Nissan Frontier overheating mystery

I will probably rent one from autozone since tool rental is free. I rent tools all the time.
Is it pretty easy to use a timing light? If I search online for the procedure, will it be the same as any other vehicle with a distributor?

Pretty easy to use. Locate the timing mark and pointer on your engine. Put some white chalk on the mark (for example, 8 degrees before top dead center). Take the vacuum hose off the side of the distributor and plug it to disable the vacuum advance. Hook up the timing light to the #1 plug wire, point the light at the timing mark and pull the trigger. If the timing is off, loosen the hold down bolt on the distributor and turn the dist. to put the timing on the mark. Tighten the bolt and you are done. Also, rev the engine to make sure the centrifugal advance is working, usually to 30+ degrees.

I bought a timing light at harbor freight today after I swapped in the OEM thermostat. I forgot to mark the notch on the flywheel Iā€™m looking for and without it marked, I canā€™t see anything. I canā€™t see a single notch on the crank pulley. Iā€™ve got to run to a cookout and then itā€™s back out of town for work tomorrow so it will be Saturday before I get to try the timing light again.

Does it help to do it at night rather than during the day?

This is one of those things IMO you need to watch over the shoulder of someone experienced.
Mark the pulley and try again.
Doesnā€™t need to be night but needs to be in shade, like a shade tree.

I found white out a good way to mark the notch for timing.

There is a white mark on the pully that I made about 40k miles ago with white out.I dont remember exactly what i marked but I did it to line up the crank pulley when changing the timing belt. That white mark is about 1" to the left of the timing arrow on the block when I use the timing light. Iā€™ll give it another try tomorrow morning.

I just finished checking the timing and it is at exactly 10 which is what the sticker says under the hood.

If this problem started after you replaced the timing belt you should double check the cam timing. 15 inches of vacuum at idle (with the A/C off) suggests there is a timing problem. Look at the long term fuel trim on your scan tool. If one bank is greater than + or - 10% and the other is near zero the belt may be off a tooth on that bank.

The problem did not start after changing the timing belt. The last thing done to the truck before this issue started was a heater core replacement.

I am very sorry that we couldnā€™t help you with this problem.

Hook it up to the battery with the red and black clips. Attach the spark plug clip around plug wire 1. The timing marks will be on the crank pulley and pointer is usually on the block. Marking pulley mark with a little white paint helps.

Was the timing checked with the TPS disconnected?

That 15" of vacuum is still a concern. When the throttle is jabbed suddenly the needle should hit zero instantly and snap back to 15" if there is no exhaust restriction. That only weeds out an exhaust restriction issue but still does not explain the low vacuum reading.
That goes back to cam timing or a compression issueā€¦

Yes, the timing was done with TPS unplugged after the truck had reached normal operating temp and revved a few times.

Iā€™m not 100% convinced I am performing the vacuum test correctly. I think at this point I am going to take my truck to local family owned shop who comes highly recommended by some local guys in my off road club. I will tell him my findings and ask him to do a vacuum test to see what he gets. Iā€™d hate to replace the cats on a hunch. They will be extremely difficult to remove from both my truck and the donor truck at the salvage yard. Iā€™m sure new ones will be out of the question since my truck has four.

It would be illegal for the salvage yard to sell used cat converters.

You could unbolt the header pipes which then removes the converters from the equation and run a vacuum check that way.

So I would separate the headers from the rest of the exhaust, not the headers from the motor, right?

Just to updateā€¦

The Main (upper) AC line sprung a leak and the AC quit blowing cold so I parked the truck for the last 4 weeks.
Last night I dropped it off at a local shop who has done some work for me before. The owner called me today and said its going to cost $340 to get the AC blowing cold again and I told him to go ahead.
He said he gathered up his technicians and looked at the truck but could not find a reason why it would be running hot except for the body lift and bumpers. He did tell me something another shop told me, which is that in his opinion, 215-220 degrees is really not that hot, and that I really should not worry about it.
I think when I get the truck back I am going to drive the heck out of it until it starts steaming or warps the head, then Iā€™ll know 100% that I had a problem and I wonā€™t be any worse off than I am right now, which from my eyes is needing a motor swap or selling it for cheap.

You seem to be putting alot of money into a truck that isnā€™t worth that much anymore. I say that you have gotten your moneyā€™s worth out of it by now, and if I was you, I would have just dumped it quite some time ago.

That 15" of manifold vacuum is still low though.

The last thing I will do is have someone else do a vacuum test, just for fun.
After talking to Nisstech in Charlotte, NC I have decided to stop chasing this wild goose and sell the truck cheap. I will report back to this thread after the vacuum test. Itā€™s a shame the exact issue will never be known. This will bother me for the rest of my life as the unsolved mechanical issue of my very first vehicle.