applied vacuum to the EGR valve with the engine idling and the engine stalled immediately. That means the ERG valve is functioning, right?

Next thing is to go to my local auto supply for a new vacuum tester. Seems my old one I have not used for like 20 years is toast.

Thanks again!

 

EGR valve is not used when engine is cold or just REVing it up, so no vacuum is normal.
Computer opens EGR valve to cool down cylinders when they are under load.
Cylinder temps get very high when accelerating or climbing hills, this increases NOx emissions, by adding exhaust gases to intake it slows down the burn rate in the cylinders, this lowers the temp and lowers NOx emissions, as a side effect it also lowers chance of pinging and knocking.
So there is only vacuum on that hose when engine is under a load.

 

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So can I assume that applying vacuum to the EGR valve with the engine (warm) idling and having the engine stall is a valid test of the EGR valve itself?

 

I have a 2001 with 80k miles. I drive 75 w/AC going and pull about 2700 RPM even on hills. Really steep hills it will down shift and jump to 3600RPM for a short period.