I'm working on a Nissan Sentra 1.8L that has no heat. One of the heater core hoses stays cold. I can get heat in the other. I have replaced the thermostat and the radiator cap. The motor temp comes up to normal. I have a bottle of radiator flush in it and have filled the system multiple times. It seems to be a never ending supply of air in the system. I have topped it off and still it will bubble out air from the system. I pulled the plugs and they are all dry and normal looking. I'm at a loss as to where the air is coming from. Any ideas?
I know what forum this is but the Nissan forum is empty and I'm not expecting to get any help there.
thanks

 

do you think it might be compression from leaking head gasket or cracked head ?
can you run a compression check ?
i guess you've backflushed the heater core both ways ?

 

I have not run a compression test. I don't think it's a cracked head or leaking gasket. It isn't smoking or over heating or sucking all of the coolant from the reservoir.
I have not back flushed the heater core. I'll do that next. But I don't know where all the air is coming from. After several hours of burping the system you'd think there wouldn't be any air left in it. But it is still got a lot of air in it. None of the hoses are collapsed so I don't know where it is pulling in air if that is what is going on. No visible leaks either.

 

backflush the heater core both ways to check/fix the circulation through it.

then put the cap back on and see if the cooling system holds normal pressure, or if it builds pressure and spits out water somewhere.

 

Ok, flushed the heater core last night till it flowed clear water. I ran the engine without one of the heater hoses connected and it was just a trickle. It never flowed a solid stream. All of the hoses on the back side are cold and are not getting any coolant movement. That's the two heater hoses and the T'd off hoses to the throttle body. There's just nothing flowing that way from either heater hose.
So I guess I'm down to the water pump now. It's been replaced twice on this car already.

 

No, not the water pump. It does build pressure and it is moving coolant. Gurgling can be heard. The hoses connect to the thermostat housing. Maybe one the passages are blocked or the gasket has failed. I can't think of where the air is being pulled from. There's no visible leak and the oil looks good and there's no cloud of smoke coming out of the rear. It's dropping to about an inch below the rad cap and staying there.

 

let me see if i understand. you disconnected both heater hoses and are able to run water through the core both ways, easily, w/ no blockage. but then when you connect the heater hoses, there's no water flow through either of the 2 hoses...

so with both heater hoses disconnected from the core, there's no water gushing everywhere ?

that tells me there is a flow control valve somewhere in the circuit, which sense whether the heater control is on or off. when off, it diverts coolant in a bypass to probably avoid putting heat in the cab. look for an electric or vacuum control... if you find it, and you can't fix it or troubleshoot it, see if you can remove it.

 

I don't see any controls at the hoses. There's the engine block, then the t-stat housing, the t-stat, then the outer t-stat housing.
The lower rad hose connects to the outer t-stat housing. The upper rad hose and both heater hoses connect to the inner t-stat housing. The rad hose comes off the front and the heater hoses come off the back. There's no switching or electrical there. On both heater hoses there is a T that connects to the throttle body. They are small lines and do not get warm either.
It's like there's a blockage where the heater hoses comes out of the inner t-stat housing. I'm gonna pull the lines off the heater core and see if I can get some flow back through them so I don't have to pull it off.

 

I pulled both lines off the heater core and flushed back through the on both. It was brown at first and then flowed clear from one to the other. We'll see how it goes today. Also pulled the carpet as best I could. No signs of a leak inside the car.

 

i thought you already did that in post #5...
i'm not quite following the flow in/around the TB, but i'd just flush everything both ways.