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Still overheating- thinking fan/sensors and/or relays?

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  • Still overheating- thinking fan/sensors and/or relays?

    Been overheating after about 30 mins of driving, from cold/op temp start. Many parts replaced, now notice the cooling fan is not kicking on. Jumped it on batt. And comes right on. So my next mission is to replace all the sensors / fan switch and see if we get somewhere. However there are a lot of different threads saying different things and pointing out different sensors and their locations. I'm under the Impression that there are three i should be worried about and then the two fan relays on top of that. Willing to replace all since I've tried (almost) everything else.. Help is not only appreciated but vital to my family's life.

    Thank you

  • #2
    instead of trying to fix a fan switch that no one seems to know where it is, you could just hard wire the fans directly to a switch that you can put somewhere in the car and have them run all the time. most people do it without any issue. you could also wire them directly to the battery and have them come on with the car if you dont want a switch.

    if you have replaced all the cooling related parts and youre still seeing an overheat issue then maybe you should look into a headgasket or maybe flushing the coolant system and bleeding it properly. too much air will also cause an overheat situation.

    if you want to try a temp fix, you can pull the thermostat out completely.

    do you have any leaks? what is the coolant-water mixture you are using? are you just using the prestone 50-50? what parts exactly have you replaced?

    if you have the fsm then look in there for where the location of the fan switch is if you really want to fix it properly. it should be in there.

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    • #3
      that mystery fan switch is up along the driver side at the front of the car near the AC condenser... there is also the ECT sensor on the block exact location depends on what year your car is... 90-91 or 92-93

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      • #4
        I have actually looked into just hardwiring to battery and that way the fan comes on only when key is in ON position correct? I've flushed the rad and heater core a few months ago and bleed the system after I do any cooling system work, although the method I use is running the car without the cap on- I've been told that should do it, don't know if using the actual bleeder nose is any more effective. Haven't had symptoms of leaking head gasket I.e. exhaust smoke, coolant in oil. The car doesn't overheat until about 20-25 mins of driving. I might hard wire fans then do another flush and bleed and see where we stand.

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        • #5
          have you done a block test for exhaust gasses in the coolant?

          These cars can have HG issues without the "normal" indications that it is there...

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          • #6
            Jump your coolant temp switch on the thermo housing. If the fan turns on replace the switch. If not look into the relays by the a/c fan on driver side.

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            • #7
              Relays are much more likely than a bad fan switch. The fan switch (AKA "engine coolant temperature switch") is located on the thermostat housing for 92+ engines and on the back of the block for 90-91 engines. Do a search if you need clarification.

              Tell us EXACTLY what is happening when the car over heats. Does it overheat at idle? Does it overheat when on the highway at constant speed? Only overheat in traffic or around town?...etc

              DON'T HARDWIRE THE FAN SWITCH!! This is stupid way to "fix" fan problems. You can troubleshoot a fan problem faster than wiring it to go on automatically or to use switch. It's a waste to have the fan running all the time and will cause it to wear out faster.

              Also, the fan might not even be the problem. If the car is overheating when you're at speed then fan is not the culprit and you should be looking at other things, namely the thermostat. If the car overheats at idle and/or slow speed stop/go driving and fan does not go on but car runs at normal operating temp when at highway/freeway speeds then the fan is most likely the culprit.
              Track Project DB2 #896
              LeMons Project DA9
              My OG DA9, Wrecked, Stripped, R.I.P

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              • #8
                Overheating occurs basically a few moments after being under load (once at operating temp) so if I'm merging onto the highway, passing someone or going uphill. So, say, I merge on to highway- about the same time I match speed with traffic, the temp needle starts to climb hot. A minute later goes back down to normal temp and stays there unless I decide to pass someone or put some load on. Same goes for red lights or stop signs- overheats after going thru 1st and 2nd but goes back down once I'm in 4th/5th cruising along. Can idle forever and never climbs. T stat is a new one AND I even took it back out and did a boiling water test.

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                • #9
                  If i drive just around town for about half an hour when I come back and park, the overflow is hissing and boiling, losing a bit of coolant from that cap.

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                  • #10
                    So no overheating at idle?

                    Make sure all those little fins in the rad are not smashed to oblivion, I had an overheating problem kind like you decribe and it took me wayyyyy to long to figure out the issue.

                    Also you can just jump the fan switch on the back of the block so the fans are on whenever the key is on, had mine like that for years and no issues.
                    Last edited by da6YO; 23 Apr 2013, 01:02:57.

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                    • #11
                      x2 on checking radiator fins, but when you say that it gets worse under load while getting on the freeway oor at freeway speed while passing... I am highly suspect of the exhaust gas scenario...

                      most mechanics will do a block test on the house or pretty damn cheap. you can also buy the kit and do it yourself..

                      ALso remove the thermostat while you are scooting around. see if you still get the issue

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                      • #12
                        Compression test.

                        Leakdown test.

                        Coolant system pressure test.

                        When I did my coolant system pressure test this past weekend, the book gave instructions on how to use the tool to diagnose a head gasket leak:

                        Hook the adapter to the radiator. With the motor cold, start the car. If the gauge on the radiator shows an immediate climb in pressure and/or the needle fluctuates with the RPM of the motor, you're getting a 'combustion leak' into the cooling system.

                        In a properly working system, it takes a bit of time for the pressure to reach normal. Operating temperature.

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                        • #13
                          Originally posted by bc_3s5 View Post
                          Overheating occurs basically a few moments after being under load (once at operating temp) so if I'm merging onto the highway, passing someone or going uphill. So, say, I merge on to highway- about the same time I match speed with traffic, the temp needle starts to climb hot. A minute later goes back down to normal temp and stays there unless I decide to pass someone or put some load on. Same goes for red lights or stop signs- overheats after going thru 1st and 2nd but goes back down once I'm in 4th/5th cruising along. Can idle forever and never climbs. T stat is a new one AND I even took it back out and did a boiling water test.
                          This sounds like your fan is not the problem. Unified 112 has some good info for you to pursue regarding a head gasket issue (which is pretty uncommon). The one thing you said which really doesn't make any sense is that you can idle the car "forever" and it never climbs. If you idle the car the fan should come on and if it doesn't then the engine should overheat. But possibly since you're in Denver it's cold enough where you don't even need the fans to go on. Regardless, all of that info is pretty clearly pointing out that your fans are not the problem.

                          When you boiled the thermostat did you make sure it opens up fully? Is it an OEM thermostat or a cheap one from the autoparts store? I've gotten them bad right out of the box before from the autoparts store. Once it seemed like it was working because it opened but later I discovered it wasn't opening enough. This caused fairly inconsistent and hard to troubleshoot overheating. Try like djzachster suggested and remove the thermostat completely to see what happens, what way you can rule it out.

                          Other potential problems could be a faulty water pump or, and lets hope this isn't it, some sort of internal engine issue. I had some strange overheating problems before my old b18a seized up on my. And it was only overheating under load.
                          Track Project DB2 #896
                          LeMons Project DA9
                          My OG DA9, Wrecked, Stripped, R.I.P

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                          • #14
                            Originally posted by Colin View Post
                            This sounds like your fan is not the problem. Unified 112 has some good info for you to pursue regarding a head gasket issue (which is pretty uncommon). The one thing you said which really doesn't make any sense is that you can idle the car "forever" and it never climbs. If you idle the car the fan should come on and if it doesn't then the engine should overheat. But possibly since you're in Denver it's cold enough where you don't even need the fans to go on. Regardless, all of that info is pretty clearly pointing out that your fans are not the problem.


                            When you boiled the thermostat did you make sure it opens up fully? Is it an OEM thermostat or a cheap one from the autoparts store? I've gotten them bad right out of the box before from the autoparts store. Once it seemed like it was working because it opened but later I discovered it wasn't opening enough. This caused fairly inconsistent and hard to troubleshoot overheating. Try like djzachster suggested and remove the thermostat completely to see what happens, what way you can rule it out.

                            Other potential problems could be a faulty water pump or, and lets hope this isn't it, some sort of internal engine issue. I had some strange overheating problems before my old b18a seized up on my. And it was only overheating under load.
                            Water pump has been replaced and the old one seemed to be in good condition but I digress.. It got replaced anyway. I will be trying no thermostat since I did buy this one from the parts store and don't exactly know how do gauge how far the t stat should be opening.

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                            • #15
                              If the rad fan is not turning on, then it is definitely a problem.

                              As mentioned, unplug the ECT switch and jump the connecter, if the rad fan turns on, replace the switch, if not, more testing of the rad fan circuit is needed. 94

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