Oil pressure diagnostic question


#1

2000 Silverado

Evergreen, CO–temp likely down to freezing overnight

The problem started about 2 weeks ago. I started the truck in the morning, cold and the oil pressure was at 0 and the message light said low oil pressure. After a minute or 2 at idle the message went away and the gauge started to climb. At ~2k rpm the gauge registered in the middle. Several days later the same thing happened only this time the gauge did not budge. I turned off the engine and back on. As soon as the engine turned over the gauge needle would bounce up a bit then return to 0. I had the truck towed to my mechanic. He installed a new gauge. All was fine for about 4-5 days then the same thing happened. As soon as I put the truck in gear and started to back up the gauge came to life and stayed okay. The mechanic said all he could do is put his pressure tester on and test the pressures at various rpm’s per the diagnostic standard. That doesn’t deal with the gauge only having a problem with a cold engine. I asked if there were any other electrical things between the gauge and the pump–no. I asked if there was a tester for an oil pump–no. So that (to me) only leaves the possibility of a flaky oil pump having trouble getting going in cold temps. That would be fine if it were not for the possibility that the pump problem might get worse with time. So that leaves (to me) and the mechanic the only solution being to replace the oil pump just to be sure vs. seizing the engine while driving at some point? Are we missing something?

Mark


#2

You have 2 issues to address here. The gauge and the ACTUAL oil pressure in the engine. Ignore the gauge for a moment.

The first test your mechanic should have done is test the engine oils pressure with a calibrated mechanical gauge. That will tell you whether you have an oil pressure problem rather than a gauge problem. IF the oil pressure tests out OK, your electric gauge has a problem. Cold starts can delay the buildup of oil pressure but recycling the key doesn’t really affect this. If the oil pump or the engine bearings are worn, cold oil pressure should be OK and hot pressure will not unless the oil-pump-bypass is blowing open and sticking that way. If the truck has been maintained, that is unlikely.

If he actually replaced the gauge and not just the oil pressure sender, have him replace the sender, it is likely bad. There are 2 parts to an electric gauge, the gauge itself and the sender. Either could have problems. Have him check the grounds for the sender as well. That is most likely your problem.


#3

He replaced the sender. I’m not sure you and he are talking about the same test. His tester would test the engine oil pressure at various rpm’s not a cold engine–which doesn’t help? If he replaced the sender then wouldn’t he have had to re-secure the grounds?


#4

Many of the gauges on vehicles today are meters that operate digitally. They have the same appearence as d’Arsonval analog gauges and mechanical Bourdon tube gauges that are becoming “Old School.” The digital gauges operation is totally different. While I don’t know whether the OP’s vehicle has digital or analog gauges it will be difficult to diagnose his complaint qithout knowing.


#5

Have your mechanic try replacing the oil filter by-pass valve.

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Tester


#6

Thanks for the heads up–I’ll have the valve replaced.


#7
That doesn't deal with the gauge only having a problem with a cold engine.

Leaving a vehicle overnight with the mechanic is standard diagnostic procedure for cold start issues.

I’ve never heard of a “flaky oil pump.”


#8

Chris wanted to keep it but I couldn’t leave it at the time. Don’t forge the sender dad replaced. So unless I’m missing something process of elimination seems to leave the oil filter by-pass valve as mentioned earlier.

Mark


#9

Right!

If the oil is too thick when it’s cold were it can’t pass thru the filter media, and the by-pass valve is sticking closed, the oil can’t leave the oil filter and enter the engine.

No oil to the engine, ergo, no oil pressure!

Tester


#10

His tester would test the engine oil pressure at various rpm’s not a cold engine–which doesn’t help?

A pressure gauge works just fine on a cold or hot engine. In fact, it should be used on both to properly diagnose any problems. It measures pressure, not oil pump flow. You read the pressure gauge while also reading RPM. Hot pressure should show about 25 psi, at least, and rise to 45-60 depending on the engine, at 3000 rpm or so. Cold start pressure may show 45-60 at startup idle and stay there until warm. The pressure gauge’s hose needs to be installed into an oil pressure port, like the one the electric sender came from. It will diagnose the bypass valve and determine if the problem is your oil pump or the sender and electrical system.

And No, he may not have tested the grounds. Replacing the sensor doesn’t actually do that as the gauge ground and the sensor ground are not the same nor likely adjacent to the component.


#11

Mustangman–now I’ve got to figure out a diplomatic way of presenting your latest post to Chris without totally pissing him off–


#12

Good luck on that one, Mark!


#13

I have an 03 trailblazer, my gauge does not work in winter months. My speedometer is also toast. My gps gives me my speed, and speed limit! After research my guage represents a calculated oil pressure, not actual oil pressure. The oil light is a true alarm for low oil pressure. The fix for mine would be a new cluster and calibrating the cluster to the tune of $500 or so. I am not sure if your gauge is the same.


#14

The oil being used is the one recommended in the owner’s manual right, and the same for the oil filter? If in question, I’d probably buy oil and filter at the dealership for a while, ask them to make sure it is the right product for the truck, see if that had any effect.

Faulty oil pump? That seems unlikely. The oil pump doesn’t have to do all that much work and is constantly bathed in oil. I guess anything is possible, but I wouldn’t place too much hope on that idea.

Engine components wearing out will cause this symptom. Usually wouldn’t occur until well over 100 K miles, unless the oil had run low or oil&filter seldom changed.


#15

I reviewed the suggestions raised here with Chris. He said he wasn’t positive but thought that the sender is grounded by the block when it’s fastened in place. He’ll verify. He now has a mechanical gauge and will test the pump etc. He said that if the problem is either the gauge or the pump that I should have the dealer do the work. From what I’ve been able to understand reading online, replacing an oil pump on this truck appears to be a huge job. He said replacing the gauge involves replacing an entire section of the dash not just the gauge.

The truck continues as it did my last post. Start on a real cold morning and the gauge stays put at zero while idling. But, as SOON as you put the truck in either drive or reverse the needle moves off zero and gets into a reasonable range within a minute. Chris is sure it’s not the pump but after just spending ~$850 on snow tires, I’m not looking to fry the engine.

I have an appointment with the mechanic for 1/4/16.


#16

Near certain it’s not the pump.
If it started with zero pressure the engine would be crying out for help.


#17

@markg2

Your mechanic is only partially correct about “replacing an entire section of the dash”

Unfortunately, GM clusters are well known for having problematic gauges. Behind each gauge is a motor, and that is the problem. They can be replaced individually. There are even youtube videos showing you how to do it. The motors are available all day long on ebay. And there are several companies that will update your cluster with all new motors for a reasonable price. Google it, and you’ll get lots of hits.

Perhaps the mechanic wants to install a genuine GM instrument cluster assembly. What gets expensive about that is you need the dealer to program the cluster, so that it will work correctly on your application.

The good news about sending your cluster out to be repaired is that no reprogramming is involved. Same goes for replacing an individual stepper motor

The cluster is very easy to remove, and you could drive around without a cluster for a few days, while you wait for it to return.


#18

You could go the Red Green route:

Duct tape a mechanical gauge to the dash, route a thin hose to the engine and put a nipple fitting in place of the oil pressure sender.


#19

If it started with zero pressure the engine would be crying out for help>>

And that would happen within a minute or two of starting and idling?

Assuming the mechanical pressure test reveals the pump ok then my only concern and reason for possibly laying out $ for a replacement is that I won’t know until it’s too late in the future should the oil pump really go south?