My 76 Eldorado revs up and down in idle.

[GrouchoDad]

My 76 Eldorado revs up and down in idle to the point to where it tries to lunge forward a little when at a stop light.  It rocks you back and forth slightly to the point where you feel your head nodding somewhat. It is a fuel injected engine.  I took it to my mechanic and he said he fixed a couple of vacuum problems.  When I drove it home it did fine and had more pick up.  I stopped and filled the tank on the way home with regular unleaded.  I did have mid grade gas in it before. Not a half a mile later it started the lunging again.  As long as you are in motion it is fine but idling is very annoying.  Can anyone please help me!  It's driving me crazy!  I just got this car and it only has 55,000 on it and has only 100 miles on a completely rebuilt engine.  Thanks in advance.
C. Gilliam

[The Tassie Devil(le)]

Do you mean "Surging", or is it just "Hunting"?   Not sure how to fix it in a Fuel Injected engine, but in a Carby one, it is a matter of adjusting the Mixture Screw.

What usually happens is when the engine is running too lean, it idles down, and then speeds up as it nearly dies.   It is "hunting" for more fuel, or a richer mixture.

Bruce.

[GrouchoDad]

It does it up and down in rhythm.  Like nodding your head slowly. It also only does it after you have been driving it a couple of miles.

[Dave Shepherd]

Vacuum leaks or a lean fuel mixture are the most common causes,( as mentioned) more thorough vacuum leak checks are in order here, throttle body to manifold comes to mind, for one, also fuel pressure should be checked.

["Cadillac Kid" Greg Surfas 15364]

Carbureted or Fuel Injection?
Greg

[GrouchoDad]

Fuel injected

[Scot Minesinger]

Agree with posts about vacuum leaks.  Replace all the vacuum hoses original or older replacements to the car.  This is an inexpensive and easy job to do which requires almost no tools, replace one at a time.  Granted you do have to remove the five bolts anchoring compressor after removing belt to get access to the one low in front of carb.

Another cause could be obstructed fuel filter.  Often engine re-builds omit the simplest minimal things.  There should be a screen at the fuel inlet on carb and sometimes this will be replaced with a filter - mistake -this also causes surging.

Good luck

[bcroe]

Fuel injected 

Try pushing down on the cold idle air valve, black plastic  on top of the throttle
body.  If it stabilizes, the valve may not be closing cleanly. 

Could also be an ECU problem.  Bruce Roe