502 mpi won't idle down
 

For some reason, my idle won't come down, about 300 rpms worth. It did this a bit last weekend, but once I stuck it in gear, it came down and stayed where it was supposed to. Now this weekend, it initially was fine. Then when started it would idle at about 1200, put it in gear, comes down to 900, pull it out of gear stays about 900. It typically idles about 750...... What happened? I looked at all the connectors I could see but didn't find anything coming apart or disconnected. I don't have a scanner. Is there anything i can disconnect (sensor) or something of the sort, and then if it idles down i know it is the culprit?

Sounds like the IAC (idle air controller) They get carbon deposits in them and stick open. The IAC is just a stepper motor that controls air going around the throttle blades. If it sticks open, it allows more air past it. Take it out and clean it with some carb and choke cleaner or some throttle body cleaner(auto part stores sell both) If it is a 502 MPI, you will have to remove the upper plenum or the throttle body to get it out. It is underneath the throttle body. It is a round cylinder with a 4 pin plug going to it. Remove it and get the carbon off the end of the plunger and be sure to clean the area in the throttle body where the plunger seats. Hope this helps, Eddie.

I'll check it out today and post results. Thanks.

I pulled the plenum, and throttle body, cleaned everything out. Removed and cleaned the iac. Put everything back together and it started and idled fine. As soon as i give it some throttle and bring it back to idle, the idle is high. Varies between 800-1100. If i take the flame arrester off while it's running the idle doesn't change. If i put my finger over the iac tube, it doesn't change. If I cover the hole on the secondary butterfly plate, it will idle down the way it's supposed to. I tried upplugging the iac and idling, which made it still high, but don't know if that's a good test.

I then put the flame arrestor back on, and it idles at 1000, put my finger over the butterfly hole, it comes down to normal.

What should i do next?

Did ou pull back on the throttle body stud where it hooks up to the cable?

Unhooking it will really tell you nothing, Need a scaner. If you have a service manual there is a test you could do by using a test light at the iac plug with the engine running to see if you are getting power to the iac motor, A few of the wires should be blinking slowly telling you the ECM is working. The test should be in this link to a service manual some place.

Service Manual #16 1993-1997 GM V8 454 - 502 Gen V & VI
Must open each section individually to save.
http://www.boatfix.com/merc/Servmanl/16/16cov2_1.pdf



You could try tapping the TPS sensor a little to see if maybe you could get the idle to change.

"FYI" Stop putting in gear till you get it fixed, Or the idle will be the last of you $$$ worrys.:eek:

mrfixxall
 

Also check tps % before replacing anything,it should be at 0% at idle,if not start with replacing the tps first..if its at 0% at idle then look forward,iac could be stuck or the threads on the iac plunger could have carbon on them,,also make sure your trmp sensor is in its values,make sure the engine is up to operating temps, last vacume leaks would raise the idle but usually if that was the case it would also be surging..

Well, without having a scanner, i'm pretty dead in the water. I either have to take it in, or i put an iac in see what happens. I think either the iac is not closing off the air flow or something is telling it to open up. Here is the last thing i noticed playing around with stuff.

1. Start it up, fairly warm yet, and it idles fine. Rev it a little a couple of times, comes back to the correct idle. If i hold the throttle up at about 1800rps for about a minute or two, bring it back to idle, it stays at 1000 rpms. I can pop the throttle up and down, it stays at 1000rpm no matter what.

2. If I start it up, and it idles fine, I then unplug the IAC. I hold the rpm's up at 1800rpm's again for a couple of minutes. Bring it down, it stays at 1000 again. But, if I pop the throttle, it will come down to 800 right away. I can reproduce these results every time.

So I'm torn wether or not to just shot gun an IAC, or if something is telling the IAC to open and let air in. I would imagine just about any shop that has a scanner would be able to check a couple of these things when the idle is high and see what is going on????? Not many people work on 502 mpi's around here............

pull the plug to the temp sending unit.

I have seen on more then one occation, the pin pulled out of the connector and did exactally what you are describing

I tried running it with the air temp sensor unplugged, didn't seem to make a difference. But again, don't know if that's a good test.

Its your TPS (Throttle Position Sensor)--there is a voltage range for it and Ill be your is at the high end of the spectrum. Drop the voltage and your idle will drop as well.