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Hello folks.

While taking 'Lil Bastard' on a test drive I noticed the temperature went up to 210 degrees then dropped down to 200 when I thermostat opened up.

Is it O.K. to have a spike for a short while?

I know 200 is too hot to run at and I suspected the mechanical guage was inaccurate. I bought a new one and this one show a contstant operating temp of 190. I suspect these are crappy FLAPS guages and I not getting boil over. In fact, I can leave the cap off durring idle and the water never boils over. I have a new themostat and the system is clean.

Are these acceptable conditions for continued use?

Thanks.

Is it O.K. to have a spike for a short while?

I don't quite understand the meaning of your question here. Since you have already trouble shot your problem, determined the gage was in error and installed a new gage that shows a steady 190.

Are you trying to say the temp still goes to 210 but now drops to a steady 190 with the new gage? If so then it would mean the thermostat is about a little stiff or even sticking a little causing the spike. The thermostat should open slowly and start at a lower then normal operating temp so that by the time it is open fully the normal temp has been reached but not overshot.

Sorry for the confusion and yes it spikes and then goes to 190. Sounds like I need to keep an eye on it.

Is the thermostat and spacer US made or off shore cheapy?

I don't know. I think I got it at NAPA. I'll run another test tonight.

Ran another test last night. It climbs to 230 then drops. This is way too hot. Another problem is that it boils over when running at 3K RPM for a few minutes.

I'm going to start out by inspecting the radiator and thermo. I have an extra civvy radiator that is new and I'm going to use that until rebuild mine. Thermo housing bolts are leaking any way so I plan on pulling it and using a better sealer.

Water pump looked brand new when I installed it.

I had a new civy radiator I wasn't using so put that in with a new thermostat and that fixed it.

Thanks everyone.

Now the million dollar question is which item fixed it?

I thought it was fixed but I just realized there is something odd happening. When I warm the engine it will climb in temperature without stopping. I shut it down at around 220 in fear. The minute I fire it back up the temp drops.

This happens every time I start the engine. It's almost like some sort of back pressure is preventing the thermostat from opening and when I stop the engine it opens up. I know this doesn't make sense but it does this every time.

This is with a second brand new thermostat. The brand is NAPA and it's a 180 deg unit. I checked the bottom hose for collapse and didn't see that it was. I once again collected the coolant and opened up the system and checked to see if the bypass hose and ports were clear and they are. I flushed water through all the opening and everything is flowing.

This may be a foolish question but are the M38 and M38A1 water pumps the same? The reason I'm asking is I got my water pump new off of a M38 block that was not finished being rebuilt so I used the pump on my m38a1 and sold the m38 block.

Crazy!

The water pumps are interchangeable. Are you positive the pump is actually circulating coolant? With the cap off the radiator and the engine warmed up does the coolant show current flow? Are there air bubble in the coolant?

Do you have the thermostat and it's support ring installed in the correct sequence and positioning?



Have you pressure backflushed the radiator and block?

Have you done a compression check to see if maybe you have a head gasket leak?

Is the timing correct at 5 Deg BTDC. You can run slightly more advanced but shouldn't go more than 10.

Hi Wes.

The bubble thing is hard to pin point. When the thermostat opens I see bubbles but not extreme and it seems like it could be from boiling inside the block. There are no bubbles before the thermostat opens.

It's hard to see coolant flow because by that time the thermostat opens it's starting to boil.

It's an M38A1 so there is no support ring.

Radiator is new and block was flushed with freeze plugs removed. I don't know how to set the timing other than with a gun and it has been set at 5 deg BTDC.

Pondering this problem last night I thought of one possible cause. When the engine is running there may be a small exhaust leak into the coolant system. This creates an air pocket and prevents the thermostat from opening. When I shut down the engine the pressure from the exhaust is gone from the cooling system in the head and the coolant can rise to open the thermostat. Then when I start the engine again the thermostat is open so the air can escape from the head.

Are there any other indications that exhaust is getting into the coolant passages? There is no smoke.

I would expect to see bubble and quick warm ups if this is the case. I check to see how long it takes to warm up and if there are bubbles.

I bet if I drill a hole in the thermostat it will fix the problem but that doesn't explain why there is air in the system.

Ran another test tonight. Temp climbed to 210 in 8-10 minutes and I shut it down. Waited less than 30 seconds, started it back up and the temp dropped quickly to just below 190.

Sounds like a bad head gasket or God forbid, a cracked block. bubbles indicate compressed gasses from the cylinder or exhaust leaking into cylinder. Leak down test may be in order here to eliminate. I know you suspect water boiling to be causing the bubbles but in all my cases of seeing bubbles in an open radiator it has been a blown head gasket. I have been through this ten or so times on modern engines. Take the thermostat out and run to see if the temperatures stay low and look for bubbles. None should be present.

Here are a few basic notes to consider.

1-Water boils at 212 F.
2-Exhaust in the coolant will smoke and smell like exhaust. This problem is rare.
3-Compression leaks will show as bubbles in the coolant. Fewer before the thermostat opens because of the restricted flow through the water pump bypass hose (you should have this small 90 deg hose installed?) and faster after the thermostat opens.
4-Before relying on temp readings of a dash mounted gauge, test the gauge for accuracy. Put it's sensor in a pot of water with a mechanical thermometer (don't allow either to touch the pot) and check their readings as you bring the pot of water to a boil. If the gauge and sender are to remain in the jeep during the test just use a propane torch to heat the pot or can.
5-If you suspect a compression leak issue the leak down test is the way to pinpoint where it is coming from.
6-Air entrapment can occur easily if there's any point in the system higher than the level of coolant in the radiator. This high point will need to be burped during the filling process.

It's fixed!

Just a final post for anyone in the future facing this problem. I drilled a .100 hole in the thermostat flange that rests in the block and it's working perfect now. I tested it a few times and it never goes above 185. I can tell the thermostat opens because the fan starts blowing hot air from the radiator and the temp stops climbing. Also, I was able to drill the hole with a long drill bit without pulling the thermostat out.

I don't know if this is a band-aid or a silver bullet but I'm guessing...

1. There was air trapped just below the thermostat OR
2. More hot coolant was needed in the thermostat housing below the thermostat to allow it to open.

I watched the coolant flow while the thermostat opened and I didn't observe bubbles or flowing of coolant. When the thermostat opened I just noticed more turbulence and water jumps out from the fill hole like pressure is pushing from the bottom.

Thanks for your help in fixing this problem.