Need Help: ZD28 Temperature Fluctuating/Red Lining at Start Up

I recently acquired a 2003 ZD28 72" pro deck. The unit runs and cuts well but I've noticed on start up that the temperature gauge gradually increases but keeps going to red line and will sit there for 3-5 seconds before quickly dropping back down. This will repeat about 5 times before settling in at the middle of the gauge.

I flushed my radiator and pipes and refilled with a 50/50 distilled water/coolant mix from the dealer. Suspecting a faulty thermostat, I also changed replaced it along with the gasket. The issue unfortunately hasn't gone away. My next thought was to replace the thermal sensor by the water pump right below the thermostat. When I looked up the part it was described as a "thermo-sensor switch". I'm not sure what it switches because the radiator fan and water pump and connected directly to the engine output.

The actual thermal sensor is located at the back of the engine and when shorting to ground the temperature needle jumps to red and when cool is around 200 ohms (to chassis ground).

The problem is very repeatable and not sporadic so I don't think it's an electrical fault. For those familiar with control loops, it resembles a saw tooth pattern before ultimately regulating itself. Any ideas on what is going on here? My fluids are all at the correct levels.

Ok, put the thermo sensor in a cold pot of water, hook up the gauge to it and start heating the pot. this will check the sensor and the gauge for movement. Should be fun hooking up the wiring for this test. ha ha

Only other thought is the wiring is corroded somewhere possibly causing more resistance in the system throws off the gauge until it gets hot enough.

Thanks for the advice, I will check the thermo-sensor. I basically did the same thing for the new and old thermostat to show that the new one is functioning properly (it opened sooner and quicker than the old part).

The thermo-sensor switch turns out to be connected to a loud horn. I unplugged the wire and tied it to chassis ground and set off the horn (scared the crap out of me!).

Too funny.

I had a mower that would not start and it turned out to be corrosion on the blades of the fuse.

New fuse and away it went, took 6 hours to find, because it controlled the fuel shut off and presented like a plugged carb.

Corrosion of wire connections and ground are common problems very hard to find.

Love the horn warning system. Imagine the surprise while mowing. ha ha ha

I ran the temperature trace this weekend and also monitored the sensor voltage to the instrument gauge. It shows the red line spikes are start-up and stablized after about 8 minutes. My white smoke is pretty brief now, I'm guessing my oil and filter change may have helped.

Here's the link to the graph & pictures. Let me know if this helps with the troubleshooting. One person had suggestion drilling a small hole in the thermostat to allow fluid to circulate all the time, and in my situation at start-up. I can see how that could fix my problem, but if there's something bigger going on I'd rather not temporarily "mask" it unless this is normal or a known design issue.

https://www.dropbox.com/sh/8g04sa7hu...iAU6G1F5Pp1Kwa

Your link doesn't work. (Error 404)

White smoke is normally a sign of unburnt fuel. Do you use the glow plug pre-heat?

Sorry, try this link.

Yes, I start by turning on my glow plugs for at least 10 seconds.

https://www.dropbox.com/s/h2fjh8azbkhvrat/ZD28 Start-Up.docx

Wow, that is great information. Well done. Never seen anything like that before.

Hole in thermostat will help prevent air pockets from causing air locks when changing coolant during the refill process. Lots of people suggest it, not a bad idea.

Still looks like a sticky thermostat to me. Reason being, the problem would continue if it were air-locks, blockage of any sort or restrictions. Even coolant flow issues with the water pump would be fairly constant if things like pump blades were broken.

The thermostat is the only thing that moves as the coolant starts to rise in temperature. Flow rate is relatively constant at idle.

Sticky thermostat is my best guess.

Just thinking here... Engine is just a pot of water trying to boil, rad cap keeps pressure up to 15psi to allow for 200+ degrees without boiling, rad hoses and rad all connected with thermostat in between to control temps. This all happens at start up and engine heats up at a set rate, nothing other than the thermostat opens and closes due to temp changes.

Unless...head gasket leak...that stops after warm up... perhaps re-torque head just in case. Not likely... (check a 900 manual, 47-50 ft lbs so the 1100 manual says, they are probably different)

Thanks for the comments SpudHauler. I'll certainly re-torque the head tonight. The thermostat was brand new, but if I have to open up that port again I'll probably disconnect my thermal sensor at the rear and flush from that side as well. When I originally flushed the system I didn't remove the sensor. I think I did a pretty thorough job but I can't really be certain how far inside the block (from the pump) I flushed. I could always drill a small hole in my old thermostat and give that a shot. I never found anything wrong with that part so I'm treating it as a spare right now.

Is it by chance like an old car where you can just keep the thermostat out and try it that way? Without the t-stat, temp should rise slowly (assuming cold engine)

I think I figured out the root cause of the temperature spikes...

When starting up the tractor the other day I noticed the radiator overflow tank was bubbling, but it was cool to the touch so it wasn't boiling. When the thermostat opened and coolant circulated the bubbling stopped. Looks like I have a head gasket leak? I took the overflow hose and put it into a bucket of water and confirmed that exhaust gases were leaking into the coolant. I'm going to assuming the temperature spikes at the sensor are due to the exhaust gases and aren't actual engine temps.

I guess the question is - if my temperature is stable when using the mower, no white smoke after start-up, and the bubbles stop once the coolant is circulating, how important is it to get this fixed ASAP? I would probably leave this repair up to the shop ($$$?).

Head gasket must be leaking slightly, it will only get worse.

Fix at the earliest opportunity will actually save you money.

Warped head and worse can happen.

They may want to plane the head even now to make sure it is flat.