1994 454 TBI High Idle

[DSnyde34]

Hey everyone,

I am currently working on my dads 1994 3/4 ton chevy with a manual transmission. It has a 454 TBI crate motor in it. The issue I am trying to solve is a very high idle. The truck will initially idle at around 2100 RPMs when it heats up sometimes it will kick down to a little over 1000 RPMs but its not consistent, it will randomly kick back up to the high idle even after it has reached temp. Another issue the truck has is if you drive it before it has reached temp it will cut out entirely when you shift from 2nd to 3rd, it sounds like the truck died but after a few seconds it will come back to life (this only happens until the truck reaches temp then this issue goes away). I am not sure if these issues are related or not. I am also getting a code 43.

So far I have replaced the idle air control valve and put a new throttle body gasket on the truck. I hooked up a multimeter to the throttle position sensor and got a voltage of .7V at idle. I have read online that this needs to be at .5V, is there a way to adjust this sensor or do I need to just replace it? My only other thought is that it could be the EGR valve but I have no idea on how to go about testing that.

Any advice or suggestions would be greatly appreciated. Thanks in advance!

 

[squaredeal91]

Could still be iac. Other things in play like TPS could be related, I would remove the iac and clean it and pull the plunger out by hand reinstall but don't plug it in. Restart engine and see how it is? If it doesn't start try giving it a little throttle to keep it alive. If you can get it to idle that way then plug it back in and see what it does.

 

[DSnyde34]

Thanks for the info,

The IAC is new so it hopefully isn't dirty but I will remove it and check. I am assuming that pulling the plunger out seals off the air, so if it starts with the plunger out does that mean I have a vacuum leak somewhere else?

 

[squaredeal91]

Another thought. Is your tps slotted or is it just one position? If it's one position and it's off on volts I'd replace it. Could also be egr like you mentioned. Remove and plug egr valve hose and Maybe try tapping on egr valve base when running and see if it closes and changes idle? Maybe someone else will chime in and have better ideas.

 

[squaredeal91]

Thanks for the info,

The IAC is new so it hopefully isn't dirty but I will remove it and check. I am assuming that pulling the plunger out seals off the air, so if it starts with the plunger out does that mean I have a vacuum leak somewhere else?

correct. If plunger is all the way out and you leave wire off it should barely idle if at all. At least in my experience that's been the case.

 

[Ricko1966]

If the EGR was coming on at Idle it would die,not idle fast,for it to idle fast it has to be getting air somewhere. Check for vacuum leaks,do a reset on the IAC. Throttle position sensor can be adjusted.Also is the engine timed correctly.

 

[DSnyde34]

If the EGR was coming on at Idle it would die,not idle fast,fir it to idle fast it has to be getting air somewhere. Check for vacuum leaks,do a relearn on the IAC. Also is the engine timed correctly.

Thanks for the info, do you by chance know the procedure to relearn the IAC? The engine has been running fine for several years so I doubt the timing jumped but if all else fails I may break out the timing light tomorrow.

 

[squaredeal91]

If the EGR was coming on at Idle it would die,not idle fast,for it to idle fast it has to be getting air somewhere. Check for vacuum leaks,do a relearn on the IAC. Throttle position sensor can be adjusted.Also is the engine timed correctly.

Thank you. Didn't know that about egr valves. Basically it would make it run like crap?

 

[Ricko1966]

I can tell you how to put the IAC in service mode,set the idle speed etc. But if you have a vacuum leak or not timed correctly your going to create more problems. Jumper ALDl A and B pins turn the key on wait 10 seconds then unplug the IAC. Now turn the key off and remove your jumper. Start the truck set your idle speed so the truck idles 125 rpm below factory speed. Turn the truck off, plug the IAC back in the ECM and IAC should bring the idle up to recommended RPM. You can disregard this now, I thought it was a fresh crate engine install.

 

[Ricko1966]

Thank you. Didn't know that about egr valves. Basically it would make it run like crap?

EGR stuck open car as a general rule won't idle,and yes it will run poorly on acceleration and power will be down,kinda like having a big vacuum leak. Stuck closed you probably wouldn't notice unless it caused preignition problems,some engines aren't as sensitive and you wouldn't notice.

 

[Ricko1966]

Thanks for the info, do you by chance know the procedure to relearn the IAC? The engine has been running fine for several years so I doubt the timing jumped but if all else fails I may break out the timing light tomorrow.

Well the way I read that I thought you recently installed a 454 crate engine. So it's an old install and this problem just started?

 

[DSnyde34]

Well the way I read that I thought you recently installed a 454 crate engine. So it's an old install and this problem just started?

Yes that is correct, the motor was installed probably around 10 years ago. I would say he has had the idle problem for a year or two now. The truck is a feed truck that only goes a couple of hundred miles a year so he hasn't had time to worry about. The truck is now currently sitting at my house so I thought I would take a swing at it.

 

[Ricko1966]

Yes that is correct, the motor was installed probably around 10 years ago. I would say he has had the idle problem for a year or two now. The truck is a feed truck that only goes a couple of hundred miles a year so he hasn't had time to worry about. The truck is now currently sitting at my house so I thought I would take a swing at it.

Check for vacuum leaks,leave the new IAC in but plug the old one into the harness. Put it in service mode as earlier,see if the IAC moves start the truck see if the IAC moves create a vacuum leak see if the IAC moves. A grain truck that barely gets used my first thoughts would be mice have chewed wires and IAC is not getting a signal or hoses,gaskets have dried up and are creating vacuum leaks. Also as per your post 7 the timing jump Im not saying it jumped,over time the chain stretches, after the timing chain has stretched you compensate by setting timing.After you get your fast idle problem fixed,sounds like the truck could probably use a good old fashioned tune up,just as maintenance.