97 Tahoe oil sending unit

[jasom]

I'm trying to locate where the oil sending unit is on my 97 5.7 tahoe. The oil pressure gauge started bouncing around last night bringing on the check gauges light.

It is full of oil and I just changed the oil about 1,000 miles ago I just hope my oil pumps not going bad. If anyone has any suggestions to where the sending unit is located or another reason my gauge is bouncing I would really appreciate it, Thanks.

[NoEcm]

The oil sending unit is right behind the distributor.

The sending unit does double duty:

1. It provides signal to the oil pressure gauge.

2. It also acts as a low oil pressure cut off switch for the fuel pump; under a low, or no oil pressure situation, it shuts off power to the fuel pump.

[jasom]

Originally Posted by NoEcm

The oil sending unit is right behind the distributor.

The sending unit does double duty:

1. It provides signal to the oil pressure gauge.

2. It also acts as a low oil pressure cut off switch for the fuel pump; under a low, or no oil pressure situation, it shuts off power to the fuel pump.

So are you saying the sending unit won't cause the oil gauge to bounce around?

[777stickman]

Originally Posted by jasom

So are you saying the sending unit won't cause the oil gauge to bounce around?

"NoEcm" didn't say that at all. He was just telling you where it's located and what it does.

It's very possible the sending unit could be the problem. Also wiring and/or the gauge. Need to put a mechanical pressure gauge in to make sure it's not the pump.

[jasom]

Originally Posted by 777stickman

"NoEcm" didn't say that at all. He was just telling you where it's located and what it does.

It's very possible the sending unit could be the problem. Also wiring and/or the gauge. Need to put a mechanical pressure gauge in to make sure it's not the pump.

Ok, Just checking. I'm not much on dash gauges anyway. Thanks

[brcidd]

All my years of studying fuel pump wiring schematics- I have seen GM use the oil pressure sensor switch "as a back-up" to the fuel pump relay- in case of fuel pump relay failure- it is a parallel circuit- and unplugging the oil pressure sensor does not kill the fuel pump-- so telling someone that the engine will shut off for low or no oil pressure is wrong (on a GM).. It used to be that way back with the Chevrolet Vegas- but not any more-- Heck I know of three M-vans that have seized their engines because of low oil- And when I researched thoroughly the wiring diagrams- I saw the parallel ciruits--It has been this way a long time.

Directly from the circuit description in the service manual for the Tahoe:.

"Should the fuel pump relay or the 12 volt relay drive from the VCM fail, the fuel pump receives a supply current through the oil pressure switch backup circuit".

[jasom]

Well I found the sending unit, Thanks. First I'm gonna change the oil thursday because I had put some resolone in to get rid of the lifters chattering when it was cold. It said that it doesnt break the oil down, but with it getting warmer I not sure thats true.

When I'm going down the road oil pressure is fine, it's when it's idleing it doesn't show much oil pressure about 7-10psi then the check gauge light comes on.

If changing the oil doesn't take care of it I'll change the sending unit. If those two things don't take care of it looks like I'll be changing the oil pump after a manual gauge check.

Thanks for your reply's I'll let you guys know how things turned out.

[j cAT]

Originally Posted by jasom

Well I found the sending unit, Thanks. First I'm gonna change the oil thursday because I had put some resolone in to get rid of the lifters chattering when it was cold. It said that it doesnt break the oil down, but with it getting warmer I not sure thats true.

When I'm going down the road oil pressure is fine, it's when it's idleing it doesn't show much oil pressure about 7-10psi then the check gauge light comes on.

If changing the oil doesn't take care of it I'll change the sending unit. If those two things don't take care of it looks like I'll be changing the oil pump after a manual gauge check.

Thanks for your reply's I'll let you guys know how things turned out.

if after the engine gets to normal operating temp at idle in gear the oil pressure drops to 10psi this is hopefully just the oil pump.... if this vehicle has alot of miles on engine i would get a high capacity oil pump.... this will hopefully produce sufficent oil flow to keep engine cool and lubricated...


if the crank bearings are worn you will get low oil pressures...after the engine gets to normal temp......



it is possible that because you used the oil additive that it may have looosened up alot of sludge and now your oil pump pickup screen is coated with goo.....if so use lacquer thinner to remove the black goo off the pan and screen before installing new pump....

[jasom]

I changed the oil and it seemed to help. I get better oil psi driving down the road than I was getting was 35 to 40 now 45 to 55 at 2k. When idleing in gear 700rpm I get 15 to 20 psi. The gauge still fluctuates a little so Later I'm gonna change sending unit.

I was using Quaker state high mileage with slick 50 then added a quart of resolone. Now using just mobile 5000.

When changing the oil I noticed a little moisture under the oil fill cap so Looks like I have and intake gasket leak to boot.

[j cAT]

Originally Posted by jasom

I changed the oil and it seemed to help. I get better oil psi driving down the road than I was getting was 35 to 40 now 45 to 55 at 2k. When idleing in gear 700rpm I get 15 to 20 psi. The gauge still fluctuates a little so Later I'm gonna change sending unit.

I was using Quaker state high mileage with slick 50 then added a quart of resolone. Now using just mobile 5000.

When changing the oil I noticed a little moisture under the oil fill cap so Looks like I have and intake gasket leak to boot.

that moisture could be from all the rain you have been getting....if your coolant level is not dropping..........


the oil pressures still a little low but acceptable....i would remove and clean oil sender.... it is possible that is , why it fluctuates....

[jasom]

I checked under oil cap today no moisture, thank god. Jcat looks like you were right on. What is a good base line oil psi idleing at 700rpm or per 1000rpm?

[j cAT]

Originally Posted by jasom

I checked under oil cap today no moisture, thank god. Jcat looks like you were right on. What is a good base line oil psi idleing at 700rpm or per 1000rpm?

if your engine has no problems/wear you normally should get 40psi in gear at idle of 500rpm.....when out of gear at idle you should get 45psi at 700rpm....this is with 5-30wt oil at normal engine temp....


your pressures are ok but it would be niece if they were at the 30-40psi range....ya it could be sender .....one way to be sure is to install a pressure gauge for testing ,,,check to see it is accurate and compare..

low pressures at idle after warm up is usually engine wear....if you had lower pressures at all rpm's that could be pump/pickup problems...



at 1500-2000 rpm pressures are 60psi. use gauge to be sure of pressures...


your idle in gear should be 550-600rpm max....check for vacuum leaks/pcv valve/hoses especially...if you have a vacuum leak this may be how moisture is getting into crankcase.....use vacuum gauge to find leaks, good vacuum is 21 inches and solid not jumping....

[jasom]

Thanks for the info. I use 10-30 for oil. I didnt realize I was that far off on psi. I'm sure they do but do they make a manual oil pressure gauge or will I have to rig one up? It looks pretty tight back there.

I know the motor has some pretty good where on it. I've changed the oil twice since I bought it a month ago. Each time I have there has been some shavings on the oil plug with sludge packed on it. It has 170k on it but for the most part it runs great just trying to make sure everything is up to par.

They took really good care of keeping it up except for changing the oil when they should. Hopefully in the the couple weeks I can round some money up and start tearing into things

Again thanks for all your input.

[j cAT]

Originally Posted by jasom

Thanks for the info. I use 10-30 for oil. I didnt realize I was that far off on psi. I'm sure they do but do they make a manual oil pressure gauge or will I have to rig one up? It looks pretty tight back there.

I know the motor has some pretty good where on it. I've changed the oil twice since I bought it a month ago. Each time I have there has been some shavings on the oil plug with sludge packed on it. It has 170k on it but for the most part it runs great just trying to make sure everything is up to par.

They took really good care of keeping it up except for changing the oil when they should. Hopefully in the the couple weeks I can round some money up and start tearing into things

Again thanks for all your input.

the pressure guage can be for water/oil/air/swimming pool filter etc.... the connection to engine is 1/4in pipe thread or similar size then use tubing to connect to guage like 3 feet long.that way you can read if you want to test while driving.. .. hardware store will have this fitting stuff.. also this can be used to check out cooling system and gas/fuel pressures get gauge that goes to 100 psi.... that way you will be able to read pressures up to 80 psi with out damaging the gauge... if you drop gauge you will have to recalibrate so don't hit/bang guage..




if you have shavings in the oil pan this is most likely the main bearings....and if so this is usually from engine running with low on oil .......

[jasom]

Well I finally got around to changing sending unit and that wasn't it still low oil psi. I checked vaccum today and it's at 21in. So Looks like oil pump or engine wear. I'll look into just changing oil pump or just finding another motor.