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Old 09-25-2009, 08:40 PM   #1 (permalink)
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Arrow Low Oil Pressure--7.3 Diesel

Hello. I hope someone can help me regarding low oil pressure. 1989 Uhaul 26 foot box truck. 7.3 diesel 5 spd. Just bought and hoping she will serve me w/o major breakdowns. Running at 50-55mph, she only is reading 32-35 psi, and when I come to a stop the pressure drops down to around 5psi. There is also a dummy oil light that flickers on and off even when driving 50-55mph. What is the normal or "ok" range for this engine? I called Uhaul and they said that it is normal for this diesel to idle at a low oil pressure. I drained out the black thin oil and the fresh oil brought her up a bit and the dummy light flickers a little less. She seems to run good......what do the experts in here say? Can I drive this truck and baby her and hope she wont pull any surprises on me? Anything else to help bring up that pressure?

Many thanks to all suggestions.
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Old 09-25-2009, 08:58 PM   #2 (permalink)
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Sounds like normal OP to me.
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Old 09-25-2009, 09:19 PM   #3 (permalink)
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A little low but not fatal . Is it a mechanical guage. I get the best pressure with Walmart 15/40 super tech. 5 lbs more than delo. I couldnt believe it.
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Old 09-25-2009, 09:24 PM   #4 (permalink)
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Fully warmed up the Oil pressure will be about 10 @ idle and 35-40 @ 3000 RPM. It sounds like your okay, but your sensor might be going bad. the dash gauge in these trucks is a joke to say the least. I have an aftermarket gauge and I can see even a 1 lb. change. DELO 400 15W/40 here and HOT I see 12 P.S.I. @ idle
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Old 09-25-2009, 09:28 PM   #5 (permalink)
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Snotzalot----thanks for replying. That is a BIG relief that you think it is in the normal operating range....whew!

Type4.......Thanks also.....I will sleep better tonight knowing the oil pressure is not "fatal". It is the standard issue oil pressure gauge in the Uhaul truck....I assume a mechanical one. However, if this pressure is fairly "normal" why is that dumb idiot light flickering? Could it be a faulty sensor going to the idiot light? That flickering is going to drive me to drink......maybe a piece of black electrical tape over it?

Thanks.
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Old 09-25-2009, 09:36 PM   #6 (permalink)
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Chuckster.......So, you think I should install a better quality oil pressure gauge, and while doing it put in new oil pressure sending units?

Also, is it ok to add a few quarts of 20w50 to thicken her up a bit? Any other additive you reccomend to help keep her from pulling any fast ones on me? Maybe give her some slick50 for her birthday?

thanks.
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Old 09-26-2009, 08:32 AM   #7 (permalink)
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dont mix oil brands--or weights--keep the same weight oil. 15w40 is whats supposed to be in your engine--
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Old 09-26-2009, 09:44 AM   #8 (permalink)
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No slick 50, if anything put in a quart of power punch or lucas oil treatment. If the pressure jumps up you may have some bearing wear. The pressure relief is in the rear oil cooler header and can be removed and cleaned /shimmed but it is a pain. DO not decide anything till you verify the pressure with a MECHANICAL gauge. I dont trust electrics, even the "good" ones.
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pyro,boost and all stock guages ignored with mechanical real ones
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Old 09-26-2009, 10:22 AM   #9 (permalink)
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Quote:
Originally Posted by Studebaker60 View Post
Chuckster.......So, you think I should install a better quality oil pressure gauge, and while doing it put in new oil pressure sending units?

Also, is it ok to add a few quarts of 20w50 to thicken her up a bit? Any other additive you reccomend to help keep her from pulling any fast ones on me? Maybe give her some slick50 for her birthday?

thanks.
Putting in a "better" gauge is your call. Mine is an Autometer electric 0-100 PSI 270 degree sweep. I would change the sending unit just to see what happens, they are cheap enough. NO DO NOT put 20/50 in it or any additives.

TYPEFOUR:
Quote:
I dont trust electrics, even the "good" ones.
Why not ?

I prefer not to have a hose in my cab in case it decides to fail. then you have oil pumping into the cab until you can get stopped and the engine shut off. What a mess, Don't ask how I know..
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ATS Turbo upgrades: 3" DP with 3" exhaust Magnaflow XL muffler: Pictures Here
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Pillar pod: Autometer C2 Series gauges: pyro,trans, boost, water, oil pressure
Hypermax Cowl induction, K&N air filter, flex-a-lite 26K trans cooler with fan,Tekonsha prodigy
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Old 09-26-2009, 01:55 PM   #10 (permalink)
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Where are the oil pressure sending units on these trucks? I want to install a decent mechanical oil pressure gauge and was wondering where to hook it up to. Thanks!
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Old 09-26-2009, 02:21 PM   #11 (permalink)
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its right behind the glow plug relay on the firewall. on any use vehicle i buy and when oil pressure looks to be low just replace the oil sensor regardless its only 5 bucks.
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Old 09-26-2009, 07:13 PM   #12 (permalink)
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94turbo/Typefour/Chuckster----seems you all agree NOT to put 20w50 or slick50 and not to mix oil type. Maybe power punch or lucas. Educate me and explain why it is not good to give the old gal a dose of 20w50.....seems if it ups her blood pressure, then why not? (If her pressure drops off alot more, I have a couple quarts of heavy weight rear-end/gear oil under the seat that may re-sussatate her. Ha.....joking).

Typefour/Chuckster.......Yes, I will have to look into a better quality OP gauge, and new sending unit. I agree....dont want oil all over my legs if a line busts......or, could put the guage outside in front of the windshield.

Final question.......still did not hear anyone advise me what to do about my Flickering Dummie Light. Bothers the heck out of me seeing that red oil light flicker for my attention as I drive. The actual gauge is reading 32-35 at 55mph, so why is that dummie light acting up? I will try replacing the sending unit to it, and if it still flickers, I guess I pull out the black electrical tape? Sound right to you?

(Thanks everyone for all your input.....really has helped)
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Old 09-26-2009, 07:19 PM   #13 (permalink)
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Ok......One last question. I have been looking, but cannot find anywhere to buy a shop manual for this truck (or at least for the engine). It is a 1989 26foot Uhaul with 5spd Intern. 7.3 diesel. Any suggestions where I can locate technical info/shop manual? Should be info someplace. Uhaul told me that they knew of no one who sold manuals for the truck.
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Old 09-26-2009, 07:32 PM   #14 (permalink)
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Quote:
94turbo/Typefour/Chuckster----seems you all agree NOT to put 20w50 or slick50 and not to mix oil type. Maybe power punch or lucas. Educate me and explain why it is not good to give the old gal a dose of 20w50.....seems if it ups her blood pressure, then why not? (If her pressure drops off alot more, I have a couple quarts of heavy weight rear-end/gear oil under the seat that may re-sussatate her. Ha.....joking).
It's never a good idea to mix oil weights in any engine. If the engine is designed to maintain an oil pressure with a specific weight oil and it doesn't, then you need to determine why. Just adding other weights will mask the problem and you may have to pay a bigger price sooner than you want to.

Quote:
still did not hear anyone advise me what to do about my Flickering Dummie Light. Bothers the heck out of me seeing that red oil light flicker for my attention as I drive. The actual gauge is reading 32-35 at 55mph, so why is that dummie light acting up? I will try replacing the sending unit to it, and if it still flickers, I guess I pull out the black electrical tape? Sound right to you?
I wouldn't do the electrical tape thing unless you can't get it figured out. After you change the sending unit, if it still flickers, I would look at the electrical connections and the ground.


Quote:
I have been looking, but cannot find anywhere to buy a shop manual for this truck
These engines are becoming dinasours. You can sometimes find a set of shop manuals on E-bay. You can also get a set of manuals from Helm inc. on line.
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1994 F-350 7.3 IDI Turbo, crew cab, E4OD,4:10 L/S, LB, Dually Photos
ATS Turbo upgrades: 3" DP with 3" exhaust Magnaflow XL muffler: Pictures Here
2012 Copper Canyon 273 FWRET w/2 slides, Air Lift 5000 bags
Pillar pod: Autometer C2 Series gauges: pyro,trans, boost, water, oil pressure
Hypermax Cowl induction, K&N air filter, flex-a-lite 26K trans cooler with fan,Tekonsha prodigy
Train Horns: Pictures here

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Old 09-26-2009, 08:08 PM   #15 (permalink)
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Chuckster........Thanks for the added input....Im never tooooo old to learn. Ok, I will refraim from grabbing the electrical tape and utility knife, and try to get the dummie light to go off voluntarily....good idea....check grounding and wire connections...and of course, the sending unit.

I will try Helm now, and see if they have manuals.

I saw the pics of your truck/trailer......that fence tried to take a bite out of the back end of your trailer! I know the feeling.....I backed up my old motorhome into a large 6x6 post.....crushed 2 bikes on the back, creased the bumper, and poked a hole in the fiberglass panel. Great way to ruin a good trip.

Take care.
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