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

[SOLIVAGANT]

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

Ha! Thanks for the reminder. I’ll have to check when I get home tomorrow afternoon. The part of the wiring harness was a plug that either went to my CKS or my throttle body. Can’t remember now.

I got so zoned in on the start issue that I totally forgot to continue checking what I started all this over. Seems like it is ending up being a coincidence that my CMS went outwent when I decided to resolve a different problem.

 

[SOLIVAGANT]

 

[SOLIVAGANT]

Okay, tested voltage at the connector with the pump unplugged and I’m getting 12V there.

But I rented a fuel pressure tester and learned a lot. With about 4 turns of the ignition to prime the system, I only saw a rise up to about 20psi. One crank and it got to nearly 30psi and stubbornly fired up. Once idling, pressure went to a steady 42psi with a slow rise to about 44psi the longer it ran.

I’m thinking my pump is bad. I believe I replaced this one many years ago, but I want to say that I’ve had the issue of needing to prime and crank once before starting for almost as long. As if the pump immediately failed. I also checked amps, and I think the FSM says something about the pump only needing around 2ish and I measured 2.7. So everything looks good to the pump. Not sure if something could’ve killed it a while back, like some other issue, or if the pump I installed was crap. Where would I buy the most reliable pump? Anyone have years of success out of a specific pump you replaced?

 

[macleanflood]

Couple years with a Rock Auto sourced Brute Power. It's cheap and it works.

-Mac

 

[SOLIVAGANT]

Couple years with a Rock Auto sourced Brute Power. It's cheap and it works.

-Mac

Hey thanks, I'll have a look at it.

Appreciate all the help.

,Daniel

 

[SOLIVAGANT]

I replaced the upstream o2 sensor and fuel pump. Went for a test drive. Everything performed great for a few minutes. Then I came to a stop and the engine wanted to die; really low rpms. Then when accelerating, it started bucking and dying. I pulled over and it was running terribly, really low rpms and dying. It gave just P0132 code. I turned it off and let it sit just a couple minutes and it started right up perfectly and I drove it about 2 miles home.

I probed the vehicle side connector and found a little over 4 volts in the sensor power supply, when expecting <1V. I’ve been pinching and wiggling the wiring harness and everything trying to find what seems to be a short to the 5V system maybe.

Anybody got helpful tips of finding this issue?

 

[ColoJeep]

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.

I had a S-10 that would do the same thing , not enough pressure to start it , but it would run great after running a minute . It would restart if still warm , but overnight needed a shot of starting fluid.

 

[macleanflood]

Unplug all your sensors and see if you have the correct voltage.

If you do start plugging them in one at a time checking until you find the problem sensor.

If unplugging doesn't get you to 5v then it's time to look in the FSM wiring diagrams. Locate the connectors and splice points and disconnect the wires and divide and conquer.

We had someone find the 5v supply wire shorting to ground on the steering wheel airbag - or something like that.

-Mac

 

[CharlesHS]

Since you have narrowed the problem to an intermittent low 5V supply voltage; there are a couple areas to check that may cause this problem.
Disconnect and inspect the three 32 pin wiring harness connectors for the ECU and wiring harness connector; inspect the pins and clips for greyish oxidation.
Not sure if this is a problem on the Jeeps with 4 cylinder engines, but this is a common problem on the Jeeps with 6 cylinders.
The wiring harness from the ECU that runs along the firewall and is routed around a threaded head stud at the right rear corner of the valve cover and is known to rub thru the plastic split wire loom and wiring harness insulation exposing the strands of copper wire which can short to ground or touch another wire in the harness that has rubbed thru.
Closely inspect this harness for signs of damage to the harness wiring.

 

[SOLIVAGANT]

Ha!! Thanks once again guys!

NOWWW the jeep is running better than it ever has in the 8 years I’ve owned this one.

When initially getting in the compartment and giving pinch and wiggles. I found my culprit. The CKP! The wiring I “fixed” at the connector, was not fixed.

So I fixed it and it’s better than ever and staying that way.

So, ended up replacing a confirmed bad CPS, replacing a not good enough fuel pump, and making that repair to the CKP plug/wiring.

Doing absolutely great. So thanks you all! I did get some low idle shuddering after having it shut down for a couple hours and then starting it back up. But no codes or anything to “deal” with.