low vacuum

[stephen p luce]

OK guys here's a tough one. I have a 89 K1500 with a 98 vortec engine running the TBI injection. Got the big bore throttle valve, 18 psi F/P, ECM has been reflashed. So here is my problem, after driving for 20/30 min, shut the truck off. after 30/45 mins go to restart the truck, it blubbers like it is super rich, which it is because the MAP is reading 1.8v at idle, after about a min or so something clicks and it smooths out and voltage drops to .9v. been scratching my head over this one. I have tried a couple new MAP sensor same thing.
so today I put a vacuum gauge in line with the MAP sensor and when it is blubbering the vac is at 14" then the click and vac comes up to 17" and smooths out. so what do you think happens? why is the vac low and then goes back to normal. I'm pulling vac off the back of the throttle.

 

[PlayingWithTBI]

what are your other sensors telling you? Maybe the IAC is sticking. Pull your PCV valve and plug the hose off. Pinch off your brake booster hose. Do you have the A.I.R. pump? Plug off that line to see if that valve is defective. Plug off your EGR valve hose. Same with your CCP line.

 

[stephen p luce]

all the other sensors are in spec, have a heated O2 sensor. I'm going to check the egr solenoid and the brake booster there are no other vac lines except the one for the canister. will be a couple days before I can get back out to the shop to test anything. will keep everybody posted. Just wierd that that vac drops and then comes back in spec.

 

[Schurkey]

Does it start running better as soon as it goes into closed-loop operation?