We tried to use the heater yesterday for the first time in ownership. Everything seems to be working correctly, vents, fans, settings but not really any heat to speak of. I tried again this morning and let it warm up until thermostat had time to open and then ran temperature up to 85. It seemed to be NOT COLD but not really hot. I checked the forum for potential issues and saw a mention for fuse #5 in engine bay. I checked it and it looks fine.

Any ideas of what to look for or try? I ran the Climate Control System diagnostic check and no codes appeared.

 

Heater core plugged? Do a search for back flushing.

 

The common problem is the aging heater pump. It is sort of hard to reach, it sits on top of the transmission, off of a bracket on the firewall. Here is my debug procedure:

• Try and mess with the vents. My car does best on recirc and face.
• Engine off, ignition on and heater controls on, the heater pump should be on. It is quiet (mag drive), but can be heard if you pay attention.
• Find the heater pump relay and measure the resistance of the pump. If "infinite", the brushes are worn out and you need to replace _them_. The pump can be repaired, you don't necessarily need a new one (> $200).
• Disconnect the plug to the heater valve. It defaults open. When fully open, you should get a _ton_ of heat in the cabin. The plug is on the firewall, sort of by the brake booster, next to the heater pump plug.
• Next, check the (stupid) hose going in the core (again, off the firewall). The hose has a built-in one-way valve, and a flat spot to accommodate it "by design". That flat section sometimes collapses and limits flow.
• Next, try and figure out the flow of heat. A infrared thermometer is useful, but sometimes the hand is best. See if heat goes in the core. This is sometimes inconclusive and difficult to draw clear conclusions from.
• Next, and this has proven _very_ effective for me, do a forward and back flush of the heater core. This is garden hose stuff off of the 2 pipes coming off the firewall. Radiator cleaner is your friend, follow the directions. Don't necessarily expect chunks or debris coming out of the core.
• Locate the fuseS for the heater blowerS (there are 2 of them, left and right). Pull them off one at a time and check if you can hear one blower at a time.

As a baseline, this engine is a high flow/low volume design. You should have _some_ heat in the cabin as the needle barely comes off the bottom of the gauge, and certainly within a mile or two of your starting point.

Best of luck, keep us posted.

 

After reading all of the helpful comments and watching several videos, I think I have it fixed. The coolant system had been flushed and refilled about a month ago and this was the first time we tried to use the heater. There must have been some air trapped in system thus not allowing heat to work.

I took the coolant system cap off, parked on a slight incline, gently squeezed the upper and lower radiator hoses to burp the system. It seems to be working much better now. I am not sure if I got all the air out but will try this for a while and see.

Glad I took the easy way out and did not try to remove pumps and valves that were beyond my abilities!!!

 

Check the coolant level in your expansion reservoir first thing every morning for the next several days. You may need to top it up each morning until it finally stays at the proper level (the bottom of the filler neck). I always have to do this any time I drain any amount of coolant in order to do any necessary repairs....

 

Now, I am not sure anything changed. It must have been wishful thinking yesterday. The heat set at HI, still blows cold. I burped the system with the car pointing down hill so the overflow reservoir was higher. I cranked and ran after squeezing the radiator hoses to move air out. Not sure now??

 

Update. I burped system and took car for a longer ride with the heat on HI. It really didn't get so hot that it ran me out of car but it did start to feel like it was starting to heat. Still not sure if its totally fixed but will wait for January and February to really test.

Opinion... Do you think this garage queen had been sitting for so long AND probably never taken out in the winter, that the heater core just did not get a good enough work out before? Maybe with more continued use, it should start to work better?

 

No doubt that these cars stay happier and healthier if they are given regular exercise all year round....

 

I hate to say it, but if you've got a 1999 XK8 and the heater pump (auxiliary coolant pump) is still functioning, you are very lucky. The brushes in the motor wear out eventually.

 

Dont wait until it is freezing cold. Fix it now! There are simple things you should check first:
1. Do the electric pump work? Located behind engine to the firewall. The relay is in the engine compartment fuse box, there are three relays in a row and the pump relay is nearest windscreen. Listen carefully or touch the pump while removing relay with the ignition on
2. Do both fans work? Easy to check, just remove fuse 11 from driver side fuse box and then fuse 11 from passenger side fuse box with ignition on
If only one fan is working you will not get any heat

 

Fans are blowing strong. I checked fuses and all are good. Yesterday, I turned on heat to HI and rode around for awhile and felt like it began to get warmer. Is it possible that previous owner just never drove car much in winter and therefore did not use heat much? It was pretty much a show car for them. I just bought and tried heat for first time. Would lack of use cause the heat core to take a while to "recharge" or start working?

 

Heater cores do not "recharge". They (a) work properly, (b) clog up and work poorly, or (c) clog up, rust out, and leak coolant inside the cabin....

 

Option B might be the choice. I knew they did not recharge like putting Freon in air conditioner. That's why I did quotes.

I am just wondering if continued use may start to clear up and work better???

 

As I mentioned earlier, a back flush might be needed.

 

One thing you could try would be to unplug the electrical connector at the valve and see if that makes a difference. When i unplugged mine, it made a massive difference. When my temp is set at say 75deg, my 97 starts blowing warm air within a half mile from the house when cold, then gets nice and toasty, then after a few miles, the temp drops off. When i turn mine to high, itll make you open the windows, almost like standing too close to a bonfire! As i was told, when you unplug the connector the valve defaults to open so you'll have constant heat, and BOY DID IT WORK FOR ME! I think my issue is possibly my internal sensor, but try unplugging the heat valve and see what happens. Pretty easy to plug and unplug

 

Dumb question. Does the white plug just pull and disconnect or does it have clips on top and bottom or on both sides. I looked at it just now and cannot really determine which way it disconnects.

 

if you press down on the front "bar" it will lift the back part just enough to clear the little nubs circled. Then it should slide off. Understand that it has probably never been disconnected, so it may take a little wiggling to get it to release 😉

 

I unplugged the water valve connector and still no heat this morning. I need to do the diagnostics check again because with the heat on, the windshield kept fogging up. Then I had to turn defroster on and that is pure air conditioning.

 

The black plug next to it is to the heater pump. Try and measure the resistance on the plug side (it's much easier to measure at the relay, but whatever). Should be a few ohms. Open circuit means the pump is not running and would explain the low heat output even with the valve open. Failed heater pump are pretty common on our aging cars.

FWIW, the pump is wired to run with the AC/heater control panel turned on even if the engine is off. You should easily be able to determine if the pump is running or not by ear, with hood open, fan off and engine off in a quiet environment. The pump is sort of on top of the transmission, just below the firewall. Just follow the short path of the hoses from the water valve in the same neighborhood. Touch the pump body with a long screwdriver or a mechanics stethoscope if you have one in case hearing it is not an option.

Best of luck, keep us posted.

 

OK, I followed your instructions and I heard a whirring noise in that general area with climate system on but engine off. I assume that is the pump running. Ran the climate control self diagnostic instruction and had zero codes come up. Water valve is unplugged and I check all fuses under the hood. Still no heat. Next?