Case of the Shakey Needle

[4rd Xplora]

Having an issue with my truck. This usually happens while driving on the interstate. After driving for a while (at least 15 minutes) at higher speeds (about 60-80 mph), when you start to decel the oil gauge will shake violently for half a second. It shakes so hard it clicks. It's been doing this for awhile now, but now it's doing it more and more. I took a good look at alt gauge and it dips at the same time for that half a second and comes back up and everything is normal. Yesterday I cut the a/c off and it did it less but still did it. I put the truck in neutral while coasting and rapped the engine a couple of times and it did the same thing. I'm thinking it may be in the alternator/voltage regulator system. Any help will be appreciated. Thanks

 

[J_C]

I haven't come across this, but a couple things I would do is use a multimeter to monitor voltage, at the lighter outlet or OBDII connector for convenience if you can't replicate it with a helper while parked, and check the engine belt and tensioner to see if there's too much slack in it so it's slipping.

 

[koda2000]

I've had this happen on 2 Explorers (a 5.0 and a SOHC). It will bang the oil pressure gauge needle so hard it will sometimes get stuck behind the stop peg. There is no problem with my oil pressure, so I assume the issue is the oil pressure sender/switch or its wire. If it wasn't such a PITA to get at the switch (behind the power steering pump) I'd just install a new one. When this has happened to me, to get the needle back on the right side of the stop I drilled a tiny hole in the cluster lens and I insert a small dental pick and lift the needle up/over the peg. Only happens to me about once a year.

 

[boominXplorer]

It's pretty easily accessible through the wheel well with a long extension. Of course you need to remove the splash shield. If that doesn't fix I'd start using thicker oil to prolong it's life.

 

[koda2000]

It's pretty easily accessible through the wheel well with a long extension. Of course you need to remove the splash shield. If that doesn't fix I'd start using thicker oil to prolong it's life.

Meh, the only vehicle we have left that does this occasionally is my daughter's 2000 Mountaineer 5.0L w/237,000 miles on it. I know it has good oil pressure and the fact that the needle gets stuck doesn't seem to bother her in the least. If I have a reason to get into the left wheel well I'll consider changing the switch, but the current plan is to replace this vehicle early next year.

 

[david4451]

I've had the same bouncing oil pressure gauge, to the point where oil pressure will zero for a couple of seconds after fast running then slowing to tick over. It will then bounce and then show oil pressure.
My Sport Trac uses NO oil between changes and runs fine and this oil pressure loss worried me a lot.
I do believe that the problem lies with the PCV (positive crankshaft ventilation valve).
There is a vacuum pull on the crank case and this is applied via the PCV and this vacuum is sufficient to affect the pressure shown on the dummy oil pressure gauge, possible due to its location at the end of the oil gallery.

I've researched this on Google and smarter than I have confirmed this is an effect of PCV on oil pressure!

 

[1998Exp]

The sender may be bad or you may have a real oil pressure problem. All that gauge can do is tell you whether the oil pressure is above 5psi (showing "normal") or below (showing 0). I would hook a mechanical pressure gauge (they are cheap) and check what's going on. If it's normal at fast idle (say 20 psi or better at 2000 rpm when warmed up), I would put the gauge on a long tube and watch it while driving.

 

[david4451]

As 1998Exp said, I did connect a oil pressure gauge on T to pressure switch on block. At cold I had 25 psi at idle. Warmed up it was 20 psi when it driven, the bounce came in after slowing occasionally. The oil pressure on gauge dropped to 5 psi and returned to 10 psi idling 500 to 600 rpm.
The oil pressure gauge was on long tube so I could monitor on dash.
This still annoyed me, as I could not understand why my Sport Trac ran very well, no oil consumption etc etc but fluctuations of oil pressure occasionally occured.

My only conclusion was something was affecting the oil pressure and I'm 95% sure it is the PCV pulling the vacuum on the crankcase.

This bouncing needle has happened to thousands with Explorers it seems improbable that so many would have this happening and all faulty gauges and not reports of damages to bearings.

 

[koda2000]

David, Interesting. Can you provide your theory as to how crankcase pressure/vacuum can effect oil pressure?

I don't see how a change in crankcase pressure should be effecting oil pressure. In an unaltered oiling system (I know your's is modified to include a pre-oiler) when the engine's oil level is normal, the oil pickup is totally immersed in oil. On the SOHC engine it literally sits in the round recess in the bottom of the lower oil pan (more on this later). The oil pump, once primed, is hydraulic and sucks up oil from the pickup. It then pumps it through the oil filter and through the various oil galleries to wherever it needs to go under pressure. The oil pressure switch/sender is located along one of these galleries and reports the pressure to the dummy gauge using OHMs resistance (or using actual oil pressure if using a real gauge). Once pressurized oil has been delivered to various engine bearings and components it gravity drains back into the oil pan to be recycled.

The PCV is never immersed in oil an it's purpose is to remove oil vapors from the crankcase and direct these vapors back into the engine's intake to be burned, rather then venting these vapors directly to atmosphere like engines used to do prior to about 1963. A faulty PCV valve can effect how an engine runs by introducing too much air (or not enough) into the intake. It can also make an engine smoke if oil is getting into the intake.

Getting back to erratic oil pressure in 4.0L SOHC engines, as most of us are aware, these engines are known for breaking timing chain related guides and when this happens the broken pieces of plastic and metal end up in the oil pan, where they sink to the bottom. These broken pieces of plastic tend to be curved, so they can block the oil pickup at times, but not enough to effect oil pressure unless the pieces line up just right. This will cause oil pressure to drop and perhaps the gauge needle to go crazy. I have personally also experienced sludge in the oil pan blocking the oil pickup, but that's another story.

I recommend anyone with a 4.0L SOHC engine occasionally remove their lower oil pan and see what may be lurking there. It's also a good idea to clean the pickup screen while you're in there. This is easy to do and the lower oil pan gasket is typically reusable. This is particularly important to do if you have any hint of timing chain noise.

As the oiling system is hydraulic, oil pressure should not be effected by crankcase pressure. General rule of thumb for hot oil pressure (with a stock oil pump) is about 10 PSI for each thousand RPM (for example 20 PSI at 2000 RPM) up to around 40 PSI.

I wonder if your loss/low oil pressure might be due to a worn oil pump or perhaps your pre-oiler system's check valve? Could it be that the hot, thin oil might push the check valve open when you quickly go from a relatively high oil pressure to low oil pressure? Turbo charged engines experience something like this when the throttle body is quickly shut after high speed engine/turbo operation and use a pressure release valve to vent off excess boost until the turbo slows down. I release this is not the same thing, as the oil pump is not not free spinning.

 

[david4451]

Hi Phil, what I read was that the vacuum pulled on the crankcase via the PCV was a bit like a sucking through a staw and this vacuum pulled oil from gallery openings, lowering oil pressure.
The vacuum would be at the max going from high to low rpm, thus the drop causing the needle to bounce.
I did not disconnect the PCV to test this as it said getting a positive pressure in the crankcase could affect oil seals, causing leaks! The vacuum provided by the PCV helped save oil seals.
This bouncing of gauge was present before I fitted the pre-oiler, so there has been no effect from the pre-oiler.
Like you my gauge bounced occasionally I also drilled a hole to move needle back to original position.
I can't for the the life of me see anything else causing this bounce on so very many explorers and not many reports of damages to bearings, when oil pressure seems to have been lost????

I

 

[koda2000]

Hi Phil, what I read was that the vacuum pulled on the crankcase via the PCV was a bit like a sucking through a staw and this vacuum pulled oil from gallery openings, lowering oil pressure.
The vacuum would be at the max going from high to low rpm, thus the drop causing the needle to bounce.
I did not disconnect the PCV to test this as it said getting a positive pressure in the crankcase could affect oil seals, causing leaks! The vacuum provided by the PCV helped save oil seals.
This bouncing of gauge was present before I fitted the pre-oiler, so there has been no effect from the pre-oiler.
Like you my gauge bounced occasionally I also drilled a hole to move needle back to original position.
I can't for the the life of me see anything else causing this bounce on so very many explorers and not many reports of damages to bearings, when oil pressure seems to have been lost????

I

Humph, well It's a strange phenomenon that many Explorers seem to experience. From what I've read the SOHC lower end (bearings, cylinders) hold up very well showing little-to-no wear when high mileage engines are torn down. Too bad the top end isn't as stout.

I've never experienced the needle bounce on my ST, or on my 5.0L EB and 5.0L XLT, but it's happened on our SOHC XLT and now 5.0L Mountaineer, requiring the hole in the cluster to be drilled. The 5.0L XLT once registered zero oil pressure (from start) for about a 1/2 mile, with no lifter noise, then the needle popped up to normal. It only did this once in the 10,000 I owned it and I blamed the oil pressure switch.

BTW, If temporarily eliminating the PCV as a test did cause any seal leaks I don't think the seals would be permanently damaged. I don't know how the engine would run w/out it though.

 

[4rd Xplora]

BTW, my truck is a 4.0 ohv with well over 300K. It burns no oil between changes but needs a rear seal. It will still run 90+ all day with no problem. Now here's why I still think it may regulator related. I found out I let a family member use the truck a while back and they boosted another vehicle with it. They connected the cables backward and tried to start the other unit. He figured out what was wrong, changed the cables and started the other car. He said my truck ran fine and so he never told me.

 

[4rd Xplora]

Fixed it. Turns out it was the IAC valve. Thanks for all the replies.

 

[koda2000]

Fixed it. Turns out it was the IAC valve. Thanks for all the replies.

So the idle speed was so low it didn't make adequate (min 5 PSI) oil pressure at idle?

 

[4rd Xplora]

So the idle speed was so low it didn't make adequate (min 5 PSI) oil pressure at idle?

Yep, that was it. The idle dropped so low at times that the truck had started to die if you revved it. I'm guessing the required voltage to the ecm was not there.

 

[J_C]

^ ??? Many people would have considered that a more significant symptom, which might have led to an earlier IAC diagnosis.

 

[koda2000]

^ ??? Many people would have considered that a more significant symptom, which might have led to an earlier IAC diagnosis.

Yeah, I was thinking the same thing when I read the fix.

 

[4rd Xplora]

^ ??? Many people would have considered that a more significant symptom, which might have led to an earlier IAC diagnosis.

Which it did. Before the only symptom was the oil gauge needle jumping, but the unit ran fine. I read numerous post on the oil needle jumping in this forum. So I posted that the truck had died once but restarted right away. Since it was a holiday week and no one was driving the truck, it sat till this past Saturday, I got serious about fixing it Saturday then posted what I found. So yeah, guess I was a little slow about fixing it since it took 11 days for me to get around to it.

 

[J_C]

I don't fault someone for being slow to fix a 20 y/o vehicle, I've done that myself. It just seemed odd that there was no mention of correlation to reduced engine RPM (unless I missed it).

 

[koda2000]

I don't fault someone for being slow to fix a 20 y/o vehicle, I've done that myself. It just seemed odd that there was no mention of correlation to reduced engine RPM (unless I missed it).

It's difficult to know what's going on when all you've got to go on is the poster's explanation to go by. I used to work in IT and was responsible for all hardware/software problems my user groups were having. I quickly learned that the fastest way to solve an issue was to go to the persons desk and have them demonstrate the perceived problem. 75% of the time the problem they were reporting had nothing to do with the actual resolution. I even had a phrase taped to my PC saying "Never believe anything a user tells you."