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??

If it has vst fuel systen then search for that , lots of threads on that.

Update: Just scanned the ECM and there are no fault codes.

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.

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

and how did you check for vac leaks and where did you check for them ?

Did you unhook your throttle cable ?

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.

^^^ exactly as I said to check for VAC leak at throttle body and at plenum but I would like to know how he checked this for VAC leaks and if new gaskets(s) and proper torque was used ???

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

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?

Swap computer from good running engine see if problem transfers.

I have a '96 7.3L that I just replaced an IAC on due to low idle and the manual said nothing about a IAC set procedure nor did the directions with the IAC. Can you point me in the area of said info?

Its in the manual 4 sure. i would have to grab it to tell you what page it is on.

Which manual are using ? Hopefully not a Seloc or Clymer

Who's IAC are you using ? you have to make sure you have the correct one. The pintle length , size, depth and shape have to be the exact as what the OEM had installed.

Power completely off anytime working on engine especially for fuel injection engines.

Anyways - new O ring needs to be used and sealing the IAC very well. Make sure all surfaces are clean including the air passage. Do not play with IAC pintle or try to jam the install with it. Install the IAC and the screws for it evenly torqued - then install Connections for that and anything else you had off. Turn power on after all of that is done.

resetting the IAC pintle position as follows

Turn the ignition key to the on / power position for 10 seconds - then turn it off for 10 seconds - water supply for engine cooling on - then restart the engine and check idle speed

I have found a boat with 1998 mag EFI's with bad intake Manifold gaskets from the factory. There was a tsb on them.I would think by now almost 20 years later they would have been changed by now but if it has low hrs ect... maybe not

Yup, a lot of GM vehicles and others do the same thing. Key to run, engine off closes the IAC, I usually wait closer to 30 seconds just in case.This way when you start it, the ECU knows the IAC was closed when started.

You have an air leak and have already found it in the IAC you replaced (the one where you plugged the airport and idle returns to normal).

Reset the IAC as BUP suggested (Key On, Eng. Off etc) and try again. IF the idle is now under control but its still rich we can look down other paths.

If the motor is still rich at idle check the yellowish tube coming off the VST and see if it is percolating liquid (fuel) into the manifold. If so that would cause your rich condition. This is an indication of an old diaphragm inside the VST chamber leaking fuel internally and out that vent.

Updates ??

Clymer manual is all I could find for '96 vintage.

I'm using a Walker Products 215-1026 IAC, new o-ring and TB gaskets used. Batteries were unhooked during removal and replacement. I did the procedure you listed above yesterday and it still stalls below having the throttle/tach. below 1k rpm. 1996 7.3 LX (FI) W/227 hrs. on it #OF884005 The bad idling engine doesn't hiss at "idle" nearly as much as the port engine, which idles solidly at 800 rpm. I guess the next step is to start testing the system electronically via the manual procedures.

Check and verify the ignition timing

Would love to but manual says to plug in a scan tool and switch to service mode for the procedure.

You can buy the timing tool as well - they are cheap. Merc sells them to.

OEM service manual lists this and the procedure as well plus the part number.

THanks, I'll see what I can dig up, the Clymer doesn't mention it. I found a scan tool, code clearer, base timing tool combo (Rinda Codemate 94008) as well for $64. May just go that route and have multiple functions.

You can also jump the 2 pins with a paper clip or jumper wire.

Got it to idle (somewhat) today.

The attached (pic URL below) male/female plug (set) at the back of the starboard valve cover was unplugged. My friend helping said he plugged it in and the idle cleaned up; however, last week I did the same when the engine was cooler and nothing happened so I unplugged it again. Note, the same plug on the other motor does not have the black jumper wire (on the connector with the yellow wire) and runs like a top. I haven't been able to decifer what the plug is for. Any guesses? Any reason why this one may be jumped and the other isn't? Harm in removing the jumper wire?

In service mode (poor idling motor) the timing mark won't hold still, keeps bouncing. It will idle at 650 rpm out of service mode but sounds like it's intermittently dropping a cylinder. Will the misfire cause the timing mark to bounce and/or are their other causes? The engine was compression tested late last year and I was told the plugs were fine.

https://im1.shutterfly.com/media/47a...D550/ry%3D400/