1969 L36 - Overheating Problem
 

Hi Everyone,

I have a 1969 L36 427 that has begun to overheat big time in recent weeks.

Apologies for the lengthy post, I just wanted to add a little history on the vehicle as of late for reference:

When I purchased the car a year ago I was told the carburetor on the vehicle was not original and that the original one would be included in the sale, which it was. The vehicle always seemed to lack the power I expected of a 427 and when I took it to a carb specialist in Huntington Beach, CA I learned the carb on the vehicle was set up for a small block which would explain the lack of power. Fast forward to two weeks ago and the car got it's original rebuilt carb mounted. The car became a rocket. However, the vehicle overheated within 20-30 mins of freeway driving - I'll admit I pushed it a bit. I took the car to a radiator shop who is currently in the process of re-coring the radiator and replacing the fan clutch with a thermo style fan. Today I was told by the radiator shop that the old core wasn't too plugged up. We will be replacing it with a higher efficiency 4-row core that supposedly will be 25% more efficient.

This news that the radiator wasn't too bad concerns me that the problem for overheating doesn't lie in the radiator. My question is if a bad distributor can cause overheating. I do know that the distributor in the car needs a rebuild as the carb shop let me know that limiter bushing likely cracked as the full advance has gone too far. Below are the timing specs the shop provided to me once they mounted the carb - 15 degrees at 650 RPM and Full Advance at 46 degrees at 3,300 RPM.

Again, apologies on the lengthy post - Just curious to know thoughts on this problem and if I'm totally off base on this distributor thought OR if the radiator likely will correct the problem.

Thanks for the thoughts and help!

If you are replacing the radiator, I suggest that you go with an aluminum "Direct Fit" from Dewitts. Have them paint it black before shipping. Get a set of radiator and shroud seals from Doc Rebuild. Remove your front license plate bracket. Make sure all the air dams are in place. Check to see that your fan clutch is working properly. Initial timing at 15 is fine if you have 36 total, including centrifugal. Are you on ported vacuum advance from above the throttle plate...or on intake manifold vacuum ?

overheating has many things that cause this, lower hose collapse,wrong timing,bad stat
air pocket,low on fluid,fan surround weather strip missing, lower air dam missing, bad
water pump, your initial timing seems high, good luck

How hot is it getting? As far as the timing goes 15 degrees @ 650 rpm and 46 degrees @ 3300 rpm are normal and wouldn't cause your engine to get hot. Timing too far advanced causes pinging and timing too far retarded causes overheating.

I have always found most "overheating" problems with the '68 thru '71's aren't overheating at all but rather the result of using a wrong sender with the "green face" OEM gauge. The sender for the green face gauge has a single round post sticking out of the sender and the wire terminal plugs onto the top of that post. If your sender doesn't have that round post it's the wrong sender. You can buy the correct sender from any of the Corvette-only vendors that support this forum.

46 degrees @ 3300 is not normal. That would mean you have 31 degrees of centrifugal advance! My buddy runs 42 degrees on his L88. How is the shop reading that...with vacuum advance plugged and a dial back or digital timing light?

Still not the cause of your overheating problem.
My L36 runs cool in summer Miami traffic...after I addressed those issues mentioned in my first post. When I first got the car, it was puking all over the place.

When the carb mechanic mounted the rebuilt original he did it with a timing light. He told me that the car had been cammed. He said it has very little vacuum at base timing which he felt was a sign the cam was quite large. He said it was likely something near 240 degrees at .050. I'm assuming that may explain why the timing needed to go way over 36. But if your buddy is running 42 in an L88 I still think 46 is way over. Only other info I know about the car is that the compression ratio is basically around 11:1.

To your point about the car puking - This car also has a history of puking radiator fluid out of the expansion tank on a semi frequent basis.

:iagree:
I also have a 69 L36 that is stock including the smog system. The original rad was leaking and when I pulled it, it just wasn't worth fixing so I went with DeWitts. I have never had an overheating problem due to timing or anything else. I run a 180 stat and it runs 180 in 100F weather.
On a 69 L36 the initial timing was set for smog at 4 degrees advance but the centrifugal advance was 24 degrees which made a total of 28 degrees plus the B360 vacuum advance can that was another 12 degrees. You could bump up the stock initial timing from 4 to 12 degrees and with the stock 24 mechanical get 36 which really would wake them up. If you have a limiter on your distributor it isn't stock and something is wrong if that is the timing you are getting.
I now use an advance limiter bushing that reduces the stock mechanical on the original distributor from 24 to 17 degrees and then use 17 degrees initial timing (34 total plus 12 vacuum) with 91 octane no ethanol for a very smooth running engine that never comes close to heating up no matter what you do.
Also, you might check the end play in the distributor shaft as slop really changes timing. Due to the helical gear every .013" of up and down end play in the distributor shaft will be good for 1 degree of timing change. Some of these old distributors have over 10 times that....

I'm surprised the car will idle at 650 with 11:1 CR and that cam. What fuel are you running?

If the car was puking out the expansion tank, you might have an air pocket. The tank should never be over half full, cold. Check for a pressure leak and any other leaks. Are the pressure cap, water pump, bypass and thermostat all ok?

I’m running 91 with octane booster. Sometimes i’ll hit a station near me that sells 101 but it’s super expensive at virtually $10/gallon.

I was told today by the radiator shop that the water pump and cap are good. They’re planning on replacing the thermostat.

Regarding the expansion tank - should that thing typically be 1/3 full cold?

I have a 68 L36. Original re-cored Harrison radiator from Lockport, NY. Never overheats. However my car is pretty stock. I suggest backing off your timing. I bet the new radiator and thermo clutch will help. I use an AC fan with 7 blades. Good luck.

I'm running '68 L36 with modified 489 cu in engine - it reads 170 - 180 all day, in heavy traffic, perhaps 190/195. I also use a DeWitt radiator. Not too sure if an extra row in the radiator will help matters - with the rad sloping back at an angle I suspect it will reduce the passage of air through the rows.
It the car has been used for a number of year without anti-freeze (or an inadequate mix) crud will build-up in the lower part of the radiator and more importantly the water ways in the block casting, reducing the volume of cooling water passing though - it may need a chemical flush. It the water pump is stock with a steel impeller lack of coolant and cavitation will possible erode the fins on the impeller and further reduce the cooling efficiency. Finally ALL air that comes through the front end MUST go through the front of the radiator and no where else - so seal off any gaps in the front radiator mount. Seal-off the fan shroud, so that the stock viscous coupled fan only draws air through the radiator and shroud. Each of the above checks will reduce the temperature by 2 or 3 degrees - find 4 or 5 of them and you have fixed it!

Thanks for the help! I just learned that the radiator in the vehicle is not even the original Harrison radiator. Apparently its some aftermarket that has an intercooler in it for an automatic - not sure why the previous owner did that given my car has the original 4-speed in it. The work had already begun on fixing the aftermarket radiator and the new core for it arrived. I was told the core that came out of it had 10 tubes that were totally plugged up and that the total count of functioning tubes was 110. My understanding is that the new core that's going in the car will have 180 functioning tubes which is virtually a 50% pickup in cooling capacity. Hopefully this solves the problem! In the future though, I probably will yank this thing out for a DeWitts.

The new radiator will solve the problem.

My original 4 core was boiled/rodded and "in good shape".... The car never ran hot, but it was hotter then I wanted on the freeway (195-205*) and would continue to climb.... I tried everything else, over looking the radiator since it was "in good shape".... Eventually I bit the bullet and installed a Dewitts and my freeway temps dropped 20-25* instantly in the same test conditions.

Just make sure you have the chin spoiler on the car and the radiator to hood foam is in place and sealing.... If it still runs on hot the freeway after that, then you have another issue that is likely engine/tuned related.

Here is my extensive big block freeway temp testing thread and results if you are extremely bored and have nothing better to do....:D

https://www.corvetteforum.com/forums...e-step-in.html

If you have the original unmodified distributor, 1111926, the factory Chassis Service Manual shows 26 crankshaft degrees of advance at 3,800 RPM. Add 15 degrees of initial and you have 41 degrees. The underside of the distributor main shaft has a pin that had a nylon bushing on it when it was manufactured. If this bushing is gone/cracked/etc. the 46 degrees total without vacuum advance is quite possible. Installing a distributor recurve kit that comes with a metal bushing would be advisable. Keep the original weights and install a combination of springs that gives you a total of 36 degrees at about 3,000 rpm. You also will need to do something with the vacuum advance to limit it to about 10 degrees.

Sure it is. The the 46 degrees is the sums of the initial advance, the mechanical advance, AND the vacuum advance. Ideally we want 50 to 52 degrees of advance at light-throttle cruising speeds so 46 degrees is still 6 degrees short of that 52 degrees.

Only if you say so. Vacuum advance is not present at 3300 rpm. Besides...I asked the OP how his mechanic was timing the engine...and if the vacuum advance was plugged. Vacuum advance only figures into the total timing envelope at idle or coming off idle, low rpm conditions.

3300 will be a pretty high vacuum area, and it will definitely be pulling on the vac advance at that point. The only time the vacuum will drop out is at WOT or pretty close to it, heavy load situations.

I actually made a limiting plate to restrict my vac advance to only give me another 5* of advance at idle/cruise. Even just with 5* additional timing (putting my total cruise advance at 36*) knocked about 4-5* off of my running temp.

From the factory, cars were set up to have 45-52* of timing advance at cruise for efficiency and emissions, but there is a point of diminishing returns, especially on todays fuel... Back then, they had 100 octane in the pumps..

Lots of these old cars today have to be careful with timing adjustments with today's ethanol fuel.

Thanks for clearing that up. I just have never tried to read the total timing with the vacuum advance connected...never in over 50 years. I remember looking at plotted graphs showing it is an area of considerable variation...sorta like the projected path of a hurricane. :D

Yeah....I usually don't either..

I normally just time them with the vac advance plugged, rev it enough to get the centrifugal advance to come in and stabilize and lock it down.... Then I would just hook up the vac advance and whatever it was, is what it was.... At least until I really started checking EVERYTHING on my 496... Now I make a point to at least check the vac advance timing and make sure its not some crazy amount like they ran back in the 60's.:D