Testing Fuel Pump
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Instructor
Joined: Nov 2021
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I had a DIY test rig and I’m surmising it was not the most reliable. I made a new one today and tested at the pierce @50, back probed at 50, really pin 86 and the rear wheel well connector. I cranked the engine each time and the rest light lit at all 4 locations. Also had 12v at each.
The only thing I’m not getting is a 2s prime, but I am getting the power to pump on cranking. I thought that was your previous suggestion?
The only thing I’m not getting is a 2s prime, but I am getting the power to pump on cranking. I thought that was your previous suggestion?
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
I’m not sure if disconnecting and reconnecting the ECM connector had anything to to with it but if you’re certain that circuit is good between the ECM and the relay is good we can take that off the table…I don’t know the ECM’s internal circuitry so I can’t say why terminal 86 is energized cranking but no 2 second prime… maybe someone more knowledgeable than me may chime in !!
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Instructor
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I’m certain the ECM to relay leg of the circuit is good. After today I am. I did question it yesterday. I’m not ruling out an ECM issue. BUT, I’m getting power while cranking at all the right spots. I don’t like the idea of no prime, but I really don’t know how to test that without giving a known good pump a try. Getting power at th rear connector today is what sold me on it being the pump. Not saying can’t be both too.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
As long as the test light illuminates at relay terminal 86 and that control side has a good ground the relay load side (87 and 30) should latch and you should have power back to the pump…you can wire that 9005 bulb into the harness at the back and try cranking and see if the bulb lights…I think it’s a 4 or 5 amp bulb…if the bulb lights ok the new pump should work ok.
Melting Slicks
Joined: Nov 2009
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From: Klein TX
2023 C6 of the Year Finalist - Unmodified
2022 C6 of the Year Finalist - Unmodified
A little tip to help with diagnosing fuel delivery problems:
Try using your Tech2 to command the fuel pump relay on. It will stay on regardless of the motor turning. This makes it much easier to test the fuel pump, relay, trigger and power circuits without having to crank the starter or worry about a "2 second prime".
Looks like you're close to fixing it, good work
Try using your Tech2 to command the fuel pump relay on. It will stay on regardless of the motor turning. This makes it much easier to test the fuel pump, relay, trigger and power circuits without having to crank the starter or worry about a "2 second prime".
Looks like you're close to fixing it, good work
Last edited by wjnjr; Mar 24, 2023 at 07:38 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,628
Likes: 4,071
From: Cape Coral, Florida
A little tip to help with diagnosing fuel delivery problems:
Try using your Tech2 to command the fuel pump relay on. It will stay on regardless of the motor turning. This makes it much easier to test the fuel pump, relay, trigger and power circuits without having to crank the starter or worry about a "2 second prime".
Looks like you're close to fixing it, good work
Try using your Tech2 to command the fuel pump relay on. It will stay on regardless of the motor turning. This makes it much easier to test the fuel pump, relay, trigger and power circuits without having to crank the starter or worry about a "2 second prime".
Looks like you're close to fixing it, good work
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,628
Likes: 4,071
From: Cape Coral, Florida
You can “command the fuel pump relay on” as you mentioned by just jumping the load side of the relay…you don’t need a Tech 2 to do this !!…not all members can afford even a Chinese “clone” Tech 2 for a few hundred dollars or a used “real” Tech 2 for a few thousand !!
Last edited by C5 Diag; Mar 24, 2023 at 08:16 PM.
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Thread Starter
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Joined: Nov 2021
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I don't own a Tech2, but will soon. I had Nissan DataScan on my laptop for all my OBD1 Nissan cars, and it was invaluable. It will most definitely be a clone.
So update, Racetronix fuel pump arrived. I jumpered it to the rear wheel well fuell pump connection and the pump primed like normal. So 100% sure my fuel pump is beat. Interesting that I couldn't get the 2s prime without a pump on the circuit. Maybe PCM as a way to check resistance or amps on the return ground to tell if a the pump is connected and initiate a prime? Feels far too complex with not a whole lot of value, but maybe?
Either way, I'll be dropping the tank and replacing. I always make it a point for my threads to have closure and spread knowledge. I'll follow with any value added information after I'm done.
Quick question: How do you siphon gas out of the tank? I bought a little hand pump, but the hose gets hung up on what I believe is the fuel pump housing inside. I can't get it past the top of the tree and unfortunately had roughly 80% full tank when it failed.
So update, Racetronix fuel pump arrived. I jumpered it to the rear wheel well fuell pump connection and the pump primed like normal. So 100% sure my fuel pump is beat. Interesting that I couldn't get the 2s prime without a pump on the circuit. Maybe PCM as a way to check resistance or amps on the return ground to tell if a the pump is connected and initiate a prime? Feels far too complex with not a whole lot of value, but maybe?
Either way, I'll be dropping the tank and replacing. I always make it a point for my threads to have closure and spread knowledge. I'll follow with any value added information after I'm done.
Quick question: How do you siphon gas out of the tank? I bought a little hand pump, but the hose gets hung up on what I believe is the fuel pump housing inside. I can't get it past the top of the tree and unfortunately had roughly 80% full tank when it failed.
Melting Slicks
Joined: Nov 2009
Posts: 3,299
Likes: 1,154
From: Klein TX
2023 C6 of the Year Finalist - Unmodified
2022 C6 of the Year Finalist - Unmodified
Good job figuring out the problem, and thanks for posting the outcome. Too often we never know what happens in these kind of threads.
Thread Starter
Instructor
Joined: Nov 2021
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Me again. So, since this fuel pump/sender replacements, I've had the fuel pump connection in the fuse block overheat. I replaced the gray block and the connector and all was well for some time.
Yesterday, go out to start it, no fuel pressure again, verified with gauge. Pulled the fuse block apart thinking maybe the same fuse block overheating pin again and all looked well. I've replaced the relay. Also the fuse is good. At this point, while I'm at a loss, what am I missing as to why I keep having fueling issues? It just seems like this is recurring too often to feel that the system is trustworthy.
So far:
New Racetronix pump
New level senders
Fixed fuse block overheat
New relays
Good fuse
What else is there - is it really just wiring now? Reason for asking, is that it's at my work and I'm limited what I can troubleshoot here. Luckily it's a safe lot.
Thanks in advance.
Yesterday, go out to start it, no fuel pressure again, verified with gauge. Pulled the fuse block apart thinking maybe the same fuse block overheating pin again and all looked well. I've replaced the relay. Also the fuse is good. At this point, while I'm at a loss, what am I missing as to why I keep having fueling issues? It just seems like this is recurring too often to feel that the system is trustworthy.
So far:
New Racetronix pump
New level senders
Fixed fuse block overheat
New relays
Good fuse
What else is there - is it really just wiring now? Reason for asking, is that it's at my work and I'm limited what I can troubleshoot here. Luckily it's a safe lot.
Thanks in advance.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,628
Likes: 4,071
From: Cape Coral, Florida
Looks like you've been trying to diagnose this for over 3 years...maybe time to bring the car to an automotive electrical specialist...you need to use a wiring diagram and start from the beginning...having a strong electrical background helps.
Thread Starter
Instructor
Joined: Nov 2021
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Things are all good once replaced, and done correctly, but then issues resurface again. I have to believe there’s a reason that fuel pump hot wire kits exist, just seems nuts to have these issues on a stock system.
Going to test the pump tomorrow. It it doesn’t work, I’ll be dropping it off at place close to me that specializes in Vettes. 🤞🏻
Thread Starter
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This morning I pulled the DS rear splash guard to test the pump. Pump is all good. Now looking at the connector, the the power wire is charred and and melty.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
Pro
Joined: Oct 2024
Posts: 725
Likes: 399
From: WI
This morning I pulled the DS rear splash guard to test the pump. Pump is all good. Now looking at the connector, the the power wire is charred and and melty.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,628
Likes: 4,071
From: Cape Coral, Florida
This morning I pulled the DS rear splash guard to test the pump. Pump is all good. Now looking at the connector, the the power wire is charred and and melty.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
Again, very interesting here how the power system seems just undersized. I'm considering the hotwire kit from Racetronix, but just hate additional harnesses where unnecessary. I bent the connectors back to make contact to just get it home and all is well again.
@C5 Diag, the system works, just not reliably. Do you have additional diagrams you can share and maybe any other suggestions? I want to check all the grounds.
If the connector which I believe is X459 you are referring to is "charred and melty" it is NOT a wiring size issue which someone had posted but a pin fitment issue...poor pin fitment or "terminal tension" as it is technically called results from loose connections between the male and female terminals...a loose fit causes fretting and that causes heat and that heat causes the melting you are seeing...the C6 fuel pump draws a little over 8 amps and the wiring is MORE than adequate...if the connection is that bad just replace it with any 4 wire connector...there are no other wiring diagrams to be looked at...if you are familiar with voltage drop testing you can check how many volts that connector is dropping on either side of it...go to Mouser.com for connectors.
Last edited by C5 Diag; Apr 1, 2026 at 10:11 AM.
Thread Starter
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The wiring is barely adequate for a stock pump and your Racetronix pump no doubt pulls even more amps which would quickly accelerate any failure mode with the wiring. The pump hot wire harness is a must IMO, even for stock applications. There is no reason not to install it especially given your issues. If you found the power wire charred and melty, resolve that issue before you go chasing grounds.
That harness also calls for the companion harness from the pump bulkhead to chassis connector, x459 displayed by C5Diag. Is that a must? I know the answer to this but just really hate dropping the tank, yet again. Hoping you could weigh in on that. I posed the question the Racetronix and was referred to their forum which is not a very productive site.
Burning Brakes
Joined: Sep 2010
Posts: 935
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OP, my stock fuel pump started leaking on my 2009 coupe in October of 2023 so I hired a race car maintenance shop at the Utah Motorsports Campus to repair it along with other upgrades.
He installed the same fuel pump as the one you installed, a Racetronix TU475-450 https://www.racetronix.biz/k/fuel-pu...-e85/tu475-450 .
Along with the pump, the following wiring harnesses were installed: FPWH-008 https://www.racetronix.biz/p/c5-c6-c...-hd/fpwh-008hd ,
IPH-008 url]https://www.racetronix.biz/p/c5-6-corvette-intermediate-pump-harness/iph-008[/url] and RTX-PBIC-001 Connector Set https://www.racetronix.biz/p/pump-bo...r-set/pbic-001 .
Take time to read the links; the information leads to the conclusion that the stock wiring is undersized for the Racetronix pump.
See this link for information on the kit that includes the Racetronix pump and wiring harnesses. The information addresses the stock pump wiring if used with the upgraded Racetronix pump. https://www.racetronix.biz/k/c5-c6-2.../rfpk-475-450e
He installed the same fuel pump as the one you installed, a Racetronix TU475-450 https://www.racetronix.biz/k/fuel-pu...-e85/tu475-450 .
Along with the pump, the following wiring harnesses were installed: FPWH-008 https://www.racetronix.biz/p/c5-c6-c...-hd/fpwh-008hd ,
IPH-008 url]https://www.racetronix.biz/p/c5-6-corvette-intermediate-pump-harness/iph-008[/url] and RTX-PBIC-001 Connector Set https://www.racetronix.biz/p/pump-bo...r-set/pbic-001 .
Take time to read the links; the information leads to the conclusion that the stock wiring is undersized for the Racetronix pump.
See this link for information on the kit that includes the Racetronix pump and wiring harnesses. The information addresses the stock pump wiring if used with the upgraded Racetronix pump. https://www.racetronix.biz/k/c5-c6-2.../rfpk-475-450e
Last edited by MissileDoc; Apr 4, 2026 at 04:22 PM.
Pro
Joined: Oct 2024
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From: WI
That’s what I’m thinking. It’s a Tu-475 which is just a stock replacement capacity pump, but they list as a turbine style, not sure if that’s drawing more amps than stock, but I will check with a multimeter.
That harness also calls for the companion harness from the pump bulkhead to chassis connector, x459 displayed by C5Diag. Is that a must? I know the answer to this but just really hate dropping the tank, yet again. Hoping you could weigh in on that. I posed the question the Racetronix and was referred to their forum which is not a very productive site.
That harness also calls for the companion harness from the pump bulkhead to chassis connector, x459 displayed by C5Diag. Is that a must? I know the answer to this but just really hate dropping the tank, yet again. Hoping you could weigh in on that. I posed the question the Racetronix and was referred to their forum which is not a very productive site.
Thread Starter
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@DarkC-6 and @MissileDoc,
I see how this harness ties in now. It runs power from the the alternator (I think) and uses the stock harness as the initiating signal for the new relay it introduces. Stock wiring gets completely unloaded and repurposed as the relay signal wiring. Pretty slick actually. Just need to make it tidy and locate a ground. $80 plus shipping (+$30 for the pump whip if I really wanted to punish myself to install it) and some light work to make it bullet proof feels like a no brainer.
I very much trust C5Diag, but I'm really curious to see if anyone has tested the amps on these pumps. I will before I install this harness. New x459 connector arrives today which I'll need to replace anyway since it was the pump side that was damaged.
I see how this harness ties in now. It runs power from the the alternator (I think) and uses the stock harness as the initiating signal for the new relay it introduces. Stock wiring gets completely unloaded and repurposed as the relay signal wiring. Pretty slick actually. Just need to make it tidy and locate a ground. $80 plus shipping (+$30 for the pump whip if I really wanted to punish myself to install it) and some light work to make it bullet proof feels like a no brainer.
I very much trust C5Diag, but I'm really curious to see if anyone has tested the amps on these pumps. I will before I install this harness. New x459 connector arrives today which I'll need to replace anyway since it was the pump side that was damaged.