Fuel Gauge Issue 04 Suburban

[TylerHagerdorn]

Alright I don't know if I've made a post about this or not. So I apologize if I did. More issues with the fuel gauge on my Burb and it's really making me angry, because I don't ever know how much fuel I have. And I've never had this trouble before in the other 3 GMT800s I've owned.

I have a brand new fully rebuilt cluster, cluster is not the issue. The fuel gauge will not sit still where it's supposed to be. Sitting at an idle it will go up and then down and then where it's supposed to be for a short time then start moving again. I'm absolutely lost.

The fuel level sensor wiring goes from the PCM to connector C152, then from there back to the fuel pump. I pulled the connectors off the PCM yesterday and found what looked like straight motor oil drenching the pins and sitting inside the PCM connector slots. I used a whole can of CRC QD electronic cleaner on it and got them pretty clean. And after that last night, got some fuel and the gauge didn't act up a single bit. Fast forward to this morning, took my boy to school and sitting at an idle it decided it wanted to act up again.

Now before this, it would just drop off to completely empty, with the DIC Range display showing three dashes. Then it would come back up and shoot past half to almost full. And would keep doing that back and forth non stop. So I suspect the oil substance was causing an issue with the connection. But it started acting up again this morning. Nowhere near as bad. The gauge just will not sit still where it's supposed to be.

So my question now is where do I go from here? Do I cut the wire closer to the PCM, and run an overlay and run it directly, bypassing C152 all together? Do I go to the junkyard and get two PCM connectors that have not been soaked in oil and splice them in? I truly believe it's a connection issue now. I really don't want to drop the tank if I can help it. Especially after putting 10 gallons in last night. And I don't have a shop, just my apartment parking lot.

Any help would be very much appreciated. Thank you guys and have a wonderful day.

 

[Blackcar]

My 02 Yukon did that when new and dealer replaced pickup in tank.
Had it happened again several years later, and I put a bottle of Chevron Techron Concentrate in and straighten it up so every year I run a bottle thru fuel system.

 

[TylerHagerdorn]

My 02 Yukon did that when new and dealer replaced pickup in tank.
Had it happened again several years later, and I put a bottle of Chevron Techron Concentrate in and straighten it up so every year I run a bottle thru fuel system.

I've seen people say that works...but it's not going to work if the contacts that ride against the resistor on the fuel level sender are broken. I don't believe it's a sending unit issue right now. I pulled the PCM connectors off again and they are covered in oil yet again. Opened the computer up and there's no oil inside. So it has to be on the connectors somehow. Some say the oil pressure sensor can cause oil wicking up the wires and get into the connectors. My oil pressure sensor is clean, and has been replaced so it's possible this has been going on for awhile. My best option right now is to go cut PCM connectors off at the junkyard and wire them into the truck. My alldata is down right now so I can't see where they go

 

[Blackcar]

Is there a way to use a shrink tube on connectors.

 

[TylerHagerdorn]

Is there a way to use a shrink tube on connectors.

I'm sure you could. I just use butt connectors on every wiring job I've ever done. Never had an issue. It's just going to be very time consuming. Heating wires is not necessarily good, because heat can and will affect wiring, and I've watched a lot of mechanics over the years use the "crimp and seal" connectors, and using a torch or lighter to get the heat shrink to seal I've seen the wires get black from the excessive heat. If others do it with success, more power to them. I just stick with what I've done for years. I also use scotch super 88 and Tessa tape when doing wiring repair. And I have some Honda rodent tape that I'll use too.

I picked up a used PCM and spotless connectors at the junkyard today. Picked the PCM up in case this one is bad, since they are the same part number. It's very strange to me that I cleaned the connectors and PCM last night and made sure every bit of the oil was out of them, and then this morning it was back again. The connectors have caked on grease and grime at the ends of them too...and the oil pressure sensor pigtail has been replaced before, as it has crimp and seal connectors on it...so that makes me wonder if it leaked out and down the harness at some point before.

 

[Fless]

I'd venture a guess that the oil pressure sender failed at some point and sent oil through the harness to the PCM. It's not as rare as people think.

 

[TylerHagerdorn]

We ended up putting a new Delphi fuel pump in it, since I still had it under warranty from last time. But the strange thing is, the fuel gauge is reading backwards. So it's showing dead empty is full.

Only thing I can think of is the PCM I got from the junkyard was from a VIN T, which is the gas truck. And mine is a VIN Z Flex...so maybe the wires are backwards on that PCM? I haven't swapped my original PCM back in. But that's all I can think it could be. We didn't change any wiring on the fuel pump.

 

[strutaeng]

We ended up putting a new Delphi fuel pump in it, since I still had it under warranty from last time. But the strange thing is, the fuel gauge is reading backwards. So it's showing dead empty is full.

Only thing I can think of is the PCM I got from the junkyard was from a VIN T, which is the gas truck. And mine is a VIN Z Flex...so maybe the wires are backwards on that PCM? I haven't swapped my original PCM back in. But that's all I can think it could be. We didn't change any wiring on the fuel pump.

I had never heard of the oil pressure sender wicking oil all the way to the PCM. That's a first for me...

I won't the the connections being backwards will cause the fuel gauge to read backwards. I think that circuit is based on resistance, so the sender acts like a potentiometer.

What year is your truck? I've got some wiring diagrams for my 04 project suburban, but I think this portion is all the same for this generation of trucks.

 

[TylerHagerdorn]

It does matter. The L59 PCM is different from the LM7...I just plugged my original computer back in, and it reads the right way.

However it's definitely bad. Because I know I have 3/4 tank in my truck, but it's only reading 1/2 tank. So my PCM is bad on the fuel level signal. The wiring is fine. Having the LM7 PCM, the fuel gauge read backwards, but it moved like it should. I have the original one back in, but I'm worried about it screwing the fuel level sensor up if that circuit is bad on the PCM like it seems. So another junkyard trip to hope and pray the L59 trucks they have still have the pcms in them. A used junkyard PCM will work without needing dealer flashed if you do the 30 minute security relearn. It'll show the vin of the previous truck on a scanner, but it'll run fine with it.

 

[TylerHagerdorn]

Well picked up a good known PCM from a flex truck matching rpo codes...still not reading over a half tank when there's between half and 3/4...so did my cluster get screwed up? I can't think of anything else it could be. My alldata subscription is expired, and I know there are resistance values for the different levels of the fuel level sender.