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88 5.7 l oil pressure gauge pegged at max


hunter4ever12
12-12-2015, 09:19 PM
So I have an 88 k1500 with the 350 that the previous owner removed and rebuilt... Didn't hook a lot of stuff back up etc... It's like a puzzle. It runs good for the most part but the oil pressure gauge in the cab pegs out when it's running. I've heard 1989 and older 350 engines have 2 oil pressure switches/sending units. I know about the one up by the distributor. But can't seem to locate the other one. Could someone help me. Also what does the one by the distributor control? And what does the other one control? If anyone had some pictures of where the other one is located that would help me immensely. Thanks in advance.

Schurkey
12-13-2015, 02:51 PM
I have an '88 K1500 with 5.7L. Pressure switch by distributor is the "backup" to the fuel pump relay. If the fuel pump relay fails, the pressure switch supplies power to the fuel pump as long as the engine has oil pressure. Symptom would be the truck won't start without longer-than-normal cranking. The engine has to crank long enough to build oil pressure, then continue cranking while the fuel pump pressurizes the fuel system.

The sending unit for the dash gauge is above the oil filter. It's a fist-size "can" that attaches to the engine block with an angled pipe a couple of inches long. This sending unit has a LONG HISTORY OF COMMON FAILURE. The thing will fill up with oil--it may slosh when you remove it--and becomes wildly inaccurate. I've had them read lower-than-true, and I've had them read higher-than-true. I've never had one that was pegged at high pressure. Wouldn't surprise me if you were to find a wiring problem on your vehicle.

Be careful when replacing the large sending unit; some are so enormous that they won't fit the engine properly. I had to return sending units from one supplier and then buy a competing product from a different supplier that I could actually install. The two sizes of sending units are seen in the photo below. I had to return the NAPA item for one purchased from--I think--Carquest. I don't have a photo of the Carquest sender or the part number, though. The tapered black piece is a shield that slides over the sending unit. It's secured with built-in spring clips that hook over the ridge at the bottom of the sending unit.

http://hbassociates.us/K1500_Oil_Sender_01.jpg

I tried to attach the larger sending unit to the engine with a 90-degree elbow and generic brass pipe, but that didn't work. The 80-degree angled adapter is the original GM part, and that is what I had to re-use along with the slightly-smaller sending unit. A real pain in the ass.
http://hbassociates.us/K1500_Oil_Sender_02.jpg

Victor62
07-17-2016, 12:03 AM
Looking for other 1500 owners with similar probs and found one here, mine does the same thing, it pegs out when I start it, and its erratic, but mostly stays pegged, I hope its the sender, my gas gauge is off too it runs out around a 1/4 of a tank, found that out the hard way ran out of gas, but alas I knew there were gonna be problems when I got it, it is a 93 k/1500 Cheyenne that originally had a 6 cylinder in it, the guy I bought it from, his dad put a V8 in it, a 350, I don't know what year the motor is, he put aluminum eldebrock intake on it and eldebrock 4 barrel carb, the motor is tight runs great its just all the other stuff that changing the motor implies, like now I have a mechanical fuel pump, which I like cause I have done enough pumps in the tank, there are just a lot of little things that aren't right, the AC is another thing, it has it but it ain't working they never hooked it up to the 350, anyway I'm goin on and on here, will try the oil sender fix, Cheers to all the Chevy truck guys, I am gonna go back to reading posts and see what else I can learn.

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