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I've also got codes for both O2 sensors but I don't know if those are failure points on this car or just bad codes. The exhaust manifolds may be leaky as well.
If you are getting a code for the heater circuits it's a possable problem with the engine computer. Just thought I'd let you know incase you replace the O2s and you still get the same result. Seems a few people over the years have ran into that happening on the MN12's, myself included. I would assume the Mark8 is the same because it has a simmilar engine computer.
 

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If you are getting a code for the heater circuits it's a possable problem with the engine computer. Just thought I'd let you know incase you replace the O2s and you still get the same result. Seems a few people over the years have ran into that happening on the MN12's, myself included. I would assume the Mark8 is the same because it has a simmilar engine computer.
They actually aren’t similar, so not necessarily. Despite being one of Ford’s most expensive cars at the time, the 93-95 Mark VIIIs used OBDI 60 pin PCMs, not the 104 pin OBDII PCMs all 94-97 4.6 MN12s did.
 

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Discussion Starter #24
I've also got codes for both O2 sensors but I don't know if those are failure points on this car or just bad codes. The exhaust manifolds may be leaky as well.
If you are getting a code for the heater circuits it's a possable problem with the engine computer. Just thought I'd let you know incase you replace the O2s and you still get the same result. Seems a few people over the years have ran into that happening on the MN12's, myself included. I would assume the Mark8 is the same because it has a simmilar engine computer.
That is highly useful information, thank you for sharing. The factory specs don't even list how many O-rings are on the car.
 

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Discussion Starter #25
I've also got codes for both O2 sensors but I don't know if those are failure points on this car or just bad codes. The exhaust manifolds may be leaky as well.
If you are getting a code for the heater circuits it's a possable problem with the engine computer. Just thought I'd let you know incase you replace the O2s and you still get the same result. Seems a few people over the years have ran into that happening on the MN12's, myself included. I would assume the Mark8 is the same because it has a simmilar engine computer.
For anyone in the future seeing this code, replacing the intake manifold gasket resolved the problem. There was nothing wrong with either O2 sensor
 

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For people who find this, and are looking for leaks:

An easy way to find vacuum leaks is to take a propane torch, unlit, and spray propane around the various areas that might leak.

If the propane gets sucked in, the fuel trims will respond on a datalog, and if it's idling rough, it will straighten out.

This is slightly dangerous, but propane doesn't burn any faster than you can move your hands, unlike starter fluid.
 
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