My Bonneville is not heating up to ~200F as it should be. The result is reduced heater output in the cabin and reduced MPG.

I have replaced the thermostat a couple of times, so I am reasonably certain that is not the problem.

I have not noticed any significant loss of coolant.

It is possible the heater core is plugged, but I flushed the cooling system a little over one-year ago. Not long after flushing the cooling system, I noticed it wasn't heating up on a consistent basis.

The odd thing is that ~10% of the time, the car heats up as it should. Probably 80% of the time, the car only heats up to 140F per reading the thermostat gauge on the dash. The other 10% of the time, the thermostat gauge on the dash doesn't even move. This may be from driving too short of a trip.

Any thoughts? Is this a heater core problem? I plan to stick the garden house in the heater core to possibly flush any blockage.

Marmot

Get a scantool on the car and find out what it'* REALLY running.

Feel the hose at the thermostat housing for heat in it. It sounds like the thermostat is not seated properly or missing the seal. Sounds like the temp gage is accurate as you verified there is poor heat output.

Do you happen to have the thermostat part number you replaced it with?

The first two thermostats were Stant. The one currenlty in my Bonneville is from NAPA. It has been a few months since the NAPA thermostat went in, but I think this is the part.

Marmot

I'll check the hose/housing for heat when I get home from work on Wednesday.

Marmot

The thermostat housing is very warm.

Marmot

It sounds like you may have low coolant in your engine, or a large air bubble that is preventing proper circulation of the coolant through the radiator and heater core. It is a fact that when coolant level falls, the heater core is among the first components to be starved of flow. So, whenever heat is good, then cool, suspect low coolant level.

These engines are prone to air blockage if they are not filled with coolant properly after the system has been opened. Part of the problem is the solid flange on the thermostat that does not allow trapped air out of the engine. The fix for this is to drill a small hole 1/16" or 3/32" through the flange as an air bleed and install the thermostat so that the hole is at the highest point.

How you refill the cooling system makes a lot of difference. Basically, you want to fill the engine at the thermostat opening, then fill the radiator with the thermostat out until coolant overflows at the thermostat opening. That will prevent big air voids in the engine that may be causing your problem.

Read a detailed procedure for refilling here: http://www.bonnevilleclub.com/forum/...=article&k=100

Refilling the cooling system could be the problem as I did flush it over one year ago. IIRC, the heat has been inconsistent after flushing the system.

Thanks for the advice and link.

Marmot

I think the no heat problem has been resolved. I bleed the cooling system per the link provided. My test run tonight resulted in the temperature gauge heating up to operating temperature and hot air coming from the floor vent. Tomorrow will be the true test when I commute 20 miles to work.

Marmot