sorry this post may be rather long, but I want to be as specific as possible.

scenario A. regular driving on highway and some through traffic car runs at about 198 deg. If longer stop at traffic light or any reg amount of short waiting may reach 205 deg. cools down fairly quickly as soon as I start moving.

scenario B. car running at 207 deg on highway and alittle higher in traffic. 212 ish in traffic if stopped too long. Purposely let idle for 5 min which I know is probably longer than average stop when cruising but reached 221 deg.

problem is I think scenario A is acceptable however car will do scenario B once in awhile and can't figure out why. for example I drove the car a good 35 miles in the middle of the day yesterday and pretty much had scenario A whole time. Later same day I drove car 12-15 miles and had scenario B. Outside temp seemed to even be cooler at the second drive.

been kind of fighting this issue off and on thought I had it fixed until yesterday. Never had a problem before and never changed anything before this started. I did put a fan shroud on the car that fits properly, put a better fan on car than originally had. put new thermostate in car I think it is a 165 deg. put radiator in car new aluminum one with plastic side tanks, nothing special but was told by local guy it should be plenty to cool the car, new water pump that claims to pump 30% more than reg pump. don't regret changing all these things as they all should be an improvement but can't figure out why I am still having this issue for now apparent reason. car runs fine just takes the fun out of it when constantly worried of getting stuck in traffic and overheating. Maybe timing next, it is not a stock 350 and I didn't build it so not sure what cam and such is in the car, so not sure on where timing should be.
well I guess that explains it and sorry for such a long post just getting frustrated and would appreciate any help. thanks

 

also don't know if gauge may be screwy the temp sensor is right next to my headers, think I tried to get a reading with a temp gun but can't remember. that was last year and hadn't had the problem this year at all until yesterday.

 

timing could be off or radiator could be to small. how thich is the radiator? should be the thicker one, like 2 inches.

 

I know the rad you have, they work fine. Here's something. Is the thermostat in upside down? Its happened, to me. Maybe the hose is collapsing.

 

I would check where the timing is at too, and make sure the advance is working. If timing is too far retarded, the engine will run hot. My car on the open highway gets up to 180, with the electric fan turned off. In traffic it gets to 190, and maybe has hit 200 once or twice. The temp sending unit reads coolant temp inside the head, the headers won't affect the reading.

 

It may all of those things, or as simple as a little air in the system. If you recently changed any of those components and haven't bled it completely, it may just take a few more drives to burp out the air. I had a similar issue, and as soon as the motor broke-in and I had a few drives on it, it reacted like it should have.

 

thanks for the responses. I am confident the thermostat is in right, but good idea. Air in the system does make a lot of sense as all of not alot of miles have been driven since most of these repairs have been done. As far as timing goes how do I know what it should be since it isn't a stock motor, do I just need to play with it until it is good. At this point I may put more miles on it and see if it is air in the system, then check the timing and if all of that is good upgrade to a beasty radiator. I can't really think what else it could be, the system really isn't that complicated. I have tried to address everything thorough but that is why I can't figure this out. I am no expert so thanks for the input, hopefully one of these suggesstions will work

 

These older car cooling systems are pretty basic, and they don't need to be bled. Checking to see if you have air in the system is as easy as removing the radiator cap and seeing where the level is.
For the timing, start with stock specs, and advance a couple of degrees at a time after a test ride. Listen for "marbles in the cylinder" while under a load, then back the timing off a touch.

 

or it may be as simple as it may be a lil low on coolant. mine would run 200-210 when idling in traffic and just a tad cooler at speed, then I put some coolant in it and it ran perfect.

 

^ lol...isn't that what I said to check?