Few months ago in California, I bought a beautiful clean '72 with a rebuilt 390 (52,000 miles on it). The dealer specialized in Porsche repair, therefore I had confidence that I was getting something reliable from someone who knew what they were doing.
Later I noticed that the engine pinged at highway speeds. In regular city traffic 0 -50 mph there was no noise and temperature stayed at normal 192 deg.
When I started my cross country trip with a small 8'X4' trailer back to NY, it pinged and overheated to 230 degrees @ 40mph. I made a detour, realizing that I will never make it to NY that way. I gave up my trailer and took the truck to a local mechanic, who "supposedly" checked the cooling system and decided to advance timing from 6 deg. to 13 deg. I had to continue on with my trip without the trailer and with pinging persisting and 205-210 temperature.
Finally in Portland Oregon, I took my truck to another mechanic who made the following report:

• Engine idles rough, surges at cruise speeds. Checked timing curve, found 13 initial w/o vacuum connected, 48 with vacuum connected at idle.
• Centrifical was 38, total was over 68 degrees.
• Moved vacuum advance hose from manifold to ported source.
• Ground wire to breaker plate inside of distributor was not connected. Reconnected the ground wire. Install new adjustable vacuum advance unit.
• Modify vacuum advance to work and fit while limiting the amount of vacuum advance total. Spark curve is now 13 @900 rpm idle / 35 (by 3200) / 40 (by 10"hg).
• Reset idle speed in gear to 600 rpm and 3.0% CO.
• Tested CO at cruise found .6% (very lean), changed from 64 to 66 jets, cleaned debris out of the carb, replaced fuel filter, retested = 1.3% CO now.
• Road tested vehicle, no more detonation or poor run found at cruise.

Recommendations:

• Have fuel curve set up on chassis dyno
• Replace fuel tank due to debris
• Recommend installing 180 degree thermostat (currently 192)
• Recommend installing a 7 blade flex

With his help, the truck did perform well on highway, but unfortunately the overheating persisted especially when slowing down to "stop& go traffic" from highway speeds or just idling in one location. In upper 80's @ 65 mph engine temperature stays at 205 and surges to 220-230 within minutes when coming to slow traffic. After regular pit stops with the hood open to cool off the engine, I did make it back to NY but do not see myself driving it in traffic until I resolve this issue. Idle is rough again and takes a bit more effort to start the engine when cold. I do not see any damage...no unusual noise and no smoke.
In general radiator, shroud, fan and water pump seem to be in order. Then again only way to test the radiator is to use it...and in this scenario everything points to it, with exception that it did not overheat before I went for my cross country trip.
Should I presume that the dealer masked problems by advancing the timing.
I have heard of cases that if the engine is overbored, it overheats...are these the symptoms of such case? If that is the case, is there a remedy to that problem... larger capacity aluminum radiator, smaller crank pulley.

 

Try a stock, solid-blade fan and see if the problem goes away

 

I do have the stock solid blade half way in the shroud. I've been told that 7 blade flex fans are better.
I don't know if it helps, but I just decoded my VIN and it shows the engine to be a 360 (Y). Perhaps the original 360 was converted to 390.

 

No, probably not, Since the 360 and 390 are identical the only way to tell the 360 from the 390 is the stoke. Same bore just different stroke. Magically a 360 turns into a 390 when listing for sale.

Try going to a car wash place that has a pressure washer and thoroughly cleaning the fins on the radiator. Replace the thermostat, get a good flush system for the radiator and heater core. Check the condidtion of both the top and bottom radiator hoses. If they feel soft or easy to pinch together, replace them as the inner coil wire has corroded and can colasp under pressure. If the radiator cap is more than 10 years old replace as proper pressure is essential for proper cooling.

 

I don't know about overbored = overheats but your head gaskets could be rotted out where they block off the passages in the front of the engine block OR they were put on backwards.

There could be a number of other causes such as ignition points with the improper gap.

The list goes on.

 

Darn....that's what I was going to say, especially as it was just re-built.....you beat me to it...LOL

Not to hard to check.....remove water pump, put finger in waterpump hole and feel up for the head gasket. Should be there both sides (both holes). Gasket has holes for similiar passages at back of block, but not at front. Since gaskets can be accidently reversed.......

 

Back again. Only time I had a really odd over-heating problem was because the block and heads had their water jackets packed with rust, scale etc. Hot tanking is the answer, however hot-tanking often only loosens all that crap. You need to use compressed air after. I've also used a magnet, but that's slow and tedious.

Anyway, if you find a lot of crap in the top of the rad, it may be because they didnt' clean the block or heads after hot tanking. I'd say it could be flushed out to an acceptable level without having to dismantle the engine.

 

Sorry guys but pulling the waterpump, pulling and hot tank cleaning of the heads, headgaskets put on backwards MAY be an answer but not the best answer. The truck IS NOT over heating while under load. Many of the fix's you are suggesting would be for restriced or blocked recirculation problems. His problem is when NOT ENOUGH cool air is passing through the radiator at slower speeds. When moving fast he HAS NO PROBLEM. If he had an obstruction or overbore, under load on the engine MORE overheating would happen.

Clean the radiator inside (A good flush system) and out (fins clogged from dirt and bugs) first; check that hoses so they are NOT soft and collasp second; replace thermostat third; check radiator cap fourth; and save all the engine tear down stuff as a VERY LAST RESORT! It seems to be running just fine until you slow down.

Would you guys rebuild the engine just because a cylinder is missing? No, I would hope you removed a sparkplug and checked to see if it was fouled first or a bad plug wire or cap and rotor problem, not assume you burnt a valve or broke a piston ring.

Once again...when you hear hoofs think horses NOT zebras. Keep it simple!!!!!!!

 

True. I missed the part about it being worse at lower speeds. Mind you, for whatever reason it is overheating, it's possible that the cooling at high speeds is enough to keep the temp under control. But yes, the old KISS principle always applies...."Keep It Simple Stupid".

 

Is the water pump pulley bigger than the crank pulley?

 

I just checked and the water pump pulley is bigger than the crank pulley. Should I go with something equal or smaller? On a side note, while idling in park and revving the engine for few minutes the temperature does drop 5-10 degrees. Interesting...

 

I'm asking out of curiosity... this subject has come up before, and the idea that the pump pulley was too big (causing the water pump AND fan to spin slower) might have something to do with it.

Any chance you can get a part (engineering) number off the pulley?

Guys? We've been over this before, but forget if anyone actually came up with an answer.

Thoughts?

 

I would say that the radiator has rust in the tubes restricting the water that transfers the heat from the engine. These trucks are old and there`s a lot of rust in them blocks. I got my 390 from the shop after a pro rebuild and noticed rust still in the block, so I got my pressure washer and hooked up a 1/16 flex hose and a 2 foot stinger w/a hydroblast tip and stuck it in the water passages while I had the block on a stand upside down and got a good handfull of rust out of the block.But it would probably help if you took your truck to a good radiator shop to have the block flushed and the radiator checked out and/or retubed/replaced. Good luck on your ride.

 

If you can't do any of the work yourself then it is going to get very expensive chasing this trouble. One of our fellow forumites, "Tinman", had a thread going about a overheating problem. Has anyone seen him around ?

His solution to the overheating (Tinman's)was a stock radiator, stock fan and fan shroud as well as a 180 thermostat.

Pit_of_Pity, what was done during the rebuild of your 390 ?

 

Sounds like a coolant flow issue. I seem to remember that some FE water pumps had a sheet metal impeller that can become loose, come off, or rust away.

Faulty thermostat (or put in backwards) and loose fan belts are other causes, although a loose fan belt usually slips at higher loads....the reverse of what you are experiencing.