[One Grand Am]

I do have a brand new 13 pound cap. And just to clarify - I've never had it boil over - it just builds heat at speed. Slow cruising its at 180 degrees consistently.

[77 Canamman]

I've never had any Pontiac with a three core radiator, ALL of mine had 4 cores from the factory, but I don't have any issues with one getting hot, either. I would say that a new 4 core would fix the issue, since you have explored everything else.

The CU165 radiator would be the one I recommend, and on E-Bay, one could be shipped to your door for around $240. I had to put a new one in my Enforcer to replace a leaky one, and it was of acceptable quality.

[Brian Baker]

My '73 Luxury LeMans, with 350/2V and factory air had but a 3-row radiator in it...go figure...how do I know this? I had to replace it in 1986 after installing headers, dual exhaust, and an intake with a 4 barrel carb. Don't know if these changes had anything to do with it, but the original radiator in it took a dump not long after. I upgraded to a 4 row.

[77 Canamman]

All of my A bodies are 400CID and larger.

[sport coupe]

Don’t know if this will solve the heat problem. Check the air cleaner damper and see if it is shut closed. Incorrect routed vacuum hose or stuck damper. Also check the heat riser and see if it is closed. Also incorrect routed vacuum hose or stuck closed.

[Alvin]

Originally Posted by One Grand Am

...It does have a 3-core rad, that's what I took out, and assumed it was o/e, and figured my new one would be sufficient...

Hello 1GA Chris!

FWIW: Our 1973 Pontiac Grand Am 400 cid, Turbo 400 with factory AC also came from the factory with a 3-core Rad.

Our T/A 6.6’s and 6.5 Litre GTO’s – all w factory A/C - came w factory 4-core Rads.

How is your progress on solving your highway speed heating problem? Is EGR working? Keep us posted.

[sport coupe]

Also curious if it was an air restriction or exhaust problem. Let us know if you fixed the overheating problem.

[One Grand Am]

No air restrictions through the condensor or the new radiator. Shroud and front small air dam all in tact. New exhaust all the way back. Once again, it has never boiled over. It just reaches 220 plus.

[Grand73Am]

Are you using manifold or ported vacuum for your distributor vac advance? I suggest manifold vacuum, if you aren't already. I've found it runs cooler and with more power that way. You'll have to lower the idle if you were running ported before.

[Brian Baker]

Originally Posted by Grand73Am

Are you using manifold or ported vacuum for your distributor vac advance? I suggest manifold vacuum, if you aren't already. I've found it runs cooler and with more power that way. You'll have to lower the idle if you were running ported before.

Pontiac engineers did things for a reason. If you use manifold vacuum on the advance, it will be at full vacuum advance at idle. Leave it on a ported source.

[Grand73Am]

But....a lot of times those Pontiac engineers called for manifold vacuum. My 73 Pontiac Service manual shows the vac advance connected to manifold vacuum, so ported would be wrong. Many older factory service manuals specify using manifold vacuum. It won't be at full advance at idle, since the centrifugal weights aren't in full swing at idle.
Older cars didn't use ported vacuum from the factory, until they started having to deal with emissions. And as we know, tuning an engine for emissions is not as good as tuning it for performance. So, I have all my old cars on manifold vacuum. They include 73 350 Chevy, 73 455(Grand Am), 75 350P, and 79 403. I've tried both ways, but manifold vacuum works best with great idle, and runs stronger off-idle and cooler with my basically stock engines. It's an easy thing to try and see if it works better, if you happen to be on ported now.

Here's a good explanation of a vac advance on manifold vacuum, from one of my old GM service manuals: "A vacuum spark advance mechanism is provided to improve performance and economy on engines operating under part-throttle conditions by supplying additional spark advance at times when a high vacuum exists in the engine manifold. This mechanism is connected to the distributor, has a diaphragm chamber connected to the carburetor so that manifold vacuum causes the vacuum spark control diaphragm to compress the spring and advance the spark....This advances the spark...but only with comparatively high manifold vacuum. Each time the throttle is opened wider for acceleration the manifold vacuum decreases retarding the spark to prevent excessive detonation. At high vehicle speed with nearly wide open throttle the vacuum is low, therefore the vacuum advance is not in operation. However under this condition, maximum centrifugal advance is in use."

As an example to show that the factory still used manifold vacuum many times in the 70's, here's a vacuum hose diagram of a 79 Trans Am 403. It clearly shows manifold vacuum going to the vac advance.

[Brian Baker]

I didn't say full advance at idle...I said full VACUUM advance at idle.

I imagine that POS 403 needed the extra advance at idle after having been saddled down with emissions equipment...best thing I ever did on my 79 T/A was yank the 403/th350 and replace them with a Pontiac 455/th400.

[Grand73Am]

Ahhh, the 403 ain't that bad. Actually, there isn't that much emissions on a 79 403. Just the converters and the egr...no more than a 400 Pontiac. Even cars from the 50's and 60's used manifold vacuum, when there wasn't any emissions equipment. Ported vacuum tends to be on cars with more emission controls.

The point is the factory often used manifold vacuum, even on 455's. My 73 Pontiac service manual shows it, as does my 75 manual. Now, some engines regulated the manifold vacuum signal with switches and valves, but they were still connected to manifold vacuum.

So, like I said before, it's an easy thing to try. Run whatever works best for you. For example, I tried ported on my 79 out of curiosity, and it didn't like it at all. Lack of power and overheating was the result.

I also have a 455/400 to put in one of my 79's if I can ever get around to it .

[Alvin]

OGA: This might be of use to you also.

The original Thunder Am and HPP articles were reprinted in a compilation titled “Pontiac Power from the Editors of High Performance Pontiac Magazine”

Cover


Article included (2) pages how to “Street-Tuning The Trans Am”

Page 1

Page 2

Re-routing hoses full size blowup here


Important Note on Layout:

If objectionable surge occurs at idle, connect distributor to ported source & reset idle speed.

Hope this helps you!

[BR57]

I was experiencing an very similar problem. My factory cluster would show 220 to 240 within 15 minutes of driving at highway speeds, it would take a bit longer around town.

My problem was easier to identify because I was running the original two row rad after swapping in a 454. For what it's worth I used a local rad shop that has been in business for over 30 years. They recommended against a traditional 4 row radiator in favour of a 3 row aluminum high efficiency radiator. I wanted an original look so he did a recore for me, sanblasted tanks, replaced trans cooler and did the three row High efficiency, all painted black to look right.

After an hour last night the guage was showing 140. Tonight I took it out on the highway and ran it hard and it looked like maybe 150 was reached. I include some photos. The traditional 4 row radiator has 104 rows or passages while the 3 row high efficiency has 120, so the high efficiency has greater cooling capacity than a four row. This is the best 360 bucks I ever spent on the car. The car runs so much better and smoother and you can easily feel that it has a higher level of performance.

[One Grand Am]

Update on heating issue. Yesterday I got my car up to operating temp. Did a hydrocarbon test and found that I am getting leakage from a combustion chamber. Did this test because a friend had suggested that it could be a head gasket issue. Looks like he's right. I'll will be pulling the heads off and having them leak tested and milled tomorrow. Then I will put it all back together and see where I'm at. Need to have it running by Aug. 25th for our local Pontiac Club's car show in Post Falls, ID.!! Thanks for all of the advice and suggestions. It is greatly appreciated. I'll be giving updates!

[Alvin]

Originally Posted by One Grand Am

Update on heating issue. Yesterday I got my car up to operating temp. Did a hydrocarbon test and found that I am getting leakage from a combustion chamber. Did this test because a friend had suggested that it could be a head gasket issue. Looks like he's right. I'll will be pulling the heads off and having them leak tested and milled tomorrow. Then I will put it all back together and see where I'm at. Need to have it running by Aug. 25th for our local Pontiac Club's car show in Post Falls, ID.!! Thanks for all of the advice and suggestions. It is greatly appreciated. I'll be giving updates!

Ask the machinist if you should have your intake milled to match the heads.