Why is my oil pressure ALLWAYS at 40psi?

driven2services

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I'm just curious about this...my last Jeep didn't have an oil pressure gauge. The jeep that I recently purchased does. As soon as I start it up...even if it's 20 degrees outside, oil pressure is almost exactly 40 psi. It doesn't change once it gets warm, nor is it any different when I start it after it's been in the 60 degree garage. It does go up (slightly) when the RPMs increase, but never more than about 45psi. Is this normal? Shouldn't there be some difference between hot and cold??
 
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My Mustang (and other 90's Mustangs) has an "idiot guage". The sending unit is basically an "idiot light" sender, and the guage is calibrated to read just over half scale at ALL pressures. Basically just an on-off switch. Maybe yours is the same?
 

Patman

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Some gauges are just not very accurate, like the ones in most Fords, which basically are just "idiot lights" and the needle won't hit the low end of the range unless you have almost zero pressure. The oil pressure gauge in my LT1 Firebird is pretty accurate, it shows about 45-50psi on a cold start, and once the oil is fully warmed up it shows about 20psi at idle. As soon as you touch the gas, the pressure gauge reacts, and it is around 40psi at anything above about 1500-2000rpm.
 

driven2services

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I just looked at the manual...it says "Normal readings of 20-60psi are acceptable", so I would think the gauge would be able to read as high as 60 and as low as 20. Some cherokees do come with only an idiot light, that's what my last one had, but I know that when people swap to have a guage they also have to change the sender.
 
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Phoenix, AZ USA
quote:
Originally posted by ZmOz: I'm just curious about this...my last Jeep didn't have an oil pressure gauge. The jeep that I recently purchased does. As soon as I start it up...even if it's 20 degrees outside, oil pressure is almost exactly 40 psi. It doesn't change once it gets warm, nor is it any different when I start it after it's been in the 60 degree garage. It does go up (slightly) when the RPMs increase, but never more than about 45psi. Is this normal? Shouldn't there be some difference between hot and cold??
This is exactly what the gage in my '99 Grandokee V8 did. It means that: 1. the engine is new or newish and not worn; 2. Jeep equipped the engine with a high-volume oilpump; and 3. the pressure-relief valve is working and set at about 45PSI. When it's HOT and idling, does it indicate a bit less than 40PSI?
 

driven2services

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quote:
Originally posted by Jeffrey Behr: This is exactly what the gage in my '99 Grandokee V8 did. It means that: 1. the engine is new or newish and not worn; 2. Jeep equipped the engine with a high-volume oilpump; and 3. the pressure-relief valve is working and set at about 45PSI. When it's HOT and idling, does it indicate a bit less than 40PSI?
1. It's got nearly 200,000 miles on it. [Smile] 2. I'm pretty sure they don't do that 3. I have no idea... When it's hot and idling it's still at 40 or even slightly above.
 
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Indy
My Viper reads 55 PSI hot, cold, idle or 6,000 rpm. Turn the engine off and it'll take 5+ seconds for the needle to drop. Very valuable information...... My 1970 Trans Am on the other hand has a oil gauge needle that could double as the tach, i.e. it moves in sync with the engine RPM as one would expect. I had suspected the same as Jeffrey... new engine, great oil pump and pressure regulation, etc. The reality is (after hooking up a mechanical gauge temporarily) the factory gauge is worthless. The mechanical gauge responded just as the 30 year old Trans Am gauge, with engine RPM and temp. After taking apart a donor Viper OEM gauge, I found a thick gummy clear gel heavily dampaning needle movement. No way I could find to remove. Unfortunately, I'd imagine all D/C gauges are similarly handicapped. Surprising that even on their premier performance car they don't trust us enough to expose the real oil pressure. Fearful of warranty claims... my O/P is 30 and his is 50.... My solution is to replace the guts of my factory gauge with a more responsive aftermarket unit in order to keep the stock look. [ January 22, 2004, 10:41 PM: Message edited by: Ron-Indy ]
 
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Some companys build in a lot of buffer so as not to alarm drivers. Other companys have a set reading that the guage stays at unless the presure readins brake some set upper and lower limit. It is not that common to find oil pressure guages in most cars and trucks bareing sports cars that accurately indicate the pressure. My 2001 Dodge Dakot accurately read the oil pressure and would move around alot with rpm. My Moms Tundra on the other hand goes up to the limit until it warms up then it just sets in the middle and does not move at all from idle to redline. If you get a manualy pressure guage you can check the oil pressure manualy to confirm accuracy of the cluster. It could be wear and tear has reduced the oil pressure some but 40psi in this engine is still a vary healthy oil pressure reading. I have to admit though that I like to see the oil pressure increase with rpm. I would not worry about it at this point!
 

driven2services

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One more thing...with 200k I thought it would be best to go with 10w40 for the first oil change. It raised the pressure by about 2psi. (was slightly below 40, now slightly above) With the pressure still at very close to 40psi with the 10w30...do you think it would be better to use that next time?
 
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Singapore
After the oil is fully warm, the oil pressure should drop significantly at idle. The oil pressure sender is most likely faulty, not the gauge.
 
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Usually you can get an aftermarket oil presure sender unit that replaces the OEM sender (the one for the idiot light). The sender sinly screws in and has probably two connectors, so you can hook it up to a real oil presssure gauge and maintain the idiot light feature.
 
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Dallas, TX
Thanks! You would think "newer is better", but my 20-year-old '82 Subaru has both oil gauge (works halfway), and voltage gauge, and an '02 Ford -- two idiot lights. I'd love to install good gauges in both, do you know of a reliable brand?
 
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Cape Cod, MA
quote:
Originally posted by Ma-fia: I'd love to install good gauges in both, do you know of a reliable brand?
AutoMeter is one...they're available from both Jeg's and Summit... All Fords since sometime in the early '90's (at least) have had "idiot gauges" for the oil pressure...I'm not sure why...my '01 GMC Safari work van has a real gauge, though...and I've replaced the fake gauge in my Marauder with a real one as well...and yes, a good gauge should read higher when the engine is cold, and lower when warmed up at idle...
 
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By Detroit
Oil pressure gauge is easy to install. Probably the best is Autometer (summit.com or Jeggs.com). You can get a Sun brand (supposed to be pretty good) at the auto parts store for $20, or go to Sun's website. They had a "garage sale" page on their website with some nice gauges really cheap. I installed a mechanical gauge in my F150 ('95 4.9L, 100,000 miles). With 5w30 I was getting 53 psi cold, 36 hot at 2000 rpm, and 16 at idle. Switched to 10w40 and now I get 53 cold, but 45 hot at 2000 and 22 idle.
 
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Virginia
I used to have a 98 Cherokee and my brother has a 96. Both had oil gauges and both were accurate from what I could tell. I've owned several Fords and my biggest complaint is the idiot factor built into the gauges. It used to cost extra to get the gauge cluster. Can you imagine that? Paying extra for gauges that don't work!!! [Mad] Anyway, our jeeps would register 40-45 under load but at idle mine would drop to near 10 and my brother's would be closer to 20. I'm almost positive your gauge should show true pressure so you may want to check/replace the sending unit.
 
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ZmOz, I was having trouble with my Cherokee oil pressure gauge, until I tightened up the connector at the sender unit. It'll only take a few minutes to clean and tighten your connection; that may fix it if you're lucky. If not, then I'd agree with the previous posters about replacing the sending unit.
 
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USA
Your Jeep engine can handle just about any oil from 10W30-20W50 no problem. The trick is to use the best oil for the temp. If you run synthetic then I would add 5W30 to the list!
 
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