big block engine overheating

[Rainer S.]

I have a 66, 427-425 big block car. When driving in normal traffic, the coolant temp is normal (needle in center). When driving in slow traffic, the engine starts to overheat and I have to pull out of traffic. When driving faster, temp. goes back to normal.
Also, after shutting car off, the aircleaner cover is extreamly hot to the touch. I was told, that big block engines run hot, but I don't think, my situation is normal. Engine runs smooth and does not miss. Engine was rebuild by previos owner. (number's matching car)Radiator was flushed, new fluid and thermostat is new.
Any ideas what to look for ?

[Jim38243]

Re: big block engine overheating

Big blocks do run hot (very common). I would change the radiator out and put in an aluminum one, they run up to 50 percent cooler.

[Mike M.]

Re: big block engine overheating

before going out and throwing dollars at suppliers, first make sure the engine is actually overheating. the orig temp guages in our beloved C-2 were not too accurate and the temp sending unit has to be an original in good working order for accurate temp reading. do you have an orig AC temp sending unit(AC stamped on the brass body of unit(may need magnification if you eyeballs are as ancient as mine)>Also make sure the H2O:antifreeze ratio is 1:1. if antifreeze is in greater proportions(ie 60 to 80% antifreeze, silicone comes out of suspension(or solution--been out of Rxy school too long to remember if its a solution or suspension) I had a 67 400 hp that definiotly ran hot(Saw 270 once before I shut it off) and i determined that silcone was depositing on the walls of the heads and block and acting as an insulator. Once i flushed the block and heads etc, the old girl made it from cumberland to bend oregon without exceeding 220F.

[Craig S.]

Re: big block engine overheating

Search back in the archives on timing for the 66 425 engines, there were several threads that related to timing with full manifold vacuum versus the ported vacuum supplied by the 3247. This resulted in better performance, running, and cooler operating temperatures. It may not be what you are fighting, but every bit helps...Craig

[Bill Stephenson]

Re: big block engine overheating

-----Do yourself a favor and check the operation of your de-clutching fan.Check the archives on this as it is a prime source of the problem you describe!.............Bill S

[Duke W.]

Re: big block engine overheating

Your L-72 has what is called a "ported" vacuum advance. The vacuum advance signal port at the carburetor is above the throttle blade at idle, which means that there is no vacuum advance at idle. A SHP engine needs 25-30 degrees of TOTAL idle timing to minimize fuel flow and keep EGT at the lowest possible level. This amount of total idle timing is satisfied with initial plus maximum vacuum advance.

Ported vacuum advance control was implemented to meet CA emission standards and because the L-72 was a relatively low volume option, GM elected to sell this configuration nationwide rather than developing both CA and 49-state verisions.

You can implement full vacuum advance by routing the vacuum advance signal hose to the choke vacuum break circuit. Buy a 1/8" tee, a 1/8" vacuum cap, and a couple of feet of 1/8" vacuum hose. Remove the OEM choke vacuum break and vacuum can hoses and fabricate a new harness using the tee to connect the vacuum can signal hose to a new choke vacuum break hose and use the cap to plug the vacuum advance nipple on the carburetor. This means you can quickly reconfigure to the original parts and configuration if you have your car judged. With the air cleaner installed, the modification will not be readily visible.

With full vacuum advance you will need to go through the idle speed and mixture adjustment procedure. Once it's dialed in, you will have better idle quality, cooler operating temperatures, and better fuel economy, and better low speed throttle response. You will also feel a lot less radiated heat from the manifolds because of the lower EGT. High EGT is good for emission control, but heats up the coolant and reduces fuel efficiency.

Check your idle manifold vacuum and set the idle speed high enough to obtain at least 14". This is to keep your vacuum can pulled to the limit at idle (The L-72 vacuum can maxes out at 12" according to the specs I have.), and you want the vacuum can to be pulled to the stop at 2" less than idle vacuum to be sure it stays put while idling. If the engine does not generate enough vacuum to keep the can pulled to the stop, the total idle timing will vary, which will lead to idle instability.

If the engine won't pull enough vacuum, replace it with a NAPA/Echlin VC1810 can (about ten bucks), which will ensure that the vacuum can is pulled to the stop at idle. The VC1810 can provides full vacuum advance at 8" and is equivalent to the OEM Delco can used on SHP/FI smallblocks.

Big blocks don't have much cooling margin. Chances are that your radiator has lost some efficiency due to deposit formation. Changing to full vacuum advance at idle will compensate for lost radiator efficiency and make the engine run much better at idle and low speed driving.

Of course, you should also do a general cooling system check to include the viscous fan, radiator shroud and sealing, but I recommend the vacuum advance modification for all L-72s even if they don't overheat.

Duke

[Vinnie P.]

Re: big block engine overheating

is that the same one that puked in the parking lot on the way to the convention in Bowling Green? The same one that you drove on the road tour to BG & got a Duntov Award with? The same one that you said "we'll have a couple of beers & let it puke & it will be ok in the morning"?

Former NCRS President Vinnie Peters

[Joe L.]

Re: big block engine overheating

Bill and Rainer-----

Yes, I think that the fan clutch may well be the root cause of the problem which you describe. If you have an original fan clutch, you can have Fred Oliva rebuild it. If you don't have an original, you could use the current GM replacement, GM #3916141. This is an Eaton-manufactured clutch that is VERY similar in configuration to the original fan clutch used on your car.

If you're really serious about idle cooling and you're willing to sacrifice some originality, the 1974-81 HD fan clutch, currently GM #12529342, is the way to go. To use this clutch you have to also use a 71+ fan assembly. Plus, you have to re-drill the fan clutch mounting flange for the 1-3/4" bolt circle used for your pulley and water pump hub (your water pump hub may already have a dual pattern, but you don't want to re-drill the pulley). Lastly, you will need a sleeve adapter to increase the pilot diamter of your waterpump shaft from 5/8" which it is now to 3/4". Install this adapter AFTER you install the pulley on the waterpump and trim its length so that the end is even with the end of the waterpump shaft. Then, install the fan clutch and fan assembly.

The GM #12529342 fan clutch is absolutely the best and highest performance fan clutch GM ever used on a Corvette. It pulls a HURRICANE of air. If the other parts of your cooling system are up to snuff, it WILL END idle cooling problems for a big block, small block, or medium block. Absolutely. Positively.

[John Lokay]

Re: big block engine overheating

I own a 66' 425 hp coupe and followed Duke's advice about getting more advance at idle to bring down engine temps and the car is consequently running better. I have the orginal 260 vacuum can that puts out a maximum 12 degrees of advance at around 15 inches of vacuum. What I did was to re-attach the vacuum advance line, from the ported source at the base of the carb, to the opening on the rear of the intake manifold via an adapter fitting. This is a full vacuum source and since the engine pulls about 15 to 16 inches of vacuum at idle I am now getting the full 12 degree of advance from the vacuumu can at idle. This summer I might try the Echlin 1810 vacuum can that Duke recommends to get more advance at idle to see if this makes a difference.
Question for Duke; What is the best method to use to adjust the idle mixture screws? Should I use a vacuum gauge and adjust each screw to get the highest vacuum reading?

[Duke W.]

Re: big block engine overheating

John - we've probably been through this before, but the specs I have for the L-72 from Corvette News Vol. 9 No. 2 say the vacuum can maxes out with 15 degrees @ 12". Did you transpose the numbers or did Corvette News?

If you are pulling 15-16" at idle then the 8" can might not make much, if any difference if the OEM can maxes out at 12". The primary criterion is that idle vacuum should be at least 2" higher than the vacuum required to pull the diaphram to the limit, so if you idle at 15" and the vacuum can maxes out at 12", you meet the criterion. This means the vacuum can will be "locked" at full advance at idle, which should give good idle stability by having a fixed amount of total idle advance (assuming the idle speed is low enough to keep centrifugal advance at zero, which may not be the case with an aggressive centrifugal curve. According to the same Corvette news the L-72 centrifugal advance is 0@900 and 30@5000, so the centrifugal probably starts at about 1000-1200.)

With all the discussions I've had with the L-72 guys, I've still not been able to nail down the actual test numbers for the max advance @ vacuum for the OEM vacuum can and the idle speed@vacuum reading when the can is connected to a full manifold vacuum source.

What idle speed are you running to pull the 15-16"? The Corvette News Spec says 750-850, but my experience with SHP small blocks indicates that Chevrolet's recommended idle speeds are at least 100 revs below the minimum idle speed to maintain decent idle stability and quality.

The "book" way to adjust the idle speed is to set the idle mixture screws to achieve maximum idle vacuum, but this could occur in a range of 1/4 to 1/2 turn depending on the carb, so it's not exact science. If you don't have a vacuum gage, then just turning the mixture screws until you achieve maximum idle speed is another method. Then you can lean them until you see a little drop, then richen until you get a little drop and set the screws half way between the two points. Or you might be able to run them at the lean limit - just a hair above where you notice a drop in idle revs.

Since the idle mixture screws also affects the off-idle mixture, I may set the idle mixture screws to solve any off-idle leaness based on the following test:

With your foot on the throttle bring the revs up in 100 revs increments to about 2000, then drop it back down in 100 RPM increments to idle speed. If you can't control the engine speed in 100 RPM increments by moving the throttle pedal with your foot, the off-idle mixuture might be lean. Leanness is indicated by the revs going up then dropping a bit. You find that you can't control the revs in precise 100 revs increments due to hesitation and surging. If this is the case richen the idle mixture. If you end up having to increase the idle mixture to the point where it is obviously too rich at idle, then drilling the off idle passages or increasing the size/shape of the idle transfer slots in the throttle bores is the only cure. The actual Rx depends on the design of the carburetor. Off idle leaness is often an issue on emission controlled engines, and the L-72 belongs to this class. It's not usually an issue on pre-emission control engines.

If you can control the revs in 100 RPM increments, then you might try leaning the idle mixture until you have a problem with the rev test, then richen it enough just to get rid of the leaness. I usually try to find the ragged edge, so I am as lean as possible without causing hesistation.

With full vacuum advance at idle, the engine will pull more manifold vacuum, and since the idle and off idle mixture is controlled by manifold vacuum, not venturi vacuum, the full manifold vacuum advance modification will tend to richen the mixture. This is why I state that once the modification is made, one must go through the idle speed/mixture adjustment procedure, The greater idle advance alters the idle speed/manifold vacuum characteristics, so the idle and off-idle mixture have to be retuned to achieve the full driveability, throttle response, and fuel economy benefits.

You can apply the basic "book rules" for your particular engine, then, by experimenting, you can usually develop a SOTP method to fine tune the mixture. It comes down to having a "feel" for your particular engine.

Duke