I have an 98 discovery and I already replaced the thermostat and I have some crazy no heat symptoms. No heat at idle, engine temp in the middle. On the highway I have heat at 50 - 55mph, engine temp in the middle, but if I accelerate to 70mph as the truck accelerates I lose the heat. Also, if I de-cellerate from 55 I lose all heat by 40mph. I do not know were to turn. Cooling system clean and I think is bled properly??? Had the exact same symptoms before I changed the thermostat. Any Advice.

 

Have you bled your system properly to get rid of air in the lines?

 

Sticky t-stat? I've got the same issue but in the cold am, the temp gauge barely get's to 8 o'clock with the heat on. I had heat before I did the cooling system re-boot so I need to pull and check the new t-stat.

 

I felt both heater hoses after idling 15 minutes with cardboard blocking the radiator. Both hoses hot, temp rose to 3/4 guage, but still no heat.

 

First check your belt routing.
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You might have a clogged heater core.
What temp thermostat did you install?

 

He is in the US, the DII did not come out in the NAS market until 1999.5.
In the rest of the world the DII was launched in 1998.
So if here were in Spain he would have a DII for example.

 

IMHO you have air behind the stat, or clog, or stat off center. The gauge sender and the ECU coolant sensor are close by, but the gauge will show 8 - 9:00 for a wide range of temps, like 140F - 240F. A data reading scanner will give you a more accurate picture of what's going on.

Did your new stat have a small "jiggle valve" in it? It will rattle. See old one below. That is designed to let trapped air pass by. If it does not, stat body is eventually out of the water and just in the steam bath. For those that think it is the same, try holding your hand over a bubbling pot of noodles. Pretty warm? Now stick your hand in the noodles. Same for stat, it needs to be wet to work best.

Old school method was to drill a small hole in flange of stat, like 1/8 inch or smaller. Position hole at top of mounting, so gas can move on by.

If heater core is clogged, one pipe to it will be much colder than other. I'll run mine and post back. If this is the case, you warm up eventually, when you increase engine load the main stat opens fully and pressure drop across radiator reduces flow to heater circuit. At lower RPM heat drops off as well, needs more pressure to push through the clog. You can try reverse flush with garden hose through the heater pipes and the core. Will post my temps when truck warms back up. When temp gauge got to bottom indicator mark (7:30), I was 139 F on lower heater hose (feed from block) and 132 on upper (return). Stat is not even open yet.

Gauge may look normal, but scanner may read low coolant temp. This can be when stat is installed, and slips down just a fraction of an inch, leaving an out-house crescent moon opening, behaves like no stat, temp can be 135 F. Stat does not leak, because gasket and RTV provide enough seal for that thin metal lip.

So IMHO, scanner reading might point toward stat, but I always by extra stat gaskets because I am so prone to scew them up.

 

Is it possible the device behind the dash that transfers between cold and hot is broken? I've no idea how it works or what it looks like but there must be something back there controlled by the dial on the dash.

 

On later model D1s and all D2, that is accomplished by moving the flaps inside the heater module. Hot water going to heater core always. Earlier D1s had valve under hood with multiple hoses. See http://www.roversnorth.com/store/c-3...g-heating.aspx, items 17 and 18. On my 97 they are not present, catalog says not used after certain VIN.

And before you start yanking it apart, see this post with pix of what you can be in for. The truck is built around the core. https://landroverforums.com/forum/ra...ht=heater+core. I'd try reverse flush and stat work first. Disco may be easier, but not as easy as a garden hose and a $10 stat.....