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98 Silverado 5.7 Fuel Pump?

12-06-2008, 11:10 AM
I have a 98 Silverado 1500 with a 5.7L. It has 92K miles on it. The other night I stopped for a 6 pack of beer :popcorn:and when I went to start the truck up it cranked nicely over but would not start:crying:. After the tow back home ($45..not too bad) and a couple of the cold ones, I noticed that ever present noisy fuel pump sound was gone. I am suspecting the fuel pump or fuel pump relay. I haven't found a fuse yet. After going through posts on this issue I have gathered the following info:

(1) Use a GM fuel pump (or Delphi at Carquest). Fuel pumps are expensive.
(2) Removing the tank or taking the bed off is a matter of preference. I am leaning towards taking the bed off. Maybe I will do some under coating before putting it back on. Since I live in San Diego under coating is probably a waste of time and money:2cents:.

OK, I am heading to do some more trouble shooting. Appreciate any advice folks may have. I'll post updates as I proceed.

12-06-2008, 02:21 PM
Update 1:

(1) No scan codes using scan tool.
(2) I could not find a fuel pump fuse, but found a fuel pump relay. No other relays had the same P/N to swap out with. If they are cheap I will buy one for troubleshooting. The relay pin numbers did not match with the wiring diagram in the 88-98 Haynes manual I have.
(3) Next step is to go to the library and see if I can get a better wiring diagram. Does anyone have a wiring diagram for a 98 Silverado 1500 5.7L? I just need the wiring for the fuel pump circuitry.
(4) No fuses appear to be blown.

12-06-2008, 05:19 PM
In the Underhood Fuse Block check the "ECM B" fuse. This fuse is "hot at all times" and supplies the power to the fuel pump when the relay is energized. The signal to pull the relay in comes from the VCM.

If you have a fax number send me a "private message" and I will send you a wiring diagram from my '98 C/K service manual.

12-06-2008, 06:09 PM
Private message on the way. I sure appreciate the help. I did manage to get a better wring diagram out of a 1998 Motors manual at the San Diego public library. My latest troubleshooting is:

(1) Pins 85 and 87 are hot pins going into the fuel pump relay.
(2) I pulled the fuel pump relay, connected 12v to pin 85 and ground to pin 86. I could hear the relay energize and I verified with a continuity check that pin 30 and 87 were now connected. I am pretty sure the fuel pump relay is good.
(3) My next thoughts were to check the oil pressure switch. My Haynes manual says it is above the oil filter. I could not find it. The only sending unit I could fine was on the drivers side between the first and second spark plug. I am guessing that is the temperature sending unit fro the coolant. Anyone know where the oil sending unit is on a 98 Silverado 5.7L?
(4) I am thinking I will pull the bed of the pickup off, and see if the 12V is really getting to the fuel pump. At least to the wire going into the tank. Not sure how difficult it is to get the bed off, but on other trucks, I haven't found it too difficult:confused:.

Again, I can't thank people enough for the help. These forums are a life saver for us DIYers.

12-06-2008, 07:16 PM
1st off I would not be concerned with the oil press sw right now. It's just a backup for the VCM signal to pull the pump relay in.

Pull the relay and jumper relay pins 87 & 30 as mached up with the relay removed.

Also check for 12v on pin 87 and a good ground on pin 30.

The pump should run when you do this.

Post back

12-06-2008, 08:25 PM
I have the 12v on 87. I do not show a good ground on 30. Digital MM jumps around, but it looks like an open. When I jumpered 87 to 30, nothing happened and the 12v was present on 30 then. I suspect the fuel pump is bad (open winding) or I have another open somewhere along the way to the fuel pump (e.g. wire, connector). I do have a good ground on 86 and 12v on 85 for the relay to energize.

12-07-2008, 06:29 AM
I would not discount the oil pressure switch. To make sure that the switch is not the problem, locate the switch (drivers side next to the distributor) and jumper the orange and grey sockets in the harness plug. The switch is in series with the relay and energizes the relay when the engine creates enough oil pressure when cranking. Trust me, replacing a $30 switch is lots less pain and trouble than a $500 fuel pump.

12-07-2008, 12:42 PM
Update 3:

(1) I jumpered the orange an grey wire on the wiring harness for the oil pressure switch. The oil pressure gauge indicated max pressure, so I am sure the jumper was working correctly. Still no start or any sound the fuel pump is pumping.
(2) The distibuter cap and rotor looked all corroded again. I'll replace them for good measure. I did check spark off the#1 plug. Looked weak, but was present. I do not think my issue is with spark. Engine cranks but doesn't even attempt to kick over. Plus I cannot hear the fuel pump at all.
(3) I was wondering if the fuel pump mayh shut off when it reaches sufficnient pressure. I opened the connections to the fuel filter and let some fuel out. There did not seem to be any pressure in the fuel lines and the fuel pump did not kick on when I turned the key on. I still believe this is a fuel pump problem. I''l also replace the fuel filter as good measure.
(4) Next step is to either drop the tank or remove the bed. Not sure which route I will go. I am leaniong towards removing the bed.

12-07-2008, 04:18 PM
Update 4:

(1) I unbolted the bed (8 bolts) and disconnected the wires for the lights and the ground wires for the gas filler neck. I picked up the bed and set it back about 18 inches. The bed is sitting on the rear tires and a couple of 2x4's. There is about 10v at the fuel pump hot wire when I crank the engine over. I am fairly certain the fuel pump is bad.
(2) In the previous post, folks have advised against after market fuel pumps. I see that Chevy used Delphi and AC Delco fuel pumps. I did not have much luck with a Delphi alternator in the past, so I am leaning towards paying the extrra and getting an AC Delco fuel pump. Rock Auto shows the AC Delco for $325.79 and the Delphi for $291.79. Napa Online has the Delphi for $359. I will check the dealer tomorrow and see what they want. Anyone know of any other sources for AC Delco or Delphi fuel pumps?
(3) I will do some cleaning up around the fuel tank and pull the pump out. Maybe I can see if the original was AC Delco or Delphi. If anyone has any feedback on AC Delco or Delphi fuel pumps, I would be interested in your experience with these products or other fuel pump brands.

12-07-2008, 05:49 PM
I'm sure San Diego has some AC Delco parts stores. They are generally much cheaper than the dealer.

One thing: Make sure the pump you get matches up with your VIN!!

10 volts is a bit low at the pump unless that's all the juice your battery has. May want to look into that.

In answer to one of your other questions. No, the pump will not turn off due to pressure. It can develop 75-80 psi with the return line blocked off and keep running.

Here's another link to try. If you go with them, once again, supply the VIN to get the correct pump.

12-07-2008, 08:57 PM
Thanks for the info. I will verify the VIN when I order the pump. Appreciate the link also. I think the 10V is because the engine is cranking when I measure the voltage going into the pump. If I jumper the oil pressure switch, the voltage is around 12v going into the pump (without cranking it over). My friend works for a local auto parts store, and says the have Delphi pumps. I am thinking around $250 is the best deal I can get searching around on the internet. Do you think there is any difference between a Delphi or AC Delco fuel pump?

12-08-2008, 08:55 AM
I woke up a thinking about the voltage being 10V going into the pump when I was cranking the engine over. A Google search found this link which says 9.6V or better is good. I also checked my school notes from an Automotive Electrical Systems class I took at Cuyamaca college in 1998. It also stated 9.6V minimal cranking voltage. I feel better about the 10V reading now, however it is hard to believe it has been 10 years since I took those automotive classes. They were a lot of fun.

12-09-2008, 10:13 AM
Anyone else experiencing fuel pump issues in 1988 to 2005 GM trucks....

It is very common for the connection between the harnass and the fuel pump to fail. (especially in snowy climates where they salt the roads). With all the aftermarket pumps I've installed they come with an updated harnass end (a 4 way flat) instead of the old 4 way square plug.

12-09-2008, 01:27 PM
Thanks for the experienced based advice. I will double check the connection. The connector is the original square shape 4 pin connector. IOt looked to be in great shape. Since I live in San Diego and the truck hasn't rolled over a 100K miles yet, there was just a little dust above the tank. I used a vacuum cleaner and rag to clean it. I will double check that plug just in case. I would hate to spend the money on a new fuel pump assembly just to find out it was a bad connection.

12-11-2008, 01:53 PM
Put in the new Delphi fuel pump assembly and the truck started up fine, however it is running a little rough. My first thought was the corroded distributor cap and rotor. I hooked up the scan tool and got a P0452 code. Books says this an evaporative system code. The tank is only a quarter full. I am wondering if it takes a little time for the tank to build up pressure or if I need to double check the o-ring on the fuel pump assembly to make sure it seated correctly. Any thoughts?

12-11-2008, 03:12 PM
This indicates the vacuum pressure sensor on the tank is bad or disconnected or some leak at that area.

12-11-2008, 05:46 PM
Cleared the codes and tried it again. This time I get a P0300 (random misfire) code. I replaced the distributor cap and rotor. Runs fine now and no service engine light. I tried to see if there were any fault codes, but the scan tool says all the diagnostic checks had not been completed. Not sure if I need to run the truck for a while of if there is a problem with the scan tool. I did turn the scan tool on while the truck was running. The book said not to do that. Hopefully that did not cause the scan tool to break. I'll try it again in a day or so. I can also double check it with another scan tool.

12-12-2008, 12:25 AM
Drove the truck 30 miles tonight and everything seems to run fine and not service engine light. The scan tool still doens't run a complete check yet. Not sure what the issue is there. I'll do a scan with another scan tool to be sure.

12-14-2008, 11:43 AM
Final thoughts:

(1) A complete scan was accomplished at around miles. Not sure if this means that it takes 100 miles for the computer to run a complete scan after clearing out all codes. The last check that needed to be accomplished was the EV (Evaporative System) check.
(2) I am really glad I chose to remove the bed instead of dropping the tank. Releasing the the fuel lines with the fuel line disconnect tool was a little bit of a pain due to the way the plastic fuel lines are molded into the fuel pump assembly. Having easy access from the top made it nice. Putting the bed back on took about 20 minutes with pneumatic tools. All 8 bolts were real easy to get to.
(3) Thanks for all the advice! These forums are great. I hope my documenting the process will help others who have similar issues with their truck.

12-14-2008, 12:50 PM
The evap test is not done by the pcm if there is over 3/4 tank or under 1/4. When this is done you will see if an evap code pops up.

12-14-2008, 05:55 PM
Thanks for the info! I would have never guessed that. It certainly makes sense though.

12-14-2008, 06:56 PM
As you noticed on your scan tool, there are several monitors that need to run and complete their tests. Each monitor requires a unique set of driving circumstances called a "driving cycle" to run and complete its test. When the driver exactly duplicates the circumstances necessary, each test runs and completes its test. This link explains what must be done to run and complete all of the monitors. With practice, you can complete a drive cycle in about 20 minutes:

Just an FYI on the fuel pump circuit that runs through the oil pressure switch: Current travels along the perimeter of a wire, not down the center. The larger the diameter, (or gauge) of the wire, the more surface area there is for the current to travel on resulting in less resistance and less heat generated. There are two reasons the circuit runs through the oil pressure switch: One is to allow more surface area for the current to travel on to the fuel pump. Secondly, is to allow the fuel pump to prime the fuel system for the next start. When the engine is shut down, residual oil pressure keeps the fuel pump running for a few seconds. When residual oil pressure drops below approximately 7psi, the contacts in the oil pressure switch open and fuel pump shuts off. That's why the fuel pump runs for a couple of seconds after you shut the engine down.

12-19-2008, 03:54 AM
Another reason to run the fuel pump through the oil pressure switch is to kill the pump in the event of an accident. When the engine dies, so does the pump and will not create a pool of gas if a fuel line is severed.

12-19-2008, 03:02 PM
Another reason to run the fuel pump through the oil pressure switch is to kill the pump in the event of an accident. When the engine dies, so does the pump and will not create a pool of gas if a fuel line is severed.

There are no safety systems for turning off the fuel pump, (inertia switch etc). When the ECM stops receiving a signal from the crankshaft position sensor, it waits 2 seconds and then shuts off the fuel pump relay. The ECM will immediately shut off the fuel pump relay when the ignition is turned off. In both cases though, the fuel pump remains energized through the oil pressure switch until the oil pressure drops below 7psi. The circuit was not designed to be any kind of safety system. Also keep in mind that not all vehicles use the oil pressure switch to power the fuel pump. On the vehicles that do use it, once the engine is running, and oil pressure is above 7psi, the oil pressure switch is the last thing that turns off the fuel pump.

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