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MG MGA - Engine over heating

This is in a Magnette, but I thought someone here might have some ideas also.

Ok, I am revisiting this issue once again. Here is the situation. 1600 block, 160 t-stat (although symptoms are nearly identical with racing sleve), 70 mph to 75 mph, outside temp - 60 F. From a cold start, after about 25 minutes of driving at 70 to 75, engine temp hits about 220 F. I do not have a tach reading since no tach. Also, I have original 4 speed. Here is the weird part. I exit the interstate and drive 40 to 45 and within 2 mintues, engine time is 205, within 8 minutes engine time is 190 and still falling. By the way, this is with mecanical guage purchased at NAPA.

I cannot believe the temp would fall so far so fast if the engine was really that hot. If the rad is clogged and needing flushing or re-coreing, why does it cool so quickly at lower RPM's? The head has been replace and is clear. I cleaned a lot of sediment out of the block while the head was off and opened up the passage leading to the block drain cock.

Is this symptomatic of a problem with the rad, or should I be looking at something else?

I am really baffeled.

Dennis in AZ.
Dennis

The basic system for cooling centres around the radiator. It is the main part and in my experience is 99 times out of 100 the reason for cars overheating. Many people get a fix around this problem by doing other things when a simple recore would be far beneficial. I have seen where some radiators have gummed up after only a couple of years of service so it is not unusual for this to occur.
Why then I ask you do you not believe that the radiator, the most critical part of the system, could be at fault. It is without doubt the first thing I do.
Bob (robert) I am coming out!! yes I once owned an MGB!!

Since you found quite a bit of crud in the block you should start with flushing the whole cooling system. At higher speeds your engine is making more BTU's than the cooling system is capable of removing. A worn water pump impeler could also be having an effect as well as retarded timing and/or lean mixture. I agree with Bob, before looking elswhere make sure the cooling system is in excellent shape.
John H

I don't know that I would say that the rad is the most critical part. If you have ever had a water pump impeller fall off the shaft, as I have had, you would find that the rad doesn't do much. A bit of trivia, Fords up until the late 20's came WITHOUT water pumps. Henry felt that the hot water would rise to the top of the rad and gravity would pull it through the rad cooling it and re-introducing it to the block to start the process over again. But I digress. The reason I might suspect the rad is not the problem is the rapid cool down. Again, we are talking a HOT block cooling 15 F in 2 minutes!! If I have poor flow through the rad, I would think that this would take much longer. In fact, I would think that this would take longer than it did to heat up. My leaning is more toward John's thinking about the pump. If the impeller is not up to par, I would think the symptoms I am seeing would be possible. IE high-RPM Flow<capacity, low RPM-Flow>=capacity. In any case, I am going to try the rad next as it is easier to remove, and since most respondents have pointed to that as being the most likely culprit. I will have it boiled out and tested and we will see what that does.

Thanks for your advice.

Dennis, Az
Dennis

Dennis, a couple last thoughts. If you can, compare your gauge to another to make sure it is accurate. Also while not common on these engines, air pockets and/or air bubbles in the coolant flow can cause strange fluctuations in temp gauge readings.
John H

Dennis,

Check your timing...I've excerpted the following from the MGA Guru website.

http://mgaguru.com/mgtech/engine/ro101.htm

Spark timing retarded to far will cause late combustion, losing the opportunity to take advantage of the most efficient part of the power stroke, at the top where the highest pressures occur. If the timing is just a little late the result is a slight loss of power and a little dip in fuel economy. But the fuel IS being burned, and the heat is going somewhere. If it doesn't go into propulsion, it goes off as excess heat. The extra heat may go into the cooling system (high water temperature), or may cause hot spots in the combustion chamber. In severely retarded timing cases and at high engine speeds, the last of the combustion may occur as the gases are exiting the engine, causing overheating of the exhaust valves, exhaust manifold, etc, all very bad news.


Much easier to check that than the cooling system....

Best,
Gene
Gene Gillam

Dennis, I've never had such a problem on an MG, but I did once have a Chevy S-10 which would run significantly warmer at 75mph than at 65mph on the highway. No other change than just slowing down a bit. I suspect that because of the engine speed the water pump was moving the water through the radiator so quickly that it didn't have time to fully release the stored heat. I would see if you could find a slightly larger pully for the water pump from some other MG model to reduce the rpm of the pump at highway speeds. Sounds like the rest of the system is easily up to the task of cooling even with a little reduction in flow rate. Usually blocked passages in the radiator or engine will result in overheating even at low speeds, especially at or around idle in traffic. Sounds like you don't have that type problem.
Bill Young

Dennis; If your radiator has a marginal heat transfer abilities ( ie, dirty, scaled, corroded,)the coolant may pass thru the radiator without transfering much heat. This gets worse as the engine speed increases as the coolant has a higher velocity and spends less time , per unit volume, in the radiator. Of course the opposite is true. Good Luck, Bill
w.g cook

I hear what you say Dennis and it is certainly a point of view. Although I would imagine if this symptom can be caused by a poor pump failing to move water around the system, surely a blocked radiator is just the same? Anyway to note the point made by Gene, I am sure you have already set the ignition timing correctly and that this simple bit is not part of the equation.
Bob (robert) I am coming out!! yes I once owned an MGB!!

Dennis, I believe that your lower radiator hose is collapsing at speed. Check your hoses. I don't believe that the rad. core is plugged, although it could also be true. Do you have the correct T-stat and sleeve?
mike parker

Dennis, above all don't get married until you correct the problem with the cooling. (See 'My wedding using wedding cars sevenoaks" thread) ;-)
Bill Young

Bill, already married, 10 years this fall. Mike t-stat and sleeve are correct. Will check hose, but I belive they are new. There may be a slight restriction in the top hose. Magnettes have 90 degree twisted bend in them. I should check that FIRST, Duh! I don't know why I didn't think of that before!! Gads!

Dennis, Az
Dennis

Took the rad to the shop today for boiling. The lower rad hose is VERY soft. Will replace. Will also correct slight kink in upper hose. Will let you know when I get it back together.

Dennis, Az
Dennis

On the A this problem is often seen due to re-coring the radiator with a denser matrix. At speed the smaller hole size does not allow very high air throughput as the grill also creates a dam effect.
I had this on a newly cored radiator and only got back the high speed cooling when I replaced with an original type.
Unfortunately the cooling around town is now back to the original poor level!!
Neil Purves

This thread was discussed between 15/03/2006 and 21/03/2006

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