Help Needed - Running Hot After Heads Replaced

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Scjarena

Newbie
Joined
Mar 7, 2023
Messages
7
Reaction score
13
Location
Houston Texas
1998 5.7 Rebuilt about 75K miles ago, running great until a few weeks ago, radiator had leak and overheated engine slightly which cracked head. I put in a new radiator, ran cool like normal, but had engine miss due to leaking coolant in head. Had 062 heads, machine shop found me 906 heads, mag checked, valve job, surfaced, etc. Put everything together, temp gauge keeps rising past 210 degrees. Tried bleeding air out system several times, ran with heater on, cracked lines, etc. Pulled thermostat, put in boiling water and it opened. Top hose going into radiator feels hot, but air being blown thru radiator doesn't feel that warm. On another car, I once had an impeller come off a water pump, but the odds of this being the problem, now? Just not too sure about that. Any ideas would be appreciated. Need to get this back on the road.
 

Scjarena

Newbie
Joined
Mar 7, 2023
Messages
7
Reaction score
13
Location
Houston Texas
Ok, I decide to take out the thermostat for testing. Start it up and immediately temperature gauge reads in the normal range. Under the hood, I can see the water flowing good in the radiator and hot air is coming out the back side of the fan. So apparently the thermostat even though tested in a pot of boiling water didn't work like its supposed to. Off the parts store to get a new thermostat.
 

Scjarena

Newbie
Joined
Mar 7, 2023
Messages
7
Reaction score
13
Location
Houston Texas
After a new thermostat, its beck to spiking up in temperature. Factory gauge up to 210 and climbing and then I have to shut it down. Removed thermostat again, took it for a test drive for a few miles, temp stayed unusually low, which is expected. I've been messing around with cars for many years, this one has me stumped.
 

TreeGeared

I'm Awesome
Joined
Apr 18, 2022
Messages
112
Reaction score
194
Location
Ohio
Bleeding air out of cooling systems can be harder than most people think. Here are a couple of tricks.
-Buy/rent a coolant system pressure checker. It is just a small pump that attaches to the radiator and pressurizes the system. If you have a leak air could be entering the system hindering proper cooling flow.
- Remove thermostadt, sounds like you have tried this.
- If thermostadt does not have a jiggle valve to let trapped air by drill a 1/16 hole in the thermostadt for air to bypass.
-Raise front of vehicle and run with radiator cap off. Should be able to hear/see air bubbles. Add coolant as needed.
-You could take it to a shop that has a machine to evacuate the system and pull a vacuum. This will also tell you if there are any leaks in the system. When they pull a vacuum they can add coolant to the system under those conditions and you should have very little if any air in the system

Hope this helps.
 

evilunclegrimace

Does not always play well with others
Joined
Sep 18, 2017
Messages
2,288
Reaction score
2,332
Location
pennsylvaina
Drill and tap your T stat housing and install a 1/8" bleed valve. That way you can bleed air out of the system while it is closed.
You must be registered for see images attach
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,966
Reaction score
13,745
Location
The Seasonally Frozen Wastelands
First Guess: Faulty thermostat. Getting to be common. The last 'stat I put in the K1500 does exactly the same thing. Temp goes beyond 210, then drops suddenly to 170. Back to 210-ish, down to 180. Up to 200, down to 190. And from there, it's fairly stable.

All the air in the cooling system moves to the radiator the first time the thermostat opens after cooling system service, at least on those vehicles where the rad cap is the high-point of the system. Drilling holes in thermostats, adding bleed valves is totally unneeded on these trucks.

However, if a person insists, Robertshaw themostats and the Communist knockoffs have an ultra-tiny bleed stamped into the thermostat housing. Passes air easily, passes very little coolant.

Photo: Robertshaw bleed opening vs. 1/8 drill bit
You must be registered for see images attach
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,890
Reaction score
7,539
Location
DFW, TX
Unlike Shurkey, in the climate we have here, I ALWAYS drill my thermostat with a 1/4" hole. I still have good heat in a little over a mile on cool mornings. 1/4" is not enough of a hole to slow the engines warm up any meaningful amount, but it keeps some coolant circulating past the thermostat allowing it to regulate temperature more evenly as well as helps stabilize the cooling system by allowing a small flow of coolant 100% of the time through the radiator. Coolant continually flowing through the radiator helps prevent the coolant from becoming ice cold in the radiator and shock cooling the engine when the thermostat opens. One of the best things the engineers have decided to do recently was adding the thermostat to the inlet radiator hose allowing bypass coolant temperature to operate it, the temperature is controlled by the thermostat admitting coolant into bypass coolant flow rather than by controlling hot coolant escaping the block.

Murray Ultra 195F drilled with a 1/4" hole is what I have in my 97 Express. Very smooth warmup and very stable coolant temps overall.
 
Last edited:

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,966
Reaction score
13,745
Location
The Seasonally Frozen Wastelands
The reason folks drill thermostats is because Thermostat Engineers AND the folks who design the engines, don't believe they're needed, so the thermostats don't come with holes in them (other than miniscule vent holes in the Robertshaw and clones.)

Some engines have "jiggle valves" in the thermostat, but they CLOSE when the water pump starts to move coolant (when the engine runs.)

Anything more than a tiny vent in a thermostat is not needed.
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,890
Reaction score
7,539
Location
DFW, TX
The reason folks drill thermostats is because Thermostat Engineers AND the folks who design the engines, don't believe they're needed, so the thermostats don't come with holes in them (other than miniscule vent holes in the Robertshaw and clones.)

Some engines have "jiggle valves" in the thermostat, but they CLOSE when the water pump starts to move coolant (when the engine runs.)

Anything more than a tiny vent in a thermostat is not needed.
Having personally witnessed no less than a dozen thermostats behaving in the manner you describe go to perfect control with a small hole, I disagree. I tried 4 different thermostats including an OE GM one in my TBI van back in the day with the same results you saw. A race engine builder that was a customer of mine suggested the hole to me. I drilled the hole and will NEVER not drill one now. The thread creator lives in Houston as well so warmups will not be changed enough to even notice. A 1/4" hole will only help by keeping a small amount of coolant in constant circulation. The 1/4" hole also prevents un-wanted air and steam pockets from forming during warmup under the thermostat which is the highest point of the coolant system during warmup. It only takes a small pocket of anything other than coolant under the thermostat to prevent it from opening at the correct temperature. GM has a similar volume of coolant flowing continuously through every GenII LT1, GenIII or GenIV LS and GenV LT engine in existence as well via the steam pipes. Even the L98 Corvette had a steam pipe off the back of the intake with constant flow. So while the engineers did not drill the thermostat to accomplish a small continuous bypass they did create an additional small continuous bypass none the less. The GM FWD 60*V6s had a continuous bypass to the degass bottle from the intake as well. Easiest way to duplicate what GM did on the newer engines is to drill a similar sized hole in the thermostat. It accomplishes the same thing with no additional plumbing to leak. Every SBC I have owned since ~2005 has had a 1/4" hole in the thermostat, they still warm up quickly and the thermostat is able to open gradually without a large temperature spike and subsequent drop along with the yoyo temperature variations you observed until it finally warms up enough to stay partially open.

As cheap as thermostats are, I would suggest he get a new one of decent quality like a Murray Ultra, put a 1/4" hole in the front of it and give it a try. The results will probably speak for themself. I 1000% believe it will cure his problem.

When I later put Vortec heads on that same TBI engine in my G20 van nearly 20 years ago now, I never even plumbed an external bypass minus what went through the 4 port heater control valve to the radiator and the 1/4" hole in the thermostat. It never had temperature spikes or the gauge yoyoing on warmups either. Just came quickly up to temp and stabilized.

These same engineer types you speak of created displacement on demand lifters, intake valve carbon choking direct injection, pellet box cats, efe grid heaters and the oldsmobile 350 diesel as well as well as many other garbage flops like the TH200C.
 
Last edited:
Top