Still looking for an answer to my Southwind problem

What engine/truck is it built on?

Is it stuck in a gear by chance? Could be going into "limp mode".

A plugged gas cap will not set a code, but a loose or missing will set a code.
Some system have an electrically operated system to vent the tank so it does not become vapor locked.
Other system have mechanical means to vent the tank so it goes back into the fuel system rather then into the atmospere.

Since replacing the throttle position assy didn't do it, I would next replace the accelerator pedal assy as that is the other half of that circuit.
Not likely to be a gas cap problem as that will just turn on the check engine light but not actuate the "limp" mode.

Clay L wrote:
There are two components in the throttle position sensor system. One is the accelerator pedal assembly and the other is a black box above it on a bracket.
Since my shop admitted that they had no way of knowing which was at fault I replaced both. That took care of it - code P1516 as I recall.

Then several years later I got roughly the same symptoms but a different code - P1518. This time when the problem started (on a trip from CO to NH and back) it would reoccur on a random basis. Sometimes days apart and once twice in one day. When we got back to CO I replaced the throttle plate assembly that has a motor that moves the throttle plate. It sits on top of the throttle body injector assembly.
That was about 2600 miles ago and so far so good.

In addition to replacing the parts mentioned above I also checked all grounds involved (there are two on the top of the engine), and unplugged and replugged all connectors involved. So either that or the replacement parts fixed the problems. Since the problem was intermittent and would not show up for many miles it was not possible to do one thing and then see if it worked or not.

Having the engine go into reduced power (1st code) or idle mode (2nd code) was dangerous. I could be stopped in a traffic lane unable to get over to a breakdown lane and that is scary to me. In many places we go there wasn't a breakdown lane or even a shoulder to speak of anyway.
Because of that I threw parts at the problems - something I don't ordinarily do.

My total parts cost was close to $1000 as I recall - I did the replacement myself so no labor was involved.

In that year, is their not also a Crankshaft Position Sensor? The symptom sure seems to fit a not failing CPS... But, I'm not up on what I assume is the 8.1GM sitting in that Workhorse...

Best of luck, and OP - please close the loop back and let us not what root cause of the problem was...
Smitty

I left my gas cap sitting on the toads bumper and never got a code. Put aluminum foil over it till I got home 900 miles later. Didn't know what to buy for a replacement so went with a chevy 3500 with the 8.1 and it fit perfect.

There are two components in the throttle position sensor system. One is the accelerator pedal assembly and the other is a black box above it on a bracket.
Since my shop admitted that they had no way of knowing which was at fault I replaced both. That took care of it - code P1516 as I recall.

Then several years later I got roughly the same symptoms but a different code - P1518. This time when the problem started (on a trip from CO to NH and back) it would reoccur on a random basis. Sometimes days apart and once twice in one day. When we got back to CO I replaced the throttle plate assembly that has a motor that moves the throttle plate. It sits on top of the throttle body injector assembly.
That was about 2600 miles ago and so far so good.

In addition to replacing the parts mentioned above I also checked all grounds involved (there are two on the top of the engine), and unplugged and replugged all connectors involved. So either that or the replacement parts fixed the problems. Since the problem was intermittent and would not show up for many miles it was not possible to do one thing and then see if it worked or not.

Having the engine go into reduced power (1st code) or idle mode (2nd code) was dangerous. I could be stopped in a traffic lane unable to get over to a breakdown lane and that is scary to me. In many places we go there wasn't a breakdown lane or even a shoulder to speak of anyway.
Because of that I threw parts at the problems - something I don't ordinarily do.

My total parts cost was close to $1000 as I recall - I did the replacement myself so no labor was involved.

Removing gas cap may solve the problem, however remember that the engine check light will come on while it is removed. It is no big thing during testing.

Have you tried running with the gas cap removed in hot weather? The heat component makes it sound like a vapor lock issue.

Check for a corroded bulk head connector which carries many wires through the fire wall under the dash. Early Workhorse 2002-2004 had the problem until they redesigned the entire connector. It can be accessed from the front and is a big job to get it open and clean/apply dia-electric grease to all the terminals that pass the wires through the firewall. Very aggrevating condition to say the least. God Luck... :)

The connector was corroding from being exposed to weather on the manufacturers lot until built into a coach which then shields it. It was redesigned in later chassis to make it more water proof.

Try having the gas cap pressure checked, or just get a new one.