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what size gas tank ?

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blazerguy 90

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I have a 90 K5, 5.7 engine, the sending unit in the gas tank is bad, the problem is that I don`t know if I have a 25 or 31 gallon tank ?, from what I can find looking in my LMC catalog my blazer has a 31 gallon tank, because they don`t list a 25 gallon tank for my year Blazer. so does it have a 31 gallon tank ?. I also plan on replacing the fuel pump while im into it that far, as far as I know the fuel pump thats in the tank now is the original one, any sugestions for a new pump or should I just get an oem fuel pump ?.
 
someone will know the actual measurement, but its something like 31 gal = 5 inches from center seam to bottom, and 25 gal = 3 inches from center seam to bottom... IIRC
 
I don't think GM offered the small tank after '88. And also, the LMC catalog lists the dimensions of the tank next to the exploded drawing. That's how I figured out the size of mine.
 
The sticker in the glove box is long gone, as far as measuring the tank, I don`t think its possible with the skid plate thats around it, unless im missing something. I can`t find any tank measurements in the LMC catalog, but everything is pointing to it being a 31 gallon tank.
 
12-3/4" deep for the 31-gallon (top and bottom half appear same size.)

10-3/4" deep for the 25-gallon (bottom half appears shallower that top half.)

www.spectrapremium.com (click on e-catalog)
 
Thanks for the link, I found the sending unit on there web site, but couldn`t find a price. any idea how much they cost ?.
 
Dude a sending unit for a TBI K5 is gonna cost you over $200. Thats the price I got from Advance Auto on one for an 87 (also TBI).

I ended up pulling one from an 87 saddle tank (truck) instead of buying a new one.
 
AC Delco pumps seem to have a good reputation. It comes with the sock and costs about $55.
Why do you think the sending unit is bad. I'd hate for you to spend that kind of money on a new one if that's really not the problem. If there is something wrong with it, it might be something simple, it's not that complicated of a device.
 
im thinking the sending unit is bad because the gas gauge reads all over the place, the tank can be almost empty and the gauge will read full, then a few minutes later it might read half or empty or were ever, the gauge is reading all over the place. I know the sending unit is expensive, ive checked all over and its about 200 dollars every were that ive looked. if there is something else I can check or a less costly fix then tell me what to check. is it possible that the gauge in the dash is bad ?.
 
If the gauge "bounces" as the fuel sloshes (when you hit the brakes, bumps, etc) then the gauge is bad. Personal experience there.

Erratic readings could be wiring or the sender.

The senders are "repairable", (more a cleaning) doesn't always work 100%, but in my case, it works much better, (more accurate readings) enough that I didn't use the new spare I have around.

Gas tank capacity with skidplate is easy. If there is more than a 1/2" gap between the bottom of the tank and the skidplate, it's the small tank. Almost no gap, 31 gallon tank.
 
I'd second the idea of checking the wiring. May be a bad ground or the sending unit wire could be shorting out and making intermittent contact with something like the frame.
If you take the old sending unit out and hook an ohmmeter up to it (ground wire and sending unit wire) it should read 90 ohms when the float is raised and 0 ohms when the float is lowered. Move it slowly throughout it's range to see if it fluctuates. A good cleaning and check over doesn't hurt either.
 
ok, the gauge doesn`t bounce around while driving, it will either read way past full or anywere in between, right now there is a little over a quarter of a tank of gas in the tank now and the gauge is reading way past full, I got under the back of my blazer just now to see what I could see, I found one plug comming off the top of the tank, I unplugged it and my engine quit, so that was the fuel pump plug. is there another plug that I didn`t see thats for the fuel gauge ?, I also removed the ground wire and cleaned it, no difference. I moved all the wires by the tank around to see if the gauge would read any different, if anything the fuel gauge is reading more past full than it was before ???.
 
Only one wire is for the fuel pump and one is for the gauge.

Sounds like a ground issue, but disconnect that plug, turn the key to run, and see where the gauge reads. Then ground the wire for the fuel gauge and make sure it pegs out. That will show the gauge is ok, and the problem is after that connector on the fuel tank side.
 
when I disconnected the plug and the gauge stayed peged way past full, plugged it back in and the gauge is still peged, were should I look now ?.

Thanks for all the help.
 
I'm not exactly sure how to do dyeager's method, he'll have to explain that. It is along the same idea as what the GM service manual says, but those instructions use a gage tester so that won't help you much.
What I would do is hook an ohmmeter up to the plug you disconnected. On the plug that's still connected to the tank connect one lead of the ohmmeter to ground and the other to the plug that has the purple wire (sending unit). The other wire (white) is for the fuel pump and has a reading of about 34 ohms. On the ohmmeter you should get a reading between 0 and 90 for the sending unit depending on how much fuel you have, and it should stay constant too. If it does then I'd say the sending unit is not to blame.
To check the fuel gage first I'd turn the key to the on position. With the plug unplugged from the fuel tank check the fuel gage, it should be pegged one way or the other. With your voltmeter connect one lead to ground and probe the plug (fuel gage side) with the other lead to figure out which side has 12V (for the fuel pump). Ground out the other terminal of the plug (NOT the 12V that you just tested :blush: ) and the fuel gage should be pegged in the other direction. If you want, you can get a couple resistors from Radio shack about $0.25 in the range of 10 ohm to 100 ohm and connect them from ground to the plug for the fuel gage to see what type of reading the gage shows and if it stays constant. If it does bounce around then you could have a bad ground on the instrument panel circuit board or a fualty fuel gage.
Disclaimer: I've never had a problem with my fuel gage so I don't know if this works, but in theory it should. :p:
 
IMO from messing with a bad sender, you REALLY have to pull it to figure out some problems. The rheostat (I think thats the right term) that the float arm uses to "send" the fuel level to the gauge gets dead spots on it. Some sort of black corrosion from what I saw.

As mentioned, 0-90 ohms should be the sweep from either end of the gauge, but the problem is you can't see if there are dead spots without moving the float arm. It only takes a tiny amount of fuel slosh for the float arm to move, which influences the gauge. The gauge is "damped" somehow, so it's response to fuel slosh should be slow, but if it goes from no reading to wherever the float arm makes contact again, you'd probably see some pretty big sweeps.

As long as the gauge changes aren't rapid (like I mentioned bounch) then I wouldn't suspect the gauge at this point.
 
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