My tbi 350 just started idling extremely rough and running very rich. I tested and replaced the tps, cts, dist cap and rotor, map, and the iac. So I tested the vacuum @ idle and the gauge is fluctuating very very iratic between 10-20"s but when you crack the throttle about 1/4 open and the vacuum gauge steadies pretty good but still fluctuates. Does anyone have any idea what could cause this? I think the vacuum problem is the source of all my issues because the map doesn't have proper vacuum. The entire top end of the engine has been redone less than 5000 miles ago. But I removed, cleaned an regaped the spark plugs and found out the no 4 plug was completely gunked up with carbon.

 

Hey racer

You need to dig a bit deeper, but based on your description vacuum readings that change quickly indicate sticky valve guides, burned valve sets, or leak in head gasket. It also depends which vacuum tap you are using to measure for your vacuum gauge. To be more specific, if vacuum gauge reading is vibrating excessively at idle but steady at higher engine speeds this may indicate worn valve guides. It also can be indicative of a bad ignition miss for one or two cylinders - rapid vacuum swings between 20 to 10 in-Hg.

My next steps would be:
1) Perform a cylinder leak down test to make sure that guides, valves are not leaking.
2) Check spark plug wires, replace them to be certain.

Good luck and keep us posted
//RF

 

It has all new dist cap, rotors, plugs and plug wires.

 

EGR valve?

 

it doesn't have one and hasn't had one since i got it running the intake i'm running is a non egr intake

 

Does you vehicle have AC......if it does have a close look at the connector for the vacuum selector. when rough hands pull this control head from the consol the fastener clip gets it's dogs broken off this leaves the 2 halves of the Vacuum selector wide open and this is the same as pulling a vacuum line off the intake.....I put a screw in between the clip jaws and pulled the vacuum selector halves back together solved the vacuum leak smoothed out my idle very nicely.....might be your is the same issue.