91 lumina has bad hesitation after warm up

[allisonmo2]

anyone???

[jeffcoslacker]

Well....when you accelerate, opening the throttle makes intake vacuum drop to near zero. Vacuum reading is highest when idling or decelerating with the throttle closed....which is when you get the CEL lit, right? That should tell you something....that vacuum, or something operated BY vacuum, is causing this. So I think you may be on the right track.

Now hard vac lines rarely ever go bad, but the rubber parts do. I've seen cars where some dipshit replaced rubber vacuum lines with windshield washer hose (which collapses under suction) and caused all kinds of headaches. Rubber lines/line ends can split, swell, get so degraded they collapse under high vacuum (like washer line), etc...

You said it coded for MAP...MAP has a vacuum line on it. Follow it and see what kinda shape it's in. I've seen MAP vac lines do all of the above and cause surging and stalling and and lit CEL....start there. I'd probably replace ALL of your rubber vacuum lines anyway...at that age they have to be pretty rough. Just make sure you get proper vac hose. There's a multi-tap where three or four vac lines are ganged together going to the throttle body area on yours, if I remember right...make sure that gang connector is fitting tightly onto the ports too.

[allisonmo2]

Originally Posted by jeffcoslacker

Well....when you accelerate, opening the throttle makes intake vacuum drop to near zero. Vacuum reading is highest when idling or decelerating with the throttle closed....which is when you get the CEL lit, right? That should tell you something....that vacuum, or something operated BY vacuum, is causing this. So I think you may be on the right track.

Now hard vac lines rarely ever go bad, but the rubber parts do. I've seen cars where some dipshit replaced rubber vacuum lines with windshield washer hose (which collapses under suction) and caused all kinds of headaches. Rubber lines/line ends can split, swell, get so degraded they collapse under high vacuum (like washer line), etc...

You said it coded for MAP...MAP has a vacuum line on it. Follow it and see what kinda shape it's in. I've seen MAP vac lines do all of the above and cause surging and stalling and and lit CEL....start there. I'd probably replace ALL of your rubber vacuum lines anyway...at that age they have to be pretty rough. Just make sure you get proper vac hose. There's a multi-tap where three or four vac lines are ganged together going to the throttle body area on yours, if I remember right...make sure that gang connector is fitting tightly onto the ports too.

that multi tap just slides in and out of the throttle body. i wouldn't say it is tight per se. mabye i'll wrap that in duct tape first and see if it has any affect. i thought i only had the hard lines. i only see the three coming off the tap and the pone coming down from the map sensor.

[allisonmo2]

well i pulled the intake manifold which was surprisingly easy and it runs well but still has a bit of a low power accompanied with some slight surging at anything less than full throttle.
now i have 3 codes
33, 34, 45

33 Manifold Absolute Pressure Sensor Circuit (High Sig.)
34 Manifold Absolute Pressure Sensor Circuit (Low Sig.)
45 Oxygen Sensor Circuit (Rich Exhaust Indicated)

somehow i made it worse?

[allisonmo2]

finally fixed all. the vacuum hose going under the intake to the diaphram on the fuel rail was off and the vacuum line going to the bottom of the map sensor was broken open. reconnect and fix the other and she runs great now. i definitely had more than one problem going on so i do not exactly know at what point all was fixed except the vacuum lines but hopefully this thread will help someone else sometime too.

thanks for all your help.