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1977 Mark V Overheating Issue

Hi Bill,

Thank you so much for your help last time with the issue I was having on my 1977 Mark V with my lights flashing on and off as I was driving. The rebuilt headlight switch did the trick and arrived fairly quickly. The fuel sending unit I ordered from you is in the tank and working great, and the front license plate holder was really nice quality and looks excellent on my front bumper. You guys at Lincoln Land are awesome, so I am back to ask for your help again.

I have recently been experiencing an overheating problem. I have the 400 engine, and here is a list of what I have done so far to try and fix the problem: I've Replaced the upper and lower radiator hoses. I've Replaced the thermostat (was 180, replaced with 192), it is installed with the spring end in the engine block. I've Removed the radiator and took it to a radiator repair shop and had it rodded out. I've Flushed out the block with the garden hose before reassembling and filling with coolant / water (about 60% coolant mix). New radiator cap. The fan shroud is the original one, and it is intact and unbroken.

I have the 1977 Ford service manuals on CD, and they have been a great help so far. Here is a list of the other things I am considering addressing as possibilities: I've got a flex fan, so no possibility of clutch failure. However, it's a 5 blade and one of the blades has the flex piece broken off, but the metal blade is still intact. Also, it measures 18-1/4" from tip to tip, but when I search online the standard size that comes up is 19-3/4". The opening in the shroud is 20-1/2", so the larger one should fit, but the one I have looks like it is original so I don't know if the 19-3/4" one will hit the shroud as it flexes at higher speeds. I'm considering replacing it with a 7-blade, which size would you use? And do you think this would be worth trying first? It is a regular rotation fan, and it is installed with the flex blades facing the engine, so I'm pretty sure it is not backwards. My Water pump (the shop I took it to for diagnosis before getting the radiator serviced said the radiator was 25% blocked, so I figure they would have noticed if the pump was bad they've done me right every time I've gone there). • I've read online that a bad EGR valve can cause overheating, considering replacing it. I've also read that timing problems can cause overheating, and that a bad EGR valve can contribute to the timing issue if there is one. I had the distributor replaced recently as well after it failed while driving. The last thing I've seen as a possibility is an exhaust restriction. When I got the car, the muffler had been damaged so I had it replaced. Later, the distributor failed so I had that replaced. When they started the car after the repair, it was running roughly. A friend at work had told me he thought the resonator at the end of the exhaust might be plugged, so I suggested they cut it off and install a straight tailpipe. The car ran fine after that. I've also recently had the carburetor rebuilt, so I doubt that could have anything to do with it. The last factor is that I live in Phoenix, and it has been between 110-115 every day for the past 3 weeks. I realize that this contributes to the problem, and I've known for a while that the car seems to run hotter than it should, but there has to be a way to get it to run at a normal temperature. It has never boiled over or had the temp light come on until recently. I'm trying to do as much as possible myself, because shops are expensive and funds are limited. I look forward to your response. Thanks again!



Hi Patrick -

Overheating engines are no fun. It looks like you have covered all of the important common areas of overheating. The fan blade that is missing the flex piece is never a good component to use on your engine. I would replace it with the largest one with the most blades that was correctly available for your engine when these cars were new.

I am assuming that your engine is actually overheating and that you are not just overfilling the system and that the hot engine sending unit is not faulty. All of the tune up specs and ignition timing etc. should of course be correct as per the shop manual. We are not familiar with the cooling system maintenance history on your engine but two other important possibilities do jump out at me in your situation. They are the extreme heat lately in Phoenix and the condition of your radiator. We have had many truly overheating engines at Lincoln Land that did have their radiators serviced along with many other parts replaced etc. that continued to overheat and were only and finally corrected when the radiators were properly re-cored with a new original style core assembly. Rodding out the original radiator core is a good repair but the procedure is only as good as the person that is doing the job. You and I were not there to watch what was actually done to each and every core tube in your radiator. Because of this and the Phoenix climate lately I am leaning towards a continued radiator problem after the core rodding. I am only strongly considering this scenario based on what you have already done to correct the issue and our experiences here at Lincoln Land. I would contact our office to discuss purchasing another fan blade to replace the one that you have with the missing flex piece for starters and then carefully considering a radiator re-core after discussing the problem with a trusted shop. If you have a good shop you can sit down with the owner and show him this reply and my statements regarding our experiences with radiators at Lincoln Land.