91 VR4 overheating issue... kooky ...

[dudewith91vr4]

I bought this '91 3000gt vr-4 from the auction, 63k original miles. When I drove it home, it didn't overheat, however, it had an oil and water leak from the front side of the engine. My mechanic changed the water pump, front main seal and timing belt (ebay cheapo package), everything ran great for about 200 miles, no leaks. Recently, I took it for a drive and noticed the temp gauge went all the way up to red. Turned the vehicle off on top of a hill and coasted down the hill, all the way home, in neutral.

Novice troubleshooting: I've noticed that the cooling fans don't kick on, even when the temp gauge is at 3/4 to red. Once the temp gauge approaches two or three marks before the red, the cooling fans kick on for about 10 seconds (tops) and turn off. Coolant does get pushed into the overflow tank, and the heater feels cold. From a cold start, if I take the filler neck cap off and observe, I don't see bubbles until the temp gauge approaches about halfway or a little bit more, at that point, the coolant begins to boil, pop and bubble... almost as if the thermostat isn't opening, there is no coolant circulation, and the coolant is just boiling in the system.

Another note: From a cold start, if I let the engine just idle, it won't overheat. As soon as I rev it, or take it out for a drive, that temp gauge will shoot up to the red zone, fast. I don't notice any lack of power, and it doesn't burn thick white smoke, although there is some white smoke coming from the exhaust which looks like condensation. The driver side exhaust provides most of the flow (75%), and exhaust doesn't seem to flow at a very high rate.

I checked all of the fuses under the hood and beneath the driver side dash, they're all good. On the passenger side, I swapped the hi and low fan relays with the horn relay (the horn works), cooling fans still didn't turn on. Pulled the dipstick and took the oil cap off, no chocolate milk from a severely blown head gasket. I replaced the thermostat and refilled the system, still overheats. I have no idea where the coolant temperature sensor is on this vehicle, can someone point out where, and how to get to it? I just don't want to go on a wild goose chase. I've seen stories of guys changing the head gaskets and resurfacing the heads, only to continue to have the same problem. As always, any help is greatly appreciated.

[sergechronos]

Cool temp sensor is on the waterneck near where the thermostat is. If you check on some of the vendor websites (3SX, Ninjaperformance, etc.) they have factory service manuals on them that you can download for free.

Are both of your hoses to and from the radiator getting hot? That would tell you if the coolant is actually making it to the radiator and back or not. You could hotwire the fan to force them to come on, but that's a bandaid to the solution.

[dudewith91vr4]

Thanks for your prompt reply, Serge. Eventually, both hoses do get hot. Would you go after the coolant temp sensor, first? I did notice that the vehicle was missing at the same time the overheating problem began to occur. It was running like a rocket, nice and cool, before this sudden kooky overheating issue.

[CoreyB]

The fans on a 91 are actually controlled from temp sensors installed in the radiated, so if the fans don't come on there is a good chance you don't have power or ground to the radiator.
The other possibility is the thermostat, have you changed that out yet?

[sergechronos]

Well, you can check the service manuals (or google) to see what kind of resistance you should be seeing out of the ECU sensor in the waterneck and out of the fan sensor in the bottom of the radiator. If you don't want to do that, you can replace them both (which would run you about $100.) That said, you could also get a tool to pressurize the cooling system and check for leaks. If you find nothing, then refill with a 50/50 mix and see what happens.

Given that it wasn't an issue before the timing components were done though, I'd suspect something that was removed or replaced during that service may be at fault. I'm not a fan of non-OEM timing components. Non-OEM water pumps frequently use cast impellers which can't flow as much coolant (and non-OEM tensioners a risk to fail.)


Edit: For clarification, the reason I would suggest replacing the ECU sensor in the waterneck is you said that it started running rough when this happened. ECU adjusts fueling based on temp of coolant, while this is unlikely to be the only cause of the rough running, it is a possibility and easy to replace when the system is drained of coolant.

[CoreyB]

I normally would point to the engine coolant temp sensor being faulty....



But it in no way controls the gauge or makes the engine over heat to the point of pushing coolant.

[dudewith91vr4]

Corey, yes, i changed the thermostat. Serge, I think the ecu coolant temp sensor is screwed into the intake, beneath the plenum, it's actually moist, i can see it through the plenum. I found an online manual for locating the sensors, will report back.

[box]

The first thing I'd be doing is checking the actual coolant temperature, either by sticking a temperature probe/thermometer in the coolant or checking the current engine data with a scanner. For all you know, it could just be a gauge/gauge sender unit fault. Fans should come on at around 95 degrees Celsius.

There are 2 temperature sensors in a 1st gen. A single wire unit on the thermostat housing which is for the temperature gauge, a two wire unit also on the thermostat housing which is for the engine ECU EFI control, and then you have a pair of two wire switches on the bottom radiator tank for fan control. One for the low fan speed, one for the high fan speed. The only sensor under the plenum is the knock sensor, if it's leaking then it should be replaced.

There are two relays in the engine bay fuse box for the radiator fans, one for low speed and one for high speed.


If you get to 100 degrees and the fans still don't turn on, pull the low speed relay out and bridge pins 30 and 87(the two big terminals) with a jump wire, this should force the fan to turn on. If it does, you know you've got a fault on the control side of the circuit.

From here you could unplug the wiring to the radiator switches (there's a white plug and a black plug off the top of my head) and bridge the terminals, this should activate the fan relays (high or low depending on which switch you bridge). If the fans turn on, you could assume the switches are faulty.....or your radiator is full of aerated water. Switches are easily tested by removing them and checking continuity with a multimeter while heating the switch body, the circuit should close when they reach the appropriate temperature.

[dudewith91vr4]

Thanks Box, and everyone else; i really appreciate your advice, I'm clueless with this car. How do i get to the two fan sensors on the bottom of the radiator, do i have to remove the entire front aero cover or is there a way to get to the wiring and sensors without doing so?

Box, pretty sure the vehicle overheats, coolant gets pushed to the overflow reservoir. So far, I've changed the radiator cap and thermostat, and noticed the heater got warm, not hot, but warm, whereas before, it was cool. One of the small hoses is leaking too, I mean little bubbles, I'll trim the hose or replace it, but first, I'd like to at least take a look at the wiring leading to the two temp sensors on the bottom of the radiator.

[box]

Box, pretty sure the vehicle overheats, coolant gets pushed to the overflow reservoir.

Never assume anything Check it, rule it out.

You should be able to get to the sensors if you unbolt the engine bay fuse box and remove the overflow bottle, you might even have to remove the front pre-turbo intake pipe. The sensors are on the RH bottom corner of the radiator lower tank, you might have a hard time unplugging them though since you can't actually see them.

Get to them from underneath if you can. You might not have to completely remove the tray, you could possibly just drop the back of it down a bit and get in behind it.