1998 Chevy Blazer Bad oil gage or sending unit?

CHIEFCATCHEMALL

MEMBER
1998 CHEVROLET BLAZER
6 CYL
4WD
AUTOMATIC
107,000 MILES

Hello all. I am new to this site. Looks like a really good site. I am just getting into engines and working on them and all that stuff. I am only 20 and want to become more knowledgeable. Anyways here is whats going on. For the past year or so I have had a sticky lifter. And about a month or so ago I decided to dump CD2 into my oil to see if I could free it. I drove it for about 15 miles and when I pulled into my destination I realized my check gauges light came on. My oil pressure read zero. I proceeded to drive home. About a 20 minute drive. When I accelerated my vehicle was reading 20 psi and when I braked it went down to nothing. Was this coincidental that it happened right after I dumped this CD2 in? I let it sit for about a month without driving it. I drove to the garage which is 5 minutes away. They flushed it and put lucas in the oil. Pressure came up to about 40psi dropped to 20 when I braked. This held steady for about a week. Now after my engine is as hot as it gets it goes down to about 20 psi when I accelerate and down to zero when I brake. I drove it for about a half hour to see if I could duplicate the problem again this morning and it did happen again. I have not heard any unusual noises coming from my motor. No clunking or nothing. Also when my car is shut off and I go out and look at it later it says I have 20 psi when its off? Could this be a bad gauge or sending unit? Clogged pick up screen etc. Hopefully my cam bearings arent shot. Thank you in advance, Andrew

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Thursday, July 8th, 2010 AT 6:03 PM

12 Replies

HMAC300

MECHANIC
48,601 POSTS

First things first, if your oil pressure really goes to zero you will hear the lifters clacking- like all of them. Loud. What you do is put a mechanical gauge on to see what you have. See what that reads. Then go to your oil pressuer sender and put a test light on it and see if it has a blinking light if it does then it's probably your sender. But ifyou don't have the blinking light the gauge is screwing up. If you have a digital dash then all bets are off as those when they screw up they need to be rebuilt. In all my years I have not seen any of the "magic" motor fixes actually do anything except line the companies pockets. AlthoughEOS that GM has is good for new/rebuilt engines.

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Thursday, July 8th, 2010 AT 6:14 PM

CHIEFCATCHEMALL

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Alright. Thank you. I am not getting any unusual noise. But its not always on zero. Only when I have been driving it for about 20 minutues then come to a complete stop. My pressure gauge is reading 15 psi when accelerating as of right now. Is it out of the ordinary to have the oil pressure gauge read 20 psi when the car is off and its been sitting? So am I safe to drive it until I get a mechanical gauge on it tomorrow then? Thanks again for your help. I really appreciate it.

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Thursday, July 8th, 2010 AT 8:41 PM

HMAC300

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Normally when teh car isn't powered up the gauges will go back to 0 except maybe the fuel guage. I would thinkit's ok to drive as long as you don't hear the lifters making noise and it would be a loud clacking noise. There are several place s you canhook up the mechanical gauge, other than where the sending unit is. It is normal when the engine is warmed up for the oil pressure to drop to about 20-25p.S.I. When at idle then go back up to about 45-50 when at higher rpms.

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Friday, July 9th, 2010 AT 9:51 AM

CHIEFCATCHEMALL

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Where is the best place to get the most accurate reading? If the dash gauge and mechanical gauge dont match up is that a bad gauge or could it still be a faulty sensor or sending unit?

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Friday, July 9th, 2010 AT 5:57 PM

HMAC300

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Yoiu can hook the mechanical gauge where the original sending unit is or down by the oil filtelr, there should be a large plug to remove and hook up the gauge tehre. If you go back to the first post I told you how to test to see if your gauge or sending unit is bad. Just make sure that there are porbably 3 wires and one will go back to the fuel pump. So if you hook in where the sending unit is lmake sure to put a "t" in there so to keep the sending unit hooked up so your fuel pupmp will work.

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Saturday, July 10th, 2010 AT 8:23 AM

CHIEFCATCHEMALL

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I couldnt find where to hook the mechanical gauge. The only place I seen that I thought might work was where the oil filter block. There was 2 lines running from it and between the 2 there was a blot. I thought if I unbolted that I could get my gauge in there. Couldnt get the bolt out. Anyways I drove it until it got as hot as it would. Same thing it would read 0 psi when I cam to a complete stop and 20 psi when I accelerated. Pulled in the driveway and I let it idol for about 10 mins on 0psi. Still didnt make any noise. When I shut it off it stays at 0psi for about 15 seconds then it climbs up to 15 psi and stays like that until I start my car up again. Figuring its alright to drive until I get it to the garage.

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Saturday, July 10th, 2010 AT 8:13 PM

HMAC300

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You seem to misunderstand me . I'm including a pic of the oil system. where it says oil cooler bypass valve there should be a plug in teh block above the cooler lines. you should be able to hook up there and there should be a plug there as it goes to an empty hole. If your truck still runs and you don't get any lights going off on your dash saying low oil pressure and your engine still runs it will probably be the gauge that is bad. it cold be losing ground or just plainn not any good.... here is the pic and i hope you can understand where it is....

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Sunday, July 11th, 2010 AT 10:54 AM

CHIEFCATCHEMALL

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Im still not really understanding where it is.I did notice and oil leak under my engine. Its not under the pan or anything. Just a small spot. Been getting that for the past few mornings.

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Sunday, July 11th, 2010 AT 3:43 PM

HMAC300

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I think this is a little beyond your abilities, take it to a pro alet him check it.

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Monday, July 12th, 2010 AT 11:10 AM

CHIEFCATCHEMALL

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I found the plug your talking about by the oil filter. Wont come free though. Soaked it in liquid wrench and started wrenching. Cant get it off. Taking it to the garage in the morning. Is a small oil spot under your engine a sign of a bad sending unit? Never had it before. Whats the cost to replace a gauge, sensor, or sending unit usually hmac?

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Tuesday, July 13th, 2010 AT 8:00 PM

HMAC300

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Figure around $100 for the sender installed, maybe less depending on where you are and what the book says for labor. The gauge I have no idea. I really beleive it's the gauge and not the sender though. Unless the sender is leaking and if you look with a flashlight and mirror on the back of the engine you should be able to see.

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Wednesday, July 14th, 2010 AT 9:19 AM

CHIEFCATCHEMALL

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Youve got all the tricks in the book dont ya? HAha thats clever. I just got it back today. They replaced the oil pressure relief valve. Drove it for an hour straight hard. Now when its completely warm and I come to a stop its reading right around 25psii. And when I am accelerating its anywhere from 45-55 psi. Cant seem to get it to go below 25 psi anymore. Jeeze did I get lucky? It seems like if I really was running it on 0psi (granted I never really drove far on 0) I would have ruined my engine.

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Wednesday, July 14th, 2010 AT 8:22 PM