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Old 01-25-2018, 12:34 PM   #1
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7.3 IDI Oil Pressure

Hey all,

I went and looked at a bus the other night. Some aspects of it seemed fine, others just seemed to be really worn more than the 150k the bus claimed to have on it. The owner had bought the bus at auction and didn't have any maintenance records, so certainly possible for an early 90's bus to have had a speedo replacement and the current owner was just relaying what he knew.

But the one thing the really bothered me was the oil pressure. It was probably in the 40's when we went to look at the bus, and when we got it fired up, it was only running about 40psi Oil pressure. We let it warm up, and took it for a short spin around the neighborhood. It was up to temp by the time we got back but was only pushing about 13psi at idle once warmed out. I thought I had read that the 7.3 IDI was more high volume than high pressure, but even so, that seemed low to me for a modern engine.

Anyone with a 7.3 IDI that can comment on whether that is something to be worried about, or just normal for that style engines.
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Old 01-25-2018, 12:38 PM   #2
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Originally Posted by Tigerman67 View Post
Hey all,

I went and looked at a bus the other night. Some aspects of it seemed fine, others just seemed to be really worn more than the 150k the bus claimed to have on it. The owner had bought the bus at auction and didn't have any maintenance records, so certainly possible for an early 90's bus to have had a speedo replacement and the current owner was just relaying what he knew.

But the one thing the really bothered me was the oil pressure. It was probably in the 40's when we went to look at the bus, and when we got it fired up, it was only running about 40psi Oil pressure. We let it warm up, and took it for a short spin around the neighborhood. It was up to temp by the time we got back but was only pushing about 13psi at idle once warmed out. I thought I had read that the 7.3 IDI was more high volume than high pressure, but even so, that seemed low to me for a modern engine.

Anyone with a 7.3 IDI that can comment on whether that is something to be worried about, or just normal for that style engines.
Sounds slightly low but its still well above critical.
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Old 01-25-2018, 12:48 PM   #3
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I should probably mention, this pressure was indicated on a factory dash gauge. While I assume its much better than the pickups that had only the L____H gauge, not sure how accurate the factory gauges tend to be. I think 40 was mid range of the gauge, so I assume it was an 80 psi gauge. I would have felt better if it was a high quality gauge so you really knew it was a specific pressure, rather than somewhere between 10ish and 15ish.
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Old 01-25-2018, 02:53 PM   #4
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Typically, they run around 15-20psi hot idle.

Is it turbocharged?

Typically, hot oil pressure with 15w40 will be around 38-42psi off idle. At idle, pressure between 10-20 is typically deemed safe. How many miles are on the oil change and what wait oil are they running in it?

Oil pressure bypass could be sticking a bit which would cause slightly lower pressure at idle.

If I were you, I'd just verify pressure with a cheap mechanical gauge. If it isn't making any ticking/rattling noises at idle... pull the oil fill cap and see how much pressure is venting at idle (blow by). Most IDIs have a bit... but it shouldn't be strong enough to push the cap away if you flip the cap over and place the flat side against the oil fill.

The IDI is a reliable old work horse... not a power house but gets the job done. The injection system (IP and injectors) are known for getting weak much over 100k miles. If it starts easy when warm, then you should be ok. Oil cooler and valve cover leaks are common but not hard or expensive to fix. Glow plug system is cheap to work on... ONLY use motorcraft or IH glow plugs ( Motorcraft ZD9 for the 7.3).

What type of bus are you looking at?
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Old 01-25-2018, 07:17 PM   #5
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Usually oil pressure on average should be 10psi for every 1000 RPM when at operating temp.
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Old 01-25-2018, 10:27 PM   #6
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Quote:
Is it turbocharged?
It was pre-turbo year.

Quote:
How many miles are on the oil change and what wait oil are they running in it?
It still had temp tags from when the current owner bought it at auction without maintenance records, so I doubt anyone knows.

Quote:
pull the oil fill cap and see how much pressure is venting at idle (blow by). Most IDIs have a bit... but it shouldn't be strong enough to push the cap away if you flip the cap over and place the flat side against the oil fill.
I had heard that I should check the blow by, but I plain forgot to do it. Thanks for the tip on how to evaluate it.

Quote:
What type of bus are you looking at?
It was a International with a bluebird shortbus body. 14.5 feet from back of drivers seat to the back wall.
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Old 01-25-2018, 10:44 PM   #7
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My work truck has right at 400,000 miles on it and I know it just needs an oil change when the pressure starts getting that low.
I usually have thicker high mileage ROTTELLA put in it and get 40-psi after warm and after a good run pulling a heavy load 25-20.
When the guage starts showing 15 or less(have been there) I do an oil change.
At 400,000 it's still pulling strong.
Not to many of those or interstate miles?
They are all highway stop and go loaded down pulling loaded trailers that are usually more than what I should.
My motto is I can pull anything I need behind this truck but it's the stopping in a hurry that is a problem?
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Old 01-25-2018, 10:50 PM   #8
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Originally Posted by Jolly Roger bus 223 View Post
My work truck has right at 400,000 miles on it and I know it just needs an oil change when the pressure starts getting that low.
I usually have thicker high mileage ROTTELLA put in it and get 40-psi after warm and after a good run pulling a heavy load 25-20.
When the guage starts showing 15 or less(have been there) I do an oil change.
At 400,000 it's still pulling strong.
So I have a genuine question here ...

The last long trip my Cummins 8.3 mechanical ran was 500 miles in moderate ambient temperatures, on the oil it had in it from the seller (assuming 15W40)

During the run the oil pressure was fairly steady at around 60-65 psi. At idle it was 20 to 25 psi.

I have seem it hit close to 100 psi only when I started it at very low temps ... well below freezing, and it soon dropped to the regular 60-65 psi.

The engine has 100k and seems to be running very smoothly. It certainly made the Nebraska to Oklahoma run without incident.

Are these pressures indicative of an engine in good condition, or could there be issues if the pressures are too high?
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Old 01-25-2018, 11:10 PM   #9
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Oil pressure was that high because 15w40 is like molasses when cold. My 4bt Cummins will peg my 80psi gauge with 5w40 in the crank case at anything off idle when temps are cold. If pressure is running that high when the engine is warm, then the oil pressure bypass valve is more then likely stuck shut. It can burst oil filters, etc.. if it gets too high.

With fresh oil, the IDI should have around 50-60psi idle cold (could be a bit lower..).. hot.. idle should be around 15-20 (I've seen guys that have idle pressures under 10 without any issue.. just typically means looser bearing tolerances.).. And anything off idle will be 35-45 when hot typically.

With no maintenance records.. I'd go through it really well. If you want an idea of some general IDI maintenance to catch up on right off the bat, check out my build thread as that is what I'm in the middle of right now.
http://www.skoolie.net/forums/f27/93...ild-18500.html

They are great reliable engines if maintained well. I would most DEFINITELY be flushing the cooling system right off the bat and refilling with 50/50 green and distilled water with the Ford or IH SCA additive. These engine are semi-prone to cavitation issues when SCAs are not used.. pinholes in the cylinder walls, etc.. and most people don't know to monitor the SCA levels in the cooling system.

On another note, these engines really like their alternative fuels. I run both of my IDIs on processed waste motor oil (when at operating temp that is..).
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Old 01-29-2018, 03:33 PM   #10
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Originally Posted by Tigerman67 View Post
It was pre-turbo year.



It still had temp tags from when the current owner bought it at auction without maintenance records, so I doubt anyone knows.



I had heard that I should check the blow by, but I plain forgot to do it. Thanks for the tip on how to evaluate it.



It was a International with a bluebird shortbus body. 14.5 feet from back of drivers seat to the back wall.
Hey man, I own one of these engines, here's my story. It's a box truck, but same engine. Unfortunately, the gauge is a fake. It is more or less a switch. I found this out a couple years ago, when one of the batteries in my truck finally died. It was pulling the voltage in the system down, and this in turn made that oil pressure gauge read below the "normal" zone. This freaked me out, and I started asking around about the relationship between this battery and my oil pressure. Turns out, that gauge simply plays the roll of an idiot light. I guess that gauge in particular is sensitive to voltage changes. Sure enough, when I replaced the batteries, the oil "pressure" was magically back in the middle. Apparently, it's because these engines don't run a high oil pressure at all, it's all about the volume. I forget the spec, but at wide open, they do flow something ridiculous. And apparently, Ford(or whoever) figured everyone would freak about the low oil pressure, and they would never be able to resell these trucks, so they did it this way. This is in a Ford truck, so I'm not 100 percent sure if the international setup talks the same language, with an actual sender. Just throwing it out there.
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Old 01-30-2018, 06:53 PM   #11
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Turns out, that gauge simply plays the roll of an idiot light. I guess that gauge in particular is sensitive to voltage changes.
From my experience with older cars, I can give you an overview of how gauges work in general, may be different for the bigger trucks, and any modern cars have moved on from these simple examples and now the ECU reads them, and communicates the results to the gauges. But in general, an electrical gauge is usually just hooked up to a sending unit that is nothing more than a variable resister to ground. This is true whether its a fuel gauge, pressure gauge, etc. its just a different mechanism that performs the variable resistance. The gauge is usually pegged at one end of the scale if you just ground out the sensor line. If the wire gets accidentally removed and doesn't ground out, then its at the other end of the scale. If the sensor is properly sized, it should cause the gauge to behave as expected. If you happen to get the wrong sensor, you could get weird readings. For instance if you had a sensor where mid range was 20psi, and your gauge was 40psi midrange, well you wont get the results you want on the gauge.

So the gauges are basically simple voltmeters, and the variable resistor sensor causes it to be somewhere on the scale, but there is the problem of how do you keep the reading consistent when the car is not running (12V) vs running (14V), because the reading of your gas gauge shouldn't change just because the car is running or not.

So what they did on older cars is they had a small constant voltage regulator for the dash, which on old cars I think it was around 10V which was supposed to take care of most fluctuation including a bad cell in your battery. But If the constant voltage regulator isn't performing well, it will drag all the gauges low or high or just be inconsistant. Another problem is when the constant voltage regulator for the dash died, you would loose all your gauges, then someone would realize, you don't need it and you can wire past it and the gauges will work again with out it (they will all now read incorrectly but they will work).

Idiot lights in theory use a sensor that is more of a On/off switch and when the switch is 'on' it makes contact with ground and turns your idiot light on. In reality, most of them also have some variability in on vs off as well. I have an idiot light on my 51 panhead which uses 50 or 60 weight oil. When starting it cold, just a kick or two will cause the molasses like oil to shut off the light, but once the bike warms up and the oil thins out, if you stop at a stoplight, the light will start to glow. If the bike is really hot, that light will be on constantly. bring the revs up off idle, and the light will get dimmer and then shut off. I think some of the crappier 'gauges' use this theory to operate a gauge off of what is essentially an idiot light sensor. I think alot of those gauges that read L ______ H tend to be like that. The manufacturer just makes sure the sensor drops the needle somewhere in the middle of that range at normal operating conditions.

In my particular case, the gauge I was reading had a large variation between cold and warm oil pressure, so I feel confident that it was behaving like a true gauge rather than a gauge that was hooked up to a idiot light sensor.
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Old 01-30-2018, 07:12 PM   #12
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So I have a genuine question here ...

Are these pressures indicative of an engine in good condition, or could there be issues if the pressures are too high?
As someone else mentioned, too high of pressure can be a problem if it is because of a blockage or because of a stuck pressure relief valve.

Good oil pressure can be a sign of a engine that is in good shape and all the internal tolerances are still nice and tight. But it is also so variable, that its hard to say for sure with just oil pressure. changing oil weight can change pressure, operating in the cold vs heat can change the pressure. Even if the engine is worn out, if the oil pump has enough surplus capacity its possible for it to keep the pressure up even with looser tolerances inside the engine.

I was concerned with the oil pressure in a 7.3IDI, but based on feedback here, it was probably considered to be more on the low side of normal. But the same pressure reading may mean something completely different in a different model of engine.


If you want a peek into the condition of your engine, I would recommend oil analysis. It can tell you alot about the current condition of your engine, although they generally need to know how long the oil has been run to be very accurate. The military got into oil analysis many decades ago, and they don't change oil unless the oil analysis tells them too (they sample monthly on the tracked vehicles) and it saves them millions of gallons of oil over the years. I believe the big rigs use oil sampling as well, but I don't know what their testing schedule is. Paying $25-30 for an oil analysis of your car is hard to stomach, considering that that an oil change isn't horribly expensive for a car. But I tried to do one every couple years just to keep an eye on the engine anyway. These big diesels have a lot of oil in them, and they may be run on a more irregular schedule than your car, so I think it would be expensive to change your oil more often than you have to, so I think occasional oil analysis is probably a good idea.

If you are trying to do extended interval oil changes, there are some beyond standard oil analysis tests (that cost more) that can also tell you the additives in your oil are adding up.

I have used Blackstone labs in the past for oil analysis, and I liked that when they return results to you, they give you all the numbers, but then they tell you what it means in English too. Also if you check their FAQ, its hilarious.
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Old 01-30-2018, 07:43 PM   #13
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high oil pressure for the first few minutes in really cold weather is normal... my T-444E. maxxes the sender out at 80 PSI. when my revs are over 1200 or so when ive just started it in super cold weather... biut after it runs and the oil temp gets above 55-60 degrees i dont see those kinds of pressures.. at 1500-2000 RPM highway cruise normal load.. engine temp 180, oil temp near the same my pressure is 42-46 (varies because of HPOP draw) .. engine has 9000 hours and 167,000 miles for reference.. seems to run fine..
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