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Old 09-20-2016, 12:55 AM   #1
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Exclamation Oil Pressure Fluctuation During Idle

Hello. I am seeing an idle oil pressure of around 35-40 PSI that intermittently flickers down to around 5 PSI. I say "flickers," because it drops for a second or so, returns to 35-40, and repeats a few times. Then, the engine continues to idle at 35-40 PSI.

I see three scenarios:
  1. Gage is faulty.
  2. Sensor system is faulty. (Loose wire?)
  3. Oil pressure is actually dropping.

If anyone could bounce a few ideas around regarding that last scenario, I would be much obliged. This bus is important to me.

This is a 1999 RE Thomas with a CAT 3126 engine and Allison MD3060 tranny.
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Old 09-20-2016, 04:34 AM   #2
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Silly question: is the oil level where it should be? I saw this on a tractor once and went to investigate. Good thing too because it turns out it had HALF the oil it needed and it was sucking air into the oil pickup when it was idling. The symptoms were exactly as you described.

If the level is up, pull the sender and thread a mech gauge in its place. See if it wobbles.
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Old 09-20-2016, 06:25 AM   #3
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this is normal on a diesel. my 8.3 will drop to 10 at idle once its hot. use your high idle switch when stationary. that should bump your idle up to 10 or 11 and your pressure up to 30 or 40. every motor is different, both in design and wear.
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Old 09-20-2016, 07:40 AM   #4
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Either loose connection if electrical, or the prv isn't seating properly and the problem is covered up at higher rpm due to higher oil flow. 99 percent chance it's a loose wire as previously mentioned. Pull the sending unit and connect a manual Guage and verify as greycoyote said.
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Old 09-20-2016, 07:48 AM   #5
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if its only doing it at idle and not at cruise id tend to suspect the gauge vs a real oil issue... if you were sucking air periodically you would see the gauge dip some during cruise esp if you went around curves or over bumps like rail tracks..

yank the gauge sender put in a mechanical test gauge as suggested and watch.

-Christropher
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Old 09-20-2016, 08:20 AM   #6
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Talking

Thank you for the helpful responses. I've formulated a game plan:
  1. Double check oil levels.
  2. Confirm pressure with external gage.
  3. Keep an eye on the oil system in the meantime.

Quote:
this is normal on a diesel. my 8.3 will drop to 10 at idle once its hot. use your high idle switch when stationary.
Thank you! I am reluctant to believe this, because it is precisely what I want to hear, but will keep this in mind and won't stress too much (particularly because this bus is new to me).
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Old 09-20-2016, 08:45 AM   #7
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You're doing it right IMHO, ie paying attention to the "funnies" and refusing to take the easy path.

I have a friend who was part of the Mercury/Gemini/Apollo/Shuttle programs. Back in the day these guys ran down EVERY "funny" until they fixed the problem or understood why it wasnt a problem. He retired, and more than a decade later they called him in to consult on a problem. He came in, talked to the lead engineer, asked a few questions, and got several "we dont knows". He went nuclear. So he grabbed the next three levels of management eschelon and chewed their asses too. His basic lecture was this: "you dont ignore funnies. You dont sweep them under the rug. "I dont know" is not an answer that should have a shelf-life measured in months. This is how you get people get killed". Then he fixed the problem for them. He was 78 years old and needed assistance just to stand, but when it came down to it he still passionately believed "the only good funny is a dead funny".
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Old 09-20-2016, 09:04 AM   #8
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im in agreement with that philosphy or at least educating yourself and determining what the "funny" is.. a used vehicle is tough because the last owner may have educated himself on all the "funnies" even if they werent fixed, but of course theres no Log book that coems with our busses that says "oh yeah the oil P is wierd.. we checked but didnt want to spend the $$ to fix it..

I build stuff and write software.. I hate "funnies" and "quirks"... ive got one Dev that doesnt think the same and it drives me crazy lolol

-Christopher
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Old 09-20-2016, 09:11 AM   #9
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Those are words to live by and a philosophy to aspire to. Thank you for sharing.
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Old 09-22-2016, 01:03 AM   #10
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It could also be oil out of grade, how old is the oil?

+1 on the funnies, machinery will tell you alot if you listen to it.
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Old 09-23-2016, 09:59 PM   #11
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The first thing is verify with a mechanical gauge. It is normal for oil pressure to drop at idle after warmup. If this is a "new" condition, further investigation will be required.
Now, understanding why oil pressure may be low is another thing altogether. Here are some causes:
1. Plugged oil filter - yes it happens. Replace oil and oil filter (see #3).
2. Pressure regulator in the oil pump sticking open at times. With this one you will see the needle "pulse" at idle as the gears in the pump work against each other. You can try something like Rislone or Marvel Mystery oil top help dissolve any varnish that might be causing the problem. Other than that, a new oil pump may be required. Judging by your first post, if it does this on a mechanical gauge, I'd say this might be your problem. The pressure regulator is sluggish and hanging open momentarily. Give the oil additive a good chance to work before dropping the pan.
3. Clearance between bearing surfaces. An engine lubricating system is based on pressure and volume. The combination of the fit between the bearings (Mains, Rods, Camshaft) and the associated shafts can have an effect on oil pressure should the pump volume be exceeded due to excessive "squeeze-out" between them. To determine if the bearings are wearing have an oil sample taken and an oil analysis done. If the bearings are going it will show up in the lab results. While this sounds scary, it's seldom the case without other obvious indicators (loud banging metal sounds, parts that should be inside hanging outside, etc.).

I did have an engine once that had 20psi running down the road and 8 psi at idle on a new rebuild. Engine lasted over 200,000 due to the increased lubrication and cooling effect by greater oil flow over the bearings.

By the way, been working on engines since 9 years old. Career marine mechanic for over 25 years.

Hope this helps.
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