It's happened 3 times now. Only on Long trips 2-3 hours of driving, my 2500 stalls at idle. No issues at speed, no issues after waiting overnight. Driving around town no issues, mechanic says "no problem."

Anyone experiencing similar symptoms?

I figure it's one of the sensors that causes rough idle, but which one would be susceptible to the long drive?

​​​​​​​Thanks in advance!

 

on such an old truck I'm going to wager on vacuum leak

 

Good call. My 1996 has multiple vacuum leak codes in the ODB2 reader, which I have yet to tackle. Both small and large leaks noted. I'll bet a 1994 does as well, if he can find someone with an ODB1 reader.

 

Hi Mountainmanjoe and jfmorris, thanks for your comments. Why would the vacuum leak only start affecting idle after 3 hours of driving, but not before? It has been 4 times now showing no problems on short drives, but only showing the problem after about 3 hours of driving. It takes about 2 hours of being shut off to clear the problem. Learning about vacuum leaks here... Thanks again.

 

All that comes to mind for me is that sustained driving for that long leads to higher under-hood temps that impact a sensor and/or exacerbate an existing vacuum leak, if there is a leak. Also could be something that fails Uber high temp downstream like a flakey O2 sensor. I’ve not driven my 1996 for 3 hours at a time in a decade or more.

I would buy an ODB1 reader compatible with the 1994, if you can find one on Amazon or elsewhere, and try to read any stored codes that can isolate the issue. And if you have the reader in the truck, you can also check it when it has driven long enough to act up.

 

Our 93 Suburban did this for about 6 months before refusing to start again one day. ICM was the cause for us

 

when an engine is cold the mixture is rich so you wouldn't notice a vacuum leak as much. But when it gets hot it might become too lean. However, you're right that this would happen a lot sooner than 2 hours.
But engine parts also expand when hot, so for example a leaky intake gasket might seal itself up.

Rednucleus makes a good point though. Electronics are definitely affected by heat.

 

I hadn't thought of electronics, but on these older trucks, that is certainly something to suspect. This is probably another reason to have a ODB1 code reader compatible with the 1994 in the truck, so he can pull over and read the codes when it happens. I found a few inexpensive Innova ODB1 capable readers for older GM vehicles (through 1995) on Amazon in a quick search.