I have a 1990 Sentra with 88000 miles and a weird overheating problem that has been going on for more than a month. I have recently replaced the radiator, water pump and thermostat and checked the hoses. The ignition timing is set correctly. I believe the temp gauge to be accurate because as it gets hot other symptoms of overheating appear such as engine knock.

The car runs too hot at low speed or idle but runs cool at high speed. I do not believe it is the fans because they seem to work fine. Also, if I rev the engine in neutral the temp gauge goes down even though the same amount of air is passing over the radiator. My hypothesis is that rust or scale from the old radiator (there was a lot of it) is causing a partial blockage in the engine's cooling chanels which is overcome when the water pump is pumping a higher volume of coolant. Does this sound reasonable? The only other possibility I can think of are problems with the head gasket but there is no smoke in the exhaust and no coolant in the oil. I have heard of exhaust gasses getting into the coolant without the above symptoms but I believe that would cause overheating at high rpms too.

I have brought the car to a number of mechanics and no one knows what is wrong. Some say there is no problem, but I know the car, and I know it is running too hot, and I don't want to wait until I am stuck in traffic on a hot day to find out I have a serious problem.

The cooling system has been flushed, so if there is something in there, it is not easily removed. I ordered a product called RMI-25 that claims it can remove rust and scale from engine block coolant chanels. Has anyone here used this before? Does anyone know any other way to check for/remove rust or scale in the cooling chanels (without disassembling the engine)? Does anyone know any other possible cuases of the syptoms I described? Thanks in advance.

Dave

Drubin,Have you made sure that ALL the headbolts are tightened to the correct specs?Loose head bolts will sometimes cause weird problems.Not only that but it's the inexpensive route for now.Never heard of RMI-25, sorry I can't help you there.

I haven't chekced the headbolts myself, but when this problem started I blew a head gasket and the mechanic who replaced it tightened the bolts to spec. (He also machined the head, replaced some valves, the water pump, thermostat and timing chain). He would look at the car for me but he is several hundred miles away and I can't get off work to bring it to him.

Since my last post, I back flushed the system with water and and a cleaner. Yesterday I was able to drive for an hour mixed highway and traffic with the a/c on while it was 90+ degrees without the slightest overheating, then in the evening on a 15 minute drive the car started to get too hot. Today after a half our of driving the car started to run much cooler than normal. The thermostat is new (replaced twice in the last month) so I still think there is some sort of blockage which is causing intermitant problems. Thanks for the advice.

Dave

Have you bleed the cooling system? There is a bleeder screw on the intake man. at #1 cylinder. It should have a gold color sticker next to it stating so. If not it's got a copper washer under it. Although it should have bleed with all the driving you've done but it may have air trapped inside.

Recently, I began having problems starting the car (had to give it gas) and occasional hesitation when accelerating at low rpms. I did not mention it because I thought it was an unrelated fuel system problem (I wanted to deal with one thing at a time), but listen to what I found:

I looked at the computer error codes and it said there was a problem in the coolant temperature sensor circuit. I replaced the sensor (the one that reports temperature to the ecm) and all my 'fuel system problems' are gone. In addition, there is no knock and the car shows no other signs of overheating. The temp gauge is still high, but the car is cleary not overheating because all the symptoms were due to poor engine control by the ecm without a working temp sensor. The gauge is controled by the same type of sensor, and I will replace that tomorrow. In addition, the fans are always on these days, and they too are controled by a similar temp switch.

It is interesting that a bad temp sensor (to ecm) and a bad temp sender (to gauge) can, in conbination, reproduce an overheating condition pretty convincingly. I should note though that I had an actuall overheating problem at first (blew the head gasket) and later a leaking water pump. I presume it was the overheating that damaged the temp sensors.

Thanks for the replies... I just found this board and its great. This is my first car so its quite a learning experience.

Dave