Overheating issue - I am stumped!

[Mattchu60]

Hey Folks, hoping for some ideas here. I have a 87' TBI 350 truck that last week i swapped a rebuilt 350 sbc into. The swap went fine and everything got hooked up ok. I am having a super-frustrating/scary issue that I cannot figure out.

Heres what happens - I start the truck up from sitting cold, it warms up to the normal operating temp and just keeps going up, up, up until she hits almost 250 degrees, hangs for a bit and comes down to 200ish for the rest of the time. Its pretty dang scary to watch it happen, once I see it start climbing past 230 I usually shut the engine off because it scares me so much.

Its not a really wild engine, I estimated the CR around 9:1, it has a pretty mild comp cam in it (12-388 i believe is the number), summit 72cc heads and flat top pistons. It shouldn't be putting out a lot of heat.

My first guess was air pockets in the system, so I sat it on a hill and ran it with the cap off to see if any air came out, no change.

Last night I decided to replace the thermostat and also the sender for the gauge to see if that made a difference. After refilling with coolant through the thermostat hole on the intake, then topping it off in the radiator (i wanted no air in the system!) i went for a drive, i was disappointed to see the same result again - climbing temps to a very high point, although now its not settling down as quickly as before.

Heres the kicker - I have the EBL flash system on my truck so I could use my TBI still. I get a full readout from all the sensors the ECM uses on my laptop - and the Coolant Temp Sensor on the intake manifold is not getting any higher than 210 degrees at the high point. This information makes me think its a sender/gauge issue for the unit in the cylinder head (TBI has one sender for the gauge in the head, one in the intake for ECM).

I am confused and frustrated. My father said to just ignore it, but its pretty darn hard to ignore watching the gauge climb so high, because I have no idea if its sending false signal or maybe its right.

Any thoughts are appreciated to help me figure this out. I may try removing the thermostat all together to see if it makes any difference.

[Front Range]

Did you make any other changes besides the motor when you did the swap? No kinks in the hoses or changes in routing, same water pump, same radiator? What temp thermostat? Is the sender for an 87 TBI smallblock?

The more detailed info you provide the more likely someone will spot the problem.

[rich weyand]

Check the instructions to see if those heads require an external bypass like the vortec heads do. Lack of a bypass (internal or external) will give that behavior. Some heads (like 882s) have them, some heads (like Vortecs) don't.

Do you have the heater hooked up? Do you have an on/off valve on the heater hoses? The heater core can act like an external bypass, but not if you have an on/off valve on it.

Which thermostat is in it? Stock is 195, but you could go to a 180.

[rich weyand]

The other question is does the temp sender match the gauge? The resistance curve of the sender unit must match the resistance bridge in the gauge. So which gauge are you reading this on, and which sender are you using?

[Mattchu60]

More info as requested:

As part of this swap I also put in a new:

- Water pump
- radiator
- 195 degree thermostat

The sender for the temp gauge was reused at first, same one that came out. Was very careful when removing the engine and all the wiring, labeled everything and reattached it in after installing new engine. Temp gauge sender is one wire that attaches to the sender on the head. Same result with the old sender and also the new sender for this gauge issue, goes to 250 and then falls back to normal temp after a bit. After it does this once, its done and temp is normal the rest of the time I drive it. Just happens when it goes from a cold startup.

heater hose is in the intake, i have been running it to try to bleed off heat/work any air out the of the system.

There were no instructions with the heads for an external bypass, none that I saw anyways. They are summit racing brand heads - 72cc chambers. Apparently they are the same as Dart Iron Eagles (thats the rumor)

[Nick_R_23]

Is your water pump the correct pump for your engine? (Forward vs Reverse Rotation)

Is the water pump impeller intact and undamaged? Physically check, don't assume because it's new.

Is the thermostat opening at the correct temperature? Verify what temp it's opening at in a pot of water on the stove.

Is the fan the correct one for your engine? (Forward vs Reverse Rotation) And is it facing the right direction?

[skeet85silverado]

Check your fan clutch.

[Mattchu60]

I am going to remove the T-stat this weekend and run without it to see if it changes anything.

Not sure how to check the fan clutch - I used the same one I took off - I know its on the right direction because it only mounts one way.

Not sure if you all saw this part - but I am monitoring the coolant through my CTS Temp sender on my laptop (I have EBL tuning software) - the CTS is showing ok temps during this issue - range is 190-210 degrees typically. CTS sensor is in the intake next to the T-stat.

I wonder if its a bad ground? I put water sealant on the threads of the sender for the head, heard that could disrupt signal, but I wouldnt think it cause this type of issue.

The odd part - this only happens during warm up, once it does this overheat issue, it settles to 200ish and stays there for the rest of the drive.

[rich weyand]

Which head gaskets did you use? They could be blocking the internal bypass.

It is basically overheating locally until the thermostat opens, and that makes me think about the bypass. The bypass (internal or external) allows coolant to circulate within the engine when the thermostat is closed. Though the heater should be functioning as a bypass as long as the heater hoses are connected to the right spots and there is no shut off in the heater hose lines.

Post a pic of the heater hose connections on the engine end so we can see what you got there.

[Dead Parrot]

Might consider buying an aftermarket temp gauge. Install its sensor in the normal place and see if the new gauge readings replicate your current setup. If not, you are chasing a sensor/wiring/gauge/corrosion issue in your original setup. If they do, you have some kind of water flow problem.

[tucsonjwt]

Step 1 for me would be to get an infrared thermometer and check several places on the engine to see how hot everything really is. I usually shoot it at the thermostat housing to get an idea of the coolant temp. You may just have a bad gauge.

If you have a standard fan clutch it should engage when the engine temp increases to the specified temperature. I have never had any luck with flex fans.

Also, I had the same symptoms on my 2001 Cavalier recently. The temperature would go up to around 230 when idling or on the highway(counter intuitive). I replaced the radiator as suggested by a local mechanic and that seemed to fix the problem.

I would use AC Delco or OEM GM parts whenever possible for the cooling system. Most mechanics around here won't install anything but a ACD of GM fan clutch (they are pricey - maybe $125, but this is not a place for an aftermarket experiment (IMHO.)

[Mattchu60]

The head gaskets are 501SD by fel pro. Here is a link to what I used:

http://www.summitracing.com/parts/fel-501sd

I am going to see if I can get a infrared temp gauge to borrow soon as well.

[Custom.Chevy]

TRY MOVING YOUR TEMP SENSOR TO THE SIDE OF THE BLOCK. THERE SHOULD BE A PIPE PLUG ON THE DRIVER SIDE OF THE BLOCK. Im pretty sure i did THIS ONCE BECAUSE I WAS GETTING FALSE READINGS. I REALIZE THIS IS NOT THE OPTIMUM PLACEMENT FOR A TEMP SENSOR BUT it may work. Also if your running long tube headers it may be possible the headers are heating up the temp sensor from the outside giving you that high reading you see, then once the thermostat opens and the cooler water flows over the internal part of the sensor its cooling it back down to the normal temp readings. This would explain why the the gauge is reading incorrectly while your laptop reads correctly. It also explains why all your equipment is in working order but not reading like it should be. just a theory. its been so long since i had this issue myself i cant remember if this was the solution but it sounds familiar.

[cadillac_al]

Isn't it normal for the cylinder head to heat up faster then the intake manifold thermostat area? By the time the thermostat sees 195 the head is most likely 230. My '70 Impala has been doing that like clockwork for 20 years with two different engines. It heats up to 220 or 230 and makes me nervous for a second and then it drops right down to 195 when the thermostat opens and it stays at 195. Yours sounds pretty normal to me except it settles at 200, The gauge could be reading 5 degrees higher than actual.

[barry1982]

A few years ago I rebuilt a 350 and the first few times I drove it had similar over heating problems. Talked to an old Mechanic buddy, and he said it was common for a newly rebuilt engine to run hotter than normal until the new parts "wear in" . Sure enough, after a few miles the temp stabilized.

[tucsonjwt]

I am assuming that the thermostat is open when checking the thermostat housing with an infra red thermometer. Removing the thermostat is not a bad idea to eliminate the thermostat element from the equation.

I got a compact Craftsman infra red thermometer for about $25 from Sears. Harbor Freight has full size thermometers for about the same price, but I think the quality might be questionable. I use that thermometer several times per year on one thing or another.

Checking temperature on various parts of the engine gives you an idea of where the problem might be. I think the exhaust manifolds and heads will always be hotter.

[Mattchu60]

Well I took the thermostat out this morning of the engine. Ran it and the temp hardly gets over 150 degrees on both my laptop and also on the guage. This means to me I have an issue with air pockets in the engine or the T-stats (2) i have had are not working correctly. GOing to boil a T-stat in water today and see what happens.

I think my plan now is to drill a small hole in the T-stat to vent air and see if it helps.

[Mattchu60]

Tested the T-stat in hot water, it opens/closes like I think it should (opens in boiling, closes when removed from the hot water).

If its not an air pocket, I'm lost. I would think the air pockets would have worked out in 200 miles of driving, but not sure. Has heater hoses hooked up so it should have a bypass with that.

[nonstop]

Drill the 2 holes in the thermostat. That should take care of it.

[Desert1957]

Originally Posted by nonstop

Drill the 2 holes in the thermostat. That should take care of it.

Bingo!

I have had the best luck with just one 1/8" hole, usually all that's needed.

Desert

[mMad Dawg]

Is your new thermostat a "fail safe " one. I have tried them in the past and they do what you are describing.Also they will stop working all of a sudden and stick open or even closed. Try putting one of the cheap ones in. Make sure it has a jiggle valve in it . That allows the air to pass through it so you dont get air bubbles in your system.Hope this helps. Gord

[KQQL IT]

I bought a high-pressure high-volume water pump they did the same thing would hold the thermostat closed

[Mattchu60]

Well, disapointting results tonight. put the Stat back in with a hole drilled in it, still overheating at the head sender. Maybe it will improve over time, not sure. I have the heater running full blast too, I can see the heater bypass hose is working when I look down into the radiator, as its pouring water in.

The water pump I got is a AC Delco professional model.

The stat is a Stant Superstat - 195 degree.

For now I am going to run it a little more without touching it, maybe its air that will work out. If no improvement I am going to try a 180 degree stat, hoping it will open sooner and get the head cooled off sooner. NOt really what I want to do but willing to try it.

[natedogg]

Dealing with the same issue with this 86 C10 that I'm working on. Just dropped in a Goodwrench 350 with new water pump, radiator, 195 thermostat, hoses and temp sending unit. Gauge was reading close to 260. Thermostat was not opening. Put in another new 195 fail safe thermostat and still ran hot! Took the stat back out and drilled a 1/8in hole in it. That helped. It still ran hot, but took longer for gauge to rise to around 260. Took the stat back out and left it out. Truck now holds at about 155 degrees. What are the odds of 2 new stats not opening? I have worked on several of these trucks and small blocks over the years and have never had this problem. I am also going to try a 180 stat and see what happens?

[rich weyand]

Originally Posted by Mattchu60

Well, disapointting results tonight. put the Stat back in with a hole drilled in it, still overheating at the head sender. Maybe it will improve over time, not sure. I have the heater running full blast too, I can see the heater bypass hose is working when I look down into the radiator, as its pouring water in.

You shouldn't have any water movement in the radiator when the thermostat is closed. Basically, the heater hoses are connected incorrectly, and you have insufficient circulation within the engine until the thermostat opens.

The bottom hose on the heater core connects to the top front of the intake manifold. The top hose from the heater core connects to the top of the water pump. This is why I asked for the pic earlier.

I think that is the problem. You are getting hot spots in the engine because you don't have the correct flow within the engine before the thermostat opens.