I've got a 98 expy with the 5.4. About 1-2 months ago it started up out of the blue. When I start the truck up, there's a pretty bad vibration you can feel. It seems like the lower the RPM's/load, the worse the vibration is. Also, as the engine warms up it smooths out. After driving for a few minutes and it gets good and warmed up, the vibration pretty much goes away and things feel normal. Any ideas? Initial thoughts were fuel injectors, but I'm thinkin if that was the case, the vibrating would continue even as the engine warms up. Any help/advice would be appreciated.

 

No chk engine lights? does the bulb work on the dash when you turn the key on?
A dirty Iac or maf.You could laso have a slight vacum leak somewhere untill it warms up.
A cop that is going bad or sparkplug boot that is burned up or cracked could cause a problem like this also.Have you changed plugs lately?

Rich