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1997 Formula with Dropping Oil Pressure Gauge


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#1 CapitalGains

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Posted A week ago

Hey all! New here and new owner of a 1997 firebird formula. Love the car, but it's been stressful. Since I've owned it, it's been hot idling at only 5-7 psi. Everyone I've spoke to said it probably needs another rebuild (has 196k miles on it, had a full rebuild at 121k). Anyways, last ditch effort I hooked up a mechanical pressure gauge in place of the oil sensor switch and she held steady at 20 psi for almost 50 mins. So it looks like a bad oil gauge and/or cluster?

Should I leave it or try to replace it? How involved is that?

Thanks!

​​​​​​​John



#2 JOE68

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Posted A week ago

I would try a new sender first



#3 Injuneer

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Posted A week ago

I'm confused (as usual).  Where did you hook your mechanical gauge?  If you removed the stock pressure sensor (not a switch), replacing the sensor would be no more difficult than what you just did.  
 

There's also an extra oil pressure port above the oil filter.  That's where the 1993 and “early” 1994 models located the sensor.

 

Was there any indication the stock sensor was leaking?  That throws the pressure reading off.  Then there's the other two fittings that the stock sensor screws into.

 

courtesy of Shoebox:

 

http://shbox.com/1/op_sensor.jpg

 

http://shbox.com/1/o...sure_sensor.jpg
 

 

http://shbox.com/1/aux_oil_port.jpg

 


1994 FIREBIRD FORMULA
FRED

 
381ci all-forged stroker (Callies Stealth, Oliver billet rods, BME 2618-T61 nitrous pistons) - 10.8:1 - CNC LT4 heads/intake - Comp Cams solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74#/HR injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11 Pro-Street gears - AS&M headers - true duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP / 800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117 MPH straight motor
 
https://www.firebird...9_698_32777.jpg

 


#4 CapitalGains

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Posted A week ago

I know, it's not a switch, yet it's called that for some reason. Makes no sense lol. Here's the replacement I installed:

 

https://www.autozone...14_1371625_8450

 

I removed that and screwed in the mechanical gauge in its place. With the mechanical gauge, it holds steady at 20 PSI hot idle. When I put the oil pressure sensor back in, drops down to 5-7 psi hot idle.

 

So the only conclusion I draw from that is that the in-dash gauge itself is faulty? 



#5 Injuneer

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Posted A week ago

The brand new Duralast sensor is reading 5-7 PSI?  What was the pressure on the gauge at idle before you replaced the sensor?  Does the pressure indicated on the gauge increase with RPM?  What is the reading at 2,000 RPM?

 

Unfortunately, new parts are often cheaply made Chinese pieces.



#6 CapitalGains

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Posted A week ago

Yes, the new sensor is reading 5-7 psi. The old sensor I assume was stock, previous owner had it for 15 years and never replaced it. That also read 5-7 psi at hot idle. So that's where I started, and then when the Duralast sensor read the same, I started freaking out thinking it was the engine needing a rebuild. 

 

In both cases (old and new sensors) when I rev up to 2,000 RPMs, PSI shoots up to about 40ish. Same as when I'm driving it, between 35-40. The only time it's low is at idle.

 

Now, I haven't driven it with the external manual gauge, but like I said, I let it idle for 50 mins and it never dropped below 20 psi.

 

So I'm thinking it has to be the in dash gauge? Unless both the old sensor and the new one I bought are both faulty? Seems unlikely...



#7 Injuneer

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Posted A week ago

Could be a wiring problem is you had the same problem with 2 sensors.  

 

5-7 PSI is not unusual in a "worn" LT1.  In fact, the dash "Check Gauges" light is set to come on for low oil pressure only when the pressure drops to 6 PSI.  For general usage, the guideline on oil pressure is 10 PSI per 1,000 RPM.  But it's still scary seeing numbers that low on any engine.  

 

Based on helping people specifically with the LT1 engine for 20+ years on sites like this, I have not seen a lot of failures of the oil pressure sensor.  When they do fail, it's usually a leak.

 

Was the Duralast part a replacement for GM/AC Delco part # 19244497 ?



#8 CapitalGains

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Posted A week ago

Yep, and a couple times the "check gauge" light did indeed come on...and freaked me out.

 

Wouldn't shock me if it was electrical...turn signals seem to be sluggish, I wasn't sure if that just how they are or not though. 

 

Yes, the Duralast part was a replacement for part #19244497. Looks like they still sell them on Amazon (from Jegs), but aren't available til September. amazon.com/ACDelco-19244497-Original-Equipment-Pressure/dp/B004JBCFSQ

 

When you say leak, do you mean a leak in the sensor itself? 

 

Here's a follow-up question: the engine "feels" extra hot. I drove it last night, and I could feel heat on my legs down by the pedals (coming from the engine). Is that normal? I mean it's a big engine in a tight space, but who knows. 

 

If it's an electrical issue, I'm wondering if the temp gauge is off too? It reads normal (about 210 or so), but I don't know if that's accurate. Could overheating create issues with oil pressure? I'm asking from a position of ignorance lol



#9 Injuneer

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Posted A week ago

OIL PRESSURE:

 

Yes, the body of the sensor will start to leak.  But unlikely two in a row.

 

According to the factory service manual, the resistance of the oil pressure sensor should be "approx 90 ohms at high pressure; approx 10 ohms at low pressure".  The circuitry of the sensor and the gauge is shown on page 8A-81-0 of the factory service manual. If you haven't downloaded the manual yet, or don't own one, free download, courtesy of GaryDoug:

 

http://www.mediafire.../?40mfgeoe4ctti

 

There is a diagnostic chart on page 8A-81-21 for "oil pressure gage (GM spelling) appears inaccurate".  But it requires that you sequentially apply a resistance load of 0, then 40, then 100 ohms across the pressure sensor terminal wire in the harness and a ground (key on, engine off, harness disconnected).  That should cause the gage to swing from 0 PSI, to 30 PSI, to full deflection

 

The instrument panel is fed 12 volts.  There is a resistor in parallel with the oil pressure gauge circuit, so I don't know what the voltage should be at the oil pressure sensor.  The pressure sensor appears to have 3 pins, but only uses 1 of them  on the F-Body LT1.  I believe there actually is a low pressure switch in the sensor as well, but that was only used on the Corvette version of the LT1, where the fuel pump is interlocked with oil pressure, 

 

 

COOLANT TEMPERATURE:

 

GM designed the engine to run at 210*F, center of the gauge.  The gauge reads a sensor in the driver side head.  The PCM reads a sensor in the water pump housing.  The stock programming turns both fans on low at 226*F and on high at 235*F.

 

Checking the temp sensors and wiring to them:

 

http://shbox.com/1/4..._tech2.html#ect



#10 CapitalGains

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Posted 5 days ago

Thanks for the step by step instructions and the invaluable resources! Bought a multimeter yesterday and will be running the diagnostics the weekend after next.
 
New "issue" came up....pulled the car out of the garage and noticed what appeared to be oil under it. Not a ton, but enough to concern me. I traced it back to a component I don't recognize toward the middle of the car, bottom of the engine. Would you mind if I contacted you privately to send you a pic of it? Can't figure out how to upload a .jpg here.



#11 Injuneer

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Posted 5 days ago

I'd prefer you post on this thread.... that way others can help you as well.

 

To add a pic to your post:

 

- click on "More Reply Options" in the lower right hand corner of the "reply" box

 

- below the bottom of the "reply" box is a paperclip and "Attach Files"

 

- click on "Choose File", should open your computer's (or phone) directory/file folder

 

- Select pic 

 

- click "Attach This File"

 

- repeat to add additional photos.

 

I can't remember is there is a minimum post count before you can upload files.

 

For photos and diagrams of many LT1 parts, this is the best online source:

 

http://shbox.com/1/4th_gen_tech1.html

 



#12 CapitalGains

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Posted 5 days ago

let's see if this works...the blue arrow is where it's wet/leaking from.

Attached Files



#13 Injuneer

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Posted 5 days ago

That appears to be the clutch slave cylinder.  Is the clutch functioning OK?  The leak could be the hydraulic (brake) fluid used to activate the clutch, or it could be leakage from the rear main seal collecting in the bell housing.  If it's hydraulic fluid, the clutch performance would be affected.  I downloaded the photo and increased the brightness, and appears there is a drip on the bottom stud.  Is it possible that is something on the back of the engine (like the oil pressure sensor) dripping on the bell housing?



#14 CapitalGains

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Posted 4 days ago

Wow, I never thought of that...you know what, it was leaking around the sensor port when I was testing it with the mechanical gauge. I wonder if it ran down the back while I let it idle. I'll check that tonight...no idea about how hydraulic fluid looks/smells compared to oil. I'll buy a small container of it so I have something to compare the liquid too. 



#15 Injuneer

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Posted 4 days ago

It’s brake fluid, same thing you put in the reservoir for the brake system master cylinder.  Clear, not particularly “oily” and dissolves car paint in an instant.  Handle with care.

 

In addition to the brake master cylinder, there is a clutch master cylinder on the engine side of the firewall, directly behind the clutch pedal.  A few inches above the clutch master cylinder is a fluid reservoir, that should be maintained full.  As I recall the reservoir is semi-translucent with a black plastic top.  My Formula was a 6-speed manual when I bought it brand new in 1994.  But I can’t go out and take a picture of the clutch hydraulic system because I replaced the T56 with a TH400 automatic about 18 years ago, and sold all the parts from the manual trans to someone to do a 4L60E —> T56 conversion.

 

Make sure there are no leaks where the oil pressure sensor screws in.  Reference the photo linked in post #3.  There are two 90-degree elbow fittings in between the pressure sensor and the engine block.  Either of those could be leaking at the threads.



#16 CapitalGains

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Posted 2 days ago

Okay, looks like it was oil that leaked down when I tested it with the mechanical gauge. Nothing is leaking anymore.

 

I'm curious...what kind of multimeter would I use to do what you described? The one I bought doesn't allow for setting an existing resistance on it. I called around and no one seemed to know what I was talking about?



#17 Injuneer

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Posted A day ago

A multi- meter typically does not include the ability to apply a fixed resistance load.  You would either need resistors with the correct values, or a small variable resistance substitution box.  GaryDoug could answer that better..... he's the resident electrical engineer.






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