Creeping temps at idle

[davepl]

My bone-stock restored 402 (which means every bit of fan shroud, heat riser, carb stove, choke, clutch fan, weatherstripping, etc is new and back in the factory spot) has been heating up at idle.

It runs around town straight on the middle of the gauge. But if you have to sit for 20 minutes on warm/hot day, it creeps without apparent limit - only I intervene once it gets to the "Red" zone of the gauge.

But it cools perfectly underway, so I though the only difference is airflow, and the clutch fan was original, so I replaced it. Not with the $23 Rock Auto unit, but to be sure, a brand new GM $100 clutch. But no change.

Yesterday I drove it about 5 miles, full operating temp, then parked and let it idle. Within 20 minutes it was up to touching the red. Turned it off for 10, drove it, and back in the center of the gauge within a mile or two.

It's not like the light would have come on, but I doubt these ran hot on the gauge in the olden days. Timing is more advanced than stock, so its not that (about 10 degrees vs 4 indicated, but that makes it cooler, not hotter).

Could be leaner than stock I guess, but it runs and idles OK. Can the idle mixture actually cause overheating?

Any ideas?

[dennislbrooks]

Do you run ported or full manifold vacuum if you have vacuum advance?

May 2016 Super Chevy says to run manifold vacuum unless idle emissions are required. If you run ported vacuum now as I am in my 71 402 bbc --10 degrees advance --- 800 rpm idle --stock 350hp cam ---- when it is about 90 out my temp creeps up to 205 or so degrees in gear stopped for a spell. Otherwise it runs about 180 -190 degrees using a 180 degree thermostat with a stock fan clutch and shroud and AC radiator - AC off.

Super chevy says that using manifold vacuum will allow you to set static timing back to say 6-8 degrees, back out idle screw to adjust for 200 more idle rpm and also may make the mixture screws to be more responsive. Idle timing will jump to about 20 degrees and allow the engine to idle more efficiently. This will help keep the engine temp from rising when stopped -or not moving.


I run a 1980 or so quadrajet from a corvette 350 sbc and at moment mine starts good hot and cold and accelerates fine does not diesel but I think I may try the manifold vacuum.

The passenger front anlged bottom port and the driver middle front port are both ported vacuum on my quadrajet so I may tee into the transmission vacuum or another spot. I checked both with a vacuum gauge -less than 5 on idle.

[davepl]

I don't want to get into a whole vacuum advance thread again, but I prefer to run things the way GM designed them to operate. It should work, the only thing that has changed is the calendar and the gasoline, and I don't know if gas makes a difference!

My shop buddy said that new motors (only 150 miles) do run a little hotter when new until they're fully broken in, but he didn't think it'd be enough to account for it.

If I could find a really shallow (like 2-3") fan I'd add it inside the heat exchanger stack as a backup. Guess it's time to look at Summit!

[Mrturner1]

This is exactly the same thing mine does, although mines far from factory and not nearly as nice and new as yours. But, it has identical symptoms. When it's sitting at idle for 10 or so minutes it creeps up slowly until it hits 200 and the fans kick on bringing it back down. I've had people tell me it does that because I took the clutch fan and stock shroud out and replaced it with electric fans.

[Coley]

Dave...do you have one of the laser dot infrared heat guns? (often used for analyzing home heating/cooling winter 'leakage' issues)
Backcheck your system with one of those as well....simply point and shoot under the hood at the manifold to ensure that both your sender and gauge are accurate.
All Good
Coley

[70STOVEBOLT]

Does it build pressure in the cooling system? My 402 will get cooler while it is idling in park, not hotter.

[1968aj]

my fully stock 402 did the same thing. I changed to a 160 thermostat . no problems after that and it was a daily driver.

[dennislbrooks]

davepl, have you had overflow when it shows hot on the gauge? Mine has not overflowed. I do not run an overflow tank and I run the stock 13 lb radiator cap. I am not sure 200-205 is anything to worry about in drive stopped on a warm day. The bbc holds a gallon more coolant than a sbc. What is ok on these bbcs?

[fwayne]

Timing can have a big impact, but only if it is running flat or even retarded. a Lean idle fuel mixture will also cause problems. Have you adjusted your idle mixture using a vacuum gage? Doing so will not only give you a better mixture and throttle response, but should help that warming trend. (you adjust for highest idle speed highest vac reading, then open slightly more to richen) always adjust at your desired idle speed setting.

Warming idling engines have a root cause. Idling should be a cooler condition always vs load. You have the fan, so I would look at radiator. if its stock, it could have plugged passages and reduced efficiency. Another place to look is the water pump. The pumps metal blades could be bad. You said its a fresh engine, so I would assume you used a fresh pump. But the simple cause could be a bad thermostat.

Another trick, remove the radiator cap (cool engine of course) and warm up. it should be cycling water very obviously. the thermostat should open, with a visible flow through the radiator. then as the temp drops due to the cooled water from the radiator it should close, this gives the water in the radiator time to cool. a stuck or bad thermostat, or a lack of one altogether, will not have a proper cycle.

[davepl]

I did notice that my heat riser valve is pretty aggressive, and still bounces closed at hot idle. Most of the exhaust comes out the driver's side pipe still.

Could that extra heat from the exhaust being forced up through and over the intake manifold and out the other side be causing any significant amount of engine heat?

On the face of it, it sounds like "Well duh, you're forcing hot engine exhaust through your water cooled intake manifold", but I don't know how MUCH heat that really amounts to. I think I'll wire it open for the summer to see, but hate doing stuff like that without knowing what I'm doing for certain.

[DBK81]

idling for 20 min? Who is idling for 20 min and why?

[fwayne]

Actually Idling 20 minutes is a great troubleshooting technique. Better to find out in your drive-way that the thing needs attention than to get stuck in a car show parade, or in traffic and loose an engine. Every year we see this occur in some of our cruise-in car shows. Someone always looses head gaskets or boils over only to find they needed a cooling problem fixed. It happened to me but I caught it early enough to avoid damage. Turns out my 45 year old radiator was about 60% blocked internally. It cooled just fine driving and short trips, but an hour long slow speed procession to get into one of our large car shows had my needle pegged.

[davepl]

Originally Posted by DBK81

idling for 20 min? Who is idling for 20 min and why?

Me, because going and getting intentionally stuck in traffic is a very stressful way to troubleshoot an overheating issue.

Since it's in stop-and-go traffic where it happens, sitting and idling at home is the safest and easiest way to replicate that environment. I'd do it in Drive with the parking brake on for even more realism but I'm too nervous to leave it sitting like that...

[DBK81]

Originally Posted by davepl

Me, because going and getting intentionally stuck in traffic is a very stressful way to troubleshoot an overheating issue.

Since it's in stop-and-go traffic where it happens, sitting and idling at home is the safest and easiest way to replicate that environment. I'd do it in Drive with the parking brake on for even more realism but I'm too nervous to leave it sitting like that...

Very true, and makes perfect sense. I come from a place where if you are sitting for 30 seconds it's a traffic jam, we just country folk, lol

[Ed F]

I have a creeping temperature story with a little different twist. When I bought my '70 C10, 350 3speed, if left idling the temp gauge would creep up almost to redline. Antifreeze looked rusty so I took it to a radiator shop to have it flushed. It was so clogged they couldn't flush it so they put a new radiator in it. Same temp readings. Changed the water pump and t stat...same readings. Put a cooler t stat in....same readings. At about that time I replaced a burned out headlight that lasted two days. That made me check voltage while idling. Started at a nice 14v..then 15..16..17..18 holy crap shut it off! Changed the voltage regulator, nice steady 14 volts and guess what... The temp gauge stayed reading cool while getting 14v instead of rising voltage. Things aren't always what they seem to be.

[davepl]

Good catch, that would be a tricky one to diagnose!

[70STOVEBOLT]

Dave, did you make any progress on finding out the cause of the creep?