Why would re-starting make a difference??

[gmrulz4u]

My 89 Supreme has been running good for months now...no problems to worry about...then, all of a sudden the following happened:

 

Last Tuesday, I was doing a lot of running around, stop and go driving etc. I got home, dropped some things off and ran back out to run some more errands. I started the car, and the second it started-up I could tell something was wrong. It was surging up and down, bogging, felt like it was misfiring etc. The SERVICE ENGINE light was going on and off. The engine vacuum was only like 8" hg instead of the usual 17" hg. So I turned it off, ran inside to call my friend to tell him that I would be late, ran back outside, turned the engine over, and it ran PERFECTLY!!!??

 

Than 2 hours later, I was leaving a grocery store and it did the same thing again! This time, without shutting it off, I decided to just drive it to my house(because I was only like 2 minutes away) so that I could grab my Scanner to see what was going on with it. I get home, and accidentaly shut if off without first grabbing my Scanner. And, guess what, I started it BACK up and it was running GOOD again!!! This got me really frustrated.

 

Anyways, it hasn't done this since. I checked the codes and the only one I got was 33 which is MAP Sensor Voltage HIGH. BUT, I'm 99% sure that this code occured because of the LOW engine vacuum due to the ACTUAL problem, and is not the MAP Sensor itself. Even AllData says that a code 33 will often occur if there's any vacuum leaks or misfires etc.

 

So here's my question. Why in the heck would simply re-starting the car make a WORLD of difference?? This make absolutely NO sense to me at all.

 

I mean, it can't be ignition related, because that wouldn't all-of-a-sudden be cured by re-starting. I don't think it could be a fuel problem because of the same reasoning.

 

The ONLY thing I can think of is an intermittent MAP Sensor giving false BARO readings to the ECM. In other words, I know that the MAP Sensor is used to tell the ECM what the outside barometric pressure is so that it can compensate for changes in altitude ie. lower altitute results in more fuel and vica-versa. So, if the MAP was telling the ECM that I was somewhere around the height of Mt. Everest, the ECM would try to compensate by taking away a whole lotta fuel...

 

Anyone!?

 

THANKS:)

[Guest Gp crazy]

? have you ever run out of fuel and sucked air into fuel rail ? that's is what i did and had the same problem.

To fix it i just got a fuel psi tester and relived all of the air out of system & that fixed the problem.

If you haven't changed the fuel filter in the last two years you might want to. you might see a diffrence. also you will help your pump last longer.

[god910]

A couple things, first MAP= Manifold Absolute Pressure. It sees the pressure INSIDE the manifold (Not outside) and provides fuel accordingly (WOT=High man. pressure (approaching zero, Or low vacuum however you wish to see it, part throttle or idle (3%)= high man. vacuum)

Since it's doing it after shutdown it could be heatsoak (Fuel boiling in the rail or lines somewhere) Could also be heatsoak to another component that is cooled by either coolant or air as the car moves. Other than that I would think it would be a fuel delivery problem. Good luck. HTH.