Dohc Ka Underheating...?

[mwknoerzer]

So recently I swapped my sohc out of my 1990 240sx due to a rod knock. After swapping I drove it around for the first day and the car would not warm up at all so I replaced the Thermostat and bled the coolant system again. The new thermostat I bought is supposed to open at 170 degrees. It warms up faster with the new thermostat but not a huge difference. So my problem is in the morning I can start my car and it takes around 20 mins to fully heat up, but while driving my temps will slightly drop to a 1/4 of the way of the stock cluster gauge or if I start driving before its fully warmed up it will never fully warm up unless i am stopped. I've driven around 15 miles and It would only warm up to a 1/4 of the way if that, but if I stop and let it idle it will go back to full temp. Is it possible I got another bad thermostat causing it to stay open, possibly air in the system? I know its not the gauge, that fully worked while I was sohc and the wiring is done correctly for dohc. Also when swapping to dohc I removed the AC is it possible that now that I don't have a thick AC condenser in front of my rad that maybe its getting too much air now and cooling it off while driving? Also its about 35-45 degrees out where I am not sure if that helps or not... Also my brother has the same exact set up as me but with AC and in an original dohc chassis and his warms up within 5-10 mins or within a lap or 2 around the block, I have to drive for a solid 30 mins or so before my temp gauge hits that very bottom dash of the gauge... My heat does work not the best but it does once its warmed up. Its gonna start getting really cold here and I'm trying to get this figured out. Thank you all in advance.

[centralcoaster33]

Hi and welcome to NICO Club!

Do your radiator fans come on at all while driving, warming up, etc.? Are you using an OEM temp sensor for your gauge? The generic ones at the parts stores don't seem to read temp as accurately. I have one and it has like 3 steps past cold. Your heater can tell a better story than those generic temp sensors can. My heater core was jammed up with some gunk. I had to get a garden hose connected to one end and let it blast through and out the other and a bunch of stuff came out. I'd recommend the thermostat be of stock temp to open and use the proper coolant to water ratio as well as a thorough purge of any air (use the bleeder screw on top of the hosing by the intake and then with the cap on the radiator with car front tilted up).

[mwknoerzer]

Yes I am using the OEM temp sensor. It is also the oem clutch fan. I've bled the system multiple times and there is absolutely no air. I checked the clutch fan has a lot of resistance when cold, isn't it supposed to free wheel? Just to try I put a piece of card board in front of the rad and it warms up like its supposed to and stays at temp without dropping. When my gauge says its up to temp my heat works. I think it's the clutch fan staying on all the time and not only when warmed up, its like its moving too much air....

[asoomal]

The fan clutch should have lots of resistance when cold. Even if the fan clutch was sticking, the thermostat should be able to regulate the temperature.

Make sure that all of the coolant/heater core hoses are in the correct location. Also, are you using an OEM thermostat?

[mwknoerzer]

Sooo... I fixed it.... kind of. So it was a defective thermostat, I got a new thermostat in it and got all of the air out, I drove it around all day yesterday and temps stayed good and heat works. This morning on my way to school my car over heated... I'm not sure if its the gauge that's off or if its actually over heating. I'm using a like oem spec thermostat from advanced. I looked couldn't find any coolant leaks. Also it over heats while driving its not like its not like it only over heats while sitting at red lights or sitting still. I'm gonna put a temp reader on it tonight to confirm if its the gauge or the actually motor over heating......

[centralcoaster33]

Does it need to pop/ burp? I mean, is it circulating? I guess it would be if your heat is working. Mine had residual air pockets and I thought I was done and it overheated the next day. Of course, I'd been watching the needle like a hawk and got it turned off in time. I had to let more air out of the radiator. Are you using your bleeder screw on top of the hosing near the intake?

[asoomal]

What brand thermostat? Most aftermarket ones are junk.

[PapaSmurf2k3]

Yeah sounds like it needs to be bled again.
KAs are notorious for this. And yes, most aftermarket thermostats don't work well in the KA.