exhaust manifold temps?

[bevans6]

How important are exhaust manifold temps at idle on an 8V-71?  I was playing around with a little IR temp gun and saw that after maybe 5 minutes of idle at low idle (I was airing up the bus to reposition it in my driveway), I had temps ranging from 125 to 185, and the whole passenger side bank was lower than the drivers side bank, on average.  I expect that there is certainly a problem with the one cylinder that is at 125, not firing well at all, but on the whole, what is the correct way to use exhaust manifold temps as a diagnostic tool?

Brian

[JohnEd]

Brian,

You know I have zero experience with D but I have a pyrometer on the Dodge 440.  I have wondered about your question in the past.  I concluded that having a large spread of temps would indicate that the rack needs to be run or the guy that did the rack screwed up......first off.  Secondly, it might flag worn injectors.  Lastly would come engine condition.  What do you think?

Exhaust manifolds used to have a device called the heat riser.  A flap in the ex manifold.  If you have more turns in one bank or the other the increased resistance to flow would certainly increase the restricted banks temp.  Now wheather you could actually see the diff caused by 2 90 degree bends is another matter.

One bank runs hotter as a rule.  Increased head temp will give you an increase in ex temp....right?  Again, how much.

Just pondering.

John

[Ed Hackenbruch]

No experience on this but i would think that the piston that is farthest away from the muffler would be somewhat cooler than the one that is closest. Seems that the heat would build up as the exhaust travels past each exhaust port on its journey towards the tailpipe.

[TomC]

Exhaust temperatures at idle with a 2 stroke Detroit Diesel has just about zero importance.  The injection rack is adjusted to be balanced at full fuel opening-meaning at idle you can have what sometimes sounds like a rough idle, lower or higher exhaust temperatures per cylinder or sides, because of slight manufacturing variances in the individual fuel injectors.  Some have tried to adjust the injectors to produce a smooth idle, then you have a rough running engine throughout the rest of the range.  The only real way of testing exhaust temp on a per cylinder basis is to either have individual pyrometers (then you have variances with 6 or 8 individual gauges) or to be the most accurate, you would put the bus on a chassis dyno and while the bus is pulling a load, THEN shoot each cylinder with your IR gun.  But personally-I wouldn't want to be near an engine that is pulling a load like that (except on my bus where the engine is a tilted V-drive and the fan is the only external thing turning since I have no fan belts-everything is direct driven) with many spinning and fan belts whipping.  Good Luck, TomC

[JohnEd]

Well Tom, so much for my logic and hunches.  Thank God I qualified my comments

I have heard DDs idle like a Swiss watch and others lope so badly they seem to quit between power pulses.  Whatever the cause I thought that idle anomaly was significant.  But I can't argue...just can't.

I also pondered running a hill and stopping quickly and running back there to "shoot" the temps.  Even shut down moments after a hard pull I would think that there would remain a temp variance that would correlate to the under load condition.  Operative word there was "think", Tom.  Looking for the answer.

Nice thing about a temp gun is that you don't have to make contact....just be within 6 feet or so.  No reason to venture into the engine compartment/bay while she is running, for sure.

[Dreamscape]

I check mine when stopped after about 2-3 hour drive. Each side is only maybe 10 degrees different. I also use an IR gun. Never did check it at idle, glad I didn't now otherwise I'd be freaking out like you! 

[Hi yo silver]

I can't help but reply to this one; I checked my exhaust manifold temps after running for a while.  I saw a difference between the two banks too.  BUT; my old two stroke 8V71 runs like a scalded hound, gets the expected fuel mileage, depending on whether I'm running 55 or 75 MPH, uses about 1/2 gallon of oil after a hard run of 1400 miles, and doesn't overheat.  My philosphy is, if it ain't broke, I not only won't fix it, but I'll use the right antifreeze, change the oil at a reasonable interval, drive it like I stole it, and not worry myself to death about it.  Disclaimer: I'm no expert, my knowledge of these old buses comes from following this BB for the last six or 8 years and my limited experience with my bus.  I can only credit my good fortune to luck rather than wisdom. 
Good luck!
Dennis 

[JohnEd]

Dennis,

Sounds like you deserve the good fortune.

john

[compedgemarine]

exhaust temps at idle on just about anything mean nothing. on all but the most expensive of intakes flow rates from port to port are different and at idle there is not enough velocity to fill the cylinders the same. as for it being different from bank to bank your cooling fan is moving air into the engine bay in different ways, on my eagle the radiator is on the passenger side and the fan blows on the passenger manifold so the surface temp is cooler than the side with no airflow. at Tom pointed out injectors are set for when the engine is up and running because that is when the air is moving through the manifolds and into the cylinders and a more accurate rate. on a big V8 gas motor we see 150 degree plus differences at idle but up at 3000 plus they all even out within a few degrees.