That's caused by a large vacuum leak.

 

vacuum leaks suck! But will begin scouting it out, thanks alloro! I did have a small leak at that heater control valve, had just found it by accident having broken the line while disconnecting. Could be all the moving things around in there has bumped something that didn't want to be bumped, or ... a diversion and the the real deal is where I'm not looking huh? lol
Thanks for the clue on what to look for

 

You never posted your year, engine, or model so I'll assume you have a 97 or later with a magnum engine. That being said, on the passenger side of the intake there is a vacuum hose that runs down to and behind the passenger headlight. There is where it connects to the vacuum reservoir for the HVAC system. This is the vacuum line you want to inspect first. Note: Removing the panel under the headlight gives additional access.

 

I see that stated in some other posts on this site, are my details not visible... the top right hand side of each reply box to a post which gives the profile details if they've taken the time to include them? I appreciate you saying that if mine aren't visible, it's a '99 3500 5.9, extended cargo van. I'll include it every post if needed, I just thought it was visible to all since I can see other posters details when they have listed them. That said,

Thanks for that starting point and on mine it's in the same general area as you said, plus the dog house snaps are buckled but not bolted down yet for easy access if need be so when it cools down a bit I may go have a look at some of those vacuum lines and the reservoir. Oh, interesting note... driving on flat highway with a/c on vents worked normal. If I gave it throttle from a stop or say entering highway or going up a grade, the vents would switch to either floor or defrost, soon as i ease off the pedal a few seconds and the dash vents work again.


made it up n down that hill today and so far as the coolant temps it was steady 202-204 this morning with 82 outdoor temp. Coming home coolant held steady n no overheating as the other day, stayed at 210-211 with 109 outdoor and back in the valley here. Likely there is a reason but it didn't run so hot before I the had this big idea to change the thermostat back to stock 195 from the 180 that had been in there. I'll just keep an eye on it, if it keeps up like this I may just go back to the 180

 

The engine should stay in the 199 - 204F range even when ambient air temp is over 110F.

As this is a 5.9L 3500 I'll assume you are hauling cargo and not just running around empty 98% of the time as some of us are.

This van can run/drive loaded with A/C up to 115F in heavy stop/go traffic without overheating but you may have to take some additional measures.

First you need to aware that there are three different fan clutches available. Standard, heavy duty, and severe service. If van is usually empty, you still probably need the heavy duty or you may want to look into changing over to an electric fan and save some gas milage. If you are driving loaded 90% of the time you'll need the severe service fan clutch and stock fan. Yes this will use more fuel but your truck will run cooler and I think in that case overall it will be less of a headache.

Second, if during typical day to day use you find yourself in stop/go traffic with limited airflow to the radiator more than half the time I suggest you add an electric pusher fan. It needs to be wired to turn on (with a relay) when the A/C compressor is on.

 

It's there I just never trust it. Way too many times I have answered posts based on that information only to be told after the fact things like, "that was my last vehicle, I forgot to change it", or "I'm talking about my father-in-law's, not mine", etc.

 

RE: Blackvan's post about electric fans
do a search on this forum for many posts regarding upgrade to electric fan

 

Thanks. I do recommend it for lightly loaded vans but after living with it for a couple years I don't know I'd go this route if I was heavily loaded most of the time. Mine works great for me ATM but if I had overheating issues I would add a shroud and possibly go bigger. That said, I love my Flexalite Syclone fan because thanks to it and couple other mods I average 18.2mpg (according to my BAFX ODB2 + Torque app) I also immediately experienced significantly improved acceleration. I can repost pics of my electric fan install if anyone feels they would be helpful.

 

Thanks for the good info black van, rsdata and alloro (it's cool, no worries I get it)

I have read some about the e-fan and like the idea but not for this van. I don't haul a days work of flooring like I used to nor drive all the miles anymore, I'm getting older and tired'er. I carry mostly just my tools and anything I load that might be heavy like big roll of carpet or hardwood is short distance. I went light duty and dug deep at the dealer doing the fan clutch but trust what other guys have learned and shared being it's best going dealer for the clutch fan

So I started the van up this morning, intention being to check the coolant level and load a couple tools needed for this floor job up the hill. It began to smell like coolant, looked underneath and sure enough was leaking, lost just a few ounces and slapped my forehead for not changing that squeeze clamp for the heater hose when I replaced the hose so that's what I ended up doing and another lesson learned. Also didn't change those clamps for the bypass hose either but as tight as that fit is and difficult as that thing was to get off n get on I'm thinking that one should hold!
Engine temps held well and right around 200 with what driving in town that was done today, a/c full blast. Now should be safe to work tomorrow.
I fiddled a little with the heater water cooler vacuum connection because i had noticed it wasn't always closing completely when the a/c was on and put just a dab of plumbers grease around the nipple and rehooked the line and it did improve the vents changing so easily n normal acceleration but not heavy acceleration. Should have time this weekend to try and inspect lines better. I had searched the erratic vents and read there are a couple check valves that could be failing? One being on the passenger side manifold where the line comes up off of the reservoir then another I'm not certain but over directly under the battery toward the cruise control valve, modual what ever it's called. Think all the abs stuff is right there in that jungle too. Wish me luck and thank you much guys!

 

Haven't check back in a few days so thought I would. Van has been running well enough with no more leaks or overheating though there still seems to be too much pressure on the upper and lower radiator hoses after driving a while and then shut down, temps will get up to right at 220 on the code reader but if I start it up it cools back to 203 or so within a few seconds.
I've done a little tugging and pulling on vacuum lines with not much success but haven't taken the time to go through everything by hand and haven't tried any other methods except by hand. I have a friend with a chevy van that has the same vents changing to floor or defrost too when giving throttle but let off the pedal and the dash vents return again.
I found a youtube video looking for suggestions on finding leaks, this guy being a mechanic had bought an inexpensive fog machine from ebay which also can be found on the walmart site and he used it to smoke test a car with vaccuum issue, seemed to work pretty good. He didn't overly pump it in the system but was enough to find the leak in the brake booster line. I'm thinking to get one of these things and give it a try, can't be any worse than pumping cigar smoke into the system or a dummy like me spraying carb cleaner on a hot exhaust and get bbq'd. lol Don't think I'll find one on store shelves till halloween time so may just order one. If this works out I'll be able to help my buddy too.