Overheating Engine (Revisited 2)

Q: Overheating Engine (Revisited 2)

Been playing with my 1969 with a new 455 (1000 miles) that doesn’t seem to have the correct power it should or maybe I got sold a lousy cam with the rebuild. Here’s part of what’s going on, any ideas out there?

Ran the car a little on Saturday. with an HEI and timing set at 12degress with vacuum module attached to vacuum source high on quadrajet body it will overheat at idle. Lowest idle I can achieve is 800 RPM. Running at street speeds keeps the temp down.

I have put new non-flex fan in, new radiator rot out, new water pump and thermostat at rebuild. and have had this problem since the motor was built. (shop has gone out of business, surprise!) Still runs slow, but has decent power, about equal to the 350, 2-bbl that was stock in it before. Could the cam be causing the overheating? or maybe there is something obstructing water flow internally? It still does not seem to run right.

When I started it up, it ran at high idle for over 5 minutes, choke was off, nothing would bring it down and then all of a sudden it just dropped down to 800RPM, maybe it needed to overheat to plug up some vacuum leak somewhere? Any ideas?

A: Do you have a fan shroud on it??? If not, then there is your problem since when the car is not in motion, it requires the shroud to assist the fan in sucking the cool air through the radiator.

One other thing you might try is retarding the timing. 12 degrees might be a little much.With vacuum advance, and mechanical advance, you might be actually running at well beyond 12 degrees at idle causing high heat.

A: First thing that comes to mind is the timing. If it is too retarded it will cause poor performance and overheating. I’m not sure whether the port that is “high on the quadrajet body” is ported or manifold vacuum, but if there is ANY vacuum advance at idle and you set the timing to 12 degrees, you’re timing is FAR too retarded. Disconnect the vacuum line from the carb and plug it. Just to be sure, also cap the inlet to the vacuum advance. Then set your initial timing to 12 degrees and see what happens. Might just solve both problems.

A: Did you add a full shroud? And what is the clearance between the fan blades and the shroud? Also, is the fan positioned in the rear of the shroud, inside the shroud, or is it just inside the rear of it? These factors are very important to provide good cooling suction. I ran my 1968 with no shroud, too small a fan for my shroud, and also improperly positioned and they all greatly affected the temperature of my engine at idle. I now have the original fan, with the original shroud, and with the fan spaced just inside the rear opening.

Take a look-see. If any of the ideas I mentioned are as you currently have it, try correcting it. It made a big difference with mine. Good luck.

What I mentioned about the timing is that if you have the dist too much advanced or too much retarded, that will also affect the temp. To time it, disconnect and block your vacuum source, and time the car, preferably at idle depending upon what your cam calls for, and yoru compression, I would think you should run at no more than 8 degrees but no less than 4. Remember, an increase in timing before TDC, such as going to 12 from 8, is Advancing it. Too much advance will create more heat, and if you’re really unlucky, detonation.

A: I’m going to ask a very silly (but obvious) question:

Are you running with a reasonable compression ratio for the fuel that you are using? I.E., 9-9.5:1 for 93-4 octane? The reason I ask is that it is possible that that 455 was fitted with smaller chambered heads off a 400 or 350. Any shop worth it’s salt would never have done this, but I think it happens. You should not run head chambers less than about 95 cc’s on a 455. Some 400 heads are OK, but the ones off higher compression engines will result in even higher CR on a 455. Higher compression with inadequate octane rating will result in overheating at idle and detonation under load at standard timing settings. (Not to mention broken pistons, rings, etc. Sometimes guys try to “de-tune” the ignition to get rid of the detonation, but end up losing a lot of power. (Been there, done that) Your CR must match the fuel rating or you are going to have problems. Get the head code off the center exhaust ports and make sure they are OK for a 455. Other than this, you been given some pretty good advice from others on the list…….

If I’m being too presumptuous, I appologize, but I ended up with engine damage and wished someone had told me so that I wouldn’t have to find out the hard (expensive) way.

A: I was reading the street machine articles written by Jim Hand and I came across something that jogged my memory. He warns about harmonic ballancer outer ring migration, which usually will throw your timing mark off into the retard region. This will result in heating problems and lost power. I had to replace a ballancer a few years ago because it didn’t line up with my timing pointer @ TDC. (I had just finished checking my cam timing with a degree wheel and had slipped the ballancer on @ TDC to check it). I had just never corrolated this to any previous problems. It is certainly worth looking at….. we usually take that little groove on the ballancer for granted when timing the ignition.

A: I have a ’72 455 in my 1968 and had a similar high heat problem. Apparently the crank pulley was much a much larger diameter than one that would be found on a 400. This was causing the water pump to spin faster than normal. At idle, the coolant was moving through the radiator too fast to be cooled very much. When driving, and with good air-flow through the radiator, the temp would return to normal. I put on a smaller crank pulley and the problem went away. And be aware that not all flex fans are the same. Some really suck.

A: Once again Ive been reading the FAQs and have come across a problem adressed that I encountered. The problem concerns a 1968 with a 455 and over heating.

I put a 455 in my 1967 and, of course, it ran hot. A friend did the same in his 1968 with the same results. Heres how we resolved the situation. I assume the waterpump thrustplate is not missing…Ive seen it frequently!!! First we made sure we had both the lower air baffle (below lower valence) and the two filler panels between the core support and the front bumper support. The filler panels were standard on 400s and on A/C equiped cars. The seal up the grill area and force the fan to pull air through the radiator instead of around it. This, for the most part solved the over heating at idle problem.

Both of use used 400 heads on our 455s and this contributed to the problem. After several broken pistons (we used early small chambered heads) we both switched to late model large chambered 400 heads. This, for the most part solved the problem, but we did end up installing heavy duty 4 row radiators…probably over kill, but then again summer in Florida can get things a bit warm.

Any proposed updates, changes, pictures, and/or corrections, please use our comment section below (may need to click on permalink to access comments feature). Information is subject to change and offered as is without any warranties or guarantees. Please review our Term's Of Use for more information.