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Turbo surging under full load
Checking to see if anyone has had a similar issue. I noticed on a run this week when the turbo is under full load (going up a hill) it oscillated from full boost to about half boost. It cycles high to low, low to high about every 5 seconds. It doesn’t do this at say 25 pounds of boost. Only when it reaches max boost for 5-10 seconds. Also hooked up a monitor to the diagnostic port and verified the turbo boost oscillations.
I went through some extensive testing today of the charge air cooler and rubber boots in the turbo system. No holes or rips and the CAC held pressure for 20 minutes without any drop in pressure. No bubbles with the soapy water test. My CAC was replaced in 2018, and I believe it to have the non-Prevost part installed.
Anyway, looking for ideas here. I’m thinking it needs to go to DD for a look-see. Thanks.
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12 Replies
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How old is the air filter?
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You might consider the turbo system rubber hoses. All hoses should be high heat red color.
Under certain conditions hose construction can cause delam. Pressure / suction reducing air flow inside the hose.
If it was my bus I would replace all the hoses in turbo system before spending any money at a shop. Complete set even if just replaced.
None of the CAC’s are Prevost parts. CAC’s are purchased from mfg per Prevost specs and cost, known as a cooling package. Where Prevost got into trouble was a cheap CAC poorly made. Vibration cracked construction seams and they began leaking. Poor performance, black smoke, fuel mileage were the result.
Major improvements were made in CAC replacements a few years ago. Gone were cheap made CAC’s due to leaking pressure. A superior supplier made a CAC offering a 7 year warranty and / or 1 million mile warranty. I have never heard of any of these failing in service.
Jon, has done major research on blue verse red CAC hoses use in buses. Jon might jump in here with his recommendations.
AL
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
Possibly a binding of the waste gate?
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You have an EGR engine and your turbo is a multiple speed turbo that shows 30 PSI, but in fact is going up to 50 PSI when the EGR valve admits exhaust gas to the intake.
I would have your turbo and its controller checked.
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The turbo speed sensor , located in the center section of the turbo can fail. When it does the ecm sees no turbo rpm and reduces the angle of the blades until it sees rpms again.
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
Thanks to all for your suggestions. I am fairly certain the CAC is one of the newer designed units. As stated, pressure checks showed no leaks. All the boots were changed in late 2018 and inspection this week didn’t reveal anything. Strange problem. Airfilter only has a few hundred miles on it.
I think its time for a trip to DD to see what they say.
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One last phone call to American Fleet in Springfield MO. might be a good idea.
DD experts…that’s all they do.
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Could be the VPOD for the turbo vanes. Do you get any codes? I had one that would do something like what you describe, and after about 400 miles it wouldn’t boost at all. DD shop here put new VPOD on. Lasted one day and it failed as well. They installed another (all reman of course) and it was fine for as long as I owned that coach.
I am pretty sure your vintage would have one. It regulates air pressure going to the control box on the turbo to change vane angles. Easy fix if that is what it is.
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I agree on the VPOD also had ours replaced with similar problem.
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Have you considered changing fuel filters?
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Thanks Badfitter…..The fuel and air filter were changed less than 200 miles ago.
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It may be the VPOD or the turbo speed sensor. To know for sure check for codes. If the VPOD is the bad guy there should be a code for turbo vanes not responding or something similar. If the turbo speed sensor is the issue it will not throw a code. The best way to catch it is to watch the live data while driving and when it occurs hit the save screen shot button then look at the data and see what the turbo speed sensor is showing for RPMs. If it suddenly drops to 0 the speed sensor is most likely the issue. This is using the DDA program.
Happy hunting…
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