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Observation / Question
I have had the great opportunity to spend a fair amount of time at Prevost-Winter Garden over the last week and had an observation. The Technicians who work on coaches do not use “high idle” when idling a bus. I was always taught to use high idle when not moving (and parked for any period of time), but I have seen many, many coaches parked outside their maintenance bays running in normal idle. I would go so far as to state I have not seen any coach (or bus) in high idle except for mine.
I asked the service manager and a few of the tech’s why, and they responded “it’s better on the engine”. When I probed further, they responded with two benefits. 1). Less heat build in the engine; 2). Gets the load off the motor, thus not on the turbo like in high idle.
They backed my coach out and let it run for about an hour when complete and it appears they may be right! I got in the coach and the coolant temperature was about 140 degrees and no turbo noise and no turbo boost pressure on the gauge. I immediately flipped to high idle and the turbo snapped to service with its usual whine with boost indicated on the dash.
Looking for other’s to weigh in, but if it’s a good practice at the service center, why isn’t it good practice in everyday use?
Thoughts? Opinions?
TNJET
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11 Replies
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I think that is total BS. Detroit used to say to never let it idle unless at a higher speed. Think about it…140 is not operating temp. In colder temp regions it would not probably even get that high. You likely have EGR on your engine. Without getting up operating temps, or as close as you can get it, you are likely not burning fuel efficiently and could be creating more gunk in your valve train and in the piston ring area. Even with high idle, we used to see issues in the winter with unburned fuel slobbering out of the exhaust after drivers would take a sleep break. That was WITH high idle on. We had to turn it up about 300 RRPM on some units to get rid of that problem.
You also do not have as much oil pressure, and depending on the electrical load, not getting the amps you would at a higher RPM. I can not think of any time I left my coaches at low idle for any amount of time. Unless cooling it down at the end of a trip I always flipped the high idle switch.
I wonder if the real reason it is not to keep noise levels down due to some local issues….??
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What Mark said. Many moons ago when the 8V92 was the standard motor home engine Prevost (at a seminar) said to never idle for long periods at low idle speed. The engine will not heat properly, the oil pressure is low, and the alternator output is minimal.
At high idle you will not drain the battery, even with OTR, the engine will heat and retain the heat, and there will be ample oil pressure.
As a practical matter we should not be idling our engines for extended periods, but if we have to use the high RPM. Unless it is essential to idle the engine such as in sub zero temps when a restart might be an issue shut it down, and use the generator if required.
What does the operator or driver’s manual say about idling? I went through my operator’s manual and it mentioned fast idle only twice. Once it mentioned to engage it during slide operation and the second mention was after a cold engine start and 2 minutes of slow idle to go to fast idle. There was no other mention of how and when to use it.
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Interesting…..I live in a southern climate and the service center also in Florida. Could be just a sloppy practice, or not as much of an issue in the warm south.
Jon….I agree. I try to idle as little as possible, but when I do, I typically use high idle. The only thing that makes sense to me is to get the motor off the turbo. I don’t like the thought of turbo boost while idling…..
TNJET
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I seriously doubt we have measurable turbo boost while idling, but if we do, so what?
A turbo is well lubricated and at low RPMs such as low or high idle it is merely loafing along with ample oil pressure keeping the bearings well lubricated.
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When I had an engine swapped out in my old ’88 coach by American Fleet in Springfield, MO (the best engine folks I know) the last thing they said to me was never leave the coach at idle. Turn it off or get it to high idle for any extended period.
I also discovered the “let it idle for 2 minutes in cold weather” problem. We were getting ready to leave MI and did not wait until I say 90 PSI on the air tanks and flipped it to high idle anyway. Went back to check the engine before leaving and found the engine fan belt had jumped off as the air bag had not yet inflated.
Won’t make that mistake again.
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Raw unburnt fuel washing the cylinder walls occurs at idle which is not good.
My DD ProDriver indicates a warning message anytime it sees an idle time longer than 5 mins. It also keeps track of % of idle time per trip and leg. -
Is Prevost the best place to get advice and counsel about the Detroit Diesel engine or Allison transmission?
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My guess is if I want engine information I am better off not asking a company that can weld a bus frame and install seats.
I’m pretty sure Detroit is not where I am going to ask Prevost questions.
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Pretty sure the Detroit owners manual says to use high idle. I will try to dig it out and post the info.
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As Don states the Pro Driver monitors idle time and provides a warning as to excessive idling.
Detroits like a lot of other large Diesel engines do not like excessive idling, it leads to low temps, bore washing and fuel / oil slobbering from Turbo.
I think that Prevost / Detroit have provided a high idle function for a good reason, i always use mine.
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Noted! I will keep using high idle as in the past. Thanks for the info.
TNJET
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