My fuel gauge has always been wrong on my 1995 Firebird. When the gauge comes off full, I know that I need to get gas fairly soon for street driving and immediately for track driving.

Yesterday through the use of a scan tool, I also learned that my water temperature reads way too low while also learning that GM has two water temperature senors in the car - one for the ECM and one for the fool behind the wheel. The ECM sensor seems correct and the in-dash water temperature gauge is wrong.

Should I be treating these two problems as separate or is it possible that these are related?

Separate issues. Our temp guages are notoriously inaccurate and can't really be fixed as far as I know. The fuel guage problem sounds like either a bad sending unit in the tank or a bad guage. Check the sending unit first.

Thanks for the feedback. I think I will simply get an aftermarket temp gauge and leave the fuel gauge until I have leisure time (yeah right!). I just was wondering if there wasn't some link between the two problems that could be caused by the cluster itself.

Thanks.

Sounds like we had similar problems. Right after getting my engine fired up for the 1st time the temp gauge quit working all together. Fuel gauge never worked either. I tried like heck to get that temp gauge to work. Even disassembled the cluster, searched for spare parts, and tried to fix it. I now have an autometer temp gauge along side my oil temp gauge in those HVAC vents in the center. I never know how much fuel I have, I just top off after every session.

All in all the autometer gauge is better, bigger, more accurate, and much easier to see.

md

The guage in your dash reads coolant right as it comes from the radiator. The sensor for the PCM measures coolant after it has been through the engine. If the two read the same under power then one would surely be broken. If you put an accurate aftermarket gauge in the same spot as the stock gauge, it'll likely read the same as the stock gauge is now.

Good information to know.

However, with the scan tool for comparative purposes on Saturday, the differences between the two didn't seem linear and the water temp gauge never really reached 180 though the scan tool at one point showed +230 degrees... With no air flow (including pulling the fan relays) sitting on a dyno, would my stock radiator really be able to pull +50 degrees out of the water temperature?

It's possible when you observed that difference, the water coming from the radiator hadn't been through the engine yet. It will vary depending upon a lot of variables.

I've found when it's settled into a nice steady state, lightly cruising so the engine isn't producing much power at all, the gauge only reads a couple of degrees below the PCM. When you put your foot into it very long, the difference gets much bigger.

I was actually planning to move my sensor for the gauge to the same spot as the PCM a while back as I think the coolant temp after the engine is the more important one to monitor. I just never got around to it. I'm glad you reminded me.

Don't get me wrong, your gauge certainly may be off. I'm just trying to point out it really is reading a different temperature, so it might not be. You could always swap the sensor location and see how it works. An aftermarket gauge would be nice, but if you're on a budget it's nice to know something really is broken before you replace it.

Let me do some searching of my notes...there's a Corvette sensor that fits where the PCM sensor is except it has a wire for the PCM and one for the gauge. I forgot all about that mod, I guess I got too busy last year. I just kept in the back of my mind that whatever the gauge said on the track was probably 10-20 degrees or so lower than reality.

Would a '96 guage cluster work in it?

If so, you can have mine ...

I don't know, mitch. For now hang onto it. I will be getting to this in the next month or so and at that point, I'll look into it.

SAE 920673 New Generation Small Block V8 Engine has a nice graph of coolant temp as it travels from the front to the back through the heads, then from the back of the block to the front (this is why the middle coolant passages in the head gaskets are mostly blocked off, to force the water to take this route). It shows a temp difference of 8 degrees from entry to exit. Unfortunately it doesn't state under what conditions it was obtained--full power or cruizing, etc. My experience inclines me to believe that's just cruizing.

Anyway, here's the info on the Corvette sensor for the water pump:

10096181 PCM/Gauge sending unit.
12102748 Connector pigtail (incl. terminals) (Superceeded by 88988301).

The new sending unit is wired as follows:

Pin A = Black, PCM sensor ground
Pin B = Yellow, coolant signal to PCM
Pin C = Dark Green, to Gauge

I'll be using this unit with the new engine as well as the old one. Switching the guage from one location to the other is as easy as unplugging the green wire from one and plugging it into the other. Once I get some data on the difference and verify gage accuracy I'll leave it in the pump and the unit in the head will just be a plug--unless the stock one really is off.

I don't know what year F-body you have, but I suspect you're talking about the '99+ F-bodies with the temp gauges that read 210* for a wide range. '98 and older 4th gens aren't like that.

The fuel guage in my 96 was very non-linear. 3/4 tank indicated was really around half, half was closer to 1/4 (or less), and below that all bets were off.

The guage in my 2001 is better, but still not linear.

I think the main problem with the fuel guage is the shape of the tank. Since the tank is bigger at the top it may take 5 gallons to drop the float a few inches. Meanwhile at the bottom of the tank, a 2 gallon difference can lower the float a similar amount. Of course the float isn't calibrated for this difference and it drops at a faster rate as the tank approaches empty.

I think you're exactly right Chris. My street Z gas gauge behaves exactly the same.

In my 96 CMC car, I installed an Autometer fuel level guage and it appears to move with the fuel level quite well. It uses the factory sender ... just attach the purple wire, power and a ground.

But in my '98, it too seemed to fall very rapidly after the first 5 gallons or so was burned off.

I wonder if it's some sort of internal resistance with the guage?