Community Forums

RV_rob · 02-20-2019, 07:26 PM

Cummins ISC 8.3:
Hey guys, got a new question about my motor. This coach is my first heavy duty diesel, we've put about 10k miles on it since purchase, and ever since the beginning, each time I start it, it is quiet for about 5-8 seconds before it starts the typical diesel "knock" sound. Again, this might be how they all run, I just want to confirm......

Then after a couple minutes (I guess when it warms up)it's like something cycles on or off and the sound smoothes out with a very distinct change in the exhaust putter, to having a deep, low pitched, almost hollow, thumping sound, and then after a few mins it changes back to just the original sound, then after a few mins the cycle repeats........... I'm thinking it maybe the exhaust brake closing and then opening due to some pre-set figure or something, but I just don't know. Again, it sounds like a normal function almost like it's going into some "long idle" setting or something. It's very consistent, and the cycle cadence never changes.

Does anyone else's do this, or maybe they all do it and I just can't find what it's called..........? You can imagine searching for this on google is kind of a chore, lol. Describing sounds is not a good recipe for a search engine.

If it's not normal, I can see about recording a sound or video and posting it somewhere.

Mr_D · 02-20-2019, 07:31 PM

Air compressor cycling?

winniman · 02-20-2019, 07:48 PM

Quote:

Originally Posted by Mr_D

Air compressor cycling?

That's my guess. It has to run the compressor until your airbags fill, and your brakes get up to pressure. You should hear it kick off when your gauges reach around 130 psi.

Mrbitz · 02-20-2019, 07:48 PM

Yup aircompressor cycling. I have same engine same sound on my Newmar.

RV_rob · 02-20-2019, 11:28 PM

okay, I will watch my air gauge and see if the timing works. To be clear, what you are all saying, is that the first change when the exhaust sound drops to the low deep thumping, that would be the air compressor turning off and the engine not having to work as hard so it changes it pitch/rpm/load or all of the above?

EDIT: apologies to winniman, I just re-read your post and you already confirmed my question I just asked. it's when it kicks off. Thanks!

YC1 · 02-21-2019, 04:34 AM

Any chance it has a grid heater and you are in cold weather where it might cycle? The grid heater puts a very heavy strain on the electrical which would put a load on the alternator.

Mrbitz · 02-21-2019, 04:46 AM

Quote:

Originally Posted by YC1

Any chance it has a grid heater and you are in cold weather where it might cycle? The grid heater puts a very heavy strain on the electrical which would put a load on the alternator.

Grid heater would do it too. Not as noticeable but it would cause a little strain on the engine for a while.

onechaddude · 02-21-2019, 04:52 AM

Quote:

Originally Posted by YC1

Any chance it has a grid heater and you are in cold weather where it might cycle? The grid heater puts a very heavy strain on the electrical which would put a load on the alternator.

My thoughts are the same Myron. My Neptune is only the ISB Cummins but the grid heaters cause it to change sounds due to the heavy load on the alternator/electrical system.

Chad

RV_rob · 02-21-2019, 03:37 PM

It does in fact have a grid heater, but if I recall it’s always sounded this way even in the summer months down in Florida. I can imagine that thing pulls high power from the alternator, the two wires running to the top of the head where it connects are pretty large gauge

RV_rob · 02-21-2019, 03:38 PM

How about the way that it starts? Where the typical diesel knock sound doesn’t kick in for about 5 to 10 seconds, is that a normal thing?

Jake21 · 02-21-2019, 04:24 PM

Quote:

Originally Posted by RV_rob

How about the way that it starts? Where the typical diesel knock sound doesn’t kick in for about 5 to 10 seconds, is that a normal thing?

My ISL 400 does this. I have heard other guys start up and is sounds the same.

mackwrench · 02-21-2019, 09:06 PM

The start up the OP describe is very normal.
On the CAPS system, the ECM adjusts the fuel trim (think like timing) to the injectors, when the engine is 1st started cold, the fuel trim is more "retarded" to allow engine easy crank/fast start. It takes the ECM a few seconds after starting to readjust the fuel trim to wherever it needs to be based on now current sensor data. When the trim "advances" the fuel delivery the engine noise level changes.
It's a emission control as well since less fuel is used to start engine,

My ISC 350 has done that since new.

rag_ftw · 02-21-2019, 09:18 PM

Quote:

Originally Posted by mackwrench

The start up the OP describe is very normal.
On the CAPS system, the ECM adjusts the fuel trim (think like timing) to the injectors, when the engine is 1st started cold, the fuel trim is more "retarded" to allow engine easy crank/fast start. It takes the ECM a few seconds after starting to readjust the fuel trim to wherever it needs to be based on now current sensor data. When the trim "advances" the fuel delivery the engine noise level changes.
It's a emission control as well since less fuel is used to start engine,

My ISC 350 has done that since new.

DITTO with my ISC 350.