Violent shaking around 1100 rpm

[matt12305]

I've noticed my ranger (1988 XLT super cab 2wd 2.9l v6) shakes the truck a lot when it idles around 1100, sometimes the idle speed goes down to 900 and it feels smooth but when RPM's are between 1100-1600, it vibrates a lot, if the hood is open the hood very noticeably shakes and you can see the engine shaking back and fourth..all this stops when it goes past 1400rpm.

Additionally the engine dies if I suddenly let off the gas pedal, even at operating temps. It's worth noting I don't drive it yet and I just got it running(has been sitting 14 years 77k miles). I also had to take off the upper intake manifold to clean the fuel injectors, could it be the throttle body needs adjusting? Or perhaps the idle motor thing?

I really appreciate any responses.

 

[alwaysFlOoReD]

I would start by looking at the spark plugs, making sure they all look the same; light tan discoloring. If they are all the same then a compression test and or leakdown test.

 

[matt12305]

Yeah I did check spark plugs yesterday, they were all black and had soot on them..the gap was way off as if it hadn't even been adjusted when they were installed. That did fix the idle from jumping up and down too.

 

[RonD]

Cold engine
Take off fan belt
Start engine, Battery(charge) Light will stay on
See if vibration is gone

Fan clutches can/do fail and they will cause RPM specific vibrations
Free test

You can only run a COLD engine without the water pump for 1.5min, 90 seconds, so long enough
This is also a good way to isolate engine noises, you will be surprised at how quiet the engine bay is without the fan turning

 

[matt12305]

The fan doesn't seem to be on a belt, it looks like it's attached directly to the upper pulley.

 

[Uncle Gump]

Belt driven pulley...