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Old 11-07-2003, 06:21 PM
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Thumbs down tbi 350 oil pressure sending unit problem HELP

HERE it goes. My father has a 1992 Chevy Blazer with a tbi 350. it has 157,000 miles on it and has had the oil changed religiosly every 3000 miles since new. The 1st problem is the oil pressure reads 15 at an idle, warmed up and up to 35 when running. which is bad but thats not the main problem. At a cold start it takes alot of cranking to get the sending unit to move and once the gauge goes over 10 it starts right up, but i mean alot of cranking. When it is warm and you crank it, smetimes it will not start, the sending unit seems to loose its prime. Ive gone through 3 sending units and have taken it apart numerous times and tightened the hell out of it, but with no luck. since the truck is only for work is there a way to bypass the oil pressure sending unit(send a false signal to the ecm) so itll start right up. i heard something about the unit reads 5 volts. Or should i put a new oil pump in it. ive never rebuilt an engine and this probably needs it but i dont want to do it and dont have the time. i really would like to bypass the sending unit.
any help or diagrams would be great

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Old 11-07-2003, 07:09 PM
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This is a neat problem.

I dont think your problem is the engine or the oil pressure sensor.

The long crank time is caused by the fuel pump relay being bad, or the control wire being bad, or computer being bad (low chance of that though).

The oil pressure sensor also acts as a switch for fuel pump that closes once oil pressure has reached 4 psi. So it sounds to me that is the only thing working.

There is a test port on the diagnostic conector that will allow you to bypass the relay. Apply 12 volts to that in morning when truck is cold bet it fires up right away.

PM me Ill send you some diagrams.
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Old 11-07-2003, 07:11 PM
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I think the problem is not the sending unit but the oil pressure. When you first start an engine it needs oil pressure before the valves will operate. The oil pressure is needed to fill up the lifters so they can open the valves so the engine will start. Once the pressure comes up the valves start working hence the engine starts.
Fix the pressure problem and I think the engine will start just fine.

Good Luck
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Old 11-07-2003, 07:23 PM
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hard start

i agree with greg, the ecm sends a 2 second prime signal to the fuel pump relay when the ignition is turned on. normally on gm products the fuelpump test lead is near the battery and is green in color. you can take a test light to this circut to watch for signal from ecm or this circut can be powered up with a 12 volt jumper to run the fuel pump if the relay has failed. the oil pressure circut comes into play after the engine has started and serves as protection to shut down the fuel pump in case of oil pressure loss.
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