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Discussion Starter · #1 ·
Hello,

Havent been on here in awhile, but still got Big Blue (the truck in my signature). Well I have been chasing a racing idle for quite some time now and every time I think I got it fixed it creeps back in. The previous threads on this issue:



The truck does not get used all that much, so I dont think that parts are going bad that soon after I changed them. So in the 4 years (12000 miles) I replaced the following:
EGR valve, ISC motor, replaced all rubber sections of fuel lines, fuel regulator, fuel filter, rebuilt and cleaned throttle body, NTK O2 sensor, new throttle cable, locked out CAD system and removed associated vacuum lines, replaced remaining vacuum lines.

During summer I am still getting that racing idle and only sometimes blipping the throttle gets it to go back down to normal. As soon as I touch the gas pedal it stays at a higher idle speed. I tried going through the engine bay with some carb clean and spraying it all connections to see if there is a vacuum leak but I am not seeing anything or hearing anything.

What I do know is that this high idle thing happens more frequently when it is hot out (85 degs+) or when the engine is warm and I shut it off and restart it before it cools all the way down. So heat is indeed a factor. From what I am reading on the forums is that it could be the ISC motor again (which I dont think, but possible), EGR valve is leaking more air into the system and thus the O2 sensor says its running leans and injects more fuel (could be possible), or some of my sensors are dirty (which I cleaned all the ones in the throttle body not that long ago).

I was pretty pissed about the situation yesterday (even more so that gas is as high as it is) I flipped open the hood and manually closed the throttle. It worked, but it was a little to easy to do so. From what I can tell the throttle level on the throttle body was touching the bolt head of the ISC the entire time. Which brings me hear today to ask a couple questions:

Given that the throttle lever was still contacting the ISC bolt head, could there be play in the ISC? is there suppose to be any? when the idle was high I could easily push the bolt head back with the throttle lever and it would go back to where it should be. So the ISC motor does seem to be retracting, but there appears to be some play which means the bolt would have to be pushed in the rest of the way by the throttle lever.

OR

My throttle return spring that is wound around throttle blade shaft is not strong enough to bring the throttle blades closed. Which almost sounds like the culprit. If the ISC is retracting to the point the bolt head could be pushed further in, and the throttle spring is suppose to be able to do so and is not, than the spring is either bad, or I didnt reinstall that spring in properly when I removed the throttle shaft during cleaning.

Does my thought process make sense? I can easily take off the Throttle body and see what is causing the binding, or try and wind the spring a little more.

Does anyone have a good picture they can post of the return spring on a 90 Dodge with the 360 and the TBI?

Thank you
 

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"""""My throttle return spring that is wound around throttle blade shaft is not strong enough to bring the throttle blades closed """""

Should be a double throttle return spring toward front drivers side corner bolt on intake bracket. That sprng likes to break. Got any pics to send to see if normal or not?

check this link for pics
 

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Might also want to see if the throttle plates are binding in the bore from carbon buildup.
 

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'79 Macho 360 Magnum, Comp 480 cam, Hughes springs, 650 Thunder AVS, Pertronix Flamethrower ignition
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Yes, definitely check your return spring(s) and your butterflies (throttle plates) for carbon build-up. I would also make sure your throttle shaft isn't binding. Check for excessive play. How many miles on the engine?
 

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Discussion Starter · #5 ·
I’ll have to check carbon build up on the butterflies.
I don’t have a picture currently of the trucks linkage, but I can get one.

however the pictures in that thread are for a carburetor truck, mine has the TBI. I have the spring on the trans linkage, but I don’t have the one going to the front of the intake. However the TBI has a torsional spring wrapped around the throttle blade shaft. Is there an additional spring to the torsional spring?
 

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'79 Macho 360 Magnum, Comp 480 cam, Hughes springs, 650 Thunder AVS, Pertronix Flamethrower ignition
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No codes? Idle air control full of carbon or throttle position sensor maybe?
 

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TBI is the same as the carb. You're missing the return spring. when I found that pic I googled tbi 1988 5.2 and got a million pics
 
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'79 Macho 360 Magnum, Comp 480 cam, Hughes springs, 650 Thunder AVS, Pertronix Flamethrower ignition
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Doesn't the computer control the idle? Try spraying some carb cleaner down the carb with the engine running. Take the air cleaner off and then start the engine then bring the RPM's up and spray the cleaner down and around each venturi for a good minute. Sometimes that works for a funny idle.
 

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Doesn't the computer control the idle? Try spraying some carb cleaner down the carb with the engine running. Take the air cleaner off and then start the engine then bring the RPM's up and spray the cleaner down and around each venturi for a good minute. Sometimes that works for a funny idle.
Sorry. Not familiar with the new Dodges. ;D
 

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The computer does indeed control the idle, but if the throttle is not at it's idle stop, the computer thinks you're pressing the gas pedal and will increase the idle. Disconnect throttle, trans kickdown,and cruise control linkages from the TBI. Then see if the idle is normal. You may have binding in other linkages. I had this with one of my 92's - the throttle bracket was bent which allowed the cruise cable to slightly open the throttle, increasing the idle..
 

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Discussion Starter · #12 ·
Thank you everyone for your replies, however I think I figured out my issue. Or at least where to start:

I was gone all weekend and finally came home last night. I decided to grab some pictures to post in this thread.

Automotive fuel system Motor vehicle Automotive tire Automotive exterior Hood


I started looking at the bracket that holds the ISC to the Throttle body to see if there was a spring mount. There is not. Thats when I noticed this bracket with a hole in the top:

Motor vehicle Hood Car Automotive exterior Automotive design


I looked straight down and it appears that hole lines up perfectly with the stud that the trans linkage is attached too. Looked in the FSM and saw this:

Book Handwriting Font Publication Material property


sure enough there is to be a throttle return spring that attaches to that bracket that goes to the stud the trans linkage is connected too. In all the years we had this truck and me looking under the hood I have never seen a spring there. It would not be the first time this truck was missing some parts. So I will go to a parts store tonight and pick up some springs and see if I can get one to fit.
 

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You will have to go thru some trial and error and the specific spring. Too little tension and the spring wont work. Too much tension and you'll need a lead foot to move the pedal.
 
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