I did the ops mod to get constant fuel flow to the IP. Good news is the fuel economy went up to around 15-16 from 12 or so. The engine also runs a lot smoother and quieter in the 55-70 range. Seems to have more power although I haven't towed anything with it yet. It makes a little puff of black smoke when I stomp the throttle while cruising down the road. I understand the more constant flow of fuel probably means a little more heat from combustion.

Many thanks to Racer55 for help and insight.

Now for the bad news, coolant temperature fluctuation. The temp regularly goes up a little past the 210 mark and then cools down to about 195-200 and then back up past 210 and back down, back up, back down, up, down, up down... The constant wide range of fluctuation doesn't seem normal. It even does it sitting in the driveway and idling. The radiator fins seem to be clean from what I can see but I know there may be some debris trapped between it and the AC/oil cooler/trans cooler units.

What do I need to look at first? I'm thinking coolant system backflush and checking the fan clutch. Should I look at removing the radiator to check for and remove any debris or will a stiff flow of water work? Is there a reliable way to check the fan clutch?

Any input with tips or pitfalls to avoid is much appreciated.

 

Take the thermostat out and check it, it might be sticking and not opening when its supposed to, it might be opening when the temp gets up to 210 and then cools back down to 195-200. Might be worth putting in a 180 thermostat. Be the first place to check. Depending on how many miles you're current antifreeze has on it, it might be worth flushing the system. But start with the thermostat.

What is the OPS mod, increase in mileage intrigues me.

 

I would say that the there might not be enough airflow to the clutch fan?

A spotlessly clean cooling stack(rad,trans,oil cooler ect) is very important for theses trucks as well as a working fan clutch.

If there are any signs of oil leakage or fur on the clutches bi metal spring-it is done.

I am a big fan of the Kennedy special calibration clutch and 20" fan.
I also prefer the 195/192 AC Delco T stats-AC Delco only for stats.

 

Thanks for the suggestions. I looked at the radiators and they appear to be clean. The engine coolant radiator looks to be relatively new. I went for a drive and checked a couple things. When I drive at lower (30-40mph) the temp fluctuates over the widest range - maybe 15 degrees. When I drive at a consistant 55ish the temp still fluctuates but only about 5 degrees. When I am driving up a long grade at slow speed (30mph) the temp guage will go a little past 210 and then back down to around 195.

I got a chance to go down a very long hill. I was able to maintain a good 50-55mph without having to push on the throttle. The temp was around 210 when I crested the hill and went down to around 185 at its coolest without pushing on the throttle.

Based on the above I think the thermostat may be a little slugish and not responding quickly enough at either end of the temp range.

I think the fan clutch is probably OK since it does get the engine cooled down at slower road and idle speeds. No oil on anything that I noticed but maybe I'm not looking in the right place.

I am going to start with changing out the thermostat since it is the easiest and cheapest. I may flush out the system based on what I see when I take out the thermostat.


As for the MPG increase after the OPS mod - my only guess is that the fuel system was working properly with the constant supply of fuel and the computer wasn't needlessly modifying fuel requirements or performance based on possible conflicting sensor inputs. I am really out of my lane on this type of technical stuff so I may be just blowing smoke. I only checked the fuel mileage over a couple half tank refills so my numbers may be a little off. I don't think the OPS mod is able to be done on '95 or later because the system is already kind of set up to do what the OPS mod does. I will let someone with more experience verify this.

 

96 or later has the OPS mod built in.

The very center of the fan clutch has a bi metal spring-look there for oil loss or dirt accumulation.
Any excess collected dirt or fuzz in that area indicates a bad clutch.

 

When looking around for thermostats I ran across some interesting write ups. It was stated that the 6.5 gets it's best performance AND economy around 195 degrees. So the thermostat is calibrated to open up around 195 and the clutch fan is calibrated to kick in around 195. it makes pretty good sense that this would cause the constant up and down of the temp gauge. The poster also said that when they put a 180 thermostat in it the temp fluctuation went away - presumably because the thermostat, clutch fan and radiator reached an equilibrium or maintaining a constant temperature.

So it kind of makes sense that my temps were low before the OPS mod since the engine wasn't getting as much fuel to burn. Once the engine started getting more fuel the temp went up because more fuel equals more heat.

So I don't really think anything is wrong with my rig. But since summer is on it's way I may drop a 180 thermostat in it and see how it works out.

Is Robert Shaw the only decent 180 thermostat on the market? Is the fan clutch adjustable? Any input is appreciated.

 

Fan clutch is not adjustable and it is not temperature activated it is a centrifugal operated fan working more so at lower RPM's the faster you drive the more air is coming in thru the radiator and the less you need the fan it's a fuel saving feature

 

You have not got it quite right.

The fan clutch is indeed engaged by temperature,there is a bimetal spring in the center that when heated expands and engages a ball and ramp type clutch.
Hot airflow through the cooling stack is required for proper operation-thats why it so important to clean your cooling stack as regular maintenance.

While it was never intended to be adjustable it can however be done:
Modifying Fan clutch - Diesel Place : Chevrolet and GMC Diesel Truck Forums

---AutoMerged DoublePost---

I prefer AC Delco for these trucks,from rock auto:
ACDELCO Part # 12T45D {#19114512}
THERMOSTAT,ENG COOL -STD LINE; ALTERNATE TEMP; 180 DEGREE

Given the choice I would stick with the 192/195's myself though,but I also have the Kennedy special calibration fan clutch that engages earlier than stock.

 

Thanks racer I was under the impression that these were completely centrifugaly opperated

 

Got a chance to tinker with this thing a little bit. I took the top fan shroud off to take a look see. Two things I noticed were that there was some debris trapped on the ledge at the bottom of the radiator. I also noticed some road debris (bugs, dust,etc) lodged in the clutch fan spring.

I vacuumed out the debris, cleaned the fan spring, and hosed out the fins on the radiator with it still in place. There was quite a bit of stuff that came out of the bottom where the debris was laying on the ledge. Not much came out of the upper part besides some bug carcasses.

I ran it around for 25 miles or so doing my normal routine and to my surprise...nothing changed

So I put in a new 195 thermostat and kind of like I was guessing most of the fluctuation has stopped. It only goes between about 195 and 200. Very seldom does it get up near 210 and never past.

One question I have is how do you get all the air out of the coolant system? When I took the upper radiator hose off hardly any coolant came out - maybe a cup (8oz). It should have been full if it is true the system will self purge over time. It has been a couple hundred miles since I replaced the bleed valve on the thermostat housing.