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Discussion Starter #1
Back history.....I have this 2007 TB with 85k miles. I purchased the TB with a blown motor. I pulled the motor and found a man who had a overhauled motor 4.2 in his garage that he was going to put in his personal car but decided not to and sold it to me. Everything tells me he did a good job.

What I have run into the a common is it appears but need to start a discussion to solve this common problem. So the car runs and idles great just after initial install of the new motor. Once the temp gets to operating temp the oil pressure switch closes and the gauge goes to zero. I verified the idle pressure of 12-13 PSI with an actual gauge. If i give it a little gas the pressure increase to 40+ for about 10 seconds and drops again.

I am not one to just throw new parts at a car. I did try and swap the oil PSI switch out with the one which came off the motor I pulled out. Exactly the same issue. Odd. I tried running a 30 weight oil to see if a higher viscosity oil would affect it. No change. I can not find any OEM data on what PSI this switch should close to drop the PSI data to the PCM but im guessing its at 12 PSI as this is the factory min.

I do not believe its the dash servo motor as they seem to be working good.

Whats odd to me is the switch closes at almost the exact same time the thermostat opens. Right when the engine is at his hottest point.

I do have a new switch but im reluctant to install it. Everything is telling me its not a OIL psi switch not the gauge motor.

Any help would be awesome.
 

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Discussion Starter #2
Back history.....I have this 2007 TB with 85k miles. I purchased the TB with a blown motor. I pulled the motor and found a man who had a overhauled motor 4.2 in his garage that he was going to put in his personal car but decided not to and sold it to me. Everything tells me he did a good job.

What I have run into the a common is it appears but need to start a discussion to solve this common problem. So the car runs and idles great just after initial install of the new motor. Once the temp gets to operating temp the oil pressure switch closes and the gauge goes to zero. I verified the idle pressure of 12-13 PSI with an actual gauge. If i give it a little gas the pressure increase to 40+ for about 10 seconds and drops again.

I am not one to just throw new parts at a car. I did try and swap the oil PSI switch out with the one which came off the motor I pulled out. Exactly the same issue. Odd. I tried running a 30 weight oil to see if a higher viscosity oil would affect it. No change. I can not find any OEM data on what PSI this switch should close to drop the PSI data to the PCM but im guessing its at 12 PSI as this is the factory min.

I do not believe its the dash servo motor as they seem to be working good.

Whats odd to me is the switch closes at almost the exact same time the thermostat opens. Right when the engine is at his hottest point.

I do have a new switch but im reluctant to install it. Everything is telling me its not a OIL psi switch not the gauge motor.

Any help would be awesome.
As I noted the oil pressure dropping in 100 percent an indication issue. The engine does maintain oil pressure.
 

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2002 chevy trailblazer_lt
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442 Posts
I have a high mileage TrailBlazer in which I suspect I have the common pickup tube O ring issue which results in a similar "no oil pressure" warning in the dash. For peace of mind and also to enable monitoring of actual oil pressure as opposed to the completely fake dash gauge I installed an aftermarket dasboard mount oil pressure gauge. I just set the up near the vin number on the dashboard where I can keep an eye on it.

IMG_20190704_080054.jpg
 

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Discussion Starter #4
This is such a common problem its stupid. Tomorrow I am going to put a vaccum gauge on the mainfold and see what data it. This being I just put this motor in and installed the intake manifold. Maybe I have a leak. Something tells me its a PCM issue because its always right when the temp hits 220. Right when the thrmostate opens.
 

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Discussion Starter #5
I have a high mileage TrailBlazer in which I suspect I have the common pickup tube O ring issue which results in a similar "no oil pressure" warning in the dash. For peace of mind and also to enable monitoring of actual oil pressure as opposed to the completely fake dash gauge I installed an aftermarket dasboard mount oil pressure gauge. I just set the up near the vin number on the dashboard where I can keep an eye on it.

View attachment 55340
Where did you port in the oil line to the motor for the mechanical guage?
 

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Discussion Starter #6
Back history.....I have this 2007 TB with 85k miles. I purchased the TB with a blown motor. I pulled the motor and found a man who had a overhauled motor 4.2 in his garage that he was going to put in his personal car but decided not to and sold it to me. Everything tells me he did a good job.

What I have run into the a common is it appears but need to start a discussion to solve this common problem. So the car runs and idles great just after initial install of the new motor. Once the temp gets to operating temp the oil pressure switch closes and the gauge goes to zero. I verified the idle pressure of 12-13 PSI with an actual gauge. If i give it a little gas the pressure increase to 40+ for about 10 seconds and drops again.

I am not one to just throw new parts at a car. I did try and swap the oil PSI switch out with the one which came off the motor I pulled out. Exactly the same issue. Odd. I tried running a 30 weight oil to see if a higher viscosity oil would affect it. No change. I can not find any OEM data on what PSI this switch should close to drop the PSI data to the PCM but im guessing its at 12 PSI as this is the factory min.

I do not believe its the dash servo motor as they seem to be working good.

Whats odd to me is the switch closes at almost the exact same time the thermostat opens. Right when the engine is at his hottest point.

I do have a new switch but i am reluctant to install it. Everything is telling me its not a OIL psi switch not the gauge motor.
I do not get any oil lights or oil pressure messages
 

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2002 chevy trailblazer_lt
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I suspect the safest way would be to use a "sandwich" adapter that goes between the oil filter and the filter mount. I didn't do that. I removed an existing plug on the engine front, very near the filter and installed an adapter there. It is very close to the belt tensioner and if the belt ever breaks it could be a mess!!

P1090082.JPG
 

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Discussion Starter #8
So I broke down and changed the oil pressure switch. Still same problem. Wasted $40. Also checked vacuum at idle and pulling 20 inches. So thats not a factor. Damn this thing. The engine has only 1 hour run time. Its not a sludge issue.
 

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2005 isuzu ascender
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This one's beyond me but could it be model year differences and how the PCM is programmed? You have a 2007 but if the engine was 2002 w 2002 sensors and mods could it be an issue? From what I've read the 02-03 were similar, 04-06, and 07+...
 

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Discussion Starter #10
This one's beyond me but could it be model year differences and how the PCM is programmed? You have a 2007 but if the engine was 2002 w 2002 sensors and mods could it be an issue? From what I've read the 02-03 were similar, 04-06, and 07+...
Thanks but most of the sensors were transferred from original motor to the new motor. I’m going to try and replace the gauge steppers tomrrow. I had a thought maybe the gauge has a dead spot at 12psi and drops off. Hell idk.
 

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2002 gmc envoy_slt
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Might be a long shot, but its a cheap thing to test. I battled this issue for 2 years on my 2002 and what solved it was switching from FRAM oil filters to genuine GM oil filters. If you are not using them, try a genuine GM oil filter
 

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Discussion Starter #12
Well it ended up being the gauge stepper motor. Apparently what I discovered is the motor will develop a dead spot and it will fall to zero if it hits that dead spot which so happens to be where it most commonly rests when hot.
 
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