The TPS going bad is VERY common on the system you have.

As has been suggested a couple times, you need to unhook the IAC after the engine is at warm idle and take it for a drive. Although spark timing is a major player in idle control, I see no possibility of it giving an 1800 rpm hang on coast-down. Is there an EGR valve still in place on the engine? All the LS swaps I do get it deleted, but in the unlikely event it is still attached to the intake -but the exhaust side is open to atmosphere- it could be driven open and cause high idle.

Have you disconnected the batt yet to clear..
like in the older units

mr. P., battery has been disconnected several times with cables being touched to drain any residual "'lectric" from the computer's inner guts. also, we did unhook the IAC after warm up, but like the dumazzes we are, our biggest mistake was to NOT write the results down so we could refer back to the results. this weekend looks good weather wise now, but i'm not sure we can get the car out due to other projects [can you say "honey do"] being "suggested". as we are both retired, maybe we can get on it next week. as has been mentioned several times, the TPS has been looked at several times and run through it's range, both on and off the car, passing [in my opinion] both the voltage and ohm tests. next time out, a notebook will be in the tool bag with my OTC scan tool. maybe then, i can positively say what results we get with what is tried. oh, i forgot to mention until it was just brought up. this conversion is used with an eliminated EGR valve. i fabbed a block off plate to take it's place, so that is one item we don't have to concern ourselves with. you guys are really trying to help out, and of this, we are SUPER appreciative !

It really does sound like its a air issue..
like its getting air from somewhere.. I know
if I mess with the throttle(move the position
I have to re-zero the TPS in the ECU. not sure
why it wants to hold the revs up.. is it always
at 1800 revs(no variance)

it's always 1800rpm or above, going down the road. sitting still, you can rev it till it hits the limiter as many times as you want, and it returns to idle as it should every time. this is why it's so frustrating to figure out why this is acting the way it is, going down the road only !

Try throwing it in neutral when this happens
and see if it falls back to idle

Check for gas residue in the MAP sensor. At 1,800 rpm, it might not be a throttle issue. Maybe the MAP is faulty causing a high timing advance condition. Check your coolant sensor also, if it dips cold momentarily, it might raise the idle speed thinking it needs to warm up some more.

If your system can datalog, maybe that will point you in the right direction. If not have a buddy watch a scan tool or your tuning program while you drive. That way hopefully you can determine if the computer is commanding the high idle (indicating a sensor problem) or not (indicating a mechanical problem). If you have a good DVOM, you can sweep the TPS and watch for spikes or dropouts. The computer can only "see" what is going on via the sensors and react accordingly. If the IAC motor checks ok, does the engine have a PCV system? Is the throttle bore clean? I've also seen PCV valves fail open before (as in the valve physically came apart), but that would be very unusual.
If you don't have a scan tool, you can pick up a code reader from your local parts store that will show stream data for around $60. This is assuming you have the standard OBD-II connector. I would think that you would want to monitor IAC counts, TPS voltage and Coolant Temp and see if there is a pattern.

I think what Mike is getting at here is taking the ecm and all the sensors out of the equation-couldn't this be something as simple as a collapsed motor mount hanging the throttle linkage partially open. Have you tried pulling the gas pedal up when the engine speed isn't coming down?

this weekend was not conductive for any testing, as other items took up our time. mr. P asked about throwing into neutral, and this has been done, but the engine still continues at the rpm it was before doing this. we also checked for interference at the throttle cable causing issues, as well as motor mounts being a cause. all checked out as being in perfect condition. the next course needed as of now is to hook up my tool and observe just single components and sensors while going down the road, logging the results to see if something stands out, or comes to light. as i said previously, i just observed the circuits for a short time to see if they were operating correctly. the biggest mistake on my part was the fact i didn't log the results. this caused me to chase my tail, as there were so many things i tried i lost track. i KNOW BETTER, and it is now biting me where i can't bite.

As has been discussed, IAC related items, IAC itself or a TPS can cause issues like this. Sometimes there's an A/C input that can offset the IAC value to compensate for compressor load but that's probably unlikely. A data log would help alot, I'd log the TPS, maybe IAC duty cycle or commanded position (depending on style), MAF or MAP depending if it's speed density or not. Any throttle switches (if there are any). Maybe even look at the coolant temp sensor and IAT, those being out of range can cause certain functions to kick in.

last night, we got a chance to look at things. it seems the MAP sensor wants to stay at 15-16hg, and the throttle position varies between 40-59%, even at idle. for some reason, the rpm at idle is a steady 650-700, yet at cruise, it jumps on my scanner between 164-1988. i know that is not correct because it is running very smooth. next up, the owner thought it might show something if it was watched from idle to shift point. and the driver side mount promptly destroyed itself ! so much for a little restraint. now until the mount is replaced, i'll take the MAP sensor off and inspect the intake port and sensor for buildup, and check the TPS for ohm readings through out it's range. but the rpm back & forth at cruise when the car is running perfectly, does that put the computer at suspect, or something else ? my scanner perhaps ? i'll check the scanner on my caravan today or tomorrow to get a ruling on it, as i am familiar with that system far more than this GM conversion system.

No matter what is giving the incorrect input value either the IAC has to be open further or the timing has to be higher. What does that data show?
Doug

I would check and recheck all your grounds and power sources, the RPM moving around could be from electrical noise which will cause all kinds of problems.

I agree with what bad340fish said, check your grounds and power sources using the voltage drop method. Too many times these problems are caused by bad power or ground connections

Why is your TPS showing 40-59% at idle. That value is completely out of the park. TPS voltage should be steady at idle since it is against the stop. The idle speed would be controlled by the IAC.

agree completely with this ! as badfish and ntstlgl-'70 mentioned, my power and ground need checked, as the pickup points for these are the ciggy lighter, so that may be an issue.
the motor mounts have been replaced, but now it's raining all the time, plus there is the grass cutting between showers my buddy has to contend with. and in the meantime, when we have a few minutes during the rains, we just built a pair of front wheels for his '64 ford galaxy. that story is one for a sit down discussion ! but we got them to .012 in both directions on the bead seats on one, and .017 on the other.