Temperature gauge dropping to almost cold at highway speeds

I will be in GA for about a week so I will check for bubbles in the overflow tank with the truck running and the cap off and might also rent a set of cooling system pressure test equipment from autozone when I get back.

I did a compression test not too long ago and got 205 PSI in cylinders 1 and 2, and and 210 in cylinders 3 and 4. These were taken with the truck completely cold and throttle fully open. I also stuck a borescope in the cylinders to check for coolant and didn’t see anything reliably well. No misfires or hard starting or puffs of white smoke from the tailpipe or anything!

Unfortunately, it seems like there’s some combustion gas leakage into the cooling system. Will have to pull the head and see what it is. Hopefully it’s the gasket or head and not the block.

Thanks Joe, for the help.

A buddy of mine had an extra bottle of Blue Devil pour-n-go, and since I got it for free I thought. “What the heck, why not?”

if it works, it beats having to disassemble the entire top end of the motor to remove the head and replace the head gasket.

I retested the cooling system with the leak tester and my first test showed no leak. Will do a second test later today to see if the leak is present or I just got a false negative.

Will also see how long this lasts. I doubt it will be more than a year, but I think the leak was probably relatively small; the last two times the truck overheated I was driving at highway speeds for about an hour.

Below is the tester after 10 minutes with the truck running.

For reference, this is generally also applicable to a Mazda 3, but obviously there will be differing steps in places.

Did the head gasket job over the weekend. Overall not terribly difficult, but definitely not the most fun. In order to remove the head, basically the entire top end needs to be disassembled. Best way to approach this is:

1. Drain the block and radiator, remove both the upper and lower radiator hoses.
2. Remove the air intake boot and air intake. In order to do this, remove the drive belt and remove the power steering pump. Just set it aside, not need to disconnect it from the rack.
3. Disconnect all connectors and unplug the PCM connector, then set wiring harness aside
4. Remove A/C compressor (same as power steering pump, don't disconnect, just set aside), alternator, and the accessory mounting bracket.
5. Remove the valve cover.

Before you do anything else:

there is a very specific sequence to follow for this engine because the crankshaft timing chain sprocket is not keyed, which is literally mind blowing. One of the most asinine engine designs of all time. You need to either purchase or rent a Ford/Mazda timing tool set from AutoZone. First, set the engine to TDC at cylinder 1. There is a small 10mm bolt at the bottom left hand side of the engine, near the CKP sensor; remove that and fully insert a timing peg, then turn the engine clockwise until the crankshaft counterweight hits it. The engine is now at TDC. I recommend to use the thickest peg in the set (there are 3). You can be certain it's a TDC by: putting a long screwdriver in the spark plug hold in cylinder 1 and rotating the engine, and by checking that the notches on the back of the camshafts are aligned. The lobes on the camshafts will also be pointing down and outwards at cylinder 1.

6. Now that the engine is a TDC, remove the crankshaft pulley. The last photo is the tool I used to hold the pulley in place to prevent the engine from rotating. You can purchase one from eBay for $25. The inserts that come with the tool are too big to insert into the pulley holes, so you will need to purchase some bolts that will fit. NB: I recommended using the thickest timing peg above in case the sprocket holder slips and the timing pegs momentarily stops the engine from rotating.

7. Remove the CKP sensor, then the front engine cover.
8. Take a picture of the position of the camshafts for reference.
9. Remove the timing chain tensioner and the timing chain.
10. Remove the camshafts.
11. Remove the exhaust manifold. In order to do this you will need to remove upper O2 sensor.
11. Remove the head. Make sure to loosen the head bolts in sequence from the middle ones outwards. There are 10. Do them in pairs. You will need a T55 or T60 bit.

Follow these steps in reverse to reassemble.

Hi Joe,

Decided to pull the engine and I’m now working on a full engine rebuild. The service manual i’m using doesn’t have anything regarding removing the main bearing caps and pistons.

Would it be possible to post the steps/torque specs for the connecting rods and main bearing caps?

Also, are the main bolts and connecting rod cap hardware TTY or are they reusable?

Thanks a bunch for the info, Joe. I was able to dissemble the the motor today and will be sending the block and head to a machine shop to have the cylinder mating surfaces decked.

I still can't find any information regarding reusing the main and con rod bolts. I also can't find them available for sale anywhere. I've tried PartsGeek, eBay, and a few other auto parts suppliers.

Might have to call the Ford dealership, but I doubt that will help.

Thanks, Joe! Will come back around if anything else comes up. I managed to find a bootlegged version of the Mazda FSM and there is a section for bolt inspection. Attached for reference in case anyone needs the information.

The saga with this car continues. I had the engine out of the car and sent the block and the head to a machine shop.

They resurfaced both the head and the block.
I installed a new head gasket.

After all of that, the car still has a combustion leak in the cooling system. From my inspection of the cylinder block, it seems unlikely that the cylinder sleeves are cracked because there is no coolant in the oil, and they look fine to my eye.

In this case, should I suspect that the head is defective? EGR valve maybe? Or should I scrap the entire engine and set it on fire?

Unfortunately, now I’m not really sure if the machine shop actually tested the head or cylinder block for cracks.

The compression is about 190, 185, 185, 195, respectively. I suppose that at worst I can remove the head from the truck and send it to another machine shop so they can see if it’s cracked, which is probably my only real alternative at this point. I’m not particularly keen on pulling the entire engine out again.

Before spending more money, I wanted to make sure that there actually was a combustion gas leak in the cooling system. The reason why I thought there may be one is because after all of the hullabaloo I've been through, the truck is still overcooling after driving for a while, especially when driving on the highway.

I've tested the cooling system with the blue block tester fluid, and it will not change color. I've tested it as soon as the car starts to overcool, after driving it around town, and when dead cold, and it won't turn yellow. Also, the intermittent misfire I'd get on startup is gone, which was probably happening because of coolant leaking into one or more of the cylinders. And less importantly, but still relevant, I cannot smell any exhaust in the coolant overflow bottle. It seems that there isn't an exhaust leak.

I'm suspecting the thermostat, since there really isn't much else to suspect. It's a MAP-controlled thermostat, since it has a connector and a wire running to it. I've heard non-OEM map-controlled thermostats are kind of trash. Any words of wisdom for me? :)