Overheating 87 TBI

Absolutely stumped on this I’ve replaced almost everything and she still slowly overheats honestly lost for words

Took heat gun to it & is accurate enough to gauge
Fan shroud is good
Stuck glove over rad cap hole didn’t blow up the glove
Have tried 50/50 & 70/30 coolant
New rad
New water pump
New coolant temp sensor
New rad cap
New overflow tank thought possibly cause there was a crack in it it could mess up the pressure
Have tried 3 195f thermostats and have tested them every time before putting them in, 1 thermostat I drilled two 1/8 holes in because I thought there could be a possibility it was airlocked, this fixed the overheating issue but engine wouldn’t get up to temp even after 2 hours of driving

Last things I can think of is head gaskets (highly doubt), fan clutch seemed fine when I replaced water pump, maybe when I put new rad in I installed hoses differently???, water pump isn’t reversed

Should mention it’s always overheated since I got her but the rad was in ROUGH shape so that’s where this rabbit hole started

When and how did this start? Is this the original tbi engine? Is the heater core hooked up?

Ricko1966 said:

When and how did this start? Is this the original tbi engine? Is the heater core hooked up?

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Original TBI, heater core is hooked up? Heat works. Bought it all rad fins were smashed and rad had a tiny drip so thought that was the reason she was overheating and then it’s just been fixing the next thing ever since but I’m about done with this it makes no sense

I would do the fan clutch to just to safe

It's a circulation issue,not fan clutch,not head gaskets etc. Otherwise the 2 holes in the thermostat wouldn't have stopped the overheating. I asked about the heater core,which you answered,because TBI may need coolant through the manifold,through the heater core and then return as a bypass. When you changed the water pump ther should have been a bypass hole drilled into the block and the water pump,I wonder if the hole in your blocknis plugged. It definitely sounds like a bypassnissue to me based on drilling holes in the Stat stopped the overheating. I'd drill a single 1/16th hole in a tstat and see what happens.

Ricko1966 said:

It's a circulation issue,not fan clutch,not head gaskets etc. Otherwise the 2 holes in the thermostat wouldn't have stopped the overheating. I asked about the heater core,which you answered,because TBI may need coolant through the manifold,through the heater core and then return as a bypass. When you changed the water pump ther should have been a bypass hole drilled into the block and the water pump,I wonder if the hole in your blocknis plugged. It definitely sounds like a bypassnissue to me based on drilling holes in the Stat stopped the overheating. I'd drill a single 1/16th hole in a tstat and see what happens.

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you’re talking about how there’s that fifth hole on the front of the block where water pump bolts overtop of may be plugged? This is the first I’ve heard of this and what you explained makes complete sense going to try drilling a 1/16th tommorow and hopefully that lets it stop overheating again but also let’s the engine actually get up to temp, I’ll let you know how it goes. thanks.

I'll find a pic of the bypass hole for you,with no bypass there is no circulation so the thermostat doesn't get hot water circulated too it so it doesn't open. A singlev1/16th hole may not be enough but you can start adding from there 2x1/16th holes sneak up on it until you find what works. Biab with a bypass pic. Got you 2 pics.

How long have you owned this truck? I am going through an overheat issue with my truck. The fan clutch failed and it would over heat on a 3-4 mile drive. It got pinned to the red on one trip and boiled out all my coolant. I had this fan clutch in for the last 6 months with no issues and the gauge went up 1/4 way like all my trucks. Since then I would have to pull over and let it cool down 10-15 minutes to get to my destination. I bought a standard duty fan clutch and now the gauge only goes up half way but makes me nervous. I am going to have to buy a heavy duty fan clutch and I'm sure that is going to cure it. I never realized how you can tune the cooling with a fan clutch.

Definitely replace that fan clutch. To the circulation issue, when I changed my freeze plugs, I was surprised by how much trash had filled the lower passages in the block. If the cooling system is in bad shape, like it sounds like yours was, a lot of rust particles can collect around the freeze plugs in the sides of the block. I spent some time digging out what I could with a screwdriver before pounding in the new plugs. I wouldn't dig out the freeze plugs specially for that, obviously, but it's something to remember if it comes up later.

Ricko1966 said:

When and how did this start? Is this the original tbi engine? Is the heater core hooked up?

Click to expand...

Should mention it’s always overheated since I got her but the rad was in ROUGH shape so that’s where this rabbit hole started

Have you made sure to burp the system and get all the air out after filling? On my truck, I have to jack the RF corner up a good ways to get it to fully burp.

Dr_Snooz said:

Definitely replace that fan clutch. To the circulation issue, when I changed my freeze plugs, I was surprised by how much trash had filled the lower passages in the block. If the cooling system is in bad shape, like it sounds like yours was, a lot of rust particles can collect around the freeze plugs in the sides of the block. I spent some time digging out what I could with a screwdriver before pounding in the new plugs. I wouldn't dig out the freeze plugs specially for that, obviously, but it's something to remember if it comes up later.

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^Leaning towards plugged cooling passages as well. Old unknown engine.
I had a 1999 vehicle recently. Heater core had probably a half lb of gunk in it. Dunno what the engine had, and never checked because it didn’t overheat. But that same gunk had to be present everywhere.
Idk how to do a good chemical pressure flush of the block but strong chance that is the issue or contributing to it. Or the water pump bypass hole. Check that first. Pull an accessible freeze plug next if clearing the bypass doesn’t solve it.
I’m partly speculating with all this but seems plausible.
Good luck. Frustrating I’m sure.

If the fan clutch isnt tight when the engines cold, its time to replace it

But that'd only affect it to a degree, i definitely.agree with circulation issues

Stock belt/accessories?

How hot does it actually get? Put a real gauge on it, and see what happens. You may be pleasantly surprised.

When does it overheat? Idle? Going down the road?

Are all the factory rubber baffles/gaskets/seals around the radiator in place? Any air that goes around the radiator instead of through it is wasted airflow. People toss those thinking they aren’t important, GM wouldn’t have put them there if they weren’t needed.

Does the truck run as expected? How is the tune?