I have a 1988 F150 with 5.0L engine. It has electronic fuel injection and EEC-IV electronic engine controls.

I have a couple of questions. One regards fan-clutch and water-pump wear, and the other regards EGR functioning and the check-engine light, and these questions seem like they could be interrelated because I have been using a thermostat with a lower-than-OE opening-temperature rating when the engine's computer may be assuming I have a thermostat installed with the original-equipment (OE) temperature rating. The OE temperature rating is apparently 192 deg F or 195 deg F, depending on what parts store I ask, and I am using a 180 deg F thermostat.

But I'll post the questions separately in the interest of keeping each question as brief as possible.

Fan-clutch and water-pump wear:

I was once advised by a auto-parts store employee that I should probably use a thermostat with lower-than-OE rating under the theory that an older engine will run better with less trouble if it has a thermostat with a cooler opening-temperature rating than the original equipment. A 180 deg F thermostat was suggested.

At first I was reluctant to change to a cooler thermostat. But I eventually decided the auto-parts guy was more likely right than not. (By the way, I would appreciate any comments to clarify whether the auto-parts guy was indeed right.)

So when the water pump went out on me about two and a half years ago, I installed a thermostat with a 180 deg F opening-temperature rating. (I also replaced the water pump but not the fan clutch at that time; I had replaced the fan clutch a little more than a year earlier than that.)

As a result the temperature gauge climbs up only half way to the bottom end of the “normal” range.

(By the way, I had also replaced the regular radiator with a heavy-duty radiator about the time I replaced the fan clutch.)

My water pump failed on me again just recently -- a leak from the weep hole -- and this time I replaced both the water pump and the fan clutch since they both have a lifetime warranty and because an early water pump failure (after only 2.5 years) suggests a bad fan clutch, whose proper operation is difficult and time-consuming to diagnose and because the water pump manufacturer recommends replacing the fan clutch at the same time if the fan clutch is defective.

It seems, from the Haynes-manual description of how to test a fan clutch (and thus how it is supposed to operate), that the fan clutch is supposed to have high friction initially during the first five minutes after the engine is started and run at idle, and then it is supposed to relax its friction so that the fan is not driven so hard. And then it seems the fan clutch is supposed to increase its friction when the engine gets even hotter. That is, it seems the friction is supposed to first decrease with increasing temperature and then reverse its trend and subsequently increase with further increasing temperature. (In mathematical terms this friction-vs-temperature relationship would be called a “convex” function of temperature.) At least that's how I interpret the Haynes manual.

So here is my first question. It occurs to me that, because the engine is running cooler than normal (at least as far as the coolant is concerned), the fan clutch may not be relaxing its friction as much as it should and thus may be driving the fan harder and longer than it should and thus may be causing substantially more wear in the water pump than it should.

Question: Is that true? Is that how the system works? Did my water pump fail after only 2.5 years simply because I have a lower temperature thermostat that caused the fan clutch to work too hard (by staying in the lower temperature range of the friction-vs-temperature relationship)?

 

I'll be the first to jump in and say that since your running a larger radiator plus the engine computer requires higher operating temperatures that you should run a 195 thermostat. I'm not sure how sensitive the fan clutch is to 180 vs 195 coolant. But what you say sounds possible. I can say for sure you need a 195 thermostat and a new water pump.