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Pete, Auto Service Technician
Category: Chevy
Satisfied Customers: 27933
Experience:  14 years GM dealer Technician.
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Chevrolet Trailblazer LT: Pete I have a 2002 chevy trailblazer

Customer Question

Hi Pete I have a 2002 chevy trailblazer with a 4.2 liter engine when I start it cold oil pressure is at 40 psi on the dash gauge. when it warms up, gauge drops & warning chimes go off. found sending unit by oil filter leaking and replaced. Same problem with new ops. disconnected connector from sending unit and reads 40 psi on start up and when it gets worm pressure drops off if I accelerate pressure will go up, but sending unit is unplugged is there another sending unit I am not aware of
Submitted: 2 years ago.
Category: Chevy
Expert:  Pete replied 2 years ago.
Hi,my name is ***** ***** I will be happy to try to assist you with this.I may need more information in some cases to help with answering your question.
Your engine oil pressure sensor/sending unit is located at the oil filter housing area and sounds like you have replaced the correct one here.
In some cases this fault can be due to an internal fault in the instrument cluster itself as the gauge itself sets off the warning you see and hear.One way to check this is to use a scan tool plugged into your vehicle that will communicate with the instrument cluster to view the cluster data to see if it is reading the correct engine oil pressure compared to what the gauge reads.Meaning if it reads 20 psi or higher and your gauge is really low around 10 or less this indicates a gauge fault with the instrument cluster needing repaired or replaced.
If this is found to be o.k. here I have seen sludge build up in the engine cause this and had to remove the engine oil pan and clean out the sludge to correct this fault.Usually though if this is the cause the pressure would be low on a cold engine as well.This can be tested with an engine oil pressure test gauge threaded into the port where the oil pressure sensor is located.
The other issue is worn engine cam shaft bearings.On cold start and cold engine the bearings are tighter to the cam shafts and create more oil pressure but as the engine warms up the heat expands the bearings causing loss of oil pressure if the gap between the bearings and cam shafts is to large.This can also be tested with an engine oil pressure test gauge.Most shops and dealers have this gauge and in some cases you can rent one from a local parts store.You don't want to see anything less then 12 to 15 psi or lower as this indicates an internal mechanical fault in the engine.
All of these will need checked into to pinpoint which is the cause here and then repaired from there.
Thanks Pete
Customer: replied 2 years ago.
Pete I have the sending unit unplugged and it is reading pressure on the gauge of 40 PSI and after it warms up it drops to zero, but when I accelerate and bring the RPM's up the gauge goes up to 40 PSI or above like you would expect to see with worn pump, bearings etc. My problem is the sending unit is unplugged a this time and I am reading 40 PSI with engine running cold and only loose it warm. With engine off gauge goes to 0 but when started it goes up I really believe there is another sending unit some where.
Expert:  Pete replied 2 years ago.
To be sure the correct sensor was replaced was this a two wire sensor you replaced with a tan wire with black stripe as well as a purple wire on it?
Customer: replied 2 years ago.
Expert:  Pete replied 2 years ago.
O.k.,how your system works is the engine computer (PCM) receives the signal from the oil pressure sensor.Then it sends this signal to the instrument cluster gauge itself.
To check and see if the sensor,wiring and engine computer are all o.k. a scan tool is needed to monitor the engine data.If when the pressure drops low on the gauge is the computer reads the pressure at a good pressure this is all o.k.
This then leads to the instrument cluster itself as the cause here.
Since the oil pressure changes with engine speed with the sensor unhooked though as it should sit at 0 to me from here this sounds like the PCM is sending a false signal causing the pressure reading to drop low and cause the alarm like this here and the computer needs replaced.
Thanks Pete
Customer: replied 2 years ago.
Thanks Pete I am warming the vehicle up again right now instrument gauge is currently reading 40PSI and scanner says 37.7 and engine data 3 reads ok at this time. I an waiting for it to warm up again to review all info when it drops out againThanks, Jim
Expert:  Pete replied 2 years ago.
Your welcome Jim.Let me know if you need anything else and feel free to rate my help above if you like.
I have to step out for awhile and get some errands done but I will be back if you need anything else.
Thanks Pete