bobbymorris26

hey just thought i would run this by some of you guys before buying parts. i got a 89 Gt with 306, i've owned the car for about two months and ever since it has not wanted to idle right. it will just surge up and down and go dead eventually. well today i pulled the vaccum line off the map sensor and the car would sit and idle fine. put it back on and it would start acting up again. I was wondering if the map sensor is bad or just what your thoughts was on this before i pay 80 bucks for a new one. thanks a lot for your help.
bobby

Joel5.0

'89 GT were Mass Air systems not Speed Density....the MAP/BP shouldn't be connected to a vacuum source.

bobbymorris26

this one runs from map sensor to a little deal like a vaccum tree , has about 3 or 4 other lines hooked up there. thats where i unhooked it from. it was sucking air from the connector not from the map sensor its self of course.

Joel5.0

Was the system modified to speed density?....just to be sure...you don't have one of these at the air duct from the air filter?



Sorry to insist, but both use that type of sensor, only difference is that the one used in Mass Air systems is left disconnected from vacuum, to measure barometric pressure.

bobbymorris26

i dont much about the speed density stuff. but i do have a mass air sensor buy the air filter which is a open k@n filter.

bobbymorris26

Maybe someone could try and explain speed density? I have tha mass air sensor at the air filter. plus map sensor back on the fire wall with vacum line to it. thanks again.

Joel5.0

Just as I thought....first off, the MAP sensor in your setup shouldn't be connected to a vacuum source, it no longer is the Manifold Absolute Pressure sensor, it now has become the Barometric Pressure (BP) sensor (it does both functions)....This is also why you disconnect the vacuum line and idle/performance is improved....keep it disconnected....and plug the vacuum line.

Speed Density systems use engine vacuum as a primary factor/measurement to calculate air/fuel, timing, load, etc.....the MAP sensor is very important in these types of systems....and a MAF sensor will not be in sight at the engine.

Mass Air systems measure the "weight" of incoming air as a primary factor/measurement to calculate air/fuel, timing, load, etc.....the MAF sensor is very important in these types of systems....and the MAP is relegated, or is now in charge, to simply measure atmospheric pressure to aid in calculating fuel/timing/load/etc at different altitudes above sea level....it now has become the BP sensor.

Hope this helps.

bobbymorris26

thanks a lot for the help u dont know how much that means to me. i have had a couple people look at it and some said i had vaccum leak one guy said it was the tps. for some reason today i just decided to pull that line off and see what would happen. i had no idea about the maf sensor. i will unplug and cap off that line tommorow its coming a pretty good thunderstrom here right now. thanks again for the help. its been pretty aggrivating driving that things last month or so.

Joel5.0

Cool........

FYI....check this Speed Density to Mass Air conversion article, and the section about the MAP to BP sensor conversion....this is if your GT was a 1988 non-California Mustang 5.0 and was going to be upgraded to the MAF system.

bobbymorris26

plugged up line going from map sensor today before work, seemed a little better, the check engine light does not come on and off like it did before but there is something still wrong with the idle surging. i'm going to recheck voltage to tps now and then undo battery cable to reset computer. the article you left for me says you may have to drive it a time or to so the computer can relearn. so i may try that out. if that dont work i may have to go over those steps one by one and see if there is a problem some where else, cause who ever did the switch over missed the vacum line step they may have messed up something else also.