OK...i'm officially stumped. This is the same engine that i showed a pic of the burned oil on the rocker arm a few weeks back. Here's the setup: 68 400. I pulled the heads over the winter & had them tanked/cleaned/hardened valve seats installed/pushrod holes elongated for 1.65 rockers. I also installed a new Performer intake as well, new Comp Cam (268H) with lifters, new water pump, & Comp Cam roller tip rockers. I also have a new full MSD ignition. I start the engine, temp goes to 160, thermostat opened cooled the engine. If it idled longer, or drove for a short period, temps climbed over 220 & up & up. I then intalled a new original 4 core big block radiator, a milodon 180 high flow thermostat, & a 16" electric fan to the front that pushes about 2100 cfm. I double checked the timing & it's right on. I started it with the changes & the exact same thing, completely overheats. I'm stumped. My last & only guess is that a water passage is plugged, possibly the head gasket or intake gasket? Any ideas???

Several things come to mind as possibilities that you haven't mentioned, in no particular order...but I'd REALLY focus on my IR gun comments, and your shrouding first. Then maybe t-stat and water pump comments next...:

Fan shroud. You say you have electric fan. What type? Is is FULLY and COMPLETELY shrouded so that 100% of the air it pulls is coming THROUGH 100% of the SQUARE surface area of the radiator, and not just at the center of the radiator in the circular shape of the fan? A photo or more complete description of your fan setup may be helpful...if not properly shrouded, 2/3 of that big radiator may not even be utilized...

Have you done any testing with an IR gun? There is no better tool for tracking a problem like this. Take some readings on your radiator when the temp starts running away on ya. This will help you narrow down the problem. Is the radiator as hot as your gauge says? If yes, this eliminates restriction possibility in flow, and leads me to back to inadequate airflow or some other reason for the radiator not transferring heat to the air. If temp at the radiator is much lower than gauge reads, then your theory of flow restriction may hold true.

With that IR gun, make sure you compare inlet temp at radiator to outlet temp at radiator. This gives you another huge clue as to whether or not the radiator is actually dispersing heat like it should. If you see similar temps in-out with little drop, it could be the result of poor shrouding/airflow, OR improper coolant providing poor heat transfer, OR coolant flowing too fast thru the radiator to dissipate heat before it exits.

"new water pump." What kind do you have? Assuming stock, you haven't said anything about checking diverter plate-to-pump clearance at time of installation. Almost every Pontiac-style waterpump needs this set closer than it is stock, in order to pump coolant efficiently. A big gap leads to very poor coolant circulation. You want a TIGHT gap. http://www.wallaceracing.com/water-pump-mods.php

Is your water pump equipped with a stamped steel impeller, or a cast impeller? Normally a cast impeller is superior to stamped and will pump more water better.

COUNTERPOINT to not enough flow from pump: TOO MUCH coolant flow can indeed be a BAD thing. But even the MOST powerful pump in the world should not be able to overcome a proper t-stat restriction. Your say you have a high flow 160 t-stat. This is not always a good idea and might contribute to your problem. It might be flowing TOO FAST and TOO EARLY, and not re-closing/restricting flow enough for the radiator to have time to transfer heat to the air properly. Sometimes a standard flow t-stat, in maybe a higher (180?) degree reading, will slow the coolant flow down enough thru the radiator that it actually has time to cool before exiting the rad and re-entering the engine. Inexpensive/easy experiment. T-stats NEED to cycle...open/closed/open/closed/open/closed. If yours is just staying open and flowing like mad because it is rated too cold and your pump is too efficient, this can indeed cause an overheat situation like you describe.

What mixture of coolant did you install? You want 50/50. Some think stronger is better. This is completely wrong. Anything stronger than 50/50 that has MUCH lower cooling capacity than a stronger mix because ironcally Anti-freeze is TERRIBLE at cooling. WATER is the absolute necessary ingredient in the mix for proper heat transfer.

Do your hoses have proper support springs internally to prevent collapsing? Have you visually checked your hoses during oveheating problem to see if they're still round and not flattening out?

How did you set up your carb? If it's too lean, it'll run too hot. MANY things can cause lean running, and not just the carb setup alone. Carb-to-intake manifold air leakage is an extremely popular and often overlooked problem, as are most other vacuum leaks. Use a propane tester and search for vacuum leaks all over.

You say your timing is "right on". How did you determine this? Factory setting? Chassis dyno-tune? Recommendation from Comp cams? Some of the mods you mention may result in a need for different ignition timing. How did you determine proper timing for your new setup?

No promises obviously that any of these are actual solutions, but hopefully it helps you think it thru and gives you another set of eyes...wish you the best in finding a solution.

comments on abv ...yes , many times it flows TOO fast...as when people remove T stat for 'better flow'...water has not enough time in radiator to cool down.

50/50 is for modern AC cars...if your car does not have AC , best cooling of coolant and water is 33 % coolant 67% water...the most efficient cooling mixture...but AC cars use a higher % for lubrication IIRC.

and other ideas...is your fan a puller or pusher? pusher fans are not good ,they also block radiator.

was this your car before the rebuild...i.e. did you drive it before w/o over heating issues?
Do you have the fillers?
the lower filler, the masticated on ea side of bottom of radiator? and the X filler plates?

Regardless of all those mixture and filler panels and air flow your car should idle stopped in the driveway for an hour before the heat starts to do anything weird unless ambient temps are high. I know this doesn't help but if it will sit still and not overheat it will probably be ok on the road.

hmmm. in Houston, in summer , idle in a driveway will NEED all the fillers to force all the air thru the radiator.....

Maybe the heater core is plugged? Try bypassing it?

Didn't say anything about the correct clearance for the divider plates in the WP, or maybe forgot to install?

When does the engine overheat? If at idle or very low speed, then it is likely an airflow problem. If at speed, then it is likely a coolant flow or other problem.

Many mentions of fillers. I tend to disagree. Those filler panels may be of no value at all for this particular car. I say this because he stated up front he has an electric fan...which is likely meaning his fan is pressed up tight against the radiator and not set back at the engine like our stocker setups are...maybe meaning he doesn't need any of the fillers at all and they may serve no purpose.

As I stated originally, really we need a photo or a better description of this cooling system to make any true judgements.

Bjorn, I agree entirely that 33/67 mixture antifreeze/water is indeed more efficient for cooling. 50/50 should also work fine, but is likely unnecessary for a collector car that will rarely be driven in below-freezing temps. But it is absolutely CRITICAL to have an adequate percentage of a/f in the mix for off season storage in cold climates.

My only point in bring that up is that so many people these days seem to think that "more is better" and just go 100% antifreeze out of pure ignorance, not realizing how critical water is for cooling purposes.

crazecars...thanks for all the great info! I'll grab an IR temp gun today & run the tests like you detailed out. The water pump is a new one I bought & was running last year, a Tuff Stuff standard water pump. I had no overheating issues last year at all...that's why I'm stumped. I'm using the same hoses, carb, & MSD ignition. The hoses I purchased thru Ames but they do not have the springs installed inside. The only changes I made where the cam, having the heads cleaned & worked on & installing the Performer intake.

The tstat is currently a 180 Milodon.

I'm not sure of the spacing with the divider plates in the WP, but I did install them.

The 16" fan is installed at the front of the radiator as a pusher, but obviously did nothing to help. I only installed this as a step to see if it would help in the cooling process, but obviously to no avail.

The engine overheats when at speed, it'll take a little while for the temp to climb at idle. I can drive it approx 2-3 miles & the temp goes above 220 & when I park it & idle it, it goes northward of 230 & I shut it down, then anti-freeze pours out the overflow tube.

I'm currently running a 50/50 mix with Be Cool additive. I tried it with just the 50/50 mix prior & same result.

The instructions for the timing was from Comp Cams & MSD.

I've tried with both the filler plates installed & without. No difference.

Longshot but: Do you have the original exhaust manifolds? I have wondered what would happen if the exhaust manifold heat riser valve (on '68s prior to spring '68) got stuck shut or partially shut. I would think that the exhaust on the right side being forced across the top of the engine would lead to overheating.

Also, do you have a 15 pound radiator cap?

could the lower radiator hose be collapsing?

Stupid question.... Did you check the thermostat before installing it?? I had one that would not open no matter how hot the water/coolant got. Best way to check it is to heat water up on a stove with a thermometer to verify temp of water and dip stat in water and watch to make sure it is working.

Yeah, I did check the thermostat before I installed it to make sure it worked. The cap pressure...i'm not sure, i need to check that. the manifolds I have on are the ram air long branch ones.

I got an IR gun & just finished starting the car & here's the readings I got. I let the engine idle to 190, watched the gauge drop to below 180 when the tstat opened, then gave it steady throttle at 3000 rpm to bring the temp up, it got to 210 & I let the engine idle & the engine kept climbing & that's when I took these temps:

top of the radiator by the hose: 236 degrees. bottom of the radiator by the hose: 200 degrees. temp at the stop [censored]: 122 degrees. temp at the top middle of the radiator: 220 degrees, bottom middle of the radiator: 150 degrees. hose inlet on the intake: 230 degrees. at the water pump where the rad hose attatches: 185 degrees. heads: 242 degrees. block: 224 degrees. water pump temp: 224 degrees. intake manifold fuel runners: 194 degrees, inbetween fuel runners: 245 degrees.

That's all the reading I got. I shut it down & anti-freeze poured out of the overflow tube & you can hear the coolant bubbling. thoughts on the readings?

Lack of water flow. Bad water pump or perhaps the lower radiator hose is collapsing (needs a spring).

The radiator is obviously doing it's job based on temps at the top vs temps at the bottom.

Look at the 185 degree reading at where the lower hose enters the water pump. That cooler water is not getting circulated well enough. Either it's a cheap, weak cast impeller, you have way too much clearance between the impeller and the divider plates, or the lower hose is collapsing.

my overheating issue turned out to be a vacuum leak. Causing a lean condition. check the simple stuff.

Also, I see nothing but bad news when people switch to electric fans.

I am a big supporter of a big OEM 7 blade clutch fan and an OEM fan shroud.

that may be so, but if its getting hot over 30mph. the fan has nothing to do with it.

Agreed. But that last trial with the IR gun was a static test. Sounds like it overheats without moving in a matter of minutes.

I do have a 5 blade flexalite fan connected to the water pump & with the OEM fan shroud on as well. The original fan was long gone when I purchased the car.

This is the second time i've heard a lean condition can cause this, i'll double check the vacuum readings tomorrow & double check the carb. it is a 600 Holley double pump.

& yes, it overheats to 230 degrees in a couple minutes if you just idle the engine at 2000 rpm's.

Exhaust gases in the coolant? That could lead to a situation where it overheats at idle and speed, and has a temp that keeps rising.

Is the Fan for the engine, I bought one once that said it was for my car and I found it was for a 6 cylinder and the engine would overheat at idle.
I also agree with PCI I found a major Vac leak that was causing my bird to overheat, it would idle fine and temp would get to normal, and when I would accelerate the temp would shoot past 220
I found two manifold ports were leaking.
I also agree the lower hose needs the spring, it is closing up,not maybe you have a new pump and the suction will close it.
Have someone rev the engine past 2500 and watch the hose? it's probably flattening out.

Timing.. Just went thru this. Back off on the timing and see what happens. You mentioned installing a cam, did the timing chain crank gear have the 3 keyway slots?

If I'm interpreting you correctly...you're basically saying that the coolant is coming into the radiator at 236, and exiting at 200? And with the understanding that this is surface temp...meaning your actual coolant INSIDE the radiator at these locations is higher than that?

You can't cool an engine with 200+ degree coolant going INTO it! So I disagree entirely with whoever said "the radiator is obviously doing its job"....or maybe I am misunderstanding the location of your temp readings...a radiator that considers 200+ degree coolant to be "cooled" and ready to shove back into an engine is most certainly NOT doing its job...

But why?

Not sure what to tell you quite honestly...other than to check everything that all people are suggesting. Frankly I see the likelihood of you having more than one problem, and one feeds the other, which may be compounding the difficulty of finding a solution...like maybe it's running scary-lean and super hot, causing rapid engine temp increase, PLUS you have maybe inadequate OR too much coolant flow, PLUS a possible airflow problem thru the radiator. You may have a trifecta of issues here.

Your fan setup has me completely puzzled. I see little reason or benefit to a double fan setup like you describe, I'm even wondering if that added fan is really screwing with the airflow of the other fan, and creating some sort of strange, extremely unbalanced airflow that is causing some air to bypass many portions of the radiator. I can easily see how a setup you describe will cram all sorts of air thru the center of the radiator, which feeds plenty of air to your engine-mounted fan, which in turn prevents ANY air at all from passing thru the portions of the radiator core that are outside of your electric fan circle! Your second fan may very well be rendering a huge portion of your radiator core completely ineffective at cooling because no air is flowing thru the perimeter areas!

Have you tried a similar test with the electric fan disconnected? Or better yet fully removed? From one of your earlier comments I believe you did run it without this second fan...but want to confirm.

Your current 5 blade fan...what's the diameter? How fully does it fill your shroud? How is your spacing...fan usually should be about 50% inside the shroud, 50% out of the shroud, with reasonably tight clearance without rubbing. Big gaps between fan and shroud will cost a massive amount of airflow to skip by the radiator. Photos would be helpful here.

Let me also be first to admit out loud that I'm taking random shots in the dark here.

crazecars...yes, the overheating problem was without the electric fan on the front of the radiator. I installed the electric fan just as an attempt to see if airflow was the problem, which obviously that's not the reason at all so since the electric fan did nothing to help, I took it off.

As for the temp readings, yes...the coolant exiting the radiator & entering the thermostat is 230 degrees. the water exiting the engine & going back into the radiator is 200 degrees, but 150 degrees at the bottom middle of the radiator which really confuses me as why the radiator is cooler at the bottom but extremely hot at the top...as in the coolant coming out of the engine is 50 degrees cooler than the coolant going into the engine at the thermostat. I'll get some pics this afternoon.

As a Ford guy I can honestly say I'm not having a ton of fun here in Pontiac land

are you sure? it seems backwards, like the radiator is heating up the coolant?
and engine is cooling down the coolant???

Gentlemen:

Reread the temp measurements carefully. I threw out the "bottom of the radiator at the hose 200" as a mistake as it would be a hard target to hit without picking up something else.

If you throw that 1 measurement out, then you have a lowering of temp consistantly from the top of the radiator (236) through the middle (220) then to the middle bottom (150 probably low) into where the bottom hose connects to the bottom of the water pump (185).

If you go straight temp change from the top of the radiatior (236) to the bottom of the water pump (185) then you have a radiator doing it's job.

Now, if that "middle bottom 150" IS correct, then this might suggest blockages inside the radiator. Which would be really odd being it's a new radiator.

I would re-torque the heads, especially if the engine has over heated and if the new gaskets require a re-torque as most do.

Since the only thing you changed were the heads, I tend to go with the "no coincidences" theory. I'd suspect a leaking head gasket and have the cooling system pressure tested.

You don't need to worry about the lower hose closing up, in my opinion. If the hose collapses when you rev the engine it only does so when the engine is cold. When the engine is hot you have pressure in the cooling system. The pressure makes it impossible for the hose to collapse.

here's pics of the engine compartment. I double checked the float level in the carb today, double checked the timing: initial 12 degrees, 34 degrees at 2800 rpm. started it back up, reved it to 2000 rpm's until the temp hit 205 degrees then just let it idle & right up to 230 degrees in just a few minutes & so I shut it down. i'm almost ready to throw in the towel on this one.

This may have already been said, but are you sure your Thermostat is not in upside down?

lol...yeah, I did make sure of that

one thing i just did notice though. I was going thru all my old receipts from when we put the heads on & I saw we purchased the Fel-Pro 1016 head gaskets, not the Fel-Pro 8518 head gaskets. Is there a difference between these two & if there is a difference could that be the problem? taking a stab in the dark here....

Boomstick,
Your fan has way to much clearance between the blades and the shroud. You should have the blades as close as possible to the edge of the shroud and the blades should be about half sticking in and half out of the shroud. You have a very inefficient fan there. You should barely be able to slide your finger between the blade and shroud. You look to have a lot of gap there.

Also the shroud looks to cover the entire drivers side of the rad which is good, but the passenger side looks to be open?? Hard to tell from these pics.

Have you checked the 2 rubber rings on the end of the tubes from the water pump divider plate? If they aren't sealing properly or have degraded they will bypass a ton of coolant from the water pump discharge back to suction.

You said on page one of this thread:

"I'm not sure of the spacing with the divider plates in the WP, but I did install them"

As was mentioned in the 2nd post by crazecars the clearance between the divider plate and water pump impeller is critical. http://www.wallaceracing.com/water-pump-mods.php If you didn't check it, there is a good chance that is your whole problem. You need to get that clearance as tight as possible without rubbing. If not your pump recirculates the coolant from the pump discharge back to the pump suction and reduces the coolant flow to the rad.

68ragtop455...

He has a 1968. And from pics I think I see it is likely original as it appears to still equipped with an 8 bolt waterpump setup. Those pumps don't have the sleeves you refer to like the 69 and up 11 bolt pumps have.

I agree with you 100% that his fan is very visibly too small and poorly fit to the shroud.

Boomstick, your fan needs to be replaced. I also can't see if your filler panels up top are missing above the radiator support. Or below. I know earlier I said the fillers don't matter in your case...but that's when I thought you had switched to an electric fan setup and was later informed otherwise. With your stock-type fan setup the fillers DO matter. But only at highway speeds. At idle, it's all fan...and your fan is poor. But because that fan is very visibly incorrect providing poor idle air flow, plus you've done hop-up mods to the motor...I can fairly easily see this motor doesn't stand much of a chance to properly cool at idle.

But it shouldn't overheat THAT quickly. So again, I believe airflow may be only ONE of a few problems you likely have.

Which brings me to ask you to oplease share your results of you pulling the water pump checking diverter-plate-to-impeller clearance with a feeler gauge.

AND...now that we have more history...MOST IMPORTANTLY: What was the result of the pressure test very smartly suggested by go2fast? A proper pressure test, in conjunction with a cylinder leakdown test, is really the only way to ensure your heads were installed properly. Your earlier different topic that showed ONE badly cooked rocker arm is another big clue. You assumed at that time it had an oil delivery problem to the one rocker, but I now believe you may have assumed wrong. When one localized spot on the engine gets that hot and cooks the oil, it's an indicator of a very localized lean/hot spot...which is a big clue of a head leak at...dare I say the #4 cylinder? (because this is the rocker that cooked). AND...when it got that hot, could it have warped a mating surface to worsen the leak? Yes. As much as i know you hate to hear this, it needs to be said: I now believe something has likely gone wrong in the assembly of this engine, and it has been overheated multiple times in testing which likely made the problem worse, and this likelihood of an internal engine problem needs to be thoroughly looked into, before anything else.

If I were you, no matter the results of the leakdown test, I'd be pulling the head on the side with the #4 cylinder and looking inside to see what's going on, and then checking mating surfaces for warpage before reassembly.

At the very least, you can easily pull rocker cover again and look at that same arm, and see if it's cooked itself again...

I have been told a over bore can do it! Not sure if its true or not

not sure if 68s are different ,(cannot remember as my last 68 I owned in 69 for only a year)...but to me it looks like the shroud is not covering the radiator...shouldnt it push all air going thru the rad. thru the shroud? plus agree with that the fan blades edges are too far from the shroud

crazecars & go2fast....i ran a cooling system pressure test today. pressure held fine at 14 psi. i released the pressure then reinstalled the tester & started the engine & the pressure did not climb rapidly as well. i'll get on getting the correct fan. I did not measure the clearance btwn the plates & the water pump impeller the last time & had it off, but you are correct that it's an 8 bolt so there are no rubber rings present. i first read that & immediately started to freak a bit but now i realize that those aren't on my year. phew. I did have the filler panels on, but I didn't put them back on when I put the electric fan in. As soon as I pull the efan, I'll get them back in.

As for pulling the head, I'm getting the feeling that the problem lies deeper as well. I'll pull the water pump first just to check the clearance, but my gut is telling me to yank the head.

I noticed you have said "plates" a couple of times in this post. Does the 8 bolt water pump have more than one divider plate? there is some good info on clearancing the water pump in the jim hand book. I used it to do my divider plate clearancing this winter. I took a few pics of it (among other things) on this page if that helps you envision the process. there was quite a large gap before I started.
Clearancing Pics
Don't give up on your pontiac engine. Cooling problems are relatively common but they can usually be solved . just keep eliminating the weakest links and eventually you will find the problem. I agree with the comments on the fan you have installed. it does not look like it fits that shroud well. And the flex fans often have mixed reviews and success. If originality is part of your goal then get an original style correctly size clutch fan and shroud. if it is not, then get a properly sized and shrouded electric fan. there is a reason no modern vehicle comes with a mechanical fan anymore. they are just not as efficient. there are lots of good articles on the net of fitting a junkyard lincoln ford e-fan assemblies so it does not have to cost a lot either.
good luck.