AH64ID

I watched the video again, I really think its just a faulty thermostat, and maybe a slightly sensitive gauge. On a side note, get an alignment and fix your brakes! Running without a fan in summer is bad ju-ju!

My guess is that is was crap from new (something I read about more often with non-OEM thermostats). That photo is from the service manual I downloaded from the downloads section.

For some reason I read your posts as you referring to that little hose as the bypass.. Whoops!The spring may look like its good, but I am quite certain its fatigued. I highly doubt water flow is stronger than the spring. When I changed my OE thermostat it looked no difference, and felt no different than the new OEM one, but it sure works a whole lot different. You may never find out what is actually wrong, other than its worn out. The reason the temps read different is sensor location in relation to coolant flow with the bypass open, or closed is a little different (at least I would presume so).

I'm not talking about your external bypass, but the internal one. When the thermostat is closed the bypass takes the coolant from the head and feeds the water pump thru a passage. When the thermostat opens the thermostat will block that bypass and force the water pump to suck from the radiator, when thusly pulls coolant from the head. If the 5.9 V8 doesn't have the same internal casting it wont operate the radiator bypass properly. $30 vs 8$.. how much do you value your time? Even at minimum wage your time has exceeded $22 worth of labor. The symptoms you are describing are classic failed thermostat, high temp spikes and swings. I didn't realize it opened late, so weak may be the wrong term, so maybe slow is better.. My original tsat was very slow to react, big spikes, but it never dropped below 180*.

Are you certain the 5.9 V8 one is blocking the bypass? If not and your at a light load that could account for the more constant temp. I don't think that the boiling test really shows anything here. It seems to me that the "junk" stat is probably weak, and thus opening full sooner than it should and blocking the bypass and allowing full radiator coolant to enter the block. The OE thermostat isn't supposed to go full open unitl 203*, and it seems yours is doing it much sooner, and then is slow to close and your getting huge temp fluctuations. The 5.9 V8 stat may work now, but I am skeptical it will operate as it should when you put a load on the motor. Honestly, put a Cummins stat in and try it.

Yeah, I'm curious as well.. That load should make you exceed 1000 pretty easily, pre turbo. 1000 post turbo is 1300-1400 pre-turbo and that's closer to where I would think you are, but a bit hot.

Looking at the poll I wouldn't say there are any clear results as to it swaying excessively. I would bet most people are missing your comment on dropping to 150. Its not uncommon to see "sway" on these motors due to the size of the cooling system. But the sway you are seeing is not normal. I know they aren't super cheap but I would try a Cummins tstat. My stock one swayed a bit, but I put a new Cummins stat in and its rock solid. The only time there the coolant drops below 190* when the stat opens is when its cold outside and I am cruising at 40ish mph, the rest of the time it opens about 195 and drops to 191 and holds between 191 and 193.

But the stock thermostat is a NAPA one right? I read about lots of issues with non-Cummins thermostats..

Sounds like post-turbo temps?

I have noticed on my truck there is a 2-3 miles difference with the front on. The intake air temps are about 30* hotter and the EGT are warmer too, it all helps the tstat open much sooner for me.

So will mine, aren't G forces cool.

If you stop the pressure comes back up?

Do you have access to a test gauge?

That hose doesn't exist on the 24V or CR heads, so I am not entirely sure of its plumbing. If the thermostat bypasses that hose I can see how the 12V's would be very slow to heat.

I am still running the stock pump. I am not 100% sold on the need for air removal, but at the price of the platinum I don't think you can go wrong.

The Cummins thermostat incorporates two pins on either side of the thermostat that allow air to escape up thru the thermostat into the radiator to bleed the system of air. From the reading I have done most folks have issues with non-OEM (Cummins) thermostats. I also have found that our engines are very hard on thermostats. The cooling system is so large that unless your always loaded the thermostat only has to crack open to keep the block full, it takes a lot of heat to get it to fully open. This causes wear spots on the thermostat, which if they get bad enough can limit the amount the thermostat can open. This also fatigues the spring and makes it slow to react, causing the huge temp swings. Before I replaced my thermostat last winter I would get to 202-206 on every initial thermostat opening and drop into the high 180's. Now I get to about 193-195 and drop to about 190 in the summer and high 180's in the winter. My thermostat was only 5 years old with 52K miles on it, nothing crazy.

Nice score on the pump!If you ever want to upgrade I am very fond of the Platinum. It has better filtration out of the box than the AD, and the FASS HD and the same as the FASS Titanium, with 1 less filter. The cost is also the best of all the filtered units.

I don't have a clue on either, sorry. If your on CF you may PM cumminalong, he is the one I found out about the BW solenoid from.

Doesn't the 12V have an accurate oil and temp gauge on the dash?I would consider fuel pressure over those two.

I would guess that greater than 75% of this is here in the Boise area. The rest is WA, OR, MT, AZ, UT.

Which brings up the question is it all the time or just when the thermostat opens?

Sounds to me like you either have a damaged neck on your radiator or a defective cap.

There's a few of em up there.