Mark Viii 4v swap to 98gt has caused overheating and I'm stuck plz help!!!

Feb 26, 2018
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#1
Hello everyone, so I have a 98gt that I 4v swapped a few months ago. I used low mileage motor out of a 97 Lincoln Mark Viii but I kept my GT wiring harness, changed injectors and called it a day. It was my first motor swap and I did it in the front yard so the fact that the car hasn't blown up yet is impressive. Anyways, one of the things that got left out during the swap was my temp sensor that controls the gauge. Which now that I am overheating (due to fan not kicking on, the car will idle fine for about 20 minutes before it overheats and fan never comes on) obviously makes things worse because I don't know until it's too late. My fuses are good and I think the fan is fine. Chances are CCRM, I did not put a thermostat in when I did the swap. This car is about to get dedicated to the track so to make things easy I am going to just wire in a manual fan switch this weekend which leaves this question...

Where is the sensor that controls the temp gauge located on the motor? From what I find online there is two sensors on each side of the coolant crossover. One is the coolant temp sensor on passenger side which I see no problem. The other is on driver side supposedly and controls gauge. A picture of the location itself would be amazing!!!

Also is the pic attached the sensor that controls that gauge? I know I will need to extend out the wires like I did for some of the other connecting wires but right now it's just floating around in the engine bay. Like I said I'm an enthusiast who is gaining more knowledge every day. I've only been working on cars for a couple years so bear with me here. Any help is greatly appreciated!!
 

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wmburns

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#2
Where is the sensor that controls the temp gauge located on the motor? From what I find online there is two sensors on each side of the coolant crossover.
To me the the sensor in the picture looks like the input air temperature (IAT). It goes into the input air stream to measure the temperature of the charge air.
Which now that I am overheating (due to fan not kicking on, the car will idle fine for about 20 minutes before it overheats and fan never comes on) obviously makes things worse because I don't know until it's too late. My fuses are good and I think the fan is fine. Chances are CCRM, I did not put a thermostat in when I did the swap. This car is about to get dedicated to the track so to make things easy I am going to just wire in a manual fan switch this weekend which leaves this question...
Can you explain exactly HOW the air was bleed from the cooling jacket? The DOHC 4V motor is VERY difficult to get the air out of. Any air trapped inside the coolant cross over will make the ECT sensor in contact with air thus making the ECT read low. The air will also block/restrict the flow of coolant.

If you really want to solve this some information is needed. For example what the PCM "thinks" the engine coolant temperature (ECT) is. Do you have an ODB2 capable of monitoring ODB2 PID's? If so, this could make short work of your problem. Even better is to have an ODB2 scanner capable of two way communication. Then you could "command" the PCM to turn on the fan. This would give a great trouble shooting tool.

Here's some information on an affordable Windows based unit. For only $30 it's almost a "no brain'er".

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
 
Feb 26, 2018
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#3
Thank you for the reply I believe youre right about that sensor. Do you know where the sensor is that controls the temp gauge so at least I can get an accurate reading before I blow a head gasket?

As far as bleeding the system, I haven't been able to get the breather to open so I am just burping the :poo: out of it. But either way that wasn't the cause of overheating in the first place because I drove it for months.
I see what you're saying with the sensor so I have a question, can I just pull that sensor? I'm planning on just wiring in a fan stitch worst case. I have a chromebook and it doesn't look like the scanner you recommended is compatible with it. I'm sure there is others that are, I spent some time googling them and wasn't able to find one that confirmed being chromebook compatible as well as capable of turning fan on/testing solenoids etc but I will keep looking. Unless by some chance you know one that is. But I am definetely interested in getting a scanner. I want my baby back on the road, car sounds so good.n Also at some point a tune will be added so this type of scanner or app will come in handy.
 

wmburns

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#4
The problem is the 1996-1998 Model year uses two different temperature sensors. One for the cluster and another for the PCM.

The ECT used by the PCM is the one that controls the cooling fan.

The separate temperature sensor drives just the cluster gauge.

I didn't answer your original question because I didn't understand WHICH function you were trying to restore. I figured that between the two senders the PCM is more useful because it actually controls the cooling fan.

The cluster temperature gauge sender wire colors are R/W and Y/R.

The PCM ECT sensor wire colors are LG/R and GY/R.

Note the two senders are NOT compatible to each other.
 
Feb 26, 2018
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#5
Exactly!! So the sensor that is on passenger side of coolant crossover is the sensor that controls fan (PCM). Which I lost my 15$ OBD tool meaning loaned it out and it never made it back. So I am going to find an OBD that communicates 2ways as previously discussed and I will see if PCM sensor throws code. My problem is the other sensor for the cluster they say is opposite from PCM sensor and I can't find it. While that is not the cause of my overheating I need to get it fixed asap so I can tell when car gets hot. I was planning on spending the weekend working on it but my boss called me into work (RAGE!!). But I am trying to get both fixed. Also my friend pointed out that with no thermostat the cluster gauge won't work. I don't think he knew that there is a specific sensor for the cluster but is that statement accurate as far as thermostat recording temp for cluster gauge? After this its fixing a control arm and then on to 5 lug swapping my fox so these two issues are really starting to become problematic
 
Feb 26, 2018
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#6
As dumb as it's gonna make me feel I need someone to take a picture of that sensor that goes to cluster so I can replace that one and go from there. Can't fix anything unless I can tell when car is getting too hot
 

wmburns

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#7
I don't have a two sensor car so I can't help you. So I guess getting the wiring colors isn't enough for you. How about you post some pictures of your wiring harness and note the wiring colors visible?
 
Feb 26, 2018
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#8
I don't have a two sensor car so I can't help you. So I guess getting the wiring colors isn't enough for you. How about you post some pictures of your wiring harness and note the wiring colors visible?



Yeah i tried looking at the car today but it has been storming where i live. I will get those pictures as soon as i can. Lwt me run this by you can I just wire in a manual fan switch? And then buy an aftermarket water temp gauge and adapt it to the radiator hose? That should fix my problem right? Other then that the car runs fine.
 

wmburns

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#9
It's your car. Feel free to hack up the wiring anyway that you want. I'm here to help people fix their factory cars. So I will pass. Thank-you.

If you don't mind me saying so, it seems to me that IF you are capable of wiring up a bypass solution it would seem that you have to skills to fix it right.

You stated the original factory harness was used. If so, the wiring should be there unless there's more to the story.
 
Feb 26, 2018
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#10
Well its for sure not a factory car, however I see what you're saying and I appreciate you trying to help me the right way. I have the stuff to wire a fan switch in but before I do that I will at least test fan with jumpers to see if it turns on. Assuming it does I will probably replace the CCRM with the one in my 98 donor. As of right now even with AC on nothing will get that fan going. Assuming one of those things fixes the fan issue I am going to just buy a temp gauge with sender and adapter for upper radiator hose. Sounds like that will get me a more accurate reading than the stock setup anyways. Also I have been doing a bunch of research on the bluetooth OBD readers so I will be buying one of those as well for future trouble shooting. This swap is going to end up at the drift track/racing events in the future my fox swap is more of a detailed swap where I want it all done specific and correct. Getting this thing dialed in well enough to beat on it at the track is the only thing standing between me and gettting to work on my fox swap, this car will never be street legal again so if I have to use the "good enough" method on a gauge or sensor I think I'll survive.
 
Feb 26, 2018
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#11
The other thing is I had it suggested to buy another coolant crossover with valve since I can't get my own to open. Even if I do all this stuff the right way it will be for nothing if I can't get the air out of the system.
 

wmburns

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Aug 14, 2009
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#12
FWIIW, many people report that the DOHC motor is one of the most DIFFICULT to bleed the cooling system. Often citing that it's impossible to fully bleed unless a vacuum purge before refilling is done.

Note, IF there is air trapped inside the motor the air will block the flow of coolant. Further the air will likely pool in the cross over. This makes the ECT in contact with air instead of coolant. This will make the ECT read low.
 

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