Thermostat Drilling

[Ecomike]

If you take the time to bleed the system properly, there's really no difference. What drilling does is allow you to not have to bleed the system manually - and it provides a small safety net if, say, you should get a compression leak into the cooling jacket (I've seen it happen on industrial equipment, and most of us use our rigs about the same way.)

Since you're supposed to leave some air space in the volume tank anyhow, you can just set the system up to bleed itself into that airspace. But, if you bleed properly (sounds like you do,) it's really not necessary - especially if you don't lose your head gasket! Once the system has purged itself of air, it shifts from "a useful two minutes" to "cheap insurance" for the 'just in case' eventuality.

You don't need to add an actual bleeder valve to complement it. If I were going to modify the engine casting proper, I'd probably look into doing a bypass around the thermostat ( Insert, Note that it does already have a bypass, see my comments below, it goes directly to the bottle side entrance) at 12:00 that would remain sealed - and do much the same thing. I just find it easier to drill out the flange, instead.

Neither idea is "right" or "wrong" - it's just what some people find easier to deal with than others. You probably automatically bleed the system after service, and have the time (even if it's only five to ten minutes) to do so. With my situation, I need to get jobs resolved as quickly as possible so I can move on to the next crisis...

A couple points of clarification. Mikeforte mentioned drilling the thermostat housing, not sure where that come from, unless there was a typo somewhere as there is no reason to drill the housing itself, unless he is calling a part of the thermostat itself a housing? If so it is confusing nomenclature since there is a seperate thermostat housing. Sorry to be picky, just trying to be real clear for future newbies!

The thermostat housing and engine block thermostat flange opening have a huge thermostat bypass that feeds the heater hose, 3/4" at that location, and when the heater hose is plumbed properly it pushes water constantly to the bottom of the heater core (5/8" hose) and to the upper side of the plastic bottle air - coolant separator (1/2" hose), thus flushing the thermostat housing's rear side of any air even when the thermostat is closed. This happens even before the thermostat opens, and continues with the thermostat open based on the elevation of all the parts involved.

With 2 bleed holes in the thermostat itself perhaps the air bleeds from the upper radiator hose backwards into the thermostat housing, then backwards through the top bleed hole, and then to the plastic bottle through the heater hose, but I think it would do this anyway once the thermostat opened, given enough air in the upper radiator hose.

Typically bleed holes in the thermostat are used to remove air from the bottom (or back) side of the thermostat so it can see, make physical contact with the liquid coolant, thus opening the thermostat on a timely basis. This is important in some engines, but the 4.0 in opinion based on my inspection of the physical design of the hardware yesterday, does not need the bleed holes, but what it does deperately need is to have the two heater hoses on the engine block plumbed properly to the water bottle!!!!!!!!

If it is plumbed improperly, i.e. heater hoses are attached backwards, all the thermostat holes in the world won't fix it, as it will continue to reintroduce air into the water pump. Lastly one should fill the system with coolant while the radiator upper hose is disconnected at the thermostat housing, and with the plastic bottle cap removed, as this will immediately remove 99% of any air in the block or the radiator during filling. Lastly, their are few warnings or picutures posted anywhere, if any, about how easy it would be to connect the heater hoses back wards at the engine block, near the thermostat housing, especially if you are not using OEM stealership hose sets. If not using the OEM hose set, there are two places where the one can connect the hoses backwards. The second place being at the metal hose T's or Y's.

 

[xjcruiser90]

If understand this, you can tell that your return hose is on correctly. With the top of the return bottle open you will see water entering through the side inlet. That way water is drawn into the system from the bottom of the plastic tank. Correct?

I’m going to assume that if the factory thermostat had a hole in it, then it’s a good idea to copy what they did. They aren’t going to bother spending money on tooling to drill a hole that isn’t needed; the bean counters would have nipped that in the bud.


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xjcruiser90
http://xjcruiser.com/xj_thermostat_replacement.php

 

[Ecomike]

If understand this, you can tell that your return hose is on correctly. With the top of the return bottle open you will see water entering through the side inlet. That way water is drawn into the system from the bottom of the plastic tank. Correct?

I’m going to assume that if the factory thermostat had a hole in it, then it’s a good idea to copy what they did. They aren’t going to bother spending money on tooling to drill a hole that isn’t needed; the bean counters would have nipped that in the bud.


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xjcruiser90
http://xjcruiser.com/xj_thermostat_replacement.php

Correct, it is most noticable, the inlet flow to the bottle from the side, when you rev the engine to say 2500 rpm.

I would not count on logic, cost savings, good engineering, or bean counters having anything to do with what ends up under the hood:exclamati

 

[Ecomike]

once the thermostat opens it pulls water from radiator and through motor then back to radiator if your system was low and needed to burp then it will have done so. your vehicle comes with a bleeder valve its called a radiator cap.and if your getting a false reading then your temp sender is bad. it is also not located at the thermostat housing.

You might want to reread this entire thread and several similar threads on the Jeep closed (87-90) and open (91 on) cooling systems, and the 3 variations over the years in sensor locations. I think you missed a few things from your comments. The 87-90 years used a closed system, had no radiator cap, but had a plastic pressurized surge bottle with a vent cap. Somewhere around 96-98 (not sure of the exact year) they went to a single temp sensor in the thermostat housing feeding data to the PCM which runs the temp gauges in the dash, and the electlric fan and feeds the ignition data input. Many people have reported having to bleed air from the block at the temp sending unit itself on older, pre-96 models, to get reliable temp data even from a brand new sensor.

 

[Davinator61]

Just finished testing out my '91 Cherokee's new thermostat installation I accomplished earlier today, including the two 1/16" holes drilled at 12 and 6 o'clock, and all I can say is WOW, that made a big difference in engine operating temperature stability. I too was dumb enough to try a Mr. Gasket High Performance Thermostat and found, much to my dismay, it seemed to open at 25 degrees or so less than its 195 degree rating. The replacement is just a NAPA/Stant Premium thermostat at about $9.00 or so. I found that my engine temperature goes up to its operating temperature range faster now than it did just this morning, when I went to pick up the replacement t-stat, and stays there more readily. Of course the test drive was accomplished during the last hour or so, during the early afternoon but the results are still promising IMHO. Just My-Two-Cents-Worth...

 

[Muddy Beast]

Anyone have pictures of the thermostat after it was drilled?

I can't seem to figure out if I drill the brass piece out or not, and what it will look like in the engine block...

~Scott

 

[TORX]

Scott, I just got done putting in a new t-stat today. I drilled two small holes (3/32nd) on opposite sides of the mounting flange. Once you get it the old one out you'll see where to drill the holes.

Quick pic of my old one (stuck open). The red dots are where I drilled the new one.

 

[Muddy Beast]

Well unfortunately I pulled the old one out last night and forgot which way everything was facing. And my new one doesn't look like my old one, as the top cap has a three way cross rather then on cross on the top.

~Scott

 

[TORX]

They're both the same. Different brands use different designs. As long as the outside diameters are the same you're good to go.

Spring side goes towards the block.