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Oil pressure - low/fast oscillating


Explorer Addict
January 24, 2010
Reaction score
City, State
Newport News, VA
Year, Model & Trim Level
2001 V8 (Eddie Bauer) AWD
I have removed (at a shop) the right side cylinder head, install spark plug inserts and put it back. New gaskets, filled with coolant and oil - Castrol 5W20 High Milage (that's the range recomended in the book for a 5.0L engine).

Now, after I drive some 20-30 minutes, at the stoplight, the needle for oil pressure starts to oscillate and hits the stopper. First it is just light bumps, but it gets worse, violently even with time (heat?). And that is fast, like 2-4 times per second. If I put the transmission in N it stopes (the RPM's are incresing slightly). If I keep driving, even that won't help - it will oscillate even on N and when I put it on drive, neddle will stay 0.5 sec stuck to the lower side, then when RPM's are over 800, will go smack in the middle. The "Oil" light will start to come on really dim at stoplight.
Water temperature is normal all the time - in the middle.

I did drain a quart of oil and added one quart of Castrol 10W40 High Milage. Feels like it did improve some but after a while it still does oscillate - maybe less violent...

Any ideas what can be wrong?

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The gauge is just an indicator.It's not completely accurate,hook a manual oil pressure gauge up to it and find out what is really going on.:D

Oil is Castrol High Milage 5W20. Replaced one quart with Castrol 10W40 - didn't help. Added one bottle of STP Oil Treatment - it did improve something but after a while, the gauge was still doing the spasms.
I did change the sender. Looks like that it was faulty - the new one doesn't do it that wildly, but still, after the heating period would oscillate a little.
Added one more bottle of STP OT - it stopped all the oscillations (for now?).

My conclusion is that, besides the bad (too sensitive) sender, the Castrol oil is garbage for my Ford. The recommended one from Motorcraft is a synthetic blend - now I realize that I shouldn't shun that info.

Also, I found out with this occasion that Fram filters that I was using till now are garbage too... way to go!

Sounds like you need a proper gauge...

To find out exactly what the oil pressure is doing here... The switch is designed to close at 7 psi and the dash gauge is only configured to be an off/on switch...If the needle is dropping to the stop then coming back up to show oil pressure you have either a defective sending unit[not likely as it is new], a loose connection at the sending unit[same as #1], or low oil pressure...

I have not heard of anyone having issues with Castrol engine oil; I have used that brand and others for decades without incident... Some people swear by Fram filters;some swear at them...I use them for fuel, tranny, and even oil occasionally and won't even get into THAT debate...

But what you are seeing is dropping oil pressure as the engine gets hot...Bearing clearances and oil leaks would affect that...

As suggested before, put a proper oil pressure gauge on the engine and monitor the oil pressure... I went the electric gauge route and mounted the oil pressure gauge in an A=pillar gauge pod...Never have to guess what the engine is doing ever again...

Essential instruments

The two essential instruments for a normally aspirated engine are the oil pressure gauge (most important) and the engine coolant temperature gauge. They give warning of potential engine damage. Unfortunately, Ford cheaped out on the oil pressure gauge.

For a blown engine (supercharged and turbocharged) two more essential instruments are boost and air/fuel ratio.

Don't forget the fuel pressure gauge...

With the history of the Ford Explorer nuking fuel pumps and clogging fuel filters having a fuel pressure gauge installed has helped me pinpoint issues quickly where I might have chased my tail looking for problems...

This isn't the first vehicle I have mounted gauges in and it will not be the last...I look at this as getting more information than the factory wants the driver to know...I have the same set mounted in my Charger and the last time the dealership saw the car the service mgr asked me how hard it was to install them...

I wonder if the cyl head work could do something to the oil pressure? Maybe something did get down to the oil pan and clogged the pump sump?
I was getting ready to adapt a real oil pressure sender from a 1985 Ford F150 (new) to my dash gauge, but that won't be calibrated. Not without an air compressor with a good a mechanical gauge.

It drives me crazy, nobody has an ideea why suddenly I have low pressure on idle after engine heats up?
I did drain the oil and went with Mobile 1 Syntetic 5W20 and still I have the small oscillations at stoplight. Sure it is better now then with the old sender that was hitting really hard the stopper.
What can make the pressure oscillate that fast now, after the head work? I would say the oil pump... but I drove it 3000 miles before that and nothing like that was happening.

Don't blame the oil, if it's not the sending unit for the guage, maybe it's the wire. I'll bet if you install a quality aftermarket guage, you'll find your pressure isn't really jumping around like you think. [Hopefully]

I would blame the wire, but it doesn't do it when I start the engine, but only after 20-30 minutes of driving.
I wonder if it was disconnected by the previous owner in purpose (it would stay at "good" position) and now that is connected back it shows a real problem :(

Do you have any pictures of the head before and after the work was done?

I ask because it is plausible that some of the built-up sludge could be in the pump pickup and might have become loosened and eventually drop into the pan...

Also what did the intake area, especially under the intake gasket, look like? I have owned many vehicles and pulled apart more engines than I can remember and when I pulled the pan off of my Explorer there was more sludge than oil there...

And the pump was dead; not from age or wear but the sludge was sucked into the pickup and the pump could not pull any oil from the pan...

This is what my pickup screen looked like...And yes this engine ran...Just no oil pressure...


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do a compression test.... drop the oil pan if you can (not sure if the v8 requires removal or other stuff removed to do).

Cyl head didn't have too much sludge there, fairly clean. Now... I don't know what the mechanic really did because I wasn't there all the time.
I just poured some "Gunk Flush" yesterday, run it for 5 minutes and drain the oil - didn't look bad.
I guess I could trow away another 5 quarts of synthetic oil now and drop the oil pan. But my Hayens book says that engine needs to be removed for that. Didn't took a good look yesterday, wife is aggravated enough already with this, looking to trade it in for a :horror gasp: Santa Fe :(

OK, it is final... I installed a 1/4" Tee on oil pressure port and on it the switch and a mechanical oil pressure indicator. Started the engine - I have 10 psi at idle/cold. So the new pressure sensor was right. Too bad Ford put a stupid switch there instead of a real pressure sender, I could diagnose this earlier.
Also I can hear that engine developed a noise in the right bank (not touched by work). Camshaft/lifters I guess?

Oil pump it is impossible to be repaired without pulling the engine out. And I can tell right now that the engine needs to be overhauled. Dealer quoted me with 2500$ new engine and another 2400$ labor. That's not gonna fly with my wife... and I don't know, maybe it would be just throwing good money after bad.

I am sad and disappointed in Ford for the first time - it is the forth Ford that I have owned. And sadly (I think) the last one.

What is the pressure at higher rpm and temperature?

10 psi at idle is not great but not a death sentence...But when the operating temp goes up and the rpm go up what is the pressure sitting at then? 10 psi/1000 rpm is rule of thumb so as the rpm go up if the pressure doesn't rise then you probably have a pump or, hopefully not, a bearing problem...

I have seen writeups of dropping the pan on a AWD truck with the engine in place...It is not as easy as a RWD truck but you do have to drop the front drive axle to get to the pan bolts and clearance for the pan to drop...My Ford service manual outlines dropping the front axle instead of pulling the engine...

The psi rise with the rpm's. I will check more tomorrow.
Hmm, you are right, the front axle it is first (big) thing in the way. Didn't took a good look to see what else it might be there.
Thank you for the tip. I have Haynes book, do you have the Chilton's?

Cold - 20PSI at idle, up to 35PSI when I rev up the engine. At 35PSI looks like the pressure limiter in the oil pump kicks in because it doesn't go over.

But warming the engine it takes it to 6-7PSI at idle in D (or 8-9PSI in N). At 2000RPM I have 12-15PSI. At 4000RPM it passes 22-24PSI.
This is AFTER I switched to full synthetic oil and then used 2 bottles of Motor Honey, because the gauge was oscillating at warm idle - now it stopped.
Any ideas? The engine makes some noise (lifters, don't know exactly), but still has compression.

It is a 94K engine - I was hoping a V8 to last more (I have a Mecury V6 with 160k with no issues or noise). Is it the oil pump???