I got a 91 5.0 with mods listed below. When i start the car and after it warms, the idle goes down to about 700rpm. But once i get going, and after i let off, it stays up around 1300 rpm and slowly makes its way back down to about 900, never really reaching 700 again. It will do this after being fully warmed up and all. I tried lowering the idle screw on the TB but then the idle starts to surge. Not sure if this is something i will have to deal with due to my cam or if there is a simple fix. any help appreciated.

also, the timing is set at 10degrees base.

 

I have the same exact problem.
Here are some things that have aided my surging issue and may help you.
1. Sounds like a vacuum problem. Check all your vacuum lines and make sure theres no hairline cracks.
2. Also remove your EGR valve and make sure the pin is able to move in and out without getting stuck. If it gets stuck grab a new one.
3. Also try removing your IAC and see how much stuff is built up in there, you might be able to get away with cleaning it but if it looks real bad just replace it.
4. Another option would be to get a new TPS sensor and make sure you install it within range.
5. Pull codes before you do any of this first. You can get the code puller for like 30 bux i think off amazon or run it over to a shop.
If none of that works let me know.

 

EGR is almost brand new and worked before the mods, TPS voltage is also good, wasn't throwing any codes. I guess I will check the iac first, then recheck all else.

 

i was messing with the tps today and the voltage jumps when you hit the throttle, but jumps back down while your still holding the throttle open, is it supposed to do that?

 

I reset my tps to .97 volts and it still takes the idle a while to come down. It might be a long shot but could my coolant temp sensor be reading wrong? Maybe it keeps the idle up because it thinks the engine is cold.

 

A COOLANT TEMP SENSOR CAN DEFINITELY EFFECT THE IDLE PERFORMANCE.
IF IT CANNOT READ THE COOLANT OR IF COOLANT IS NOT GETTING TO IT AND IT SENSES AIR. IT WILL SEND SIGNALS TO THE COMPUTER TO COMPENSATE.
I WOULD ALSO BE SURPRISED IF YOUR NOT RUNNING RICH AS A RESULT OR IF YOUR TEMP GAUGE IS NOT ALREADY BEHAVING IRRACTIC.
CHECK IT ALL N SEE. THIS IS A PROBLEM THAT I HAD TO DEAL WITH TO CORRECT MY IDLE. NOW IT IS ALL OK AFTER REPLACING MY THERMOSTAT (TAKING TOO LONG TO OPEN/CLOSE) AND REPLACING MY COOLANT TEMP SENSOR.

 

is there any way to diagnose a bad temp sensor by disconnecting it?

 

This is funny, I just installed the same heads and cam as you, and I have idling issues. I cracked my temp sensor during the install, but the wires inside were still together, so I jb welded the porcelain together. I thought it was working fine, but now I think I'll be replacing it.

 

let me know if that fixes it.

I was messing with the tps and iac and got it to idle decent. It wouldn't hover around 1200 after letting off. then i noticed that i forgot to plug my iac back in, do i even need it? seems like its only good for idling your car up when its cold(or in my case, whenEVER).