First post on this, or any other site forums. My apologies in advance if I am not using proper protocol. I have a 97 w/ 4.6 Romeo that has been giving me trouble for since I purchased it almost 2 years ago. It has about 280,000 miles, and no mods. About a year ago it started BILLOWING white smoke and was getting about 4 mpg. I knew it needed work when I purchased it, so I was prepared to put some work into it, but the main issue I had before all that work was the temp gauge that will spike to hot as soon as you turn the key. I replaced the following:

Plugs, plug wires, and both coils, (Not COP's),
Camshaft and Crankshaft Sensors,
Starter,
2 sensors located just behind the alternator, which I was led to believe are for coolant,

After I did the plugs, wires and coils, she wouldn't start, so I pulled the starter and had it tested at Autozone, and if failed so I replaced it. I determined that it was cranking, but had no spark, and eventually gave up and had it towed. $400 dollars of testing later, they must have cleand out some corrosion, because they were able to get her to start. She still smoked heavily, and they told me that the #2 injector was stuck open due to a short in PCM, and wanted about $600 to replace. Knowing that I was able to get a replacement for about $135, and 10-15 mins of labor, I couldn't see spending that much so I drove her home to do it myself. After replacement, she was running much better, but still have the temp gauge problem. After much searching, I found another sensor just under the intake manifold that I was able to reach by removing the alternator, and tried to test it. The Haynes manual says to pull the plug and see if it goes to cold with KOEO, but it is still pegged. She seems to run much better, but tries to stall when throttle is applied. I found some info here as to the "limp-home mode", if the temp reading was causing the rough running, but it has never thrown a code for that. The only code I get now is a P0118, ECT circuit high input.
Not sure where to go from here, as I'm not even sure I have the right sensor. The wire colors match the Haynes manual description for the Cylinder Head Temp sensor. I believe there is a short somewhere in the system, as I followed basic directions to test for voltage drop with a multimeter.

Any advice would be greatly appreciated. I am not working and cannot send her back to the shop. I am very new to any automotive repair, and have been able to do everything up until now by using the search engine here. Thanks

 

White smoke would indicate a water leak into the combustion chamber. Got an antifreeze smell coming out of the exhaust?

 

No, the smoke smelled like gasoline, and after I replaced the PCM and ran a few cycles it seems to have stopped. The garage had told me that the #2 injector was stuck open, due to the short in the pcm, and that seems to have cleared up. Just need to determine why the gauge is spiking, if that is what is causing it to run rough, and figure out if there is a short somewhere.

 

I think there are 2 coolant sensors on that engine. CHT & ECT the one your describing is the CHT behind the alternator not sure but I think the ECT is on the block on the left side..

 

I have found a total of three sensors that are in the area. The 2 just behind and to the left of the alternator have been replaced, and I just located the one under the intake, but have not replaced it yet. I was prepared to change it out (according to the colors on the wires it appears to be the CHT) and the manuals testing procedures are to disconnect the sensor, and turn the key on, and check to see if the gauge goes down, and even unplugged, if I turn on the key, it still pegs. Actually, if I turn the key on and unplug any of those 3 sensors, it pegs hot as soon as I turn the key.
Am I dealing with a short to ground somewhere? If so, do you know how I would go about locating and repairing it? I have a multimeter, but I'm not sure how exactly to use it.

Thanks