CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

5.2L V8 Low Oil Pressure?

Hi, I have a 1994 Dodge Ram 1500 with the 5.2L V8 (and 46RH AT), with 147,000 miles. Here’s what I’ve been noticing over the past few months: when the engine is up to full operating temperature, and the truck is in gear but idling, the oil pressure (according to the oil pressure gauge) gets very low, and in fact, the needle often “bottoms out”, going to a reading of “0”, and the “dummy light” comes on. When this happens, there’s no change whatsoever in how the engine is running; it runs perfectly fine (which is why I haven’t bothered to fix it). Also, as soon as the engine is given a little throttle, the pressure reading goes back up and behaves normally under driving conditions (unless at idle). As mentioned, this only happens when the engine is completely warmed up. So my question is: does this sound like a bad sensor, or does it point to some other issue? I check the oil regularly, and it is always within the acceptable range. If it’s a bad sensor, how important is it to replace it? I try to take good care of my truck, but if a bad sensor isn’t hurting anything, I’ll probably just leave it alone. Hope I made everything clear; any advice is appreciated! Thanks!

I could not in good conscience drive around with an inoperative oil pressure or engine temperature warning system.

The problem could be due to a faulty oil pressure sending unit or there could be more serious problems such as engine wear and/or oil sludging; all depending upon the oil change regimen and so on.

You can simply replace the sender and see what happens or have an oil pressure test performed to verify if it’s the sender or not. Senders are generally inexpensive and easy to replace and hopefully that will cure the problem. If not, an oil pressure test should be performed with an external gauge. Hopefully it’s the sender.

(I will add that a major problem in this area can exist even with the oil level on FULL and apparently running fine.)

“I try to take good care of my truck, but if a bad sensor isn’t hurting anything, I’ll probably just leave it alone.”

Wow, if that’s taking good care of your truck I can’t even imagine what poor maintenance means to you.

Your truck is telling you the engine has an oil pressure problem. That means NO LUBRICATION is flowing inside the engine. You’re asking if it’s okay to just ignore this warning.

Sure, maybe the warning is in error. If it’s correct though, and it probably is, you’ll be trashing the engine very soon and asking “how could this happen when the truck has only 148,000 miles”. You’re trying to ignore the most important thing this truck could possibly tell you.

@ok4450: Thanks for the advice; I have wondered about sludge built up on the screen, but I’ll try the sending unit first. I have mentioned this issue to 2 separate mechanics (who are trustworthy), and both said the sending unit is highly likely (and neither was worried that my engine was running with “NO LUBRICATION”, which I guess is why I wasn’t worried). Even if the sending unit is the problem, I still think a pressure test would be a still be a good idea, to give me a little more insight into the health of the engine.
@JayWB: I came on here (first-time poster) for some opinions, which I guess you gave… and I think through all your attitude that you’re actually trying to help… so thanks.

The problem with an inoperative system like that, and assuming it’s a sensor problem, is that if a real problem does develop you would never know it. This could lead to a catastrophic engine failure.

Sensor failures on many cars are also a common problem with age.

What weight oil are you using?? Sometimes high-mileage engines benefit from a little heavier oil, say 10w/40 or even 20W/50 which will improve oil pressure when at a hot idle…

I had the same problem occur on a Ford Maverick that I owned. I was on vacation and noticed when I came to a stop, the oil pressure light would come on. However, the engine ran just fine and the hydraulic lifters weren’t clattering. I took off the oil cap which was on top the valve cover and observed that oil was being sprayed around the rocker arm shaft as it should. I decided to take a chance and keep going.
When we got back, I had the oil pressure checked with a mechanical gauge and it was fine. I had the sensor replaced and that solved the problem.
In your case, I would have the pressure checked with a mechanical gauge. If the pressure is o.k., then replace the sensor. If the oil pressure is too low at idle speed, then you need to look further for the problem.

If you actually had no oil pressuer at hot idle the lifters would get noisy.

Thanks everyone, that’s kind of what I was thinking; my parents had a Dodge Ram van with the 318, which, by the end of its life (close to 200k, and still running when they scrapped it) drank oil like crazy. You could always tell it was getting low by the “chatter”. Obviously I don’t want my truck to get to that point though! And I definitely see what @ok4450 was saying, that a defective sensor/gauge prevents you from what’s going on, and I understand, as @JayWB said, that it’s not really a good risk to take. I’m planning on going and buying an oil pressure test kit (they’re cheaper than I thought)… I’ll try to remember to post what I find! By the way, @Caddyman, I’ve always used 10W30 regardless of weather (I live in Ohio… actually pretty nice here right now!) or other conditions, since that’s what’s recommended in the owner’s manual.

At 147K miles, sometimes you have to compensate a little…Your bearing clearances are somewhat larger now than they were when it was new and 10-30 would hold pressure as designed…