Low Oil Pressure at HOT idle... use heavier oil?

[ratdude747]

While driving my Ranger on a 2500 round trip to Colorado and back, after a lot of driving (engine hot, but not overheating!) my oil pressure was dropping off at idle to the point of tripping the check gauge light (note- I have a modified gauge and sending unit per the tech article I wrote years ago). As soon as I give it gas (up to 1000+RPM) the pressures come back to normal.

I have nearly 208,000 miles on it currently... which for a 4.0 OHV I didn't think was that excessive. I'm not looking to pull the engine for a rebuild yet (as much rust as I have, I'm kinda holding out that I find a better long bed 1995-1997 to move upgrades/parts to), so perhaps running some 5w-40 oil (summer) may be in order here? Or just leave it... I wans't clattering valves or anything (not even after idling for several minutes in a traffic jam). Currently I'm running 5w-30 pennzoil platinum high mileage. The oil level is OK (not low oil).

I'd upload a video to YT but moron here forgot to pause the tunes and I'm sure I'll get content matched. Likewise, since coming home I haven't had a repeat of the issue... must require 1.5-2 hours of driving to get the oil hot enough to cause the issue.

Edit- this issue happened on another long drive in-state several weeks ago... still have the "oh crap" oil I bought in the back thinking it was low.

 

[dvdswan]

I don't think you'll get any change with the change from 5w30 to 5w40. How many miles are on the current oil change? If it's due, change the oil and check for a change. You have 200k on the engine, have they been a lot of city miles or highway miles? You could have an oil pump that's tired.

 

[Shran]

I've never heard of low oil pressure causing the check engine light to come on... have you scanned it for codes? If so what were they?

I'm just wondering if something else is going on, electrical issue maybe? In the summer it shouldn't take hours for a hot engine problem to appear, your engine and oil will be at operating temp within a few miles and should stay there. And you're not hearing valve train noise so you've got at least SOME oil pressure. 5-10 pounds at idle is pretty normal for many engines, especially high mileage ones. I would think that if this was indeed oil pressure related, it would not be an intermittent issue.

 

[ratdude747]

Clarifications:

1. No CEL. That was a typo. Check Gauge light... the light that comes on if any of the gauges are in the red.
2. Per my records, the oil was changed this spring at 204081 miles. The first of the issues were seen around 205,200 miles (based on a fuel record from earlier in the long in-state trip). It was the first long trip with a lot of hot idling (traffic jam) of the summer season. I usually bi-seasonally change my oil (spring and fall) due to running longer-life synthetics.
3. I did have issues on the way out of Colorado after 30-45 minutes of driving (stuck in a traffic jam). I've heard oil can take that long to fully heat up, longer than the cooling system takes to heat up.
4. No idea on it's past life (It had 168k or so when I bought it roughly 5 years ago). It's seen a mix of driving; it was my daily driver for a number of years but also saw a lot of long trips. It sees about 40% of my daily driving and about 60% of long trip driving these days (duties split with my 1984 F150).
5. The only valvetrain noise I have is some brief lifter clattering on cold winter starts. Probably needs pushrod TLC. But that's not a new or unheard of issue.

Any chance Pennzoil gave me the wrong oil? I've read the 4.0 was the only 1998+ engine kept for a long time after that wasn't re-rated for 20 weight due to needing a higher volume pump to maintain adequate pressure... almost seems like I got 20 weight by accident? Not that I have the oil analysis tooling to tell...

 

[RonD]

Normal oil pressure at idle would be 7 to 12psi, under 5psi is when lifters would start to "tick, tick, tick"
So if you are not getting lifter noise then you are fine
General Rule of Thumb is 10psi pressure per 1,000rpm
So 800rpm idle is 8psi
But should top out at about 50psi even at 6,000rpm


Remember, oil pressure is the oil the engine CAN NOT USE at that moment, its the back pressure as oil builds up between pump and main oil passage OUT to the bearings and valve train
You need about 5psi to push oil up to the top of the engine, to the valve train

Oil pressure goes up with RPMs because oil pump pumps even more oil than is needed, but the higher pressure does push oil thru bearings faster for more cooling which is needed as higher RPMs generate more heat/more friction
But oil pressure thats too high, above 70psi, can cause "washing" of the bearings, it's no longer cooling the bearings its actually blowing off the oil coating because of the high pressure

 

[Eddo Rogue]

Maybe a little aftermarket oil pressure gauge temporarily installed to see what its doing? I did that with a temp gauge when I was having cooling issues, it helped with diagnosis.

 

[ratdude747]

Normal oil pressure at idle would be 7 to 12psi, under 5psi is when lifters would start to "tick, tick, tick"
So if you are not getting lifter noise then you are fine
General Rule of Thumb is 10psi pressure per 1,000rpm
So 800rpm idle is 8psi
But should top out at about 50psi even at 6,000rpm


Remember, oil pressure is the oil the engine CAN NOT USE at that moment, its the back pressure as oil builds up between pump and main oil passage OUT to the bearings and valve train
You need about 5psi to push oil up to the top of the engine, to the valve train

Oil pressure goes up with RPMs because oil pump pumps even more oil than is needed, but the higher pressure does push oil thru bearings faster for more cooling which is needed as higher RPMs generate more heat/more friction
But oil pressure thats too high, above 70psi, can cause "washing" of the bearings, it's no longer cooling the bearings its actually blowing off the oil coating because of the high pressure

The idle in question was (per torque pro) around 550RPM. Maybe that's the "issue", combined with possibly a bad low range on my sending unit? It never hit the "L" mark... got close (ish) before dropping off entirely. The fact the check gauge light came on tells me the sensor went to (or acted like) zero PSI... likely not just a needle issue?

 

[Shran]

That is likely part of the issue, why was it idling at 550rpm? Normal idle speed on our trucks is 750-800RPM.

 

[Blmpkn]

Without numbers on the guage to give you a sense of what your pressure actually is...... it's hard to say anything either way.

Modified dummy guage so it gives you a better idea sounds cool, but still not as functional as a 'real' guage

 

[RonD]

Yes, the 550rpm was the issue, 5.5psi

V6/V8's Manual trans idle at 625-650rpm, no lower, automatics 750 in Park/Neutral, 800 in gear
4cyl are about 50-75rpms higher

And you get a double whammy at 550rpm as well, the alternator voltage gets very low which can drop gauges a bit, so check gauge light comes on

Clean the IAC Valve

 

[ratdude747]

Yes, the 550rpm was the issue, 5.5psi

V6/V8's Manual trans idle at 625-650rpm, no lower, automatics 750 in Park/Neutral, 800 in gear
4cyl are about 50-75rpms higher

And you get a double whammy at 550rpm as well, the alternator voltage gets very low which can drop gauges a bit, so check gauge light comes on

Clean the IAC Valve

Last time I cleaned the IAC I ruined it and a used replacement (and it threw me for a two week loop of misdiagnosis). The valve was replaced around 4-5 years ago, not long after I bought it. Other than don't touch the plunger, any suggestions on how to clean it without breaking things?

Also, it only idles that low when hot. Idles around 1000rpm when cold. I figured that was normal... although the incident several weeks ago did include a voltage drop not seen on this trip. Weird...

 

[Eddo Rogue]

Last time I cleaned the IAC I ruined it and a used replacement (and it threw me for a two week loop of misdiagnosis). The valve was replaced around 4-5 years ago, not long after I bought it. Other than don't touch the plunger, any suggestions on how to clean it without breaking things?

Also, it only idles that low when hot. Idles around 1000rpm when cold. I figured that was normal... although the incident several weeks ago did include a voltage drop not seen on this trip. Weird...

I replaced my IAC with a generic from my local parts store. It didn't work. Replaced it again with a Motorcraft ordered online. It worked.

 

[RonD]

Yes, Ford uses a True Solenoid IAC Valve, 3rd party are usually stepper/solenoid type valves so will act up when used in Fords
Motorcraft or Hitachi and the onlt IAC Valves you should use

1,000-1,200rpm cold start is normal
Then slow drop to 750-800 when warmed up, for automatic

Next time its hot see what its idling at and then unplug the IAC Valve, should drop even more or engine may stall, either means the computer was setting that low idle, so IAC Valve was not stuck but working
If idle doesn't drop then IAC valve is stuck closed, computer couldn't get to to open more


500-550rpm is what a warm engine should idle at with IAC Valve unplugged
On a 1995 there should be an Anti-Diesel screw on the throttle linkage, looks like an idle screw, thats for fine tuning TPS but also can be set to prevent stalls if IAC Valve should fail

 

[ratdude747]

Yes, Ford uses a True Solenoid IAC Valve, 3rd party are usually stepper/solenoid type valves so will act up when used in Fords
Motorcraft or Hitachi and the onlt IAC Valves you should use

1,000-1,200rpm cold start is normal
Then slow drop to 750-800 when warmed up, for automatic

Next time its hot see what its idling at and then unplug the IAC Valve, should drop even more or engine may stall, either means the computer was setting that low idle, so IAC Valve was not stuck but working
If idle doesn't drop then IAC valve is stuck closed, computer couldn't get to to open more


500-550rpm is what a warm engine should idle at with IAC Valve unplugged
On a 1995 there should be an Anti-Diesel screw on the throttle linkage, looks like an idle screw, thats for fine tuning TPS but also can be set to prevent stalls if IAC Valve should fail

Mine came from O'Reilly. Worked fine and fixed my issue back then (it'd idle at 450RPM and stall).

 

[RonD]

Mine came from O'Reilly. Worked fine and fixed my issue back then (it'd idle at 450RPM and stall).

Not working any more