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It looks like it's time for a new fuel pump - '97 STS


KHE

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The starter is fine - it spins the engine at the proper speed but the engine takes longer to start and come up to the fast idle speed. Other than that, it drives fine. On very rare ocassions, it will stall.

I connected a fuel pressure gage to the fuel rail and with the key on, engine off, the fuel pressure was 27 psi. Definitely lower than the 42-48 psi spec. in the ship manual. Once the engine started, it came up to 42 psi but dropped off when the engine was revved. The pressure should not drop off when it is revved up - it should increase slightly since the drop in vacuum would keep the regulator from opening.

I turned off the engine, clamped the rubber portion of the fuel return line to prevent any fuel from being returned to the tank - that will result in the maximum pressure for the pump. Key on, engine off: 27 psi fuel pressure. I'm thinking that the fuel pump is weak.

I need to verify the pump connections to make sure there is a full 12 volts there but it sure looks like I need to replace the fuel pump. It looks like the whole sending unit needs to be replaced.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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You might check the fuel filter before you drop the tank. It's just possible that water in the gas has the filter partly clogged.

If you turn the key off and on again a few times and the fuel pressure comes up to 42 psi, it could mean that the filter is all you need.

But I don't like the occasional stall. That's a ringer for a bad fuel pump.

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-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I have never had a fuel filter in a multiport fuel injected car plug enough to cause issues. The fuel pressure does not reach 42 psi until the engine starts.

I removed the fuel return line and stopped off the line. With the engine running, I was able to get the fuel pressure up to 50 psi but that is still below the specified value of 48-55 psi.

I need to check to make sure the pump is getting full 12V at the pump. I would hate to spend $200 on a new pump module assembly only to have the same issue. I'm also going to run the test on my '05 Deville to see what its key on engine off pressure is for comparison.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I checked my '05 Deville (that has no issues whatsoever) just for comparison and it left me with more questions than answers.... Key on, engine off: 40 psi. Engine running: 36psi. I need to do more investigation before I replace the fuel pump on the STS.

Maybe the calibration of the gage is off?

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Fuel pressure readings when the engine is on can be misleading. I'm thinking that your problem is elsewhere. I would try a can of Techroline and see of the problem goes away.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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You might check the fuel filter before you drop the tank. It's just possible that water in the gas has the filter partly clogged.

If you turn the key off and on again a few times and the fuel pressure comes up to 42 psi, it could mean that the filter is all you need.

But I don't like the occasional stall. That's a ringer for a bad fuel pump.

If several key on-offs help to start, it could be also a bad fuel pump check valve.

The saddest thing in life is wasted talent

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I checked my '05 Deville (that has no issues whatsoever) just for comparison and it left me with more questions than answers.... Key on, engine off: 40 psi. Engine running: 36psi. I need to do more investigation before I replace the fuel pump on the STS.

Maybe the calibration of the gage is off?

KHE,

I have extra made in USA sealed fuel pressure gauge. I will gladly ship it to you for a back-up. Free, of course. :) Let me know.

The saddest thing in life is wasted talent

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I checked my '05 Deville (that has no issues whatsoever) just for comparison and it left me with more questions than answers.... Key on, engine off: 40 psi. Engine running: 36psi. I need to do more investigation before I replace the fuel pump on the STS.

Maybe the calibration of the gage is off?

KHE,

I have extra made in USA sealed fuel pressure gauge. I will gladly ship it to you for a back-up. Free, of course. :) Let me know.

Thanks for the offer. I really haven't had time to troubleshoot the problem but hope to take a look at it in the next couple of days.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I went through the troublshooting steps in the manual and the test results point to the fuel pump but..... I checked the voltage to the pump and it was only 10.5 volts so I need to find out why the pump voltage was so low.

There are a few connection points between the fuel pump relay and the pump connector - I'll disconnect each one and test the voltage and work toward the relay until I find the one that shows battery voltage. It could even be the relay contacts that have high resistance.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Check the voltage at the relay, if you can get to it without pulling the relay. The fuel pump relay is the most likely point of failure other than the fuel pump connector or fuel pump itself.

Better yet, just change the relay. It's a under-$10 part (on RockAuto, $14.03 on Amazon), and one of the two (with the A/C relay) that is put out front for switch-and-replace on the underhood fuse and relay center, which tells me that these are things that were planned to be replaced occasionally. When mine went out, I got a sack of them to save on shipping; I still have two - but I find that my 2011 CTS-V uses the same part number.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I used four jumper wires to connect the relay to the panel so that the terminals would be exposed so I could measure the voltage while the pump was running. I have several Omron relays and a replacement had the same exact voltage drop (20mV) across the contacts. I am now going to jump the fuel pump so that it stays on while I monitor the voltage at the pump connector. I'm thinking the voltage doesn't get to the battery voltage because the pump only runs for 2 seconds or less. I am also going to measure the pump current at the relay to see what it is.

I want to be absolutely sure the fuel pump is bad before I purchase a new one and go through the work to drop the tank.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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If you have a 20 mV drop across the fuel pump relay, that's not the problem. What was the voltage there? The idea is to find out where the voltage drop is.

The schematic is in the FSM page 6-254. Power from the ignition switch goes through the 20 Amp FUEL PUMP fuse to a PNK wire to the relay. A WHT wire goes to splice S213 and a GRY wire goes from the splice to the fuel pump. There are two splices along the way, S185 and S481 that don't have any other wires on them in this diagram. The BLK wire goes to G401. There are two splices on it, S420 and S402.

G401 is under the rear seat on the right hand side. The wire to G401 breaks out of the wiring harness bundle that goes across the car. From the diagram it's hard to tell whether that is inside the car or under it, but G401 looks like it's bolted to the brace for the back seat, so I think it's under the car.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I checked the voltage drop at the pump on the negative side and it was 80mV. I then checked the voltage drop on the positive side by running a lead straight from the battery and the drop was 1.3V. That indicated a problem. I then jumped the pump to full battery voltage and the fuel pressure still did not meet spec. It's looking like the pump needs to be replaced.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I gathered some additional data this evening. There is a .31 V drop on the one side of the fuel pump relay contacts and a .34V drop on the other side of the contacts so I know the drop is between the relay and the pump connector. The problem is the car is a 1997 and the shop manual is a 1996 and the fuel pump schematic in the engine controls section shows a C101 and a C415 but it does not state the location of those connections. Jim, can you post the wiring diagram from the 1997 car if it shows the locations of the two connectors?

I then checked the pump current draw and it was 2.5 amps. This seemed low to me. I then checked the pump current draw on my '05 Deville and it was 6 amps. I'm thinking the pump has some high resistance or the wiring in the pump module has some high resistance. This data seems to point at the fuel pump module.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I suspect this has already been checked, but the 92-97 gm's are known to have wiring connecter problems and the new fuel pumps come with a new pig tail from the pump to the wiring harness connecter. I reread these posts and cant be sure the connecter was checked, though the resistance was tested.

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The connector looked fine - no discoloration or arcing/melting. I suspect it is the wiring that is between the pump connector and inside the tank.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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The wiring does burn up inside the tank. Look at rockauto and ebay for a pump as I picked one up for my 02' Deville for 60$ complete with the updated harness on ebay.

97' Seville. leave the exhaust in place remove the left hanger for the tank support the tank and remove the 13mm bolt for the right tank and and massage the strap towards the front of car and it will slide right out. Don't forget the supply hoses on left side of tank and the evap line on the top.Piece of cake 1/2 hr job.

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The wiring does burn up inside the tank. Look at rockauto and ebay for a pump as I picked one up for my 02' Deville for 60$ complete with the updated harness on ebay.

97' Seville. leave the exhaust in place remove the left hanger for the tank support the tank and remove the 13mm bolt for the right tank and and massage the strap towards the front of car and it will slide right out. Don't forget the supply hoses on left side of tank and the evap line on the top.Piece of cake 1/2 hr job.

Thanks for the tips. Was the pump you bought on ebay an OEM pump? Rockauto has a Delphi pump for $188.00 and Delphi is the OEM supplier. For $20.00 more, I can get the same pump in an AC Delco box.....LOL.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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It's not just a GM problem. Anything with a electric pump can suffer over heated connectors. The in tank harness is a common area....common enough many pumps now also come with a new harness....mostly because of the labor involved to fix the problem a second time....and fuel pump returns.

Most over heated connectors will not be as obvious as this picture..

post-2-0-49991200-1412172641_thumb.jpg

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My 96 had green corrosion on the crimp connection on the wire pigtail which plugged into the elec motor. It was only visible when fuel pump assy was removed from tank. I tested the pump and it worked fine. Than I threw it away. After 12 yrs, time for a new pump. That 6-8yrs ago? Hard to believe 18 yr old elec fuel pumps still work. But they do.

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It is not always obvious....green and over heated pump wires.

I found a better picture....it can be almost be invisible with the wire mounted inside the plastic connector.

post-2-0-94706300-1412196884_thumb.jpg

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It is not always obvious....green and over heated pump wires.

I found a better picture....can almost be invisible with the wire mounted inside the plastic connector.

Are these the wires that are in the tank? The 4-wire connector outside the tank looked fine - no burning or arcing.

There was a 1.3V drop at the positive terminal of the pump connector. The ground side had only .08 V drop. Should I be concerned with the voltage drop on the positive side of the pump? I would hate for a new pump to fail due to this.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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It can be any of the wires....inside or outside of the tank. It can be at the top-of-the-tank connection or the harness inside the tank between the connection and pump.

Nothing worse then dropping the tank....install a new pump. Only to have to drop the tank again to replace the harness.

It can be at any of the yellow arrows in the picture....it can also be in the (body side) plug that connects at the top of the tank.

post-2-0-11269600-1412216767_thumb.jpg

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