Wow, I'm impressed by the intelligent answers to this question. Usually I see a lot of platitudes and urban legend type answers to a question like this. Nice job, guys.

I wonder if the engine's tune is causing it to run hot; the EGR is not working and the advanced spark timing is causing the engine to work harder.

As I recall reading on this forum, the tune of the 400 was a bit tricky. To reduce NOx emissions, Ford; retarded the cam to reduce low speed compression, enriched the carb's jets to reduce combustion temperatures, and, of course, used EGR. The use of EGR with the rich mixture is a problem - the EGR spacer thingie under the carb gets clogged by unburned hydrocarbons from the rich mixture.

Most of the guys on this forum know that when an engine is tuned for EGR, the vacuum advance is increased a lot to allow for the slower burning of the mixture. (The slower burning of the mixture is caused by the dilution of the mixture with exhaust gas.) If the EGR plate, or EGR valve itself gets clogged by carbon from the rich mixture, the extra spark timing becomes a problem.

If your engine still has an EGR valve, I would recommend that you check to see if it's clogged by carbon from the rich mixture. If your engine had an EGR valve setup, but it was removed, you need a new distributor with a different vacuum curve.

You might want to get a carb that's tuned a bit leaner, too.

The optimum setup is to get the cam set to 2 degrees advanced, lean the carb to a proper mixture, and use an iron EGR spacer. Lots more power, better economy, and still have a clean burning engine. You'll have slightly higher NOx emissions, but overall, that's probably a lot better for the environment than a rich running, engine with a short life.

 

As for me, I have never ran a EGR on my engine. But in the coming months I am putting on EFI with out a EGR on the engine. I doubt I will ever need one as this bronco is coming more and more trail only rig. But I do want to find out what my millage is going to be for the EFI over carb once it is all tuned and running.

 

The EGR is still in place, but the vacuum line to the EGR is not present. Thanks for the information.

 

I wanted to supply an update to this thread. Replacing the thermostat with a Robert Shaw 333-192 seemed to solve the overheating problems and fluctuating temperatures on up-and-down long grades. The supplier, however, specified a 370-195 thermostat for my '78 F-150 with a 400 motor. I couldn't convince them that the 370 is the incorrect application. The 370 is 2 1/2 inches in diameter, which would put the lip of the thermostat about 1/3 of the way into the thermostat housing bolt holes, not to mention not fitting the recess in the block for the thermostat. Thank You everybody for help and ideas.

 

Thank you for the update. It's informative to hear what the resolution to the problem was.

 

Just another link explaining the bypass function.
http://www.are.com.au/feat/techt/thermostat.htm

 

BrianS, that is really good information. That is exactly what I was looking for in terms of an explanation about the skirt on the thermostat. Thank You.

 

My 78 F250 was also running hot after a rebuild. After reading numerous posts on this site I ordered the Robertshaw 333 192 thermostat and it did also resolve my temperature issues.

 

This post has me a little scared...so are NONE of the thermostats offered at a place like advanced auto the correct ones? I'm getting this list for the 351m (some are for the windsor):

http://www.partsamerica.com/ProductL...cement&ptset=A

Do I have to get the 333-192? Its coming up as $24 shipped, vs the standard $7 thermostat. I'll pay that much if its necessary...

 

In my case, having the knowledge then that I have now about thermostats would have saved me about $700 in cost(parts only) for completely replacing the cooling system. In my mind, the $24 would have been a mere pittance compared to what it cost to find out that 95% of parts houses will sell you a thermostat that they swear will fit your 351M/400 engine. It will fit, but it does not close the internal bypass on the back two cylinders when the thermostat opens.

cwheel, the link that you provided for partsamerica lists a prestone thermostat that is the correct application. The other source that was given to me by another fte user was flowkooler.com. Some of their application literature is incorrect, however. Make sure you get the 333-192, and not the one that they suggest.

 

So would a thermostat for a 78 351M be the same as a 70 351C? I have always put in your average thermostat and I noticed that partsamerica doesnt list the 333-192 for the application on my 70 351C but does on my 78 351M.

 

Try the flowkooler.com website. You can cross-reference thermostats based on make, model and engine. They do list some 333 robertshaw thermostats for a 351C.

 

The t-stat for a 351C would be the same as for the 351M/400.