Help with my 5000 BTU A/C in my camper van: starts cold then goes warm.

Background:

The microwave in my Van died so I installed a 5000 BTU AC in this location. I've installed a 2nd vent in the side of the van (it's up high in the fiberglass cap) this is in addition to cutting out the wood so that the unit could also exhaust hot air out the vent for the fridge.

The AC blows cold for about 30 minutes or so then goes warm. If I let the unit cool off it blows cold again. As a test I tried a small house fan in attempt to further exhaust the hot air but this doesn't seem to help. Note it's not physically possible to mount the AC stick out of the side of the van. The space the unit is in is about 4" wider on the sides and 10" taller than the unit itself. 

The simple solution would be to just get a roof mount AC unit. The hitch is that my van fits though the tunnel at the track, a roof mount AC would prevent me from using the tunnel. This means having to wait to the end of the day to exit the track. Additionally my generator is only a 2000 watt units so anything beyond 7000 BTUs would eg an issue.

I'm clueless when it comes to HVAC concepts the I to thing I can think of is that I need some small fans to pull outside air in via the lower vent and another set to exhaust it out the upper vent. 

Have you tried it outside of the van to see if it's maybe the AC unit itself?

Sounds like it's not getting enough air to cool the condenser core. It takes a pile of air to cool one of these. Most small units need to draw cool air in the sides and exhaust hot air out of the back. If you installation is allowing the hot air to be drawn in the sides, it will overheat and stop cooling. 

It sounds like you're just sticking the condenser end of the unit in a big box with a smallish vent?  You're going to need to get lots of cool air to go over the coils, and I don't think you're going to get the airflow you need.  It's a pretty simple system - you're just picking up heat energy from one side of the system and moving it to the other side.  But nature is lazy, so if the condenser coils are soaked in heat, the energy that's been pumped to them isn't going to want to go anywhere.  You could try to duct the air path and add some fans to push more air, but I think you're going to have a hard time giving the coils enough airflow.  I think you need some sort of split system, if you can't use a one-piece unit.  Alternately, one of the portable A/C units might help.  It sounds like you've got a fairly small area to cool and there are some places to mount the exhaust tube.  The big problem with those is that they rely on taking some of the cooled air from the room to blow over the coils and then out the vent which means you're pulling in fresh, uncooled air from the outside which makes them fairly inefficient, but I've had pretty good luck with a portable in an RV, so a van would probably work great.  The trick is finding some place to put the thing...

OK now that I know how the unit works ; I.E. it pulls air from the sides and out the back I understand my problem.

I had also tested the unit by simply placing it on the sofa and it ran for over an hour without issue.

Sounds like I need to come up with a way to separate the back of the unit from the sides. I can also vent the sides of the cabinet if need be.

Carboard and tape/staples should be pretty ok as long as the unit itself cant slide any during 'maneuvers'. 

Could you duct the vents and use a simple 12 volt computer fan to circulate cooler outside air to it?

Update:

I ran an experiment tonight; I took a bunch of foam and stuffed into the compartment housing the AC so as to separate the back of the unit from the sides and top. I let it run for 2 hours with the outside temp at 102 degrees when I started and then falling to 95 degrees. The cool air coming from the unit stayed steady at 71 degrees. 

Unfortunately the van itself barely cooled off until the temp fell to 95 degrees. Additionally I used the curtain that separates the back third of the van to give the unit a smaller area to cool and that seemed to help. 

There is an existing exhaust fan in the roof that is currently inoperable but I plan to rectify that in the next couple of days. That should help a little. 

I could also block off the cab of the van fairly easily which again should help.

Addtiianlly I'll have to seal of the exhust from the unit a bit better as one of the wall panels was up to 112 degrees. The back of the AC is showing 130 degrees. 

End of the day I'm thinking a 5000 BTU unit isn't up to the task of cooling down a camper van when the temp is 102 degrees outside much less our frequent 107 degree days.

A car ac is 2-3x that capacity on average, if i remember correctly. RV rooftop units are usually 8-13000 btu as another reference point.  Vehicles have a huge heat load compared to a room in an insulated building. But, a lot of the heat load of actually driving the vehicle is from road heat, exhaust heat, and engine bay air exhausting under the car and at least you shouldnt have to deal with that while parked. I would think the 5000btu unit could probably keep up while parked if you do a little work mitigating the heat the interior takes in. 

But just to be sure, is the whole front of the unit exposed? The air is sucked into the larger grille that makes up most of the front of the unit and then shot out of the obvious vents at the top. If that lower section is partially  blocked for some reason it's going to majorly hamper performance. 

If you end up wanting to upgrade it, you should definitely have some room for growth from your generator. The 5000btu window units only use 450-500w while running, probably slightly more at startup. 

My 1978 20' Winnebago originally came with a 13500 BTU roof A/C unit. When I retrofitted a 2.5kw generator, the A/C would start fine, but the generator labored badly when the A/C cycled. I swapped the 13,500 for a 11,000 A/C unit I happened to have. They were happy together.

Hope this detail might be helpful.

Some other things you may be able to do to mitigate it - reduce solar gain.

Reflective windshield shades, even if the cabin is blocked off.

Paint the roof white

Park in the shade or put an EZUp over your van during the heat of the day.  

Definitely look into the efficiencies of the ducting to the unit as well.  Without pics its tough to help a whole ton.

Guys cool down their little campers built out of enclosed trailers with 5000-7500 BTU A/C units BUT a lot of those guys have the benefits of using them at night and adding superior insulation.

A 5000 BTU window a/c works well to cool off my bedroom at home, but I'm guessing the body of a metal van holds a lot of heat - especially if you haven't insulated it yet.

Something like this would make a world of difference. 

Keeping in mind that part of this is driven by my inate cheapness but I really want to keep the ability to make it through the tunnel at my home track. A 9200 BTU roof unit would get the job done but that's a $700 upgrade, whereas I'm into the current unit for $90. 

I need to get the cab AC resolved; we're trying to find the leak in the system. Having the cab cooled off would also help. 

I've given thought to installing a second window unit as my 2000 watt generator would just handle it but that also seems a bit silly. 

I forgot to mention the van is white. 

I think if you resolve your hot side ventilation issues, you will find that the 5K btu unit should do a pretty fair job of cooling things down. Maybe not cold but at least bearable. If there isn't enough ventilation, the efficiency of the units fall off rapidly. If you have to, use ducted fans to get the heat off the back of the unit or blow cool air into intakes.