steve meltzer

My repro radiator cap on the '66 427/390 leaked so I bought a Murray 16# from O'Reilly's. It leaked by the tester (car overheating and boiling over) and I tried another Murray. Same thing: car spews coolant and that cap leaked from 16# to about 13# by the tester. I think this is a cap issue as it's a new motor, no sign of stock w-pump leaking, good relatively new lower hose, fan clutch OK, radiator tested and cleaned prior to engine install. Timing is 8° BTDC and shroud is sealed to radiator with AC tape. I don't want another repro cap; the car is a driver. What cap and how much pressure should I shoot for? Will it tolerate 16#, and if so, is the AC Delco a good choice. thanx s

Vet65te

Steve, how 'new' is the new motor? Was this new engine running fine without tossing coolant prior to the radiator cap issue? The cap(s) could be defective, that's a possibility but I noticed that on my Stant Coolant Pressure Tester, the adapter that is used for testing the caps is seriously chromed and very slick and unless wetted a bit, the caps were all appearing to be leakers and not able to hold pressure but once I added a bit of water, they held. Just a thought.
Mike T.

midyearvette

you could be buying caps for an "open" system
check the package and look for the word (vented)
if it says that, you have the wrong application
it will not say that on the cap itself

and no, i don't mean the lever type vent....(release)

nassau66427

Overheating - What (verified) temperature?

Puking - Are you filling it to the fill line, or more?

steve meltzer

The motor is brand spankin' new. For all intents and purposes this started with the motor rebuild and all of the components only have 100 miles on them. It's the correct fan for an AC car and also has a "pusher" electric fan, which should more than solve the issue. This stuff happens at slow speeds and has been verified (IR gun) about 230°. There's no fill line (of which I'm aware) on the radiator, but it's passed "seeking it's own level", as corroborated by the increased temperature. I assume this is for a non-vented system, tho' I assume that it's right, as it's been catalogue checked several times. thanx so much.

Vet65te

Steve, when the engine heats up to 230, is it doing that rather quickly or at a slow steady rate?
Mike T.

nassau66427

If you have a stock radiator, there is a fill line on the pass. side end about 3"-4" down from the top.

Are you using anti-freeze? With a 50/50 mix and the recommended 15# cap, it shouldn't boil over 'til about 265*.

Is your vacuum advance hooked up and functioning properly?

If the car is a "driver". s**tcan the radiator and get an aluminum one.
My car doesn't overheat even being stuck for literally an hour or more in not going anywhere soon traffic.

MikeM

Your primary problem is not radiator cap problem.

steve meltzer

I live in Houston, so the ambient temp of 100° ain't helpin' any. On the freeway, it runs about 210, but zooms up to 230 in slow traffic, boiling over in my driveway. I didn't know that there was a fill line. If the cap were bad, wouldn't that lower the boiling point significantly, secondary to a loss of pressure? (P-chem was awhile ago!) There may well be another problem, but I can't find what it is. Maybe install an electric sucker fan? thanx s

steve meltzer

Oh yeah, 50/50 mix and yes the vacuum advance works well. (rebuilt by Clupper). When the engine was built, there was a sl. leak from one of the brass freeze plugs and I used a packet of Barr's leak. They did that at the factory, so I doubt it's plugging a water passage. s

Vet65te

There are a number of possible causes to an engine overheating and hopefully you'll find it's something simple but the reason I asked if the temps rise quickly is because that can sometimes indicates the presence of combustion gases in the cooling system and with the boiling-over occuring right after the engine being rebuilt, if everything else checks out alright, you could use a kit like this one below to check for those combustion gases. On the box they list some of the possible causes and on a newly rebuilt engine, improper torque on the head bolts could also be a reason for the overheating.

Mike T.

steve meltzer

i've seen that kit, but don't have one. Can't you pressurize the system via the radiator (same Stant kit that tests the cap) and see if it holds pressure overnight? Should it fail, might indicate leaky head gasket. I suppose it could also leak from hoses and stuff, but with no hot liquid, under pressure, leaking at this time, a failed test would be highly suspicious of a leaky head gasket. argghhh. s

Vet65te

The pressure testers don't put much pressure on the system compared to what a combustion stroke of an engine can do. That kit is available from NAPA I believe, if you decide to give it a try.
Mike T.

steve meltzer

thanx. i'll let you know. s

silvercamaro

One issue I've had several times is new radiator hoses sometimes don't have the internal spring inside and collapses while driving. Something to check, anyway.

Bluestripe67

Correct me if I'm wrong...please, but any "brand spanin' new" engine is going to run hot during breakin. I think some miles need to be put on this car for a reasonable observation of the cooling system temp. Dennis

MikeM

I think if a new rebuilt engine has enough internal friction to overload the cooling system to the point of overheating, something is bad wrong with the engine.

JMO

tjstarduster

i live in dallas and it's been over 100 for several days. when i dropped in my 496 bb, i went with a champion 4 core aluminum radiator. with a 160 thermostat and 11.5 c/r, my engine stays between 160 and 180 all the time, even idling in parades. throw away that copper and go with aluminum, best decision i've made with the new engine. tjs

steve meltzer

OK, I put on a new fan clutch (Hayden) and discovered that the fan was backwards, so I fixed that. The car runs about 210 on the freeway, but rises as you slow down the airflow, so at idle, after slow traffic driving it's at about 221, this, confirmed by IR gun. That still seems high especially since it has an electric pusher fan operating with the AC on and the correct (and expensive) 7 blade factory fan. Reconfirmed that the timing is correct. I'm gonna see if it's got a bad head gasket, but it seems like if it had a head gasket leak it wouldn't get that much better with improved airflow. With the car freshly started (well 2 minutes of idling) you can see tiny bubbles accruing in the radiator (cap off). I thought this was a sign that combustion gases were escaping into the coolant passages. Is this a reliable test for cracked block, bad head gasket, etc? thanx s

Vet65te

Steve - I've used that Block Check tester on quite a few cars and some had what I'd call a little combustion gas leakage into the cooling system and a couple that had so much leakage that the dark blue fluid turned pure yellow very quickly. Since you've put a few miles on the car by now, I wouldn't expect to find a lot of bubbling going on but anything's possible. As for a combustion leak issue not being obvious during highway driving with improved airflow, my 66 Coupe's overheating story would disprove that. Right after getting the L79 fired up and driving again, I took it around the neighborhood, typically during cool early morning weather and low rpms. We're only talking maybe a mile or so. Then out on the freeway and it seemed to run in the 165-180 range with no problem while doing about 60 mph but as soon as I took the offramp and the rpm came back to an idle and the car was sitting at a traffic light, the temps rocketed up to 210/230 and I mean it moved 'fast'.
Once I got the car moving again, the temps dropped down, almost as quickly as they came up. The block had been decked and we surfaced the heads but when it came time to torque the heads down, I kept the torque readings towards the low side of the listed 60-70 foot pounds in an attempt to minimize pulling any of the old block threads. Wound up pulling the heads, cleaning things up, adding new head gaskets and this time torqued them to the higher number and it's been fine ever since.

The fact that your temps aren't climbing real fast now is a good sign that it might not be a combustion gas leakage problem.
Mike T.