So you can't figure out why your AAZ is overheating - FAQ/Tech Tips/Please Read First - VWDiesel.net The IDI, TDI, and mTDI source.

• So you can't figure out why your AAZ is overheating by jimfoo on 15 May, 2008 21:06
• Mine had this problem, running very hot at times. Once it finally died, I converted it to a M-TDI. In the process I had decided to modify the old thermostat and add a second oil cooler(air). My thermostat had a failing rubber seal around the moving part, and when I pulled the rubber off, I found it had a bunch of small holes, which is something I wanted to allow more flow. I also worried about the block off plate maybe not closing the recirc port on the block, or maybe the spring for the plate being too strong. So I took the plate and spring off, put on a much weaker spring and turned the plate around  so it would close sooner, riveting it in place with a washer. The TDI ran much cooler, maybe too cool in winter. So while working on it recently, I decided to buy a new thermostat to get rid of the extra flow.  I got an 87*C. I put it in, and on a short test drive, the temp got up to 230 on a cool day. The radiator was cold. I took it out, put it in a pan along with my old stat and one out of a 1.8 that's in my garage. My old one (87*C also) started opening at 165*F or so, maybe that's why it ran cold, but both of the other ones opened where they are supposed to. I reversed the plate on the new one and replaced the spring, drilled a couple small holes to make filling easier, and put it in. Now it runs fine, temp between 180-190 on normal driving, 200-205 on the hill that I got up to 230 on.
  I don't know if the strong spring on the block off plate is too strong, if the thermostats listed for AAZ's are the wrong ones or if the block off plate is just too far from the port, but this mod fixed the problem. So if you can't figure out your overheating problems, try this as it just might help. Also if you are careful and depending on the design, you can bend the brass in on itself to get the block off plate off, then carefully bend it back, heating it first might help keep it from cracking.
  
  Stock on left, thinner spring and reversed plate on right.
  
  Stock on left, rived back on on right. Use an aluminum rivet or brass screw to avoid the rivet rusting off.
• #1 by stopping on 10 Jul, 2008 19:00
• Are you serious?
  
  This would be a crazy oversight on the part of VW.... I will try it!
  
  I am using an AAZ to push a Vanagon... not over-fuelled but over heating.... new rad... no blockage.... timing is straight not advanced.
  
  I will try it if I can't find some micro combustion gas leaking into the coolant.
• #2 by jimfoo on 10 Jul, 2008 22:50
• When I converted my AAZ to a TDI, then had to replace the head due to me not tightening the IP bracket bolts and the valves hit the pistons and screwed up a lifter, I put a new t stat in. I tested it before and after, and it opened at 190 ish, but the truck would get to 230. I put the old stat, which I had modified, back in, and it brought the temps down to normal. I modified the new t stat and put it in, and I have been fine.
  I'm not sure if it has to do with the thinner spring I put on the blockoff disc, or the fact that the disc is closer to the port, but I swear it did fix my overheating problem in both the AAZ and TDI(AAZ block)
• #3 by zukgod1 on 16 Jul, 2008 17:20
• I wonder if this would be the same on our 1.6's?
  
  I have a OE t stat in it now and the plate on the end looks like the one you have with the stiffer spring.
  I HAD a lower temp t stat in it that had the lighter spring on it but I swapped it for the OE one as the area where the water passes through on the OE one was quite a bit larger compared to the STANT one I had so I swapped to gain flow.
  
  I've been thinking of going back to the colder one and flipping the plate as you did to see if it helps.
  I just don't have the air flow required to keep my car cool while going up hills is my problem but if I can make an improvement in the cooling system I see no reason not to try.
• #4 by wildenbeast on 06 Oct, 2009 15:44
• Has anyone else tried this?  I am also living in the CO Rockies and trying to run cooler oil temps over some of the high passes.  I have a 1.9TD installed in a Vanagon with a high top.
  
  Bill
• #5 by red_method69 on 30 May, 2016 05:36
• If you have a vanagon and swaped the engine from 1.6 to 1.9, you probably left the original 1.6 exhaust system.
  The van is overheating bechause the exhaust is too small. Same problem happened to me 1y engine and 1.6 1990 vw van exhaust
• #6 by theman53 on 30 May, 2016 12:28
• Jimfoo eventually added another t stat to his upper hose to "fix" the fixed problem. Search it