Idle Overheating

[Glenn B.]

Have been away from the forum for a while - busy with other things, with just the occasional summer drive for the C2. Got back at I this weekend and the top of my "to do" list was to investigate the idle temperature.

I bought and calibrated an IR gun. I have a stock 67 L36. I took a series of temp readings this aft. At 75F ambient, temp at idle (after maybe 40 min) measured at the thermostat housing with hood open rose to 260. The temp at the rad outlet was difficult to measure but looked about 145F (when tstat at 260)

I have looked through many of the relevant posts and eliminated a number of suspects as follows:

1. Idle timing = 29 degrees (@700 RPM)
2. Coolant = full
3. Fan blade centered (front to back) 50% inside top of shroud
4. 
   
   
   I'm running out of ideas - thinking I might reverse flush the engine next
   
   Glenn

[Dino L.]

Re: Idle Overheating

My 64 365 hp had a cooling at idle issue.....I chased it down.....timing......do research in the archives and find out the details.......vacuum, 2" rule, correct timing, etc....

[Mike T.]

Re: Idle Overheating

Glenn - What was the reason you are doing this investigation? Are you seeing coolant overflowing? Something sounds off on that 260 temp you're reading if your also seeing 145 at the bottom of the radiator. Out of curiosity, what does the dash gauge read when you're seeing 260 via the IR gun?

[Leif A.]

Re: Idle Overheating

stuck thermostat??

[Duke W.]

Re: Idle Overheating

29 degrees total idle timing doesn't make sense with a 32 deg. centrifugal distributor and a 12 deg. VAC. That implies that initial advance is 17 deg. and total WOT advance is 49 deg. NO WAY! It would detonate to death unless the CR is less than 8:1 and you are using premium. The data doesn't make sense unless the initial is way too much.

Look at the spark advance map specs in the AMA Specs or service manual, and tell us what they are.

Does the engine have an aftermarket cam?

What is the idle vacuum at 700?

What's the number on the VAC? Have you tested it for conformance with specs?

It's just not possible to have a 120 degree difference between the radiator inlet and outlet unless most of the water pump impeller blades have been ground off and the coolant circulation rate is barely measurable. Twenty degrees is typical if the coolant circulation rate is as designed.

At idle speed the coolant flow rate is in the neighborhood of 10 GPM, and 80 GPM in the upper rev range, so coolant doesn't spend much time in the radiator and doesn't loose that much temperature. The theory on engine cooling is to circulate the coolant quickly to maintain even temperatures in the engine. If the coolant is overcooled in the radiator, the engine is subject to thermal shock. High coolant flow rate and 10-20 degrees difference in radiator inlet-outlet temperature is normal for most automotive engines.

My Harbor Freight IR gun reads the average of a 4:1 cone, and most others are probably similar, so you need to have the gun very close to the surface you want to measure of it will "see" colder or hotter surrounding surfaces and give an average reading.

Get the spark advance map squared away, and if it still runs hot remove the thermostat and put it in a pan of cold water on the stove with a meat thermometer. Turn on the heat and watch for the themostat to start to open at the opening temperature rating, which should be stamped on the part. It should be full open by 30 degrees above the initial opening temperature.

Duke

[Glenn B.]

Re: Idle Overheating

Duke, thanks for your message and Qns.

I did a full rebuild of the distributor when it ate the tach gear in2008. I have a B25 vac can with 16 degrees advance, from my records - I set the initial at 12 - so the 29 degrees I measured today is bang on where I had left it 7 years back. When I mapped it back in 07 - centrifugal was only adding 13 at 3000 RPM - so I still don't think the timing is my overheat at idle issue?. I have the 1111247 dist - I'm not sure I know why my centrifugal is not 32 (stock) as you note it should be - I seem to recall springs were not stock probably explains advance curve diff.

I had a hard time reading lower rad temp -I have a 50:1 IR gun, which I thought would be OK from distance - but my lower numbers are suspect. I think need to go back and carefully check outlet temp - may have a tstat stuck issue?

I need to also check the heat riser valve.

Good info on flow rate and cooling.

Glenn

[Duke W.]

Re: Idle Overheating

OE full centrifugal doesn't come in until about 5200, so you would have to rev it at least that high to set total WOT advance. Get a spring kit and install some lighter springs, then rev the engine until it stops advancing and set total WOT advance in the range of 36-40 - as much as it will tolerate without detonation, which would put the initial in the range of 4-8, and the total idle advance should be in the range to 20-24 with a 16 deg. VAC.

You said B25 VAC, but do you mean B26? What about idle vacuum? Does the B26 meet the Two-Inch Rule? It will with the OE cam, but how do you know what cam is installed? It's very common to find higher overlap aftermarket cams on rebuilt engines.

I can't tell you how many guys are in denial about the effects of the spark advance map on coolant temperature. It's like I have to beat their heads into the ground to get them to take it seriously.

Part of the problem is often in their measurement technique. Common Internet advise is to set the total WOT advance at 3000, but if full centrifugal isn't all the way in until above 3000, the engine will be overadvanced. You have to determine where the centrifugal starts/stops. OE on your engine is 900/5200, so it's best to set the initial below 900 and then quickly rev the engine to 5000+ to ensure it fully advances. But you first have to test to at least find the start point of the centrifugal because it may have been modified in the last several decades. Then set initial below that speed

On 30-30 cam engines the centrifugal start/stop is 700/2350, but a 30-30 cam engine won't run long enough below 700 before it stalls to set initial, so the easiest way is to set total WOT advance at 36-40 at 2500-3000. Most OE spark advance maps don't see full centrifugal until 4000-5000, and centrifugal starts a few hundred revs above normal idle speed, so it's easiest to set initial at below the speed where centrifugal starts, then quickly rev the engine to test for full advance; 30-30 cam engines are an exception.

Get your spark advance map squared away, and if it still runs hot, look for other causes, but about half the time, correcting a bubbaed-up spark advance map solves the hot running issue, especially if cooling system components are fairly new and known to meet or exceed OE performance.

Duke

[Glenn B.]

Re: Idle Overheating

Duke, good advice on setting my curve back to stock. Need to add that to the to do list. I suspect its only a matter of replacing springs and readjusting initial. I have the vac and timing tools to do so. I simply do not know what cam is in the engine now - but as it will idle quite smooth and reliably below 600 - so it can't be too radical a cam.I have always read the too LITTLE advance was a cause of overheat - I did not believe that too much could also be an issue at idle?. Is it possible that a reduction in idle advance from 29 to say 20 would reduce my idle temperature?Determined that my heat riser is not moving smoothly beyond first 30 degrees of travel - so I will first fix that and will accurately re-measure rad outlet temps as my next step in this process.Thanks

[Leif A.]

Re: Idle Overheating

Duke, good advice on setting my curve back to stock. Need to add that to the to do list. I suspect its only a matter of replacing springs and readjusting initial. I have the vac and timing tools to do so. I simply do not know what cam is in the engine now - but as it will idle quite smooth and reliably below 600 - so it can't be too radical a cam.I have always read the too LITTLE advance was a cause of overheat - I did not believe that too much could also be an issue at idle?. Is it possible that a reduction in idle advance from 29 to say 20 would reduce my idle temperature?Determined that my heat riser is not moving smoothly beyond first 30 degrees of travel - so I will first fix that and will accurately re-measure rad outlet temps as my next step in this process.Thanks

That heat riser not opening fully will certainly add to your overheating problems. I see by your profile you live in Canada. Down south we either remove those things or wire them fully open. You might try wiring your new one fully open during the warm season (or, permanently if you don't drive it in the cold weather).

[Duke W.]

Re: Idle Overheating

Duke, good advice on setting my curve back to stock. Need to add that to the to do list. I suspect its only a matter of replacing springs and readjusting initial. I have the vac and timing tools to do so. I simply do not know what cam is in the engine now - but as it will idle quite smooth and reliably below 600 - so it can't be too radical a cam.I have always read the too LITTLE advance was a cause of overheat - I did not believe that too much could also be an issue at idle?. Is it possible that a reduction in idle advance from 29 to say 20 would reduce my idle temperature?Determined that my heat riser is not moving smoothly beyond first 30 degrees of travel - so I will first fix that and will accurately re-measure rad outlet temps as my next step in this process.Thanks

You said you have a vacuum gage, right? Measure manifold vacuum at idle speed, which you should choose between 600 and 700. Install the vacuum gage signal hose, in parallel with the vacuum advance line by using a tee and a short piece of vacuum hose to connect the tee to the VAC, like this:

VAC-T---------carb. vac. port
.....vac.
.....gage

Specify the exact idle speed that you measure idle vacuum. This data will tell whether you have an OE cam or something close.

The two biggest factors in idle vacuum are valve overlap and idle speed. For a given idle speed, greater overlap will reduce manifold vacuum, and for a given overlap, the higher the idle speed, the higher the vacuum.

Knowing idle vacuum will also allow you to determine whether the installed VAC meets the Two-Inch Rule, and the installed VAC should be tested to make sure it starts and stops at the nominal specified vacuum input.

You haven't confirmed the installed VAC. I'm not familiar with a B25 that you said is installed. The correct replacement for your engine with an OE cam is B26. What's installed?

Neither I nor anyone else can offer much useful in diagnosing and solving a problem unless we are provided accurate installed part information and test data.

Duke

[Glenn B.]

Re: Idle Overheating

OK, I did the easy things first. I found the heat riser was sticking and not moving freely, so I wired it open this morning. I took a new round of temp measurements (the rad temp differential looks OK - see below) I am now going to dig into the vacuum and timing situ. - will post when I collect the data


July 26, 2014 67 427-390 Temps @ Idle (700 RPM, 75 deg F Ambient)

Temp @ TSTAT Housing Deg F	Temp @ Upper Rad Inlet Deg F	Temp @ Lower Rad Outlet Deg F	Inlet To Outlet Delta Deg F
110	73	71	2
140	75	71	4
160	78	71	7
170	120	80	40
180	171	165	6
193	193	176	17
200	200	175	25
205	205	191	14
212	209	198	11
215	214	203	11
221 UPDATE...	221	209	12

228	228	212	16
233	233	221	12

Vacuum is 16 inch at Distributor Can, 730 RPM timing at 22 degrees (vac is from carb - not manifold) I don't have fittings to connect to manifold vac, but this does not seem related to my issue.

Temp continues to climb.

Next step... maybe service rad?

[Duke W.]

Re: Idle Overheating

I can't correlate your data to anything because you still haven't confirmed what VAC is installed - the data stamped on the mounting bracket. You said B25, which is not OE equivalent and not one that I recommend. I recommend a B26 for your engine. Please confirm the number.

If you teed into the VAC signal line as I explained in post #10 you should be reading manifold vacuum and 16" @ 730 indicates an OE cam. You should be able to get it to idle at 600 with 14-15".

I also think your fan clutch may be weak. It should be very stiff at over 200 compared to less than 180, and if you free rev to 2500-3000 RPM you should feel substantially more air flow and hear much more fan roar at over 200 than at below 180.

Point your timing light at the fan. At idle over 200 it should appear nearly stationary, which means the clutch is not slipping. As revs increase it will appear to move increasingly faster backwards as fan speed falls below engine speed. Maximum fan speed is about 3500 when the viscous clutch is fully tightened regardless of how high you rev the engine.

Does it run over 190 at steady speeds on level ground at 35 MPH and up?

Duke

[Glenn B.]

Re: Idle Overheating

Well, I stand corrected, took my distributor cap off tonight and my VAC is indeed a B26 - seems my notes from when I did the rebuild were incorrect (prob a typo). The data I could find on B26 says 16 deg at 13". I measured 16 inches of vac teed into the line that feeds the vac - so it seems this is not my idle issue. I took the car for a run and it does not respond as it should when the pedal is pushed to the floor - I'm sure my timing curve is off - but that's for another day...

The car runs cool - est 180 at highways speeds, even fine at lower speeds - temp is just an issue at idle

Your suggestion to use the timing strobe on the fan is good one , I had the same idea myself this aft. I can say it is clearly running slower than the crank at idle.

Is there a simple and reversible way to lock up the fan clutch?

I'm thinking a Robertshaw 330-180 high flow thermostat could not hurt also?

[Timothy B.]

Re: Idle Overheating

Have been away from the forum for a while - busy with other things, with just the occasional summer drive for the C2. Got back at I this weekend and the top of my "to do" list was to investigate the idle temperature.

I bought and calibrated an IR gun. I have a stock 67 L36. I took a series of temp readings this aft. At 75F ambient, temp at idle (after maybe 40 min) measured at the thermostat housing with hood open rose to 260. The temp at the rad outlet was difficult to measure but looked about 145F (when tstat at 260)

I have looked through many of the relevant posts and eliminated a number of suspects as follows:

1. Idle timing = 29 degrees (@700 RPM)
2. Coolant = full
3. Fan blade centered (front to back) 50% inside top of shroud
4. 
   
   
   I'm running out of ideas - thinking I might reverse flush the engine next
   
   Glenn

Glenn,

I am not sure there is really a problem here. You stated at idle after 40 minutes the idle temperature rose to 260*. I think most of these cars would overheat if idling for 40 minutes. Somewhere else you stated the cars cools fine at speed so what are you trying to cure.

[Mike T.]

Re: Idle Overheating

Glenn - Can you add some detail to your mention of the radiator being 'rebuilt' 8 years ago? Was is simply rodded out or was a brand new core installed? Also, you said this was at the top of your list, how long has it been heating up?