502 mag mpi idle issues
 

I have some of you have been following my venture with my crownline but long story short 454 mag mpi blew andi replaced it with a 502 mag mpi short block. I used all 454 mag mpi parts and had the ECM tuned to a 502 gen vi stock tune.

Engine fires up beautifully and runs fine but after it warms up I get a issue im not sure how to deal with. It will idle all day around 750 rpm until i rev the engine. Then it idles at 1500 rpm and stays there until I cut off the engine. Soon as I start it back up its idling at 750 rpm again. I checked the timing to make sure it's at 8° btdc. If I advance it to 5° atdc it will surge a bit but settle back to 750.

IAC comes to mind but I wasnt having problems with it last summer.... only on this engine. Any chance one of the 454 mag sensors are throwing off the ecm? I've done some research but everyone with similar issues never post the fix.

Video of the problem. I cant cut it off and it starts back all over.
https://youtu.be/riLgfxmrovo

I was driving when you messaged me earlier, with FA off, put your thumb or finger over Iac hole or air feed horn when its idling high or "uncontrolled", IF it immediately drops, your iacs not cycling/sticking etc. Set timing at 8 degrees with timing tool or paper clip plugged in and leave it. Make sure you have no vacuum leaks, especially the hose going to PCV, for now you could just put a cap on intake port to eliminate it, if THAT makes a difference, you found your issue

I will try it tomorrow. She is sounding to good not to go on a lake test... lol

So to re-iterate, I think you told me before you DID have your injectors flowed and cleaned by someone? If not, you SHOULD (they are NOT causing your high idle BUT can melt a piston IF you have a low flowing one. On the early Tb you will find a little brass horn thing that feeds the IAC, newer ones just have a hole. This is where you will block it, running too high to see IF idle drops. You also asked "where you cap the intake pcv vacuum port", On front, you should have a 5/6 or 3/8 vacuum port attached to your rubber hose going to PCV, just plug the port wiith a cap for now

Do everything they said above. You have a vacuum leak due to 1) IAC that may be stuck or hangs up, 2) a true vacuum leak, cracked vacuum line, bad gasket on manifold etc. or 3) check to see that your throttle linkage is allowing the throttle body blades to return fully to idle. It may be binding.

Does it have a TPS to inform the ECU engine is in idle mode?

Only and idea, what if the IAC is unplugged when hi idle?

I have not had the injectors out or flowed. Never had a problem with them before. Technically I don't have a PCV just a elbow and baffle that are routed together but does let into the flame arrestor like the picture in the manual. The 5/16th line on the intake is already capped and has always been that way that I can remember.

I will try closing off the hole in the feed horn or throttle blades this afternoon. Then Ill try unplugging the IAC to see if that helps. Seems very plausible it could be the IAC resetting after I cut off the engine so if I disconnect it while its running high shouldn't it close and idle return to normal?

I confirmed the throttle linkage is not causing high idle by disconnecting it. I guess its possible there could be a vacuum leak but it should run that way all the time if that was the case. I can kill the engine and start it back up and it idles fine again. Soon as I rev it its back at 1500.

https://cimg0.ibsrv.net/gimg/www.off...d93d1825b3.png

Check the secondary throttle blade/shaft for slight dragging. Or possible spring not applying enough force.

I run into this with vac secondary carbs with light diapragm springs and GM tbi units with light shaft springs on bigger than OE engines. So....just a suggestion. :)

And yeh, shut off engine, restart the rpm goes back to normal idle. :)

Does this engine use a TPS? I had a very similar problem on a 525efi and tracked it down to the weather pack connector for the TPS. Check all connections as well to ensure the pins are contacting and stay tight.

Yes it does. Ill check tonight.

I pulled the whole intake back off, removed the throttle body, removed the IAC sure enough it had carbon build up so I thought this has to be it. Cleaned it all up looking good as new and put it all back together with the exact same results.

When I put my finger over the air tube feedhorn the engine dies very quickly but when I put my finger over the hole in the throttle blade it idles smooth and clean at 650 700 rpm.

I can reproduce the same results everytime I cut the engine off and start it and it will idle all day at 800 but one push of the throttle and it remains at 1500 rpm until I cut the engine off.

I checked timing, sprayed around looking for a vacuum leak, looked at all connectors on the intake for something corroded or bent.

I noticed in the picture below it states not all mag engines have the throttle blade with a hole in it. Is there possibly slightly different parameters on a ecm tune for this difference?
https://cimg7.ibsrv.net/gimg/www.off...eafdd1d768.jpg

try disconnecting throttle cable and do the rev up w/ hand and let go,,see if its a cable adjustment messing things up.

In the tune there is a target rpm, iac minium position for restart, iac reset key up etc, You adjust minimum throttle blade position manually unitil your IAC counts on a scan tool warmed up are in the 30 to 40 range. Ive had to drill holes in TBs with big cams to get enough airflow for iac to have a good range and NOT have tb cracked open so far it send tps past its minimum range, ive had to plug holes. Your tune has ALL the stock idle characteristics a 502 uses as far as iac max, reset, etc for THAT style tb but fueling PW for a gen 6 cam . The extra 2 to 3% fuel it gets in higher rpm pw bands ISNT making it idle too high. These things are usually a combination of problems when looked at in person, when you said THIS, was it diling at the 1500? " When I put my finger over the air tube feedhorn the engine dies very quickly but when I put my finger over the hole in the throttle blade it idles smooth and clean at 650 700 rpm.", If so, adjust throttle blades closed more or run a very short screw, tight fitting screw in that hole with red loctite.

I've done that as described above. However it does do it after revving like it doesn't know where ri reset the throttle.

I would bet your iac counts are near or AT zero WARM , when you rev it up, it gets into a little more timing, IAC counts go to ZERO, idle NEVER drops as NOTHING is going to make that happen. When you restart it, it gets less timing, IAC barely has control of it warm though. That would be MY guess. Does it also idle at 1500 cold if you rev it a tiny bit 15 or 30 seconds after starting it? IF it doesn't , I can tell you its DEFINITELY the IAC NOTt having control warm, IF it DOES, then IDK for sure

What is your fuel pressure at idle? Is engine hard starting when hot at all?

When I had a bad fuel pressure regulator and my idle fuel psi was quite low, one of my engines was doing something very similar. I had a very surge-y idle and often it would not drop below 1500 rpm as you stated until key off/key on/restart. If you say idles good when hole in TB blocked off, indicating engine is happy with LESS air, your idle fuel pressure may be low. Engine was also very hard starting when warm on mine though. Fixed FPR issue and that solved the idle and hard start issues I had.

It doesn't do it when cold. I can rev it and it settles back to 800 but as soon as the engine is warm it does it. If it's not having control warm what would that lean to?

Anyone know a GM part number to replace the IAC? As I understand it the IAC is the same as the car engines but I do want ot make sure I get the right one and since they are much cheaper it will definitly at least let me test it.

No its starts up at the rub of the switch everytime easily and sounds great. I havent checked fuel pressure but I will. Good thought. Havent had a problem with it before but I would also think if fuel pressure was the issue it would stay high when I start it back up. maybe when I give it throttle its changing fuel pressure. Ill have to check.

[QUOTE=articfriends;4738550 when you said THIS, was it diling at the 1500? " When I put my finger over the air tube feedhorn the engine dies very quickly but when I put my finger over the hole in the throttle blade it idles smooth and clean at 650 700 rpm.", If so, adjust throttle blades closed more or run a very short screw, tight fitting screw in that hole with red loctite.[/QUOTE]

Yes it was idling at 1500. I can put my finger on the hole in the throttle blade and it idles at 650 700 rpm on the gauge. Looking at the throttle blades they look completely closed to me. I cant find a vacuum leak but Im going to rent a vacuum gauge today. I dont expect it will be very helpful considering that its seems like it is getting to much air. By putting my finger over the hole tells me its getting to much air somewhere. Surely its not from that hole. Those holes were there from the factory and serve a purpose but carb cleaner around the intake and other port hold show no results of leaks. Im literally at a loss here. What is the best way to test the IAC that it is functioning?

Who tuned the ECM ? Smitty ?

I dont think your IAC is necesarrily bad, I think when its warm it has no control as its in its "minimum position" (unless its NOT cycling as far as it should). When its cold, it needs MORE air which is usually throttled by the IAC, so, when you rev it to 1500 cold, your Iac still can "throttle" the air enough to drop the idle back. As it warms up, the IAC counts go down, ie, minimum position. When you rev it, it hits a little higher timing table, idle takes off uncontrolled, your iacs probably in its minimum position and can no longer drop the idle back to "normal". Back throttle body stops on both primary and secondary a 1/4 or half turn back, see IF it changes things, if not, look for vacuum leaks (uncontrolled air feeding motor causing IAC to NOT have control) or plug that hole in the throttle blade with a very short, tight screw and red loctite

you can pull the IAC and watch it by cycling the key on and off to see if you're getting full range of motion

Yes Smitty tuned the ECM and did an amazing job. The engine fired off within 2 seconds and idles perfectly all day until i give it throttle. I believe the problem is vacuum related just finding it is the problem and figuring out why this wont go back to pre start condition. I will try and back off the throttle body stops to see if it helps. I feel like the hole in the blade is there for a reason and should be Id really like to find the culprit. I will try some carb cleaner around all the intake ports and hoses again as well. I like the idea of pulling the IAC off and checking for movement but have no way of telling if its actually all the way out or in. For a completely bone stock engine its giving me a little fit and I just want to float.

Im starting to think its not the IAC at all. Since it only works during idling and this occurs after a rev maybe a TPS? If the TPS doesnt think the throttle is all the way closed cant the RPM be increase by the ECM?

Bad IAC!!!!! Went to oriellys and bought part number# 2IAC2. Went back and took off the old one and it didnt move when I cycled the engine on and off. Put the new one on and it cycled. Started the engine and idles around 650 700 rpm revs and it idles back at 650 700. It surges once but may need to learn the IAC?

Bad IAC!!!!! Went to oriellys and bought part number# 2IAC2. Went back and took off the old one and it didnt move when I cycled the engine on and off. Put the new one on and it cycled. Started the engine and idles around 650 700 rpm revs and it idles back at 650 700. It surges once but may need to learn the IAC?

If the TPS doesn't think the throttle is all the way closed cant the RPM be increase by the ECM?

I'm going to say yes.
How many wires on TPS, and is it just closed/open/closed or variable ohms from idle to WOT? I'm not familiar with the EFI system but basing on my auto knowledge.
Can you post the wiring diagram showing wiring from TPS to ECM?

Based on the manual cable was using its a MEFI 1 and here is the wiring


https://cimg1.ibsrv.net/gimg/www.off...fc4ffcd7fa.jpg


https://cimg2.ibsrv.net/gimg/www.off...b088704b53.jpg

That was a tough problem without a computer to read the ecm. I had this boat on the water last summer and didnt have a problem with idle or anything really. So having it down over the winter and changing the engine I didn't think it could be anything sensor related. My mind was fighting me on this. Thinking back the old engine ran rich at idle and kinda stumbled from 3500 to 650 but I assumed big block kinda normal. Sometimes I'd have to run it to get it to clear up. Now it idles smooth and low.

If anyone remembers my ordeal with this boat from my past post you will know by now I believe she may be cursed so if anyone has any advice on how to uncurse a boat I'm all ears. Im thinking I need to change her name maybe that will help.

Glad its fixed

Glad you found it, we knew the IAC didn't have control of hot idle, we just didn't know why. The fact it reset though to idle target hot on a key cycle has me puzzled though if it wasn't cycling at all. Must be when it hit the higher timing after revving it up as to why it wouldn't return. Glad you can move, Smitty