Expert: Sean Halliday - 12/26/2008

Question
Hello, 89 suburban 1500 with 350 the RPM will tidal from regular idle speeds to almost dying and sometimes does stop running (can't drive it anyway). I have found that it will run just great for a short while at an idle when the MAP sensor is disconnected from the vacuum line (large vacuum leak). I bought a new MAP thinking that was the problem, but it didn't do the trick. I have changed the fuel filter and checked the fuel pump pressure (plenty of pressure at the throttle body; it's not the fuel pump). I checked for codes and there weren't any. I have checked for any vacuum leaks and there were a few questionable hoses that have all been replaced.  The EGR valve is functioning just fine and the timing is in specs. There is fresh gas in the tank, new plugs, and usual tune up stuff.... I think that possibly the fuel injectors are not spraying the fuel as fine as they should be, so I will try to clean them with some concentrated dump in the fuel tank cleaner before I make another high dollar purchase (like injectors). I know I can check the TPS next but just wondering what I am missing. Just asking for suggestions on what to do next, Thanks

Answer
The vacuum must be tested with a gauge and the fuel pressure is the same.Its ok to say there seems like good pressure,as this must be established to eliiminate each component as you go.Fuel pressure must be 45psi approx at the rail and vacuum will be steady at approx 15/20.If the vacuum needle fluctuates,then there is a problem either with the intake or vacuum system.The vacuum tester will have a list of variations on this in the instruction book it comes with.The injectors will not require being replaced as this is not your problem or cleaning will not help.Neither is it the TPS.It is most likely a bad idle air control solenoid .Do the tests with the proper testers and see what you find.Usual tune up stuff?Did you change the wires?