Vehicle in question is a '94 Caravan with 3.0 and OD auto.

Heater blows cold. I'll try to be systematic about this:

1. temp gauge reads normal and no reason to think it's not reaching normal operating temp

2. I believe blend door is moving as I can hear it thump when all the way to cold. We popped off dash cover around HVAC controls and all appears well; the cable does not look broken and does not appear to bind in the cable housing.

3. both heater hoses seem equally warm at the firewall.

4. owner (my father) reports problem started after head gasket replacement by a shop in Omaha NE. He also reports sometimes heat will work when on flat ground/coasting, but not accelerating, sooooo maybe vacuum eh? :

I see an inline vac-actuated valve on one heater hose, not sure EXACTLY what that does? However during troubleshooting I reached behind it and was able to manually actuate the vacuum valve, which didn't make a damn bit of difference, still no heat at the dash. So, if it were vacuum, not sure why this didn't do anything?

5. if the heater core were plugged would I expect to see equally warm heater hoses at the firewall?

This shouldn't be that hard, and I really want to figure it out JUST 'CUZ. Suggestions? Try flushing the heater core anyway and go from there?

I found one thing online about the front motor cover eroding, causing for poor circulation by the water pump impeller which will cause poor flow thru the heater core, but that was on a 3.3. Seems like a longshot regardless, and not sure if the 3.0 is even realistically subject to this?

 

I going to guess the t-stat is staying open. You may want to replace it and see if you get heat.

 

Keith, hi.

I'm going to stick out my neck and suggest that an answer has been given, the head gasket job. May just be some debris got into the heater lines. Draining the system, forcing open the heater vacuum valve (use one of those vacuum pump test tools) and running/forcing water backwards through the heater core might get you running again.
Both hoses warm? If the heater were delivering good heat, with the blower running at full blast, I'd expect that the return hose would be cooler, like the difference between the feed and return hoses on the radiator.
Also, look at any nipples, had an old Buick with really poor heat, turned out a steel nipple was clogged with spongy rust, rodding it out did the trick.

Good hunting,

Stan

 

Yeah, this sounds plausible. Appreciate the suggestion. Not sure the heater core would shed enough you'd see an appreciable temp drop on the outlet side, but this would vary by vehicle for sure.

Also appreciate the suggestion from Master Tech of t-stat. Even with an open t-stat I'd think the motor would get near operating temp - it would just take longer. As such I'd expect to see lukewarm heat, but we're getting nothing. However, it's an avenue we'll eventually have to explore if nothing else solves this.