General Questions About Overheating Imperials


Imperial Home Page -> Repair -> Cooling System -> Overheating

Tips from Ken:

If you are having problems keeping your Imperial cool and have tried the standard tricks without success, here's a suggestion that helped me on my 413. I tow a trailer and need to have all the help I can get to keep that engine cool. I've done all the standard things - re-cored the radiator, flushed the cooling system, replaced the fan clutch. But I recently found a relatively simple change that gave that engine some real relief. It turns out that there are two standard water pump replacements for a 413. One is listed for non-air-conditioned cars and one for air-conditioned cars. The A/C one has several more impeller blades and moves a lot more water through the radiator. The pumps are the same physical size and interchange exactly. My car has A/C, but somebody (probably me!) put the non-A/C pump in many years ago. With the new, correct pump in, I pulled a 4000-pound trailer over a 2000-foot pass at 86 degrees with the A/C on and never went past the high end of the normal range on the temperature gauge. A marked improvement over the past! If you're having difficulties keeping your engine cool, this might be worth a try.

Question from Philippe (1958):

I've some overheating problem with my '58.  When the car runs at slow speed or if I wait at a red light the gauge climbs to 3/4 high. At normal speed the gauge is at N or slightly higher depending exterior temperature. At high speed (70-80 mph) the temp. climbs again. Note that the radiator is new, the thermostat is not removed.  I've tried to make some temp. tests: after driving, I parked the car, temp was 3/4 high so very hot.  I turned off the engine and remove (slowly and with care !) the radiator cap. As you (and I ) expected, a lot of steam, some coolant spilled from radiator and the coolant in radiator boiled. I put a jam thermometer in the radiator and I read 200 F (gauge was at "H" mark).  A bit higher!  I immediately run the engine (w/o the cap and thermometer always in radiator) and the temp. decreased to 180 F (which correspond to the 2nd "leg" of N on gauge).  I let the engine drive some minutes and surprisingly the temp didn't increase. So why with cap the temp is higher than without?  Do I have a problem with the cap which doesn't allow a too high pressure to decrease (I never saw the coolant spilling from overheat tube)?  What do you think of an expansion tank?


From Phil:


I hate to say this, Philippe, but I think you have a head gasket going bad. My '63 300 did the exact same thing in the exact same way. I changed the thermostat, removed it, and recored the radiator. Nothing helped much until I bit the bullet and pulled the heads. It's also a good time to flush out the block, cause you can access the rear somewhat thru the holes in the block. I think why the temperature gets higher with the cap on, is that it puts enough pressure on the head gasket for leakage to start.


From Dick:

I have added expansion tanks to some of my cars which did not originally come with them, because I live in an exceptionally hot area, and often had the experience of a car "burping" out some coolant through the overflow tube after a hard run. This does not necessarily mean the engine is excessively hot, as it will also occur when the coolant had been overfilled when cold (higher than 2 inches below the cap, usually).

Expansion tanks have been provided on some cars starting in the 30's; it was a good idea then and now. I added one to a 1956 car (has to be nameless, here) which had this "burping" problem prior to a 7000 cross country trip during the hot summer months, with AC going full blast most of the trip, and the car did not use one drop of coolant for the whole trip. Pretty impressive. I found that if I checked the expansion tank after a hard run, there was about 1 quart in it, which disappeared when the engine cooled down.

Be aware that to add an expansion tank, you have to get an expansion tank type radiator cap. They are different, since they must seal to the top flange in addition to the inner flange. They are labeled as to application in the Stant Catalog, but if you don't have access to one, let me know and I'll look up the correct one to fit your car with an expansion tank.

If your present pressure cap is holding pressure, and is the correct one, I think you can assume it is OK, as the design is pretty "fail-safe", meaning when they fail, the failure is such that it releases pressure at too low a threshold, not too high. Removing a pressure type radiator cap when the engine is hot will almost always result in some coolant loss, as the hoses will collapse slightly, adding to the expansion of the hot fluid and propel coolant out the top of the system.

200 degrees won't hurt your engine at all, in fact modern engines are designed to run above 200 as a normal practice. You seem to indicate that "H" on your gauge is around 200, and with 50% coolant/water mixture and a 14 pound cap, you are not going to be boiling fluid unless the temperature goes above 240 or so (actually I think it's 258), so if you never see over "H" on the gauge, and that only after shutting down after a run, I am not convinced you have a problem. Since even idling in hot weather only produces a 3/4 indication on the gauge, I don't think you have a serious problem.

One thing you said worries me, though, and that is the fact that the gauge creeps up at high speed. This could be a consequence of the aerodynamics of your front end shrouding letting air bypass the radiator, but it could also be a collapsing lower hose at high speeds. Even with a pressurized system, the water pump spinning at 3500 RPM can pull an actual vacuum on the lower hose, which can cause it to pinch off the water flow to trickle until you slow down. Lower hoses should always have an inserted spring to prevent this, but many are not supplied with replacement hoses, you have to transfer it from your old hose. Also, many mechanics seem to misunderstand the purpose of this spring, and discard it. If you can feel the spring in the lower hose, relax about this, but if you can't, I'd replace it before any more high speed driving.

I cannot come up with an idea as to why your car appears to run cooler with the cap off, perhaps some of our newer, brighter members will have an idea for us.

From Bob:

One check to make is what your water temperature actually is. One of my cars continually seemed to run "hot" after major engine work and I finally took it to the radiator shop that re-cored the radiator. They used an external temperature gauge/sensor (infra-red?) and checked all the hoses, radiator, etc. and got lower/normal readings. They also then used an accurate dial temp gauge to check the water temp - all normal. Their conclusion was "no problem".

I got a new, inexpensive water temp sensor and now my temp gauge reads more "normal" and I worry less. Conclusion: If you have a good radiator shop in your area, get an opinion from the "pros".

Question from Bill (1959):

Today was the first really hot day of the season here in Southern California, with the mercury exceeding 90 degrees, and I decided it would be a good test for the Imperial for the months yet to come. I decided to take it to work and see how good the cooling system was really working. My drive to work is about 20 miles, mostly freeway driving. I started out and the temperature was staying around 160 degrees for about half the trip, but then steadily started climbing to 190 degrees, where it stayed until I exited the freeway and took surface streets for the final three or so miles. While at stoplights I noticed temperatures going up to 200 degrees, and by the time I got to work the temperature was around 210 degrees while stopped. On the way home tonight temperature stayed at 160 most of the way, climbing to 190 by the time I was exiting the freeway, and staying there till I reached home. My question is, is it normal for these types of fluctuations in hot weather? I was getting a little uneasy when temps started soaring to 210. I am seriously considering taking the car to a radiator shop to have it routed out.


From Jim:

Sounds like your radiator may be clogged. I'd take it to a reputable shop to have it cleaned. Here in small town Florida it costs about $60.00 if you take the radiator to them and its just like new. I'd also go for a new thermostat. One of the high flow racing types is better than the little two dollar special off the rack at the parts store.

From Elijah:

Some fluctuation in temperature is normal, and the temperature will definitely "spike" for a bit after you exit a freeway (at highway speeds, you've got 70+ mph worth of airflow over the radiator, as opposed to the effort exerted by the fan at idle or under 30 mph).

DEFINITELY take the radiator to a shop and have it rodded out and cleaned. The last couple of times I've done this, the cost has been around $50. After you do this, you'll probably notice that your overflow tank (definitely a good idea!) gets less use as well.

Sounds like you're on the right track to be ready for driving in warm temperatures.

From Steve:

Before you do anything I would bypass that filter with a regular hose. If it is stopped up it could be causing your issues.

From Larry:

I have had this happen to me also. I went to the local Caterpillar dealer and bought some 30 day flush. just add this to water and drive it. this flush works great and have had success with it. just remember to drain it out after 30 days shot the radiator upper hose first with infrared gun and lower hose to check split in temps to see if it was cooling correctly. after about 1 week of driving I check it again and had lower results and this told me it works. No need to rod out radiator if all you need is flush.

Question from Steve (1960):

My 1960 has a cooling problem that has me baffled. When driving slow or in traffic the car does not heat up. When attempting highway speeds, it overheats quickly. I have replaced the water pump (was leaking), thermostat, hoses and fan clutch. The fluid seems to be flowing easily in the radiator. Does anyone have any ideas?


From Mike:

Generally, overheating at highway speeds indicate a circulation problem, like junk clogging up water passages in the block or a collapsed lower radiator hose (check the lower hose for an internal spring).

From Dick:

An often overlooked item, the lack of which causes high speed overheating, is the spring that should be inserted into the lower radiator hose. At high speeds, the pressure in the lower hose can actually go below atmospheric, causing the hose to collapse. That is why all our cars were designed to have a spring inserted in the hose, but most mechanics don't know beans about physics, so they discard the spring when replacing the hose. Some hoses come with a new spring, and some do not. If you cannot find one any other way, use one of the ribbed hoses that are designed to fit many cars - these incorporate a spring in the hose construction.

Question from Mike (1967):

I've noticed my own '67 running a bit on the warm side now that the daytime temps in our nation's capital have broken the 90 degree mark. At idle, its fine, however the faster I go, the warmer the needle registers. This seems counter-intuitive, since there would be more air rushing past the radiator at, say, 90 than at a stoplight (Not that I EVER drive, not me.) At idle, the gauge will return to and sit at the mark indicating the bottom of the "operating range" on the gauge, but above 55 or so it will begin climbing. It stays within the
"operating range" but gets into the top 1/3 of it, which I consider a bit too warm. My old '67 would stay in the same place regardless of driving conditions.

My own thoughts tend towards something wrong with the engine itself, ie timing wrong, plugs, mixture, etc, rather than a defect in the cooling system. Am I on the right track here?


From Steve:

I would lean toward a clogged radiator. When they start to clog up it does ok at idle or slow speed but can't dissipate the heat generated by hiway travel.

From Chris:

I had a similar problem once on a 73 Plymouth I had. As the car reached operarting temperature, I felt the top and bottom of the radiator. The top was a lot cooler than the bottom, and indeed the radiator had a clog in it.

Before you get too drastic, have you checked the radiator cap? Over time the gasket begins to give way, and it doesn't hold as much pressure causing the temperature to rise.

(Remember the relationship: Pressure X Volume = Temperature)

From Chris:

In addition to the obvious cooling system issues (the previously mentioned thermostat and such), you are indeed wise to check the timing and plugs. Pinging causes the car to run much hotter, and I was able to cure most of my '67's hot-running ills by switching to the Autolite 85 spark plugs I am now so fond of. Switching to the Autolites cured my car's pinging tendency, which brought the highway driving temperature down to the beginning of the normal range. Once I recored the radiator, the car runs, as others have noted, between C and the left end of the normal range, under nearly all conditions.

Try the new plugs and setting the timing to spec and let us know what you find!

From Gary:

Chris is correct in stating that as radiator caps age they often do not hold their rated pressure. I recently tested the cap that was on my car when I purchased it. Placing a section of heater hose in the heater water circuit along with a pressure regulator allowed me to use compressed air to pressurize the system. The cap held 4 psi. I installed a new 14 psi cap (which is proper for my '56) and retested it. The system now holds 12 psi. That's close enough for me.

The reason we run these cooling systems above atmospheric pressure is to delay phase change (changing from a liquid to a gas) as the coolant temperature rises above its standard atmosphere boiling point. The only way a lower pressure cap can cause overheating is if the engine gets hot enough to cause the coolant to change phase and blow of the remaining liquid coolant. Coolant, in its gaseous phase, is a very poor conductor of heat to say nothing of the fact that the water pump can't circulate it.

While it's true that in an adiabatic system when a gas is compressed it increases in temperature (as in Rudolf Diesel's remarkable invention), liquids are very resistant to pressure induced temperature change because they are, for the most part, incompressible.

From Demetrios:

I doubt these old gauges were that consistent. Before you conclude the car is running "too hot", measure the temperature. As you recall, after we ran hard my LeBaron this January at speeds well over 120 and then stopped, the temperature gage was in the middle, but your temp measuring thingy only showed 190 (coolant temperature may have been 210, this car has a 190 thermostat). I think what is happening in these old cars is that they were designed such that they run a bit cool at low speeds. This may have been good for back then, so that they give a chance for the oil to cool down after a strong highway run (remember, the 440 does not hold a whole lot of oil considering the engine size). Now however, modern oils can take the heat much better, so its better to keep the engine warmer than cooler.

From Dick:

Check your fan clutch before you go off on a wild goose chase. High speed driving will cause the engine to heat up. I know this is counter-intuitive, but I've seen it enough times to believe it. Apparently the aerodynamics are such that too much air gets around instead of through the radiator if the fan isn't doing it's job. Of course there is a lot more heat rejection at high speeds also (that extra gas goes partly into heat).

Another cause of high speed overheating is the lack of an anti-collapse spring in the lower radiator hose. The average gas station mechanic doesn't know much about physics, and discards that spring when replacing the lower hose. It is important - the water pump inlet side can actually go way below atmospheric at high RPM.

So squeeze your hose to see if you can feel its ribs. If you can't - you've found your problem.

Even a well maintained car can produce a vacuum at the water pump inlet at high RPM (of course, as he notes, a failing radiator cap will make this happen more easily). He is definitely wrong, however, when he states that once the car leaves the factory there is no further need of the spring (or whatever he wants to call the coil of wire inside the hose). This is easily proven by removing it and trying a high speed blast down the freeway - most cars will have a problem with the lower hose collapsing, especially when the hose is a few years old and getting a little soft. On Packards, the problem was resolved by using a length of solid tubing, with rubber hose only used to connect at both ends to the radiator lower outlet and the water pump inlet.

Question from Jim (1968):

My 120,000-mile '68 LeBaron has recently been running warmer than usual. Not overheating, but the needle on the temp gauge, which until recently always rested comfortably between the "C" and the first notch of the gauge, unless I was climbing the Tejon Pass at 70, is now moving to the middle of the gauge when I'm sitting in traffic only briefly or if I climb a hill. It isn't overheating to the point of steam billowing out and the needle all the way to the "H", but it's definitely running warm. And when that happens I'm getting coolant running out of the overflow tube.

All coolant components are new within the past year -- water pump, hoses, thermostat, radiator.
So, I'm just looking at a cheap easy fix like a thermostat, right? Right?


From Ken:

Thermistats do not last like they useto so change that first.I change mine every year;seems it needs it. Drain your radiator and put new 50/50 anti freeze also to much anti-freeze or to little acts the same.

From Tom:

Last year I had a similar problem and used some Water Wetter in the radiator of my '68 convertible which seems to have solved the problem.

From Paul:

If it has a manifold heat control valve (not sure if that was still there in '68) it could be stuck closed. That would also have about the same amount of an effect on the temperature as you describe. The valve is suppose to be serviced regularly, but almost never is.

From Dick:

This sounds to me like a failed thermostatic fan clutch. To investigate this, when the engine is as hot as it gets, open the hood and rev the engine in neutral to see if the fan is really making a lot of noise. If it is just idling along, your fan clutch has failed.

If you haven't heard the fan making a big noise lately, it has probably died. When they die, they just allow the fan to turn slowly all the time and not make the whooshing sound. The car will still stay pretty cool as long as it is moving along at moderate speeds, but under severe conditions, it will run warmer than usual. I had the fan clutch die on me when I was touring Death Valley in 3 digit temps with my whole family aboard (in my '68) - I temporarily bent up some coathangers from the motel closet to force the blades to turn with the pulley all the time. Noisy as hell, but it sure cooled the engine down!

Question from Bob (1981):

My '81 Imp is a factory carburetor conversion. I live in Las Vegas and the temp here gets to 120 degrees sometimes. But even on the cooler days she runs hot. When I say hot I mean 210,220,230.And forget about A/C. NOW: the cooling system is new. The radiator is a new 3 core, new water pump, new thermostat and housing, new H.D. fan clutch, new belts and hoses. The thermostat is a 160.I have an Stewart Warner gauge set up and it's accurate to the T.I had the cooling system power flushed and the Antifreeze is 50/50 and clean. HELP!!!!!I am to the point of parking it and taking it out in the winter only.


From Dick:

You should clean out your block by removing the core plugs. It's a messy job, but essential if you want your car to run as cool as it did when new. You'll have to buy a coolant filter to catch the crud you stir up doing this, but they're cheap, and will prevent transferring the loose bits back into your radiator!

From Bill:

I may have missed a few things along the way, but the overheating problem is either in the block or the radiator, if the water pump and thermostat are working fine.

As Dick Benjamin pointed out, the block could be clogged up and need to be cleaned out. I went through that with my 1983, although it was done as the core plugs were leaking, including the ones hidden by the transmission case. It runs great now, but it will be a little while before the credit card gets back down to normal!

Have you checked the radiator? There is a very simple test to check the flow in your radiator - all you need is the palm of one hand. When the car is at operating temperature, feel the surface of the radiator core - but be extremely careful when putting your hand on the radiator surface. If it feels hot before your hand touches the surface, go no closer - you know it's hot. If the radiator is plugged up, you will find cool areas and that temperatures across the surface (and thus inside the radiator) vary.

And in a radiator in good shape, the temperatures across the surface should be the same - at least to the touch, Temperatures should vary from top to bottom, though, as the coolant is supposed to cool as it progresses through the radiator.

From Rick:

Check your heat riser valve on the passenger side exhaust manifold. They stick closed and can overheat a motor.

From Stuart:

On my '81 with carburetor, I installed a temp gage. It runs 210 degrees. I changed the thermostat and even tried another temp gage. I just runs 210 degrees. I concluded that it isn't overheating. The oil still looks ok after 4000 miles. The car starts easily when hot. I glance at the temp from time to time. A sudden rise in temp would mean something is wrong. When I shut off the motor the gage goes WAY past 240. It drops back to 210 when the engine starts. Running the AC makes little difference.

Question from Rob (1983):

Overheated my converted '83. (Small block )The heater hose rubbed on the a/c and split. Idiot light didn't come on. The car died. I let it cool down and bypassed the heater. The car ran ok, but seemed hot. As I went to shut down the idiot light came on. Car dieseled, shut it with it in gear, pulled battery cable, etc, wouldn't stop. I left it. Went back today, checked the water a little low, oil level OK, but smells burnt.

Started it up, no water leak, Then I looked under the car and there is a massive oil leak. It is pouring out of the oil filter adapter. Is this real bad? Or did it just blow the gasket? Maybe the heat expansion caused the leak? Either way, it remains on the side of the highway. What do you guys think?

Reply from Dick:

Drain and refill the crankcase with fresh oil, and replace the oil filter. Refill the cooling system and start 'er up!

Then decide what if anything else you need to do. These engines are pretty tough, you might not have done much damage.

Watch the oil and coolant levels closely, and keep smelling the oil - if it seems OK, it is OK.

Fix the idiot lights - there are two, one for oil pressure and one for coolant temperature. Probably the temperature one is the one that has failed. If the light doesn't light when you crank the starter, the bulb is blown. If it does light when you crank it, but did not come on when it overheated, either the wire is off the sender, or the sender is bad. This one is extremely difficult to even see, and much harder to change; it is right next to the dip stick, almost under the AC compressor.

The oil pressure light should be on when the key is on without the engine running. If it is not, the bulb is blown or the sender is bad. Ground the engine end of the sender wire to see if it brings the bulb on. If it does, the sender is bad. If it doesn't, either the wire is bad or the bulb is blown.

The bulbs are a BEAR to get at to change, but if you have a very small friend with tiny hands who is also a contortionist with a built in miner's lamp in his forehead, get him to do it for you. Please don't ask me to do it!

Follow-up from Rob:

That's what I'm hoping. Looks like another roadside repair horror. What do you think about the oil leak though? It's coming from the oil filter adapter (small block). I'm getting a new gasket, but how big is that bolt? It's more than one inch! I already checked the bulb (you posted the technique before) It works, but I think because there was NO water it gave no reading.

Reply from Dick:

Lack of water wouldn't prevent the sender from indicating the heat - it would still sense the temperature of the surface it is screwed into. It sounds like your sender is bad to me.

I don't recall the details of the oil filter adapter - but I'd guess there is a die-cut sheet gasket between the adapter and the block, and only the filter's gasket between the filter and the adapter. Am I misunderstanding your question? There isn't any large bolt, other than the fitting the oil filter screws onto, and that has no gasket other than the filter itself. The oil filter adapter gasket is probably not available separately, you may have to either buy a complete set for the engine, or make one yourself out of sheet gasket material. This is most likely just a heavy paper type gasket, not rubberized cork or anything thick, but copy the original as closely as you can.

This page last updated June 22, 2004.  Send us your feedback, and come join the Imperial Mailing List - Online Car Club