Strange Heating System "Hiccup"

Was driving Superblue along today on a cold day (40s) here when I suddenly noticed her heater was no longer blowing warm air but pretty much ambient level temp air. :open_mouth: I kinda panicked, wondering if maybe I had a sudden coolant leak in the system (e.g. heater or coolant hose), but the temp gauge never budged from her usual reading and no signs of smoke, steam, etc. coming out of the engine compartment. :confused: So, I kept driving, and turned the temp control (heater valve switch) off for a few minutes. When I cranked it back up again, the usual warmed air flow returned. This kept on for about 10-15 more minutes, then got cold again. I repeated my previous action, with same result as before. By then I had reached my destination O.K…

Checking the coolant level, it’s topped up, so no leak. I don’t think the heater valve is blinky, as I r/red it just a couple of winters ago (with the same brass type as OEM, not plastic). The small vacuum actuator hose to it was also tight, with no signs of cracks or holes.

I’m wondering if maybe I had some kind of debris passing through the cooling system that temporarily stopped up one of the heater hoses, or something, but the coolant looks normal in appearance. Tomorrow I may try taking the cap off and watching all the coolant flow by with the engine running, to see if I spot something odd in it. :thinking:

I don’t think this is related, but the past few days too I have noticed my MPG has really dropped - from the usual city average of 14.0+ MPG to barely 13.0. Checked the tire pressure, and it’s up to snuff on all of them. She still has the “blinky” heated O2 sensor (per the radio display) she has shown for a long time now, but other than that no new error code.

If Superblue were a V-12, I would worry maybe somehow an “air gap” has formed in her coolant, but she’s a 4.0. Moreover, I haven’t added or r/red the coolant in her.

Thoughts on this, anyone? :confused: Hopefully not a harbinger of some future disaster to befall “us”. :grimacing:

On older cars it’s a clear symptom of the A/C Amplifier failing.
Don’t know if it applies to your 94.

And why do you think it would not be the cause?

When the heater gets cold, you can remove the vacuum line to the control valve and see if it opens?

The control valve is NORMALLY OPEN and vacuum is required to CLOSE the valve.

not sure if my 88 with the Mark3 system is the same as yours, but since i’ve owned my car, i’ve determined that heat is blowing from lower vents, but upper vent blows ambient cold air most of the time, so overall heating is almost nothing. Meaning a vacuum line is not shutting off the upper vent when heater is on. I have yet to rip out the dash to fix it. Not too cold here in Seattle, so no rush.

And I can’t BELIEVE they would use vacuum lines to control the heating/cooling system. These cars just have too many vacuum switches for my liking. :frowning:

On my ‘88 (which I believe has a working Delanair III), the dash side vents are controlled by the lever below the radio when in heat mode. When the lever is fully to red (hot), the dash side vent temp matches the floor outlets while heating.
Likewise the dash side vents match the floor vents in cooling mode with lever fully toward blue (cold).
Nice feature as I don’t generally like hot air blowing in my face, but like warm feet.

What i get is cold outside air blowing from top center vent with heater on. I was wondering if superblue has same problem?

I get nothing from the upper center vent in heat, only in a/c. I just googled it, there is a vacuum operated center vent door that works in conjunction with the defrost door. You might try cycling a few times from defrost to heat to see if it is just sticking.
Looks like actuator is on the right side of unit.

The MkII had more of them. OTOH, in the MkIII, the servo motors were more notorious for failures than the vacuum switches.

When was the last time the thermostat was checked?
Change in economy might be the result of coolant temperature.
Try pulling vacuum host to heater valve to see if it moves.

Many, many makes and models use vacuum to control all or portions of the the HVAC. It’s not just a Jaguar thing.

Good point, motorman (and equip) … I’ll try that and see what happens … :+1:

b/c it has had the same error code showing for a LONG time. Unless suddenly the O2 sensor somehow got a lot worse (assuming it is in fact defective to begin with and not a false code at work) I don’t see why the MPG would suddenly drop that much. :confused: