Overheats at low speeds

[oldrusty]

My 83 4wd runs pretty good. Just passed smog with the help of a tune up. The problem is that at speeds of around 20-30mph on hot days or on low speed hills it starts to overheat. Turning on the heater doesn't lower the temp. But its fine driving down the highway even up hills. The fan is on its own switch so i know the fan is working. I replaced thermostat and flushed the cooling system. I even soaked the radiator in vinegar trying to unclog it. The radiator doesn't look that bad and flows pretty good. Now I'm wondering if its the head gasket. I sometimes gets very light smoke out the tailpipe-barely noticeable. I did a compression test and the cylinders are all around 165. I did a test on the coolant to see if the head gasket was bad but it passed the test. I know the radiator i have is a single row,should i get a double row? or just replace the head gasket? or could it be the water pump?

[marlinh]

If the fan is wired separately, maybe it is running backwards. I think there was a forum member who had that problem. Do you know what the actual temperature is at the head? Those handheld temp guns are indispensable. Even though you replaced the thermostat, they are known to be bad right out of the box. It would save you a lot of guessing if you can buy or borrow a temp gun and confirm there is a problem. The temp of the hose right beside the thermostat housing should match the rating of the thermostat. I have seen the upper rad hose reach 200 F before the fan kicks in. The lower should be around 90 (guessing) and that would tell you if the rad is working properly. Best to try to diagnose instead of throwing parts at it.

[oldrusty]

the cooling fan blows towards the radiator. I don't have access to one of those heat guns. Wouldn't it be overheating all the time if the thermostat isn't working? I have felt the hoses and the thermostat opens up when its warmed up.

[marlinh]

It may only be opening partially. You could stop by a shop and ask someone to check the temperature for you while it is idling, since that it when you are worried about it. They may do it for free, or something nominal. Your gauge may be inaccurate and you are chasing a problem that isn't there. It would be worth the ten minutes of diagnostics. Just my opinion.

[Petros]

Welcome to the forum Oldrusty,

The single row radiator should have plenty cooling capacity if it is working correctly (not plugged up). if you have limited cooling capacity it shows up when you have maximum load on the engine, going up hills, at near full throttle conditions, etc. not just at low speeds. A bad head gasket by itself will only cause overheat after all of the coolant has leaked out, if there is enough coolant to keep it cool it will do so at both low and high speed. I can not see how a leaky head gasket would cause it to get hot only at low speed.

It is extremely rare that a bad water pump causes cooling problems. Sometimes the impeller can have some of the blades rust away and not flow enough coolant at lower speeds, but I would not pursue that until all other possibilities are eliminated.

Your situation is odd, overheats at low speed but not at high speeds? perhaps not enough airflow through the radiator? Double check the fan is blowing the correct way. Also, could be a partly blocked radiator, but that usually gets worse at higher speeds. A radiator shop can test the cooling capacity of the rad and also use the heat gun on it to look for "cool" spots (meaning places that are plugged up).

Marginal thermostats can fluctuate temp, though they tend not to go by car speed. Cheap ones will fluctuate right out of the box, and than shortly after fail all together. that is why I do not buy cheap ones. Also, check to see if your lower radiator hose is closing down when you rev the engine, if they are soft the suction from the pump can close them down. Time for a new lower hose if it is soft and swollen.

[dlb]

The problem is that at speeds of around 20-30mph on hot days or on low speed hills it starts to overheat. Turning on the heater doesn't lower the temp. But its fine driving down the highway even up hills.

i agree with marlinh to first verify the temp and rule out the gauge as being the problem. if the gauge is accurate, the fact that the engine overheats at slow speeds makes me think that the rad is clogged and can only cool sufficiently with lots of air movement over it. you mentioned it seems to flow well but it could still be partly clogged. the hot upper rad hose/cool lower rad hose is the only other test i can think of for a clogged rad. neither test is definitive, unfortunately.

besides that, i would replace the rad cap (unless already recently done) since they're cheap, remove the thermostat and test it in a pot on the stove, and watch for coolant consumption. make sure the overflow reservoir is filled to the line, go on some big drives, and then check it again when it's cool. if it's consuming coolant, it's probably the head gasket.