I'm totally stumped (mech oil gauge)

[Cheapamanche]

I recently installed a mechanical oil pressure gauge in my 88 comanche 4.0 and I don't understand what my oil pressure is doing. My oil pressure is really good at idle, it's about 40 psi cold but with the slightest touch of the throttle or just incresing cruising rpm to about 1700 it climbs to about 80, hangs for about 1 seconds and then drops back down to about 50-60. Sometimes it spikes OVER 100 and then drops, almost like it builds up pressure and then something gives and blows off pressure. In a previous post I though it was a blown head gasket because I was losing coolant but it looks like its just a bad overflow tank cap. I also pulled the spark plugs and none of them look washed out or wet or oily. Does anybody have any ideas? I'm usually pretty good at researching and figuring things out but the ONLY search results I get for 4.0 oil pressure problems are either, "just replace the sender" or "it only needs 10psi for every 1000 rpm" I'm stumped guys, please help.

I'm using 10w30 penzoil platinum. I also changed the oil filter once thinking it was a bad oil filter. Didnt help.

[Cheapamanche]

Is there a relief valve in the oil pump that could be sticking?

[Dzimm]

The only vehicle I've used a mechanical oil pressure gauge on is my Suzuki Sidekick and that fluctuates a lot with different throttle inputs.  Well over 80psi in some cases.  I'm not really sure how it would behave with easing into the throttle at cruising speeds as its usually only put to the floor on this thing.  

 

I know for the 4.0 stock oil pressure gauge in the cluster it tends to not move all that much.  I have read that Jeep may have done something so the gauge stays put as to not freak people out with a gauge that's bouncing around but I have no idea if that's true.  But it has been my experience that the mechanical gauge moves a lot compared to stock gauges.

 

That being said, you may have nothing wrong with the motor.  It's possible that spike is just a pressure spike when it's trying to get all the fluid moving.  I'm not all that knowledgeable on fluid dynamics but IIRC pressure is the resistance to flow.  Meaning the more resistance, the more pressure you have.  An object at rest tends to stay at rest and it takes more force to get something moving than it does to sustain that movement.  I have a feeling this may be the phenomenon you are seeing here a quick pressure spike when the fluid is starting to move/speed up then it settles into the pressure for sustained movement.

 

Keep in mind this is just my arm chair keyboard jockey thoughts based on observation so take it with a grain of salt.  Hopefully someone who has a mechanical gauge on a 4.0 motor will jump in and offer up their experience.

[phatton]

I wouldn't worry about it - oil pressure fluctuation with a mechanical gauge is pretty common.  I'd worry if it started showing extremely low or no oil pressure.

[Cheapamanche]

Thank you guys for the reply. I did a little experiment last night. I drained about 1 quart of oil and added a quart of Marvel in its place. I took it for a ride and the pressure never went over 70. I think that the penzoil 10w30 is not suited very well for my winter enviroment. I think I'm going to switch to a 5w30 for winter.

[MiNi Beast]

if you ensured all air has been purged from line then you should be fine. 20-60 psi is normal. inline engines have a pressure build up different then typical v6 setups and part by design with spikes. i would switch to 5w30 for sure. but also consider age of your rings as well and if get bored, pull your pan and check the journal's on your crank. otherwise run it till she don't run no more  enjoy.  

[cruiser54]

I know you changed the oil filter, but which brand are you using? 

[gogmorgo]

If you don’t have absolutely all the air bled out of the mechanical gauge, the bubbles can act like a little spring, making the gauge bounce around a bit. You’ll also see a spike with an increase in rpm beyond the nominal “10psi for every 1000rpm” which will go away, this is the surge of extra volume that takes a bit to accelerate the oil already in the passages, and the spike subsides once everything’s moving at the same speed.

 

Constant high pressure may indicate plugged passageways, but I wouldn’t worry about spikes with change in rpm.

[Cheapamanche]

On 12/4/2020 at 4:28 AM, cruiser54 said:

I know you changed the oil filter, but which brand are you using? 

Original filter was a WIX, second was a red Purolator.

 

23 hours ago, gogmorgo said:

If you don’t have absolutely all the air bled out of the mechanical gauge, the bubbles can act like a little spring, making the gauge bounce around a bit. You’ll also see a spike with an increase in rpm beyond the nominal “10psi for every 1000rpm” which will go away, this is the surge of extra volume that takes a bit to accelerate the oil already in the passages, and the spike subsides once everything’s moving at the same speed.

 

Constant high pressure may indicate plugged passageways, but I wouldn’t worry about spikes with change in rpm.

I didn't even attempt to bleed the line. I have heard this before but when I kill the engine the oil bleeds back into the engine. won't this introduce air again?

[cruiser54]

I've never bled an oil pressure line.....

[JMO413]

I've also never bled or heard of bleeding one.

 

[elycomanche]

The fact is that the pressure inside the capillary is the same either its filled with oil or there are bubbles of air inside

[elycomanche]

1 minute ago, elycomanche said:

The fact is that the pressure inside the capillary is the same either its filled with oil or there are bubbles of air inside

Better to use glicerine filled gauge to avoid the needle vibrations

[gogmorgo]

They do sorta self-bleed. It’ll still be accurate, the needle will just bounce around a little if you don’t. It’s not usually that big a deal and probably not the issue here, just a thing that can happen.

[AZJeff]

Oil pressure on an engine running normal motor oils (10w30 and the like) will NOT rapidly fluctuate.   Yes, it will go up and down as engine speed changes, but advancing the throttle from idle to 2000 rpm and holding it there should cause the pressure to climb (with a slight delay) and hold.  Likewise, releasing the throttle from 2000rpm back to idle should cause the pressure to drop, again with a slight delay.

 

If the oil pressure is "bouncing around" with a mechanical guage, something is wrong with the oiling system (assuming you have confirmed it with a second guage).  If the oil pressure is rapidly fluctuating with an ELECTRICAL guage, then the issue is either (1)bad sender, (2)bad guage, or (3)varying voltage being supplied to instrument cluster.

 

Mechanical oil pressure guages do NOT need bleeding to get accurate readings.