inconsistent heat

[LightlyToasted]

2007 4.8i.

Symptom: air through vents alternately blows warm then cool in roughly 30 second intervals.

Test: Crank up the temperature to max, choose auto mode, and go for a drive. Cabin never really gets up to required temperature. When it decides to blow warm the blower speed will increase, then 30s later it will decrease and the air is immediately cooler. The warm/cool cycle will repeat. Both driver and passenger sides are either warm or cool at the same time.

Coolant level is OK, coolant temperature gets up to 102C and holds steady so thermostat seems OK. Electric fan was replaced 2 years ago and operates normally.

No IHKA or any other relevant fault codes.

Auxiliary water pump was replaced last year, and appears to be operating OK when driving and during rest mode.

Heater control valve was replaced 2 years ago. IR thermometer shows input and output hoses are around 76C but it's hard to get a precise reading as they're crammed in the engine bay.

Any ideas? Perhaps a faulty interior temperature sensor is causing the mixer flap to go back and forth? If so, is there any way to test it via ISTA?

[80stech]

So the V8 has an auxiliary pump? There must be a little fan and grille to draw air in for the IHKA (like the E53) have you checked that? I would think you should be able to run and monitor everything in ISTA. You're saying that engine coolant temp stays steady when this is happening?

[LightlyToasted]

Yes, V8 has an auxiliary pump, located below the power steering fluid reservoir at the front left of the engine bay. The pump feeds the heater control valve.

Engine coolant temperature stays steady at 102C.

The air temperature grill on the IHKA seems clear. The ETK shows that there is also a temperature sensor in the heater core. So far I cannot find any way in ISTA to read the values of either sensor.

I'm going to go for another road test to see if I can hear any relay or flapper operating when the air goes from warm to cool.

[80stech]

That's all readable in DIS and INPA for the E53 anyway AFAIK so I would think ISTA would work too. I haven't used ISTA but I have heard some stuff isn't obvious (not as hidden as some things seem to be in INPA though--I hope!

[LightlyToasted]

Hmm, couldn't find it in INPA either, but I'm not very familiar with that tool.

Apparently there are L and R vent temperature sensors, L and R heater core temperature sensors, and a cabin temperature sensor. Would be nice to be able to read them.

[80stech]

I'm a huge fan of DIS 57 but I'm not sure if it works on the E70. DIS can read and actuate pretty much everything and run diagnostic routines, come up with repair plans, TIS bulletins and wiring diagrams and AFAIK ISTA is supposed to be the equivalent but maybe not ??

[LightlyToasted]

ISTA performs a similar function, but I haven't yet found a way to read the individual temperature sensors in the heater core nor air vents nor interior. The coolant temperature and outside temperature can be displayed on the instrument cluster via the hidden menu.

I think the IHKA believes it needs to quickly ramp up heat so it ramps up the blower and blasts warm air for 30s, then it lowers the blower and reverts to cool air for about 10-15s. Then the cycle repeats. It's either getting bad data, or is itself faulty.

[LightlyToasted]

Hmm, I may have found something in ISTA to allow me to read the temperature sensors. I'll try this out tomorrow morning and report back.

[LightlyToasted]

I was able to use ISTA to read the temperature sensors by selecting the IHKA module, selecting "Call Up ECU functions", "Diagnosis scan" tab, "Status - IHKA high", then selecting whatever sensors I wanted to read. I confirmed that the L/R vent temperatures and cabin temperatures matched my IR thermometer.

I cranked up the temperature to max and went for a road test. Once the coolant temperature reached 82C things started to happen: fan speed ramped up (auto mode), L/R heater core temperatures rose, L/R vent temperatures rose, and then the interior temperature rose. All good so far.

I pulled over to watch the readings. Then things seemed to go wonky. One or both heater core temperatures would drop very quickly sometimes to as low as 5C, the vent temperatures would follow, then the IHKA would react by lowering the fan speed. In this state it wasn't putting out any heat even though it was cranked up to max and the cabin temperature was cooler than that.

AC on or off, recirc on/auto/off, etc. had no effect either way.

When I revved the engine, or got back on the road, I could sometimes get the heater core temperatures to climb again and the IHKA to add heat to the cabin. Ordinarily I might suspect that the higher water pump pressure from revving the engine was pushing more coolant past a dead secondary pump, but that secondary pump is only 1 year old, and it seems to work fine in rest mode.

Not sure where to go from here. I wish I could confirm the state of the heater control valve solenoids and the secondary water pump state.

[80stech]

Well that is interesting, this now sounds a little bit like the other thread with the 3.0L engine. Does the V8 have a manual water pump and not an electric like the 3.0L? It does sure sound like in this case that the aux pump would be suspect. Any history of cooling system work? DIS would give choices for diagnostics and bulletins based on symptoms even if there are no codes maybe ISTA will too??

Heater valves and secondary water pump should be in the ISTA as well.

OK I see you listed some history in the first post, anything else maybe? Was this the same when the aux pump wasn't working ??

Head gasket can give those heater symptoms but you would think there would be more going on and I think both those engines are pretty bullet proof as far as that goes ??