First trip out in 110 heat.

[Miles]

I went to Lake Havasu for the 4th. 110 was the norm. Hot. Anyway, my 2500 dmax/ali sounded like it was trying to suck more air than it was getting. I could hear a definite sound change the way the motor draws in air. I have a Amsoil air filter and I was thinking I might want to try the stock one again? Anyway, it did this comming out of a rest stop on the way up and then again in town. On the highway it would labor for air for a few miles and then act normally. In town it would labor until I stopped the truick and let her cool down. When the truck was breathing hard it felt like I had lass power. Not good. I was not towing anything this trip and the bed was empty. Any ideas?

[Dawg]

It most likely was your thermostatically controlled fan engaging. Makes a noticable loud sound. If it was the fan, perfectly normal.

[SoCalDieselNewbie]

Loss of power?

I have the fan turn on every now and then...

it is loud...

it doesn't sound right...

it makes the uninitiated believe that something wrong with the vehicle.

As a matter of fact, on my just completed road trip to Montana, both of my buddies woke me up the 1st time they heard the fan kicked on and both wore similar looks of concern on their faces.

[ 07-08-2002: Message edited by: SoCalDieselNewbie ]</p>

[DMax_Doug]

Sunday I finished a 3000 mile trip to Oklahoma towing my 10k travel trailer and experienced the same thing. With an outside temp over about 90 degrees, I guess the engine fan engages quite often, particularly on hills. It can outright scare you when the Dmax downshifts to 4th with the fan on - it's that loud. That fan must draw some power too - I got 9.4 mpg on one tank driving in 98 degree heat with the engine fan running constantly (3hrs) driving 70-72mph. On the same run at night ~78 degrees the fan never came on and I got 12.4mpg.

Another interesting thing is that the engine temp never moved a tick off about 185 either before or after the fan engages, so I guess the parameters for engaging the fan are pretty narrow.

[Lawnboy]

Well, I guess none of you complaining about cooling fan engagement noise have ever had a '73-'00 GM truck then. Those ARE loud. When they engage, the truck actually goes FASTER :eek: due to the suction of that propeller er. ...Fan.

My Dad just installed one of JK's Duramax fan and reindexed thermal actuator clutch on his '95 'Burban. Wow is that fan ever quiet! And pull air!!!

Better to hear the groan of the fan then to see the needle on your temp gauge rising out of controll!!

[01_Duramax_Dually]

Miles,
Another thing to consider along with the fan discussion is at 110 degrees, I bet the air was at 6500 feet or better. I race in Las Vegas so I know the air gets that high with heat. Forced induction or not, thin air makes things work alot differently.

Carry On... :D

[windjammer]

DMAX-Doug,
I'm glad you posted about the fan engaging when the engine temp is normal. I just pulled my 10K 5th wheel in the hill country of Tx and was very surprised at how (1) frequently the fan engaged, (2) how frequently the allison shifted down when running with cruise on and tow-haul on. I next tried tow-haul on and cruise off. This was much better. Do others turn cruise off to eliminate the frequent down shifting? It didn't seem to matter whether I ran 60, 65, 0r 70, with cruise on, I experienced frequent down shifting.

[DMax_Doug]

Windjammer,
Yes, it is interesting that the fan engages when the temp gauge is still at about 185, but as Lawnboy said, it's better than overheating.

As for the cruise & tow haul situation, you'll find it's been covered considerably in these forums. I've read on this board that the engine will downshift when the actual speed falls off 10% from the speed set (i.e. cruise set at 60mph, downshift occurs at 54mpg). There may be an software update available to improve this condition.

My truck was built in Oct 01 and my experience is that the tolerance, at least on my truck, is somewhat less, closer to about 4mph at freeway speeds. What I discovered (on I-40 in Texas by the way!) was that if you take cruise off and begin a hill, you can learn to feel how much pedal you can give before causing a downshift. Just get out of the pedal enough to keep it in 5th and you can lug it over most shorter grades in 5th at about 60mph when you start it at 65-70mph. On the few longer grades I hit, I just let it kick down to 4th and drove up the hill at 60-65mph.

[TLA]

The fan senses the temperature of the air not only from the radiator, but fromthe trans cooler, the A/C condenser, and the turbo intercooler. That is why it cycles in and out with out change in the coolant temp gauge. Get a really hot, humid day when you are running hard, and it will be engaged most of the time.

[TBC]

Frequent downshifting while towing in cruise is caused from the lack of having the trans computer updates not loaded. It will fix that problem. The fan clutch is mechanical and senses air temp thru the radiator and other coolers in front and therefore will engage the fan clutch even when the engine temp is running normal. I was in Vegas and AZ about 3 weeks ago with the 110 deg temps pulling my 32 ft trailer (about 8600 lbs plus my tools etc in truck) and never had one temp problem. The eng stayed below 200 and the trans 180 to 200 the entire trip except the one following incident. The trans temp went to about 210 and the eng went to 230 on a grade of about 7-8% for about 10 miles and I was pushing the RPM at 3000 (3rd gear and the only time the whole trip it went into 3rd). As soon as I saw the temp up that high I backed off to 2500 and the temp immediately dropped to normal. You have to really push this truck hard to heat it up. It ran great with fantastic power. I love this truck.

[Miles]

Ok, good tip on the fan. Where is the fan located? I want to get out of the truck and see that it is indeed the cause. It makes sence.

[SoCalDMAX]

Miles,

Quote: "Where is the fan located?" LOL!

It's the engine cooling fan. It is temp controlled with a hydraulic clutch, just like many V8 engines. It's located behind the radiator, in front of the engine. Beneath shroud which states: "CAUTION FAN".

Boy, do I feel like I just got suckered :D

Regards, Steve

[Marty Lau]

Jeff;
Your thought process is correct about the effect of heat on the opperation of internal combustion engines acting like they are at altitude. It's called density altitude. Standard temp. is 59 degrees at sealevel. For every 3.5 degrees above standard temp you add ONE THOUSAND Feet of altitude. So assuming sea level, 110 degrees is 51 degree's above standard which meeans your truck will drive like it is at 14,500 foot altitude. Then take into account the intercoolers don't cool the compressed turbo air as well and further affects preformance. Many airplanes have crash here in the west because of desity altitude. It's something aircraft pilots are taught and tested on, and are expected to calculate. Thats also why some times airline will say they are filled and they have empty seats, as because of desity altitude they have to lower their take off weight and in extreme cases aircraft can not take off period.
Sorry to go on just my aviation bakc ground coming out. ;)

[xwing]

I hear the fan on after first starting the car in the morning sometimes. "vrroooom" sort of sound. It shuts off after maybe 2-3 blocks.
I remember driving Army Hummer diesels...seems like they were working sorta hard to get up around 65mph, and when hot and going uphill and the fan kicked on, would lose maybe 5+mph speed because of the power the fan sucked!