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2004 GMC Yukon Denali V8

The car can’t go any faster on freeway then 40-50 mph.
Any ideas what can be wrong with it and what can be done such as major service, transmission service , engine tune up ? Or it could be some serious mechanical problem that needs to be fixed and very expensive ? It has 260,000 miles on it.
Any advices will be greatly appreciated …before I had to the dealer or a private mechanic shop.

What happens when it 'can’t go faster"? when you push on the throttle at 50 MPH, does the engine rev up? or just start to miss? Manual or automatic?

Is there a check engine light? did you read the codes?

What happens at idle? how does it run if you push on the throttle in neutral, does it rev up OK?

Service history? ever had any major work done on the engine or transmission/

It’s automatic. No engine light. It runs at idle. I’ll check on throttle at 50 mph again. It doesn’t miss nor stall. No major repairs on engine and transmission.

By doing a google search I found this:

It is probably your Vehicle speed sensor. It is mounted on the tailshaft of the transmission and is held it with one small 8 mm bolt. Your speedometer wont work right and it will probably shift wrong or wont shift at all because the transmission relies on the speed signal to know when to shift. The tahoe/yukon use the 4l60e transmission.

is the yukon shifting 1-2-3-4?
do you watch the tach or can you feel the shifts?
what rpm are you running at 40mph? <2000 rpm?

Have the diagnostic codes read out. There could be some even though the CEL remains unlit. Also verify the CEL does come on with the key in “on” but the engine not started.

Yes, with 260K miles on the engine that’s definitely possible. Low engine compression could cause low power for example. However for that you’d usually notice the engine was also hard to start though, especially when cold. And you might notice an unusual amount of oil loss. Is it ever hard to start? Are you using any oil to speak of? Any smoke coming out the tailpipe beyond what’s normal?

Other common possibilities beyond what’s mentioned above by other posters are a clogged air filter, clogged fuel filter, bad fuel pump, and clogged cat.