Fuel Pump only getting 7v after new relay and jumper test - 1998 - xc70

Texasbrainstew · Post by **Texasbrainstew** » 04 May 2026, 08:19

Hey all!

Was trying to revive a 1998 xc70 that'd been sitting for a while.

I'll try to keep it nice and simple:

-No pressure at the fuel rail: so we replaced the fuel pump. (No cutting the floor)
-Powered the wagon on with the battery and with the key in second position, pump is not activating.
- Replaced the relay with a jumper and then tested, as well as with a new relay. Nothing changed.
- I'm getting 7 volts at my fuel pump plug (with the black and red wires going to the pump supplying power) with the key in position 2. Drops to zero when off
-When I manually power the pump it pumps fuel as needed without leaks
-Full gauge stuck on full even after emptying some gas out of the tank (at least 3-4 gallons)
-Checked the 20amp fuse in the "main fuse" box under the hood labelled fuel pump and ignition coil or something similar(cranked to see if we have spark and we do.)

So I'm not fully sure where to start, everything else seems fine but I'm curious if I have to replace the unit on the other side of the saddle tank (drivers side). The 7volts doesn't seem right to me at the plug, and I hadn't brought the tank back up so no wires have been pinched.

Can anyone tell me how many volts I should be expecting at:
Plug at fuel pump w/red and black wires
Between the relay jumper points
And at that 20amp fuse

Power is technically making it to the pump when the key is in position two but it isn't enough to get the pump running

Post by **abscate** » 04 May 2026, 09:06

The pump voltage should be at battery voltage minus a bit of loss (say battery 12.5 Volts, Pump voltage 12.3 volts) on Key on. It only runs for a second or two to build pressure.

You can test the supply by jumping the fuel pump "relay" in the dash fuel box between terminals 15 and 87

7 volts is probably leakage through the fuel pump relay (which is more than just a relay)

Post by **abscate** » 07 May 2026, 02:58

Welcome to the psycho p80 XC resurrection club ! The 1998-2000 XC models are special and require. BIT OF INSANITY to undertake service

Scot850 is President ( his sonic blue v70R is a legend here) , I am staff psychologist with three personalities on call…the sick, the dying, and the dead

Texasbrainstew · Post by **Texasbrainstew** » 06 Jul 2026, 16:07

Abscate- I cannot stress enough how many times I said I would never...... AND HERE I AM. HAHAHA somehow have TWO!

SO GUESS WHAT. The damn pump came with the wrong plugs on it! I was attempting to connect the power to the sender wires and vise versa!! HIJOLEEE. Only found it was wonky because I took photos of the plugs before taking things off. Made my own connections thanks to Matthews fuel pump guide on the xcs!

SO since then I had drove the car home and was able to boost and everything. (Was hardwired with a switch originally to get it home before discovering the wrong plugs). Got the pump wired correctly after getting home.

Then it slowly started to happen. Alllllll the craziness. If I need to make another post to follow the rules I will.

****I'm somehow pressurizing air or boiling fuel in the fuel rail. ****

Symptoms:

Car now looses power and cuts/stalls.

Schrader valve shows air or boiled fuel in the rail.

Diagnosed/replaced:

Removed the new pump from the housing, tested with rubbing alcohol a rubber hose and a jar to ensure it didn't run dry and watched the flow with a m12 Milwaukee battery. It pushes constantly with good voltage and the other housing seals look good

Used the schrader valve to check pressure after it cuts:
Has PRESSURIZED air with spurts of fuel in-between WHEN HOT. Otherwise has pretty decent fuel pressure and had a hard spurt when running when first started and idled for a while before driving. When primed with the key, had a pretty damn good pressure compared to what I started with in the very beginning.

In those moments of checking the schrader valve when it cuts, you can audibly hear some sort of backward suction. I don't believe it's from bleeding or opening the valve itself. I've had plenty of p80s and have done so much work requiring removing the rail and have never observed or heard this behavior before. It sounds like it's taking gulps of fluid and allowing air to pass up to where I'm at.

I have tested flow: Out of the pump, out of the filter, out of the rail (where the 13mil connector is).

I've used a smoker with a pressure gauge and pressure tested the line from the rail to the pump. It held around 14psi solid with no issues.
***I just now remembered I have not tested the return line for pressure or leaks.*** Also don't know if 14psi is adequate for the fuel pump check valve.

When trying to figure all this stuff out originally; I may have run the pump backwards for no more than 3-5 seconds at a time and always with liquid involved. No dry running. Not sure if that messed with the check valve internally or not. Also being ignorant to my sensitive fuel system there may have been a time or two where I cut the fuel pump and let the car die since I had it hardwired to a switch.
It's embarrassing but I did do it and have to mention it and I will likely be ordering a new pump again just to rule that out.

I have consistent voltage when the car begins cutting out. I had a volt meter plugged in and had the cut floor board exposed while it was choking out. Solid 12 volts.

Replaced:
The fuel pump with main seal under the cap, sender assembly (drivers side) with main seal, injectors, fuel rail with new dampener, fuel pressure regulator - (Bosch brand new) New fuel filter, vacuum hose from the intake manifold to the FPR. Few of the turbo vacuum lines.

Think the last detail is: the fuel rail is not hot enough even with the engine being hot to vaporize fuel.... It was at 163-170max. The vacuum line from manifold to FPR was 183 but no fluid goes through it. Rubber at the manifold then turns to a hardline back to rubber to the FPR.

Quick question, is there REALLY a coolant line that connects in the same spot as the line going from the vacuum side of the tree on the FPR to the intake manifold? Makes me believe that's why they felt like routing a coolant line to the manifold in the very next year of engines was smart

I added a boost gauge to see if I'm creating enough vacuum to have the FPR work properly, that's one of my next targets.
I'm getting -12/13 when I should be somewhere in the -18/22 range on the vacuum side of the boost gauge. Also unable to create boost entirely.

Im at the point where I think either the check valve to the pump has been worn due to neglect and ignorance from yours truly, or it's the lack of proper vacuum that's causing the fuel to become vaporized instead of injected and or sent back.
That or somehow the return line is clogged: causing the pump to run and create a vacuum in the tank which when turned off begins sucking backwards to bring pressure to equilibrium. Those are my guesses and I'll be making videos to better describe what's happening.

What are y'all's thoughts??

Vova585 · Post by **Vova585** » 06 Jul 2026, 20:17

Please excuse my lack of knowledge/memory. My 98v79xc days unfortunately ended in 2010.
If I remember correctly, fuel pump has 2 hoses connected to the sender unit. 1st is supply to system and 2 is supply from left side of the tank. I would question if your issue is due to defect of ejector pump. Is your issue present at all times or with tank being 1/2 full? I would try to disconnect the line from left side and through connector(fcp sells fule line repair kit from skandix for 35 that you can use here) bypass it into a clear container. Then "bridge" fuel pump relay(preferably with battery maintainer on connected) see what is coming into the bottle from the left side. If you have good steady stream without air pockets- your return line from engine to tank, ejector pump work great. I saw an issue on A8 where problem was in the ejector unit and car drove great up to 1/2 tank at which point all the fule was in the left side and fuel pump was sucking air and ofcourse was dying frequently.

Texasbrainstew · Post by **Texasbrainstew** » 06 Jul 2026, 21:17

I began having issues with a full tank. With both the sender and pump submerged confidently. But I do want to test the return somehow.

Vova585 · Post by **Vova585** » 06 Jul 2026, 21:34

What is your residual fuel pressure? Given your test of supply line, it logically should be around 10psi which is good for check valve. Would be great to test the return line with smoke as well. You are in Texas, so hopefully no rust surprises

Texasbrainstew · Post by **Texasbrainstew** » 06 Jul 2026, 22:03

Definitely something I need to check! I have a pressure gauge but my adapter for the Schrader valve side is missing the pin in the middle. Once I get another adapter I'll be putting a gauge on the rail immediately.

Like I was saying before it holds pressure pretty well regardless if it's gas or liquid. I just don't have a number for you sadly. I wonder if it will show the gulping effect I was talking about.

I definitely think smoke testing the return line is going to be the next test, also I want to square away any vacuum leaks so I'm not concerned about vacuum being a possible issue for the FPR.

Want to make sure I understand this correctly is my return line dumping into the sender side of the tank? Let me know where I'm wrong on my drawing

Vova585 · Post by **Vova585** » 06 Jul 2026, 22:51

1998 xcv70 return fuel line goes to? from engine https://share.google/icF99PbqBZ50KyfVB

Hopefully this one can help. It has a link to a video. I never had to do such work myself, so cant answer if the line goes directly to the sensor der unit.

Texasbrainstew · Post by **Texasbrainstew** » 07 Jul 2026, 05:06

Ahh well I promise I've already been through the 98 xc70 info on here. I've gone through that post already. Regardless thank you!