I've had my '77 for 2 years. The previous owner said it ran steady at 220 degrees. It did that for me for awhile. Then as summer hit, and I drove further (an hour or so) it climbed to 240, 260 and stalled out (not running the AC, by the way). This is at highway speeds only; it cools off on secondary roads. This pattern is always the same. On cooler days and shorter highway drives, it hits 180, then 200-210 and everything seems fine. But last week it hit 220 on the highway, sunk to 200 on secondary roads and started stalling when I braked at stop signs and lights.
Here's what I've done in the last year, which hasn't helped: New stock radiator, 2 new thermostats (160), new fan clutch (it was loose), checked for combustion gas (none), checked pressure (steady), added chin spoiler that was missing when I bought it, and had timing checked. My next and last option seems to be a high-flow water pump. The current water pump was changed in 1993. Any diagnoses or suggestions out there would be greatly appreciated, before I give up on highways forever. Thanks.
Rick
:confused:


[Modified by RickP, 10:40 PM 6/11/2002]

 

You may also want to check your air/fuel mixture. Too lean of a condition can also help an engine overheat. Seems like you've check pretty much everything else, what is your timing set at? I would probably try a different water pump, there is no way that your engine should be running that hot at highway speeds.

BTW I would not recommend letting your temperature get to 260 degrees. Good luck, let us know what you find.

 

Rick, several things.
- Make sure the lower radiator hose has the spring in it. Higher RPM's can collapse it and restrict the flow.
- Make sure all the seals are around the radiator and the shroud is in place.
- A higher flow water pump will definitely help, but may not be the whole problem.
- Check your timing out for initial, centrifical advance, vacuum advance and total timing at 3000 rpm's not just idle.
- What do your plugs look like? You might be running too lean.

For a small block unless you have made some serious modification to the engine, 220 is too hot. Even a stock radiator should cool adequately.

 

As posted earlier check radiator hoses and seals around the radiator and fan shroud. Air, at highway speeds, will take the path of least resistance, which will be around the radiator rather than through it if the seals are not in place.

 

Thanks. I'll check that. Only once did the temp approach 250, actually.

 

Most of the shroud seals are in, but there are some gaps. I will pack it in better with pipe insulators. Thanks.

 

OK, you did not mention the year of your car, but everyone referrs to the chin spoiler, so I assume it's a plactic nose shark....
everyone SAYS that chin spoiler is necessary for at speed airflow,....ME, sorry but on a friends car some years ago, we didn't have it on because it was broken up like all of them from driveways, and plain intertia...well anyway, her can never overheated here in Florida highway driving for lack of that chin spoiiler...so I submit it's a red herring...not the source of problems....
at any rate, what is the rear gear set...anything over 323 or so is troubles on the freeway, check that spring in the lower hose, keeps it from collapsing on the suck side of the pump.....it's not the pump...allmost bet on it...check the flow.....drill holes in the t-stat plate there, about two 1/4 inch diameter holes....that allows air trapped to burp it way out.....
also just because it's a new stock radiator don't mean much...is it at LEAST 3 cores deep??? if it's somcehint like 2 cores, forgetaboutit...won't do the job....now if you have an early chrome bumper shark, remove the front tag assy, mounting plate and all, try it that way.....

GENE

 

Gene

I see the two holes in the t-stat a lot but fail to see how it matters once the it opens. Wouldn't the air pass through once it opens without having to drill holes in it? Just curious.

Thanks

Chris

 

Miranda runs hot when she's low on oil and/or transmission fluid. I think I've fixed the last trans fluid leak, so I'm looking forward to the performance of a full transmission.

Check your oil level - it takes heat out of the engine also. GM

 

Admittingly you have replace almost all of the cooling system. Read all the suggestions and take one stop at a time to diagnose the problem(s). As I said before if you replaced the radiator with a "stock" unit and with a 160 or even a 180 it should cool. Couple of quesions.

- What is your rear axel ratio? Or what crusie RPM's are you running at at 60? Maybe the old owner has some tall gears in there.
- Pull several plugs on both banks, clean them up and run them for 50 miles or so. Read them and see what your A/F is looking like.
- Install a temporary mechanical temp gauge (about 20 bucks) in the head with the emission temp sensor. Think it right side on a 77. Then you can tell what temp you are running at various RPM's or idle.

The holes in the T-stat are to let steam pockets and air pockets out. It also increases the flow somewhat under all conditions. I run a BB with a super stat and have 4 - 1/8" holes drilled. It lowers the temps by about 10 degrees when fully warm. I have tried with and without - it works.
Good luck.

 

Still not buying the air pocket theory since when it is open what is keeping the air in? I could see if it was concave then you would get a tiny air pocket but how much air could the top of a thermostat hold? Must be something more to it. Can't argue your results though, 10 degress is a nice drop.

 

I tend to agree with Chris. The holes in the T-stat allow air to escape while filling the system. They are out of the equation once the T-stat opens. I've never had a problem with air entrapment. Except on a SVT Cobra. There is a vent screw on them specifically for this purpose on them.

 

I'm having a similar problem with my car. I suspect part of it is a lean condition.

Last year I was running WAY hot on the freeway. Even earlier this year I was running pretty hot (220ish) on the freeway only. 3500rpm @75 with 3.70 gears.

Put on:
New Chin Spoiler
Rad support to hood seal
Rad to Rad Support Upper seal

Removed:
License Plate & Bracket

Yesterday in the heat (90-ish) I was running on the high side of 210 or so. That's hotter than I like, but better than it was. My next move is going to be to drop my coolant/water ratio to mostly water & add some water-wetter and see what happens.

Oh yeah, I also have a big block radiator in my car. It should cool great with no problems. AND I have an auxillary electric fan that was on while I was on the freeway. Airflow shouldn't have been an issue either with that and the new seals & stuff.

 

After checking your lean/rich condition and ensuring your air flow is good through the radiator and not around it, I would look at the water pump. I replaced mine with a higher flow and it helped keep my temperatures down greatly, esp. in traffic. I wonder if changing back to a 180 degree thermostat might also help? If you are running that hot, the thermostat is never closing - you might as well remove it, and we know that's not a good idea :nonod:

Are you sure your timing is correct? Any chance your balancer's outer ring has slipped?

 

It's a 77, and it does have a spring in the hose, about 2 inches back from the tip. The radiator's a 3-core brass job matching the original. I don't know what the rear gear is set at. I'm not even sure how to check that.

 

I'll have the ratio checked. I think I read almost 3000 rpms at 70 mph.
I checked out the couple of plugs I could remove without killing myself, and they look pretty good.

 

I had a similar problem with my '75. The temp would run fairly normal on
back roads, but creep past 220 on the highway.

I traced my problem (finally) to a missing shroud extention. It bolts to the
top of the fan shroud. I couldn't believe that such a small piece would
make much differnece, but it cured my problem. Good luck.

 

Great discussion. We all know that a stock cooling system on a vette in perfect tune is right on the edge of running hot. Fragil beast ar'nt they.

Anyway, I want to interject a comment on the holes in the T-stat and their merit. Stewart Waterpumps Co. makes 3 stages of pumps for street cars. They make the largest majority of pumps for NASCAR. The only T-Stat they use is a Robert Shaw (not recommending it in anyway). Guess what they do to each T-stat? Drill 3 3/16" holes for flow control for pressure etc. Just some information.

 

I have not seen in your posts if you have an auto or 4 speed. Assuming it is an auto, 3000 at 70mph says the gears are kind of tall. Stock ratios were 3.08:1 for autos which would turn about 2500 at 70. I have a 73 w/auto and installed 3.55:1 and at 70 am turning about 2800. Suspect you have the 3.73:1's.

Increased RPM's at 70 will contribute to cooling problems but is not the total cause. I still think it is a combo of timing, running a little lean, maybe not advancing properly and flow of the waterpump. You already said the radiator is new, new fan clutch, new T-stat, spring in lower hose. Change to a 180 if you can. Consider a high flow water pump. Read the other posts and try to draw a conclusion.

 

Typically, a water pump is not at fault...the only time I've ever replaced a water pump is because it started leaking through the seals. You have replaced everything that needs to be replaced.

Here's what I would do in your case. Remove the t-stat and see if you still have a problem...removing the t-stat would allow even a weak water pump to flow enough volume to cool the engine.

If it still gets too hot, then I would suspect the engine is creating too much heat. Generally this is due to either too much friction, or weak combustion. Worn bearings, lifters, rockers, etc. will create more heat than they did when new. Also, improper air/fuel mixture and poor ignition (including timing as well as weak spark) will create more heat. Normally, the spark is adaquate if you hold a spark plug wire at a ground location (like the block) and see a blue or white spark...a yellow or red spark is not sufficient. There's many components that could cause a poor air/fuel mixture, such as a worn or incorrectly connected EGR valve, and vacuum leaks.

I would block off the EGR valve and plug the vacuum line...this valve allows hot exhaust gases back into the engine...it may be allowing too much in, thus resulting in an engine that doesn't have the proper air/fuel mixture at high speed cruise. I would also plug all vacuum lines, except for a ported vacuum line to the distributor, and the manifold vacuum lines to the fuel evaporation cannister, and PCV (replace the PCV while you are at it), and of course the vacuum line to the power brake unit if so equipped. Keep in mind that this is just for testing...I am not suggesting that you remove smog equipment for operation of the vehicle if it is required by law to be connected.

Also, you might want to get a dyno test with an air/fuel mix test to help eliminate a possible air/fuel mixture problem. There could also be a problem with a melted catalytic converter...or other type of plug in the exhaust...perhaps testing with chambered pipes might detect this problem.

Wisdom says, that if you shackle a race horse then it will get very hot at you...lose the shackles and see if your horse runs cooler.