I have a 1990 L98 with about 103K miles.  It is running hot at lights and city driving.  I've seen some older posts about this so I can see that its a known issue.  I noticed that the fans do not come on however, they do come on when I turn the air on.   So far here is what I did:  Had the coolant system flushed.  Replaced the thermostat with a 160 degree one.  I replaced both fan relays.  Replaced the coolant temp sensor (now after reading not sure this helps with the fan issue)   I am currently in Germany so finding a mechanic is difficult and getting parts can take long.   I am not a mechanic by no means either.  Is there a cooling fan temperature switch I should replace?  At about what temp should the one or both fans come on? Of course any help would be greatly appreciated.  

Cooling fans are direct wired to the A/C circuit so they automatically turn on when the circuit (including defrost) is turned on...that is normal.

The fact that yours turns on when A/C turns on indicates your fans operate correctly.

If the temp sensor is good (new as you stated), and fans works when A/C circuit is activated, my idea is to go to the fan controller itself, and wiring from sensor to controller to be sure all is properly connected and not burnt or broken.

If all wring is correct, r/r fan controller...which; is common to replace.

Thanks for the info.  So it appears that I can buy a cooling fan switch.  The 1990 and 1991 models were not equipped originally.  There is a plug on the drivers side between #1 and #3 spark plug where I would install it.  Then I also have to get a wire harness that connects to the 2 relay switches.  Do you think that this sounds about correct?  

What specific temps are you seeing that feels like are too hot? The main cooling fan typically does not come on until around 226f or so. The secondary comes on around 235f....which is hot, for sure, compared to what we normally see in other vehicles. Fans will come on with the A/C, as long as the A/C system is actually working.

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Had the same problem with my 91. It overheated and I had to replace head gaskets and have the heads machined, yet it still ran too warm around town. I di not get into any of the controls or wiring, I have read that going with a 160 Tstat messes up the computer as it is looking for 180. The issue resolved upon installation of a new radiator. I did that and the temp guage has not crept past half way since.

Suggest before you get into fiddling with wires and relays and switches , check the space between the radiator and the air conditioning condenser. This space can fill up with debris such as leaves , paper, nesting materials and must be clean. Then I'd replace the thermostat back to the original spec and replace the radiator. My experience has been that a 30 year old rad is gummed up and flushing it doesn't work. If the engine has not been modified there is no need for anything bigger than a stock rad. Hope this helps, It worked for me.

From experience with overheating in other vehicles, I tend to agree with Dave.  In most every case I've dealt with the culprit has been the radiator.  Flushing alone doesn't always do the trick.  And depending on where you live overhauling an old radiator can be almost as expensive as getting a new replacement (can't speak to being overseas).

When getting a replacement if there is room under the hood going with one that has one or two more rows of tubing adds more cooling.  And with all the sensors, circuitry, and computers do go back to the correct thermostat for the car.  Running it too cold does mess with things.

Jim Olson

 
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Thanks for the info.  I will have to see about getting a radiator sent here.  I think I found another mechanic that I can work with.  The last mechanic said that he did a pressure test and the radiator was still good which is why he recommended doing the flush.  He didn't say anything about replacing the old therm with the 160.  As far as I know, it could have had a 160 in it before.  I did order that switch already so I just might install it.  Its getting cooler here so its been running at max 1/2 way on the temp gauge with the defroster on.  Without the defroster on it comes very close to that shaded area.  On the autobahn it runs about 1/4 way.  Soon I'll be storing it for the winter.  Oh, all the debris was removed and I check it periodically.  

I can tell you now that I’ve done all of same things as you have and it came down to being the water pump. It would work but it had a leak, the leak wasn’t noticeable easy at all due to no dripping underneath the car. The leak was not a coolant leak but was in fact air leaking INTO the engine cooling system through a small small hole that are on near all water pump manufacturers for these engines and only a very very small amount of moisture would every now and then seep from the whole when there was no suction near the pump housing. Physics I tell you can be a huge pain in the A** when it comes to diagnosing problems on an engine lol but after all you’ve shared, you’ve left me with feeling very confident that your having the same issue that I experienced. Let me know if that solves your problem after replacing the pump cause it cannot be rebuilt. Very cheap fix by the way and not to difficult to swap out as well.

oh and one other thing, don’t let this fool you to think your fan relay switch or temperature sensors are bad because most likely they are not bad. The steam that an engine will build up due to air getting caught in your coolant system will get extremely rapid hit and cause an over heat but also cause sensors to not get hot enough and the reason for that is the steam pockets a lot of the time right near where GM likes to put there temperature sensors, therefore the coolant doesn’t fully contact the sensor enough to trip the relay and signal the fans to come on by themselves WITHOUT having to turn in your A/C.