Cranks, won't start after checking voltage at fuel pump plug (throws P0340 now)

SOLIVAGANT · Oct 15, 2025

Main Issue:

- Cranks and starts with a few 'priming' key turns on and off and maybe on the second crank.
— Long term problem.
— Recently felt a few bouts of stumbling at highway speed, randomly, and then on steep grades.
— Suspected fuel delivery issue.

Troubleshooting:

- Probed voltage to fuel pump at plug behind rear left tire.
— Voltage read about 7-8V when ignition turned on. Suspected bad probing.
— Check for fuel at fuel rail. NOTHING
— Try starting to get fuel flowing.

New Issue:

- No start. Many cranks, many 'primes.'
- Throwing P0340 code (Camshaft Position Sensor Circuit Malfunction)

More Troubleshooting:

- Jumped fuel pump at relay sockets, found fuel at fuel rail.
- Power to relay sockets.
- ASD and Fuel Pump Relays tested good every which way.
— Ohms, switch, and voltage/voltage drops.

So, yeah. Hi! I want to say thanks for any help/guidance you can provide! I really don't want to start replacing parts and sensors if I don't need to in order to fix this. Maybe it's mostly because I haven't eaten before writing this and my brain is on low juice, but I feel stumped and baffled. Could my fuel pump be the culprit still, even though when I feed it all the voltage it pumps? I can hear it spin up every time I turn on the key still. What am I missing?

-Daniel

Mitchrb1 · Oct 15, 2025

When I had a similar code on my 98. I had to replace the Pickup coil sensor in the distributor. And soon after it threw another code and I replaced the Crank Shaft position sensor.

When yours cranks do you see the RPM Jump up on the dash gauge?

Hoping someone else can chime in on the fuel rail issue?

ItchyDong · Oct 15, 2025

Have you verified battery voltage? If battery voltage is good and you’re only getting 8 volts in the back, you’ve got a nasty voltage drop.

TJ4Jim · Oct 15, 2025

SOLIVIGANT said:
Main Issue:
- Cranks and starts with a few 'priming' key turns on and off and maybe on the second crank.
— Long term problem.
— Recently felt a few bouts of stumbling at highway speed, randomly, and then on steep grades.
— Suspected fuel delivery issue.

Troubleshooting:
- Probed voltage to fuel pump at plug behind rear left tire.
— Voltage read about 7-8V when ignition turned on. Suspected bad probing.
— Check for fuel at fuel rail. NOTHING
— Try starting to get fuel flowing.

New Issue:
- No start. Many cranks, many 'primes.'
- Throwing P0340 code (Camshaft Position Sensor Circuit Malfunction)

More Troubleshooting:
- Jumped fuel pump at relay sockets, found fuel at fuel rail.
- Power to relay sockets.
- ASD and Fuel Pump Relays tested good every which way.
— Ohms, switch, and voltage/voltage drops.

So, yeah. Hi! I want to say thanks for any help/guidance you can provide! I really don't want to start replacing parts and sensors if I don't need to in order to fix this. Maybe it's mostly because I haven't eaten before writing this and my brain is on low juice, but I feel stumped and baffled. Could my fuel pump be the culprit still, even though when I feed it all the voltage it pumps? I can hear it spin up every time I turn on the key still. What am I missing?

-Daniel

Wile checking on startup you will only see 12V at the fuel pump connector for a few seconds. Check the pump power lead to any ground and also the connector ground to any ground for continuity.

Also do a thorough visual of the wire leads into the loom side of the pump conector looking for any corrosion or damage.

macleanflood · Oct 15, 2025

If you want to eliminate the fuel pump as the problem disconnect the plug and jumper in or provide 12v and ground on the appropriate pins while checking fuel pressure at the rail.

As stated start with battery voltage. Clean the terminals. Then check all your grounds...firewall next to the battery and engine block. Both speaker covers. Driver's side kick panel. Hell check both grounds next to the headlight buckets.

My guess is you have a short to ground somewhere. Grabbing the factory service manual wiring diagrams and start ohming and continuity checking everything from the fuel pump connector back to the PDU and PCM connectors.

-Mac

SOLIVAGANT · Oct 15, 2025

First off, Thanks to all of you who have put your heads in it to lend a hand!

Mitchrb1 said:
When I had a similar code on my 98. I had to replace the Pickup coil sensor in the distributor. And soon after it threw another code and I replaced the Crank Shaft position sensor.

When yours cranks do you see the RPM Jump up on the dash gauge?

Hoping someone else can chime in on the fuel rail issue?

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.

ItchyDong said:
Have you verified battery voltage? If battery voltage is good and you’re only getting 8 volts in the back, you’ve got a nasty voltage drop.

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?

TJ4Jim said:
Wile checking on startup you will only see 12V at the fuel pump connector for a few seconds. Check the pump power lead to any ground and also the connector ground to any ground for continuity.

Also do a thorough visual of the wire leads into the loom side of the pump conector looking for any corrosion or damage.

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt).

Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?

What would that tell me?

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.

macleanflood said:
If you want to eliminate the fuel pump as the problem disconnect the plug and jumper in or provide 12v and ground on the appropriate pins while checking fuel pressure at the rail.

As stated start with battery voltage. Clean the terminals. Then check all your grounds...firewall next to the battery and engine block. Both speaker covers. Driver's side kick panel. Hell check both grounds next to the headlight buckets.

My guess is you have a short to ground somewhere. Grabbing the factory service manual wiring diagrams and start ohming and continuity checking everything from the fuel pump connector back to the PDU and PCM connectors.

-Mac

Shoot. I'll have to eliminate the fuel pump as the issue I guess. It's weird that it isn't starting now. I drove it a few days ago, with the same struggle to start it that has become my normal with it for years. Very suspicious that it won't start after I probed the plug (male).

I can still hear the pump spin up when I turn on ignition. I can't rule out that today was just the coincidental day it finally 'broke.' That P0340 code is brand new, have you or anyone experienced and issue throwing that code resulting in crank/no start situations?

TJ4Jim · Oct 16, 2025

SOLIVIGANT said:
First off, Thanks to all of you who have put your heads in it to lend a hand!

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt).

Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?
What would that tell me?

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.

Shoot. I'll have to eliminate the fuel pump as the issue I guess. It's weird that it isn't starting now. I drove it a few days ago, with the same struggle to start it that has become my normal with it for years. Very suspicious that it won't start after I probed the plug (male).

I can still hear the pump spin up when I turn on ignition. I can't rule out that today was just the coincidental day it finally 'broke.' That P0340 code is brand new, have you or anyone experienced and issue throwing that code resulting in crank/no start situations?

Testing the power lead to any ground and testing ground to ground proves you have proper ground continuity in the circuit.

SOLIVAGANT · Oct 16, 2025

TJ4Jim said:
Testing the power lead to any ground and testing ground to ground proves you have proper ground continuity in the circuit.

Got it. So testing power to ground (frame), I found no continuity. Testing ground to ground (frame), I found continuity. This is what I want to find right? Checked those leads on the female plug running back from the engine compartment.

TJ4Jim · Oct 16, 2025

SOLIVIGANT said:
Got it. So testing power to ground (frame), I found no continuity. Testing ground to ground (frame), I found continuity. This is what I want to find right? Checked those leads on the female plug running back from the engine compartment.

Yep.

SOLIVAGANT · Oct 17, 2025

So, I pulled the Crank Posn Sensor and am giving it a deep cleaning. Can’t find continuity through it, but I’m hoping at the moment that’s because it needs the flywheel to be influencing it.
Crappy looking grounds. Not the circuit I’m worried about. Can’t hurt to replace them.
Peeling back the harness to clean up oil spilled in the looms millenia ago.

CharlesHS · Oct 17, 2025

How to test CPS operation when part has been removed.

How to test a CPS when part is still installed.

Intimnasc · Oct 17, 2025

Not sure if this applies or not but years ago I had a 94 F150 with dual tanks. One fuel pump would not start the truck but I could switch to the other tank and truck would start then switch to other tank and it would run. Basically pump was going bad and wouldn't make enough pressure to start the truck but it had enough to drive. Make sure you use a pressure gauge when checking the fuel pump.

SOLIVAGANT · Oct 17, 2025

CharlesHS said:
How to test CPS operation when part has been removed.

How to test a CPS when part is still installed.

Nice, thanks. So I tested both my cam and crank posn sensors, and neither of them passed these tests. Did both of them following the first guy's videos that he has for each of those sensors. Crank sensor never shows a significant voltage rise. Cam sensor shows sub voltage rise for a second only when connecting to the sensor output.

This begs the question: What could have caused them to both go bad? The jeep would've thrown a code or not run if one of them went bad before the other right?

CharlesHS · Oct 17, 2025

Three most common issues are: engine heat causing resistance of Hall effect, loss of 5V supply or bad/dirty ground.

SOLIVAGANT · Oct 17, 2025

CharlesHS said:
Three most common issues are: engine heat causing resistance of Hall effect, loss of 5V supply or bad/dirty ground.

Copy that.

I just triple checked everything. The plug I used for my 5V supply was only giving 4.85V. Found another plug that supplied 5.12V. Using that plug, I found that my crank sensor is good using the test from the first video. Cam sensor is still showing the same; never 5V. I'll replace.

Can I assume a bad CMS would prevent start up?

CharlesHS · Oct 17, 2025

Not 100% sure if it 4.85V will stop the engine from starting, but certainly wouldn’t run properly.

macleanflood · Oct 17, 2025

I'd measure the resistance of the wire that's providing the lower voltage... essentially the PCM is the 5v supply...it converts 12v from the battery.

Some folks have bypassed a failing PCM converter using a 12v to 5v converter often used as a power supply for USB devices.

Also understand that the 5v supply wire hits many of the sensors...it really makes a tour...so one spot with nicked insulation or a corroded connector... problem.

-Mac

SOLIVAGANT · Oct 17, 2025

Sorry guys, the voltages I was talking about were for the tester that I made out of a usb cable and block that it plugs in to the wall with.

I used 2 different blocks to plug in to the wall socket to test the CKS and CMS. First block I used only put out 4.8V. I tried a different block that gave 5.1V.

So since I retested both with the 5.2 usb block/plug and the CKS worked but the CMS still didn’t, I’ll replace that CMS.

I am excited to test a brand new CMS to quadruple check my “bench” test.

SOLIVAGANT · Oct 17, 2025

Jeep is running again. Better than ever.

I bench tested the new camshaft sensor and it worked perfectly vs. what I saw with the old one on the bench.

Found a really melted together few wires on one of the plugs. Forgot which one already. But I fixed that! Found another bit of exposed wires, but I’ll get that eventually. I also want to check the fuel pressure soon.

In addition to replacing the sensor, I deep cleaned and refurbished a massive section of the main wiring harness involved, replaced the spark plugs and distributor cap and rotor. I also cleaned out a lot of the plugs of corrosion and oil.

Runs incredible now, like one of it’s “good days.”

Thanks for all the help, everyone.

swtrailboss · Oct 17, 2025

So are you now getting the 12V +or-.5V measurement at your fuel pump connector after repairing the wire harness?

Cranks, won't start after checking voltage at fuel pump plug (throws P0340 now)

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First off, Thanks to all of you who have put your heads in it to lend a hand!​

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.​

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?​

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt). Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?​

What would that tell me?​

​

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.​

TJ Expert

First off, Thanks to all of you who have put your heads in it to lend a hand!​

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.​

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?​

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt).​

​

Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?​

What would that tell me?​

​

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.​

Shoot. I'll have to eliminate the fuel pump as the issue I guess. It's weird that it isn't starting now. I drove it a few days ago, with the same struggle to start it that has become my normal with it for years. Very suspicious that it won't start after I probed the plug (male).​

​

I can still hear the pump spin up when I turn on ignition. I can't rule out that today was just the coincidental day it finally 'broke.' That P0340 code is brand new, have you or anyone experienced and issue throwing that code resulting in crank/no start situations?​

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First off, Thanks to all of you who have put your heads in it to lend a hand!

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt).

Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?

What would that tell me?

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.

First off, Thanks to all of you who have put your heads in it to lend a hand!

Gave it a rest for the day. Didn't notice anything with the tach while I was cranking. I'll take a look tomorrow.

Verified battery voltage everywhere but the battery itself, like at the relay sockets. Getting 12V there. The 8V feels too good to be true for me to have an extremely accurate hunch. I should chase it though, huh?

I wish I was seeing 12V at the pump, but I only read 7-8. I was having a hell of a time even trying to get a reading by probing the power supply (Gr/Wt).

Are you recommending checking pump power lead to a ground for continuity AND the connector (plug?) ground to a ground for continuity too?

What would that tell me?

They seemed good at the connector, I was all up in it's business today. I may have to chase them front and probably drop the tank anways and then check.

Shoot. I'll have to eliminate the fuel pump as the issue I guess. It's weird that it isn't starting now. I drove it a few days ago, with the same struggle to start it that has become my normal with it for years. Very suspicious that it won't start after I probed the plug (male).

I can still hear the pump spin up when I turn on ignition. I can't rule out that today was just the coincidental day it finally 'broke.' That P0340 code is brand new, have you or anyone experienced and issue throwing that code resulting in crank/no start situations?