Leaner fuel mix = hotter engine ?

[Zoltan]

I was playing around with the smoker setting and noticed a significant increase in engine running temperature when I "dialed" in a leaner setting. Granted, it was too lean (1/4 turn) and the truck became sluggish. Just wondering if the increase in temperature was due to the fact that I had to press the gas pedal harder to get on the hill and perhaps I was running higher rpms. Any thoughts?

[asavage]

On a diesel, less fuel should always equal less heat, so I have no explanation for your observation. You can't run a diesel "too lean", you just get less power out of one if you reduce the fuel quantity.

Contrarywise, overfuelling a diesel leads to obnoxious smoke (particulate matter), melted parts, cracked parts, lube oil dilution, and other Bad Things.

[philip]

I was playing around with the smoker setting and noticed a significant increase in engine running temperature when I "dialed" in a leaner setting. Granted, it was too lean (1/4 turn) and the truck became sluggish. Just wondering if the increase in temperature was due to the fact that I had to press the gas pedal harder to get on the hill and perhaps I was running higher rpms. Any thoughts?

As Al said, more fuel means more heat or ... less air with the same amount of fuel means more heat.

Turning the Smoke Setscrew clockwise will "lean" (weaken) the fuel mixture. You need to watch the exhaust in your sideview mirror for smoke while the engine is under full load and higher RPM. I like 36-39 mph in 3rd gear and full throttle.

I would look elsewhere for the percieved and coincidental increase in coolant temperature. (I'm hinting)

[Zoltan]

It was definetely not just a perception. Here's what happened. I turned the screw clockwise 90 degrees and took it up the Pali for a test drive. This is a 3-4 mile uphill drive on a mountain that separates the windward side of Oahu from the Honolulu area. By the time I got to the top, the temp needle reached the upper 2/3 of the safe zone. Normally it would stop at half by the time I get on the top. The truck was definitely sluggish and had to step on the gas. Could this explain why it got hotter? Perhaps -- with leaner fuel mix --to achieve the same speed the engine had to run at a higher rpm than usual?

For the most part I cannot see black smoke, but once in while there are a few puffs between shifts. I think I ought to test the injectors and have them rebuilt if necessary. The truck has 155K.

[philip]

It was definetely not just a perception. Here's what happened. I turned the screw clockwise 90 degrees and took it up the Pali for a test drive. This is a 3-4 mile uphill drive on a mountain that separates the windward side of Oahu from the Honolulu area. By the time I got to the top, the temp needle reached the upper 2/3 of the safe zone. Normally it would stop at half by the time I get on the top. The truck was definitely sluggish and had to step on the gas. Could this explain why it got hotter?

The way you control a diesel's power output is by regulating the fuel delivery. This is true of all diesels including our SD diesels using a pneumatic throttle governor. So ... back off the fuel ... engine produces less power ... which also reduces heat output.

Perhaps -- with leaner fuel mix --to achieve the same speed the engine had to run at a higher rpm than usual?

Were you in fact running the next lower gear to make the Pali grade?

For the most part I cannot see black smoke, but once in while there are a few puffs between shifts. I think I ought to test the injectors and have them rebuilt if necessary. The truck has 155K.

A puff of black smoke the moment you lift for a gear change indicates the pneumatic governor is lagging behind manifold vacuum. This can easily occur when the leather diaphragm stiffens from age or using a preservative oil on the diaphragm that engine heat has turned to tar.

[Zoltan]

Were you in fact running the next lower gear than usual to make the Pali grade?

Well, yes, I had to run in 3rd gear a lot longer than usual before I could shift into 4th, and in 4th too I had to push much harder.

Thanks for the diaphragm tip, that was on top of my list to check out. I wondered why there's a lag between pushing the pedal and throttle response. I guess I have to see the dealer for a new leather diaphragm.

[philip]

Were you in fact running the next lower gear than usual to make the Pali grade?

Well, yes, I had to run in 3rd gear a lot longer than usual before I could shift into 4th, and in 4th too I had to push much harder.

Ah. So the effect of "turning down" the pump had an unintended consequence ... a compensating driver behavior. Higher RPM at lower vehicle speeds for a longer time period. These are the things responsible for the elevated coolant temperatures you witnessed.

Thanks for the diaphragm tip, that was on top of my list to check out. I wondered why there's a lag between pushing the padle and throttle response. I guess I have to see the dealer for a new leather diaphragm.

See thread: Pneumatic Governor Diaphragm