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Old 01-25-2012, 10:01 AM   #1
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Loss of power over 45 mph

2003 SMB 4X4 w/ 5.4L engine 160,000 miles. The van has been running very well until two days ago. Yesterday there was a slight loss of power over 45 mph; under 45 mph it runs fine. At 50 mph and over, there is a new vibration or more of a "low growl" heard - something like a bad "U" joint. The "growl" is heard when the loss of power is felt. Cannot hear the vibration with the window rolled down. No vibration at idle, when stopped and no performance loss when accelerating. All fluids are good. Any ideas?
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Old 01-25-2012, 11:45 AM   #2
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Re: Loss of power over 45 mph

Idea- plugs or plug wires

Idea - does it happen when overdrive is engaged? May be transmission trouble.
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Old 01-25-2012, 01:25 PM   #3
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Re: Loss of power over 45 mph

dhally - Thanks for the response. I had three new plugs and coils put in three months ago. When the coils started to go the engine experienced a similar loss of power but there was no unusual sound (vibration or slight growl) at that time.

The transmission was rebuilt and upgraded in July. Trans and transfer case fluids look good and the trans is shifting normally. The new sound occurs above 45 - 50 mph whether the trans is in or out of OD. I can run the engine up to a similar rpm in first gear but don't notice any drop in power or unusual sounds. Same when it is in neutral. The rear diff was rebuilt last year (outer bearings inc) and the front hubs had new bearings installed two years ago.

The alternator is the only thing that was warm after a 10 minute run down the street this morning but I am not sure that that is not normal. All of the other devices that are driven by the serpentine belt are cool to touch after a 10 minute run from a cold start.
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Old 01-25-2012, 03:32 PM   #4
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Re: Loss of power over 45 mph

PS The U joints are tight and the A/C and power stearing seem to be OK.
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