VP 8.1GXi Fuel Pressure / Hunting at Idle

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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby cbsmith » Wed Jun 13, 2018 6:01 am

I had a vacuum leak on an old 1990’s era fuel injected Chevy car and it causes an uneven idle like you are describing. Once any throttle was applied it smoothed out.
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Wed Jun 13, 2018 7:01 am

Thanks for the tip - measuring vacuum shows a consistent vacuum but I do suspect an intake manifold air leak. I will let everyone know once the engine is fixed; I can hardly wait!
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby fatmattyd » Thu Jun 14, 2018 6:47 am

These are interesting tips... measuring vacuum shows consistent readings, but a bad sensor could still throw the ECU off the deep end.

I think we're pretty much all rooting for you at this point, it seems you've been through the ringer!
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Thu Jun 14, 2018 9:57 am

Most of the sensors were swapped with the starboard engine which did not result in any improvements. Readings on the computer seem to show accurate sensor data as well. All the time my marina and I invested in attempting to solve the problem should indicate a deeper issue than the standard issues. There are so many things that can cause hunting and surging at idle; that alone makes it somewhat complex. Ultimately, I would have been happy (but sad) if it was a quick fix that I missed so I am reserved but happy about the delay. For now; I have a floating condominium but I am happiest when driving the boat and that has only happened once in the past 10 months. This weekend, the marina that my boat is at is having a big party with a band so my wife and I will make the best of of it. Happy boating!
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby gofast24 » Thu Jun 14, 2018 10:56 am

JNelson90 wrote:Thanks for the tip - measuring vacuum shows a consistent vacuum but I do suspect an intake manifold air leak. I will let everyone know once the engine is fixed; I can hardly wait!

Assuming you measured the intake manifold vacuum on the port (surging engine) why not try measuring the vacuum on the stbd and compare the reading? A +/- 2% or so should be OK, much more than that (higher vacuum on the stbd engine than the port very well could indicate a intake air leak downstream of the throttle body. What is the vacuum scale on your vacuum gage? Inches of water (probably not) or in-Hg, PSI?
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Fri Jun 15, 2018 7:48 am

I did measure vacuum on both engines and they were very close (+/- less than 1 inch). Volvo Penta sent me a document indicating that the vacuum should be 11 inches (because of an aggressive cam per VP) but I was seeing about 14 inches on both engines. Still no updates from the marina; I can hardly wait to hear what the fix is - assuming they can actually fix it.
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby gofast24 » Fri Jun 15, 2018 10:58 am

JNelson90 wrote:I did measure vacuum on both engines and they were very close (+/- less than 1 inch). Volvo Penta sent me a document indicating that the vacuum should be 11 inches (because of an aggressive cam per VP) but I was seeing about 14 inches on both engines. Still no updates from the marina; I can hardly wait to hear what the fix is - assuming they can actually fix it.

A inch of water column (not really significant, there are 34' of water (308") or a inch of Hg (Mercury,29.92" "very significant) at sea level! These numbers are relative to "0" pressure if your in space so typically pressure or vacuum reading in the engineering community are referred to PSIG (the G meaning gage ) pressure. Anyway, sorry to bother you with this technical stuff:)
Good luck, hope you figure this out before winter!
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Fri Jun 15, 2018 12:23 pm

No worries at all; I like the technical stuff. Thanks again for all your amazing insight!
Kindest regards, Jeff

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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby gofast24 » Sat Jun 16, 2018 11:09 am

JNelson90 wrote:I did measure vacuum on both engines and they were very close (+/- less than 1 inch). Volvo Penta sent me a document indicating that the vacuum should be 11 inches (because of an aggressive cam per VP) but I was seeing about 14 inches on both engines. Still no updates from the marina; I can hardly wait to hear what the fix is - assuming they can actually fix it.

With VP indicating you should have 11" (of what, inches of mercury or water column?) and you are measuring 14", just as a last resort remove your flame arrestors from the throttle body's and re measure manifold vacuum at idle! Had to pull our flame arrestors after buying our vessel as both were seriously plugged with pine pollen, dust, some paper towel chards, oily from crankcase oil vapors and other stuff you wouldn't normally find in a flame arrestor in a boats engine room , but the previous owner never cleaned anything even with over 700 hours on both engines!
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Mon Jun 18, 2018 10:59 am

Just referenced the VP email and it was 12 (not 11) and listed as [12 inHG]. I like the idea of running without the spark arrestor in place. My engine compartment is really clean and the spark arrestors look clean as well so I am not optimistic about it but certainly worth trying as it is so simple to test. I called the marina yet again this morning and I am still waiting to hear back on the status of the repair.
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Re: VP 8.1GXi Fuel Pressure / Hunting at Idle

Postby JNelson90 » Fri Jun 22, 2018 10:55 am

I finally heard a possible diagnosis for my engine problems from the marina with the certified Volvo Penta mechanics. They performed a smoke test using an endoscope and found smoke getting into the the intake plenum which probably indicates a bad gasket seal. They were not able to confirm where the smoke was getting in but they suspect removing the intake and replacing the gaskets should do the trick. I had suspected this all along but was unable to prove it. I will continue to provide updates as the repair continues to progress. I have been without a running boat now for just over 10 months now; so what will a few more days mean - actually its killing me not having a working a boat and yesterday would not be soon enough to have it repaired.
Kindest regards, Jeff

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