502 MPI idle variation

My 2000 502 MPIs idle in gear roughly 600-700 rpm and If I try and make slight increases they jump up to about 1000 rpm. there seems to be no in between. Ordinary this would not be a problem but for slow speed salmon fishing it’s a huge deal. I need to make slight adjustments in low end rpm to make slight adjustments in speed. I changed the idle air control motors/valves thinking they might just be sticking but this did not help. Is this normal for these motors or is there another suggestion out there for me to try? I previously had 454 MPIs and never experienced this

My understanding is that the MEFI controller is using the Idle Air Control Valve to constantly adjust the manifold air pressure by allowing in more or less air. This in turn controls the idle speed. The controller software has given it a goal of 650 rpm, based upon the rpm signal it is getting from the crank sensor and it will try and deliver that regardless of what you do. So, at first blush, it would seem that there is no way to beat the system and get an adjusted idle speed. I recommend you get a MEFI Diagnostics Manual that will explain the system as well as contain a lot of other good information about your engine.

There was a thread some time back on this forum about your very problem (salmon trolling) and you should search for it. As I recall a lot of neat ideas around this issue were discussed, such as adding a manual control over the stepper motor IAC valve but I don't recall if it was believed to be feasible. Others here with better memories or new ideas should weigh in.

Thanks! I will search for it!

i can understand how this works and why it’s difficult. Have also heard others with the same issue.

My curiosity comes into play as to why my 7.4L MPIs were able to idle at any rpm without issue but the 8.1s and 8.2s of the same vintage seem to be more finicky

The clean way to change your idle speed is to get the ECU reprogrammed.....

as far as the level adjusting RPM, there should be control above idle...similar to your 7.4's...

If there's a 'jump in RPM' with the lever, I'd check to make sure the cable isn't a contributor.....in other words, can you control the idle by hand.

If the controllers have no effect, it may be worth getting a scan tool hooked up to see what's going on. It could be carbon build up on the throttle plate/throttle body housing, or the TPS may be a little 'sticky'....the MAP factors into the air-fuel curve but I doubt it would make a big difference below 1000 rpm...

Thanks!

I do not believe it is a cable issue as it happens exactly the same on both motors. Increasing idle a can get the engines to 800 rpm but very short term. The motor automatically increases to its seemingly happy high idle of +/-1000 rpm. Same thing in reverse - 1000 down to 800 it will sit there for a short period of time them decrease to its “happy” low idle of +/-650 rpm

I did notice the old IAC valves were pretty black and looking inside did seem like a good amount of carbon build up inside. Next step will be clean up and check the TPS

Another thing to check for is were around the throttle plate shaft....if its leaking air, the engine will speed up...same for any other vacuum leak....

Its also quite possible there is another issue and the symptoms appear to drive the idle RPM....the scanner would be beneficial in doing a more thorough check...and not all issues will set a code.....