502 MAG Idle Problem
03-06-2016, 01:55 PM
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502 MAG Idle Problem
I just bought this boat that has two 502 MAG MPI's one motor ran great across all RPM's and the other didn't like to idle and smelled very rich but ran fine at all other RPM's. I had the injectors rebuilt and install them and still had the same problem. I just installed a new Idle air motor and now it wants to idle at 2,000 RPM's. I swapped the MAP sensors, and the whole throttle body between the two engines and still had the same problem with high idle. You can plug the airport for the idle air control and it brings the idle down and will stay there after pulling your finger off, until you shut the motor off and restart the it goes to 2300 RPM's. Any ideas of what is going on??
03-06-2016, 05:20 PM
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You need to post the serial number of your poor running engine and year modle
GM IAC for MEFI control systems really do not give as many problems as the Ford IAC - You have the GM. they are 100 times more reliable.
The TPS are more prone being the problem than the IAC for those apps
AFTER INSTALLING THE NEW IAC DID YOU SET IT ??? The GM IAC needs to be set after the install
Have you check for the throttle plate position ??? Unhook your throttle cable and see if high idle comes down
Do you have a VACUUM LEAK - IMO possible VAC LEAK from throttle body and gasket ??? Did you install a new gasket after the swap ??? And did you torque it to spec ??? Try testing for a VAC leak especially around the throttle body and plenum.
Rich condition can take because the MAP is over compensating for the VAC LEAK as most people will think ONLY A VAC LEAK CAUSES LEAN CONDITIONS. Just some added info for whomever.
Whats your MAP readings at idle ??? I can less about that you have no codes. MEFI 1 thru 3 contollers do throw out that many codes anyways.
MEFI 3 only has 17 codes at the MAX to throw out anyways and MEFI 2 less than that with 14 codes and MEFI 1 has only 7 codes to throw out.
GM IAC for MEFI control systems really do not give as many problems as the Ford IAC - You have the GM. they are 100 times more reliable.
The TPS are more prone being the problem than the IAC for those apps
AFTER INSTALLING THE NEW IAC DID YOU SET IT ??? The GM IAC needs to be set after the install
Have you check for the throttle plate position ??? Unhook your throttle cable and see if high idle comes down
Do you have a VACUUM LEAK - IMO possible VAC LEAK from throttle body and gasket ??? Did you install a new gasket after the swap ??? And did you torque it to spec ??? Try testing for a VAC leak especially around the throttle body and plenum.
Rich condition can take because the MAP is over compensating for the VAC LEAK as most people will think ONLY A VAC LEAK CAUSES LEAN CONDITIONS. Just some added info for whomever.
Whats your MAP readings at idle ??? I can less about that you have no codes. MEFI 1 thru 3 contollers do throw out that many codes anyways.
MEFI 3 only has 17 codes at the MAX to throw out anyways and MEFI 2 less than that with 14 codes and MEFI 1 has only 7 codes to throw out.
Last edited by BUP; 03-06-2016 at 05:41 PM.
03-06-2016, 05:34 PM
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The serial number is OL053055 When you say to set the new iac after installing it are you speaking of adjusting the primary throttle plate to get desired voltage to iac? If so no i did not, but when removing the entire throttle body and swapping it it runs exact;y the same and that throttle body comes off an engine that runs perfect, am i missing a step here? throttle cables are are fine and i checked for vaccum leaks. thanks for the input
03-06-2016, 05:53 PM
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Pull the upper plenum, there are 4 square gaskets around each runner for the lower intake. If those are broken they cause a vacuum leak and high idle when all else is good. If the screws are loose, tighten them first and try again. You had to have the plenum off to access the injectors, maybe the gaskets got damaged. The should have an orange line on the gasket.
03-06-2016, 06:27 PM
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Followed vaccum lines to start with found 3 one from the upper to lower intake one from upper intake to a tee that there goes to the fpr on the fuel rail and one regulator that from looking at it i believe is regulating pressure at the pump. there is also a a larger hose coming from the upper intake to the pcv valve, all the hoses look good and plugged them at the intake to double check. the throttle body gasket is new and yes I removed the throttle cables. Another thing that is really strange one time you can start it one time and it idles around 800 and sounds and smells like it is running rich, the next time it may start up and run a 2200 rpms. Also we just noticed that the port motor the bad running one the fuel pump runs longer than the starboard when turning the key on before starting. thanks
03-07-2016, 03:44 PM
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Update: I just replaced the upper intake gaskets and cleaned the throttle body very good and it ran the exact same. I swapped the throttle body with the known good one and it still ran the same. Any ideas?
