intermittent high idle when hot

[johnthegoalie]

truck will run great and idle down correctly when started cold and driven. once shut off and restarted when engine is hot the idle will go high and will not drop. go a few miles or so and itll drop and act normal just for it to act up again.

it did it after a long trip back in december, i reset the computer and new tps and it hasnt done until last weekend.

replaced the iac, tps and disconnected battery.

 

[10-1=9]

Check for vac leaks

 

[johnthegoalie]

No vac leaks. Cleaned tb, intake. Put new battery as it was dropping to 9.2v when starting. When it decides to not idle down. If I turn truck off and immediately restart it'll idle like it should.

 

[10-1=9]

You should also check the engine harness for pinched or melted wires

 

[seethrough]

My first guess would be the IAC but you stated that you replaced it. Did you clean the port that the IAC mounts to - the plunger has to seat against a channel to seal so if you have buildup on this spot it could cause the issues that you are seeing. Is it the stock throttle body? Can you pull the air filter tube off of it and manually open the throttle from closed to open position without it catching or binding? The other thing that caught my attention was the battery replacement due to the low voltage. Low voltage causes all kinds of strange PCM issues. You might try to pull the battery leads for 15 minutes, reset the throttle position sensor and then take the truck for a 30-60 minute drive to give the PCM time to relearn everything.

 

[johnthegoalie]

Everything is clean, nothing binding. Reset computer and took it for 50 mile drive... So far its OK. Keeping fingers crossed...

 

[johnthegoalie]

Idled stupid once today after all that. Went OEM on the tps this time and had shop learn the pcm and check tps and IAC operation.... Hoping this is it

 

[oldcolt]

Do you have a stock TB or aftermarket? Either way, take it off and clean it. BBK's can screw up the idle a bit.

 

[johnthegoalie]

Aside from exhaust and a flashed pcm, all stock

 

[johnthegoalie]

So far, its acting correctly... There's something to be said for OEM parts...

 

[texastonka]

Mine did this as well, I'm running a jtsvp TB that I got back in 2010, been great for years until recently. Replaced IAC and TPS as well, cleaned TB. Throttle would stick at whatever rpm, would not come down at all. Started out you could 'goose' the throttle and it would drop to normal idle. Got to where it would stick at redline. After the new IAC and TPS etc it worked great for a bit then started to act up again. Throttle closes at idle but you can go in there and manually push the blades closed more and it idles fine. Buddy at dealership had the star scan tool monitoring everything and all sensors are getting proper voltage etc.
do the springs in the TB assembly get weak over time?

I do run a Paxton, I just don't want to drop coin on a new TB and reuse the same parts to have the same issue. Any new TB come complete with new springs etc and are they any goo, or just try and rebuild my old jtsvp with new oem springs, blades and shafts?

 

[VENOMOUS1 RACING]

Mine did this as well, I'm running a jtsvp TB that I got back in 2010, been great for years until recently. Replaced IAC and TPS as well, cleaned TB. Throttle would stick at whatever rpm, would not come down at all. Started out you could 'goose' the throttle and it would drop to normal idle. Got to where it would stick at redline. After the new IAC and TPS etc it worked great for a bit then started to act up again. Throttle closes at idle but you can go in there and manually push the blades closed more and it idles fine. Buddy at dealership had the star scan tool monitoring everything and all sensors are getting proper voltage etc.
do the springs in the TB assembly get weak over time?

I do run a Paxton, I just don't want to drop coin on a new TB and reuse the same parts to have the same issue. Any new TB come complete with new springs etc and are they any goo, or just try and rebuild my old jtsvp with new oem springs, blades and shafts?

Many have had to sand away the high spot that catches the blades. I've had to do it on a few other TBs brands as well. Possible the heat cycle expands the alum and changes tolerances IMO.

 

[texastonka]

Many have had to sand away the high spot that catches the blades. I've had to do it on a few other TBs brands as well. Possible the heat cycle expands the alum and changes tolerances IMO.

Yes, I'm sorry we did do this with 2k grit I believe. It wasn't much, just enough to clean it up. Could not see it rough but could feel a little bit, smoothed that out.

 

[VENOMOUS1 RACING]

I don't see why a stronger return spring wouldn't help. They're a pretty weak spring. Of course that makes the gas pedal stiffer in return.

 

[seethrough]

You state that you have a Paxton - is your blow off valve working correctly. If not you could have your throttle blades trying to close against boost pressure from the supercharger.

 

[texastonka]

You state that you have a Paxton - is your blow off valve working correctly. If not you could have your throttle blades trying to close against boost pressure from the supercharger.

Yup, blow off is working 100%. Have full custom water cooled set up with a Tial BOV and paxton specific spring from Tial. There is zero pressure issue with this monster Tial open.