I'd have to say "suddenly". You didn't mention the thermostat as a
possibility? Could it be partially opening or something like that?
Several people have mentioned the fan, but I don't see how that can be
a factor when the rad is not even hot...
Tom

On 8/14/08, Jim Felder <jim.felder@gmail.com> wrote:
> Did this problem appear suddenly, or did it slowly get worse over time?
>
> I can think of two things. One, you have something in your radiator
> that is slowing down the flow of coolant through it. Moving at highway
> speeds could overcome this. You might try a radiator flush for that,
> but it won't be conclusive if that doesn't work because of how
> difficult these radiators seem to be to clean. Some on the list have
> taken their radiators out and had them professionally cleaned, but the
> general consensus seems to be that new radiators are cheap enough that
> it doesn't make much sense to do anything but replace them.
>
> The other thing that occurs to me is, and this is more likely in my
> opinion, that you have a failing water pump that isn't efficient
> enough to move water at low speeds but when the engine is turning
> faster it does the job.
>
> One final thing that probably doesn't apply to your situation is
> something that happened to me last summer when I had a cracked head.
> The engine did seem to overheat at low speeds and run OK above 35.
> Something I never figured out, I guess having to do with the dynamics
> of the crack and the compression.
>
> Jim
>
> On Thu, Aug 14, 2008 at 8:23 AM, Tom Rowsell <86westy@geoexploits.org>
> wrote:
>> The fan is working fine. At first I suspected it, so I wired a switch
>> to it to turn it on manually. Running it on high does not help because
>> the rad is not hot anyway.
>> Both the front and rear heaters are producing lots of heat. If I reach
>> under the van, the 2 heater lines are scalding hot, but the 2 big rad
>> hoses are hardly warm.
>>
>> Thanks
>> Tom
>>
>> On 8/14/08, Ryan <easywind1975@hotmail.com> wrote:
>>> Tom,
>>>
>>> Confirm your radiator fan status using Ken Wilford's write up on his web
>>> site http://www.vanagain.com/How2fan.htm.  The 86 and up should self
>>> bleed
>>> after a few cycles (I think).  Does your heater blow heat?
>>>
>>> Ryan
>>>
>>> ----- Original Message -----
>>> From: "Tom Rowsell" <86westy@GEOEXPLOITS.ORG>
>>> To: <vanagon@GERRY.VANAGON.COM>
>>> Sent: Wednesday, August 13, 2008 11:37 PM
>>> Subject: Overheating at idle
>>>
>>>
>>>> My '86 recently began overheating but only at idle...drive all day on
>>>> the
>>>> highway and it's fine, but stop and idle for a few minutes, and the
>>>> temperature climbs. Rev the engine and the temp returns to normal. Even
>>>> after a 15 minute drive: stop and idle, and it'll get hot.
>>>> I recently had a leak, so it might be an air lock problem ? I'm thinking
>>>> the
>>>> water pump or the thermostat. It's not the fan since the radiator never
>>>> seems to get hot enough to kick that on. I guess tomorrow's job is pull
>>>> the
>>>> thermostat and check it? There are no unusual noises from the water
>>>> pump...
>>>> As usual, all advice and critiques appreciated  :)
>>>>
>>>> Thanks
>>>> Tom
>>>>
>>>
>>>
>>
>