Odd vacuum issue (1 Viewer)

[Hank6760]

I have an 82 with 135k on the 2f, recently the idle went low and to get home from work had to run with the choke pulled out slightly to keep from stalling. I changed the fuel pump two months ago and the fuel filter, spark plugs and distributor cap and rotor last week end. That has not helped. In looking at it this afternoon I found some loose bolts on the intake/ exhaust manifold, tightened those to no effect. What is odd is when I pull the cap from the vacuum tube at the base of the carb the idle goes up and there is gas in the cap. In the past if that cap leaked the idle dropped or it stalled. It acts like a vacuum leak, sort of, but I can't find one ( spraying starting fluid at suspect areas) . What the heck?

 

[CaptClose]

I've had those same symptoms in the past. It's always been a vacuum leak. And sometimes, spraying carb cleaner won't find it for some reason.

My guess, is that your manifold is warped and perhaps your gasket needs to be replaced and possibly doubled up. Having your manifold machined, would be doing the job right.

You can pull your hair out looking for something else, but this is where I would start.

A compression test could tell you if you're losing vacuum internally. I suppose that's easy enough to try first and rule out a compression issue or head gasket problem.

 

[Landpimp]

Idle control selnoid would be my next check.

 

[peterabbitcook]

Measure your intake vacuum at idle. "Normal" vacuum pressure depends on your elevation, but if it's very low (say below 14 inH2O?) you've likely got a leak. Check vacuum hoses - there's some hard to see vac nipples that might have loose hose - carburetor advance ports, a couple on the metal piping below the air cleaner, BVSVs on thermostat housing, ports below passenger air cleaner, brake booster & AC vacuum switch (firewall side of intake manifold). PCV hose & grommet often come loose on my rig (especially after shaky off-roading).

If you've got normal vacuum sounds like it could be an idle cut solenoid issue - test your idle cut solenoid with a +12 V source, and follow its vacuum hose from the carburetor to the vacuum pipe to the vacuum switch.

If it's a manifold leak, check for warping. If warped take the manifolds to a machine shop to get grinded/belt sanded flat & put a new gasket on. Tighten & Torque manifold bolts in correct order(attached image).

 

[Hank6760]

I guess I should have said the truck is desmogged with a trollhole carb. Checked idle control selonoid and it seems to be working( clicked when plugged and unplugged, stalled as soon as unplugged while running). I guess I'll have to check manifold this weekend. I will look more closely for vacuum plugs. Still puzzling that the idle would go up at removal of cap at the base of carb and there is gas in the cap.thanks for the ideas

 

[Hank6760]

I replaced the manifold/ exhaust gasket today and it seems to have fixed it, as well as fixing an exhaust leak. The cap now when removed causes the engine to stall rather than increase idle. Thanks for advice, folks! Would probably still be stumped withou it