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volvo 740 non-turbo overheating

deathlocdeathloc Member Posts: 3
edited April 2014 in Volvo
got the car and the second day it over heated. two years ago the engine was replaced. I met the mechanic and he's a moron. No thermostat swollen hoses of which I have replaced most of them. I took the car to get it inspected and the heat sensor blew out of the radiator while I was waiting in line. Does anyone have any suggestions? Temperature sensor was mid range when this happened.


  • VolvoRWDMechVolvoRWDMech Member Posts: 2
    1) If it "overheated" when the "temperature sensor" (I assume you mean the temp gauge) was mid-range then there are a few possibilites: The gauge reading incorrectly is a possibility. Very common on 740s to have a wonky gauge. Many times whacking the dashboard will make the gas and water gauges read correctly for a while. This problem is caused by loose contacts in the gauges and can only be fixed by taking the instrument cluster apart, taking out the gauges and tightening all the little screws in the gauges, then reinstalling them.
    Also possiblility is low coolant, where the temp sensor (which is on the cylinder head) is actually below the coolant level, so it reads wrong.
    A head gasket leak could also cause the coolant to vent (which might be interpreted as "overheating") resulting in low coolant and swollen hoses even when the engine was not really hot. Test for this by removing the coolant cap WHEN COLD and running the engine. If bubbles come up in the coolant tank, you have a leak in the head gasket or a crack in the head itself. Serious $$$.

    If the water level is OK and no head leak then the next question is:
    2) Does it only overheat when you are stopped? This is caused by a worn out thermostatic fan clutch, which is the aluminum thingy at the center of the plastic fan. They are supposed to let the fan "slip" when cold to spin the fan slowly, but tighten up when hot to increase airflow to make the radiator work better. They do fail and mystify people since they do see the fan spinning and think all is well.
    To check this start the car cold and wait. After about 15 minutes the fan should start making more noise when the fan clutch kicks in. If the engine is fully hot and then TURNED OFF the fan should be harder to turn by hand. If it is still easy then get one from a junkyard.
    3) You mention the "heat sensor" blew out of the radiator. I am assuming you mean the thermal switch at the top right side rear of the radiator, with the 2 wires for the ELECTRIC FAN, which is supplemental cooling to prevent overheating. (Hmmmmm....). The thermal switch could be bad. or the fan or (another common failure) the fuse for the fan has melted the contacts and there is no power for the fan to run. Test this by disconnecting the 2 wires, starting the engine, and touch the ends of the two wire terminals together. The fan (in FRONT of the radiator) should run. If not, either the fan is dead or the fuse (behind the ashtray) is faulty. If the fan does run, the switch may be bad.
    4) The lower cover under the front of the engine is not optional. It prevents air from being blown behind the radiator when driving which would reduce the air flow thru the radiator, causing poor cooling when driving at highway speeds.
    5) Your radiator could be clogged. Hard to check by yourself. Not so likely in a side-fed radiator (unless some bozo put in some radiator sealant.....)
    6) Every car has a spare mini-radiator. It is called the HEATER. If you open all your windows and put the fan on full and the heat selector to full hot, it will help cool the engine (while cooking you, unfortunately). This could help you get to where you can stop safely or just limp home.

    I would suspect #2 or #3 to be the most common problems. ALL 740s I have seen get flaky gauges when they get old.
  • deathlocdeathloc Member Posts: 3
    I knew about the gauge and was using the wacking the dash method. I'll check for the head gasket problem this weekend.
    Yes the thermal switch may be bad the fan works... and I'll check the fan clutch... this seems like a good target.
    flushed the system when I replaced the hoses... but didn't get the heater core flushed. system definitley had sludge in it.
    and that's how I got the car home in the first place...
    However, I was leaking fluid from behind where the thermal switch blew out is there an expansion plug there or something?
  • VolvoRWDMechVolvoRWDMech Member Posts: 2
    The thermal switch has a rubber gasket that it sits in and is held in place by a threaded ring like a screw-on bottle cap with a hole in it for the terminals to protrude thru. If the switch blew out the gasket probably exited the premises. Dealer item or junkyard only.
    Your car should be able to run at idle with the radiator reservoir cap loose without overheating if the outside temp is reasonable (below 70).
    Gunk in the system is not a good sign. Most radiator "flushes" are not back-flushes. and do not clear the junk out of the inlet side of the radiator (the thermostat side or upper hose side). Recommend removing the radiator and back flushing with a hose into the lower hole (outlet) to the upper hole (inlet). If your heater works no need to flush it.

    Another possible cause I forgot in the last post is: low flow from a rusted out water pump. This can happen when the impeller of a cheap after-market pump rots away due to not keeping fresh anti-freeze (and anti-corrosion) in the system. Everything is spinning but no water is pumped and so the water in the head boils. This pushes the water down and can uncover the temp sensor causing erroneous low readings and the whole slew of overheat problems. If your heater works this is probably NOT your problem as the pump is needed to force the coolant thru the heater core.

    BTW, whacking the dash is known as a "percussion adjustment" ......
  • deathlocdeathloc Member Posts: 3
    Thanks fixed those problems had to press in a new gasket and sensor... now I have to figure out how it's supposed to be wired... previous idiot disconnected the power line and I cannot find the other end.
This discussion has been closed.