any ideas what would cause steady but low vacuum 12" and very rich conditon? 1990 5.8 efi compression 130-150 psi on all cyl.

 

let me guess...noby's had a truck with low vacuum and puffing black smoke???:o

 

Check for vacuum leaks esp. between upper and lower intake manifold behind distributor. Gasket will break and cause a vacuum leak. Also, have you checked fuel pressure?

 

Ya, fuel press. 39 psi at idle, sprayed front and drivers side of intake w/ wd40 no change, new IAC, ECT, timing chain seems OK (dist rotor move immediatly when I turn crank) motor does have 184145 km (115000 mi) thanks for reply

 

Do a leak down test.

 

great idea..I just don't have any way of getting a tester at the moment, I'm not sure what I'll find though b/c the vacuum reading holds steady as a rock at 12 in hg, any possible valve or head gasket problem and it would fluctuate...the only possible thing I can think of (unless I'm missing something, which is very poss.) is a vacuum leak, but where?? funny thing is though if I unplug the line to the brake booster or any other major line, creating a major leak -the idle actually smooths and goes up, but the vacuum reading stays the same.

 

How does it run while driving? Low on power? How many Miles on it?

Have you checked valve timing?
Bad gas maybee?
Clogged converter?

Need more info.

 

You have a device called a MAP sensor.
When the vacuum goes low from a leak, bad hose etc. it runs the motor rich.
It's supposed to do this under normal running conditions because the system does not have mass air so is the only way to get the EEC to add fuel for acceleration and power in a Speed Density system when the throttle is opened and the intake vacuum changes with power demand.
Find a bad hose going to the MAP, bad MAP etc..
This one should be easy to find.

 

115000mi, I'm not driving it 'til its fixed (no insurance)..fresh gasin a new tank, no cat, headers, magna flow, o2 in left pipe stock distance from engine...engine runs better w/o map plugged in..for some reason map is causing rich burn (new map) thanks

 

first thing I check and replaced...then applied 20in hg vacuum to new map, truck ran a little better............seems something else is causing steady low vacuum I just cant find it, tore off upper intake and resealed it, removed all unneeded vac. hoses and plugged...no difference

 

Next to look at is ignition timing. Is it retarded?
Has the timing chain jump a tooth at that mileage and changed valve timing and distributor timing?
Is there vacuum at the hose where it attaches to the MAP?
Is the fuel regulator diaphram ruptured and letting gas into the intake?
Got to be one or more of these items.

 

that's what I'm thinking as well...timing is at 12btc, havent checked for jumped chain, but the truck revs fast and smooth, there is vacuum at the map...just not enough it seems and the fuel pres. reg is fine....this has got me stumped to the point of tearing the engine out and putting one I have in another truck in (probably to find the same condition, which is why I'm obsessed with fixing this) that and the other motor is an 1987 351HO not quite sure how it will work with the SD system (was rebuilt, bit of a cam)

 

In my case it was a kinked fuel return line near the tank.

 

Replace the fuel pressure regulator until you find a regulator with pressure of 35 or lower when the engine's running (39 with the engine off). It's getting too much fuel.