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Oil pressure ?


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So I've got a 4.0 with about 130k, recently did a gauge swap, so I could have more than just a warning light letting me know that armageddon is about to occur.

 

So to my question: I'm reading 20 psi on my gauge cluster no matter what. If the engine is cold when start up, I reads zero for a split second and then goes to 20 psi, if I'm driving on the highway 20 psi, if I'm wot from a light 20 psi.

 

Is my sensor gone bad? Or r the chariots of fire blazing towards me?

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My 4.0 did the same thing when I did the full cluster swap shortly after buying her. I found out 2 things: 1. the 1st sending unit had a piece of garbage inside that didn't allow it to register real time pressure changes (engine internals were dirty) and 2. when verified with a mechanical gauge I really only did have 20-25psi at 2000 RPM. I dropped the pan, scraped the sludge, did RMS and oil pump and replaced the sender. Now it's 40PSI at stone cold startup, 2.5-10psi at hot idle and 20-25PSI on the highway. I'd love higher oil pressure but what can one do? She runs good, the oil is getting cleaner all the time, and I've put 11K miles on her like this. :dunno:

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Yes, but before you start dropping the pan, be sure that's the problem. The oil pressure SHOULD be higher at road speed than at idle, by at least 10 psi or more. It should also read quite a bit higher immediately after a cold start than after it warms up. If yours is reading a constant 20 psi, there may be another explanation.

 

Step one is to verify that the needle on the gauge goes through the full range. Quick, crude test:

 

* Turn key to ON (don't start the engine). Not Accessory - ON.

 

* Remove wire from oil pressure sensor and place it somewhere so it is NOT touching any metal. You now have an open circuit. The needle should go all the way to the right (80 psi)

 

* Ground the sender wire directly to the block. The needle should now swing all the way to the left (zero psi)

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The 4.0L on my '88 Pioneer does the same thing. Figured that with only 78K miles on it, I'd have some prem-o oil pressure, but when I swapped in the full gauge cluster I was let down. At cold start-up it hovers around 40 psi, but once it's warmed up, best I can do is maybe 20psi. Usually sits between 10-15psi. Also verified it with a mechanical gauge, so I'm thinking I have bearing issues. Just haven't had the time to drop the pan and take a look for myself. If the engine didn't run hot all the time, I'd probably just do what 1987Comanche does and just drive it till it quit...

 

Now, you could also have a bad cluster. The first full cluster I put in must have had some sort of water damage and all of the gauges except for the tach read too low. The oil pressure gauge never moved a bit no matter how much I revved the engine, just sat around 0psi, even with a brand new oil pressure sender. My mechanical gauge was showing 35-40psi at cold idle. Even passed the test that Eagle described above, so that's not the end-all-be-all of determining whether or not your cluster is fine.

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so i just did the test as describe about

 

removed the wire from the sender- went all the way to the right past the 80 psi mark

 

ground the wire- couldnt get a wire to stay in the connector(don't have a second person with me to do this) but with it hooked up to the sending unit, with the engine in acc mode, the needle went to zero.

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Okay, that at least proves that the needle on your gauge is free to move, so the consistent low pressure isn't because of a stuck needle.

 

That's useful to know, but it means the problem is more than likely worn bearings.

 

Eagle, I feel real queasy about this, but..........

 

All that proves is that the needle in the gauge moves from one limit to the other and that the wire pathway is probably OK from connector to dash cluster. What about a bad sender? Perhaps a check with a real oil pressure device would yield better info? I view the dash gage as an indicator only, not a precision device. Maybe the oil is really sludge and just needs a good draining? Too many other questions to ask without more information.

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Well to give more info to help in this diagnosis, I am having an oil leak from the pan area, there is also oil on the bellhousing, I was hoping it was from the Rms, and not the seal on the tranny oil pump, reasoning being I don't want to pull the motor, I'll give a stab at replacing the Rms ,oil pump, and pan gasket, to see if it helps my oil pressure,

 

Where do I hook up the external oil pressure gauge to get a accurate reading?

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