The whole cooling system is controlled by the coolant cap. I'm wondering if the 20lb cap on the expansion tank makes everything in the cooling system wear out sooner than say a 16lb cap would. It seems that the biggest problem with the car is everything in the cooling system wearing out and failing. It seems to me like 20lbs is overkill. I don't care if I have to wait another 20 or 30 seconds for the heater and car to warm up. A V8 Mustang cap of same vintage is the same design and is rated at 16lbs, which seems sufficient to me. Any feedback would be great.

 

all it does is determine the point when it vents. particularly when the engine is stopped

in normal conditions it does nothing

 

The normal condition, at least with my car is that the system keeps building pressure unless the cap releases the pressure. You can tell by squeezing the main hoses. they should be squishy, not hard as a rock. That is when the problems start. I even had the upper radiator hose pop off the radiator before. I recently replaced the water pump, hopefully the new one won't build up as much pressure as the old one did.

 

right

you have the choice between ejecting coolant early or retaining it. both have their ups and downs

 

Interesting point you raise.
The higher pressure on the system of course the more stress on all the associated components in the cooling system; namely the hoses. Who knows with this car, maybe even the DCCV may have a longer life as well!
If you have a fault in the system and you have a 20psi cap you may end up getting to one more set of traffic lights as compared to someone who may be running the same car with a16psi cap..
To drop 4psi from 20 is not a huge difference and even with a modern car I don't think it would make any difference but you never know what may be affected with modern cars..
Probably a greater issue would be the reverse scenario but read on for the relevance here to your query.
When I bought my 1976 Triumph Stag, I noticed that the cap was below the manufactures official 20psi rating. It had the earlier model's 13psi cap. I duly changed caps. Soon after it started to develop leaks around the inlet manifold (V8) and if I remember one or two other areas..
I did have to remove the manifold and reseat it in place; a skilled job on a Triumph (Stag) V8. So in this case more care was needed to get a very effective seal in that area with the higher psi. So really the lower psi the better. Been running the 13 psi cap ever since.
As I mentioned however, you never know what could be affected with modern cars.

 

Besides the DCCV, the whole coolant system gets more stress and especially the plastic parts, thermostat housing, etc. will fail quicker with more pressure, so it seems. People fail quicker with more pressure too.
I looked at a couple different Ford cars' coolant caps. The mid 2000's V8 mustangs have an identical cap with 16lbs and the 2005 modern Thunderbird that shares the same chassis with the S Type and has a Jaguar sourced V8 has a cap with 16 lbs also.
I'm going to see if I can find out why the pressure is at 20lbs from some mechanics. I'll bet it's so the car warms up quicker, or something like that. maybe a Thunderbird cap is sufficient and easier on everything?

 

I hope that planned obsolescence isn't the reason for a higher pressurized system with the cars.

 

A while ago I went to the local Ford dealer and got a 16 psi coolant cap for a 2005 Thunderbird that has a Jaguar engine, a V8 Mustang cap is the same and have been running my car with it instead of the 20 psi cap for some time now and the car runs fine. It doesn't leak or anything, the car takes a little longer to warm up and the boiling temperature may be a little lower than before, but I don't live in the desert. After replacing just about everything in the cooling system I thought that a little less pressure may make things easier on the system and make it last longer. When I go to a hot climate or go auto crossing I'll probably put the 20 psi cap back on. For this reason I do recommend switching to a 16 psi cap, especially for people that haven't yet replaced everything in their cooling system and want the plastic components, hoses, and radiator to last longer.

 

I think I may have mentioned that on a faulty cooling system, 4 psi more would probably get you to one more set of traffic lights before you'd have to pull over.
I'd leave it and not put the 20 back; if the system's been running and got used to the 16 for a long time, bumping up the pressure may then not be a good idea.