• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

O2 sensor on startup

H

Hickey

NAXJA Forum User
Location
Magna, Utah
Would a faulty O2 sensor prevent or seriously hinder startup? Would a bad O2 sensor always throw a code?

MY 92 XJ with 4.0 and auto sometimes starts right up with little cranking, but lately, most times it will crank for quite a long time(minutes) finally start, run rough for a few seconds, then quickly smooth out. On accelleration, it will bog and surge. Seems if I keep a steady throttle pressure on a level road, it will run fine.

I can hear the fuel pump when I turn the key on, but not everytime. Once it primes, is there something that tells it to stop pumping? I have already bypassed the Ballast resistor. I have also changed the filter. I can feel the fuel line from the tank to the filter vibrating while it is running, it isn't perfectly smooth, has some slight surging/pressure changes to it when the engine is running.
 
Last edited:
Sounds more like a choke issue to me.

but that's just me.
My 92xj 4.0 cranks for about 5 seconds, then starts.
From what I understand, the fuel pump is on if the key is on, nothing should tell it to stop pumping, just the fact that the fuel doesnt have anywhere to go it the injectors aren't firing keeps it from "over pressurizing" Might want to try changing out the pump, could be allowing the fuel to back flow when turned off, but that would also mean you have an air leak in your fuel system somewhere, or vacuum pressure would keep the fuel in place in the line.
:repair:
 
The O2 isn't used at all until it warms up, so that wont affect startup.
Actually the fact that it runs well at steady throttle and bogs on acceleration say that the O2 is the one sensor that *is* working.

I dont know the particulars of the jeep system but fuel injection pumps
generally are set up to stop pumping if the engine fails to start.
(for safety, in case the reason it isn't starting is a busted fuel line)
If you have doubts about the pump get pressure gage - its a very easy thing to check.
 
Pflumingo said:
Sounds more like a choke issue to me.
Click to expand...
There is no choke on fuel injected XJs -- only on the 84s and 85s with carburetors.
 
What is the pressure supposed to be at before it hits the fuel rail? Say, just before the filter? I know it is suppose to be about 39 at the rail.
 
Hickey, the pressure will be the same, at all points in the line from the tank to the FPR.

The fuel pressure at idle should be about 31 psi (with vacuum hose connected), and with the vacuum hose disconnected about 39 psi.

Your symptoms sure sound like a faulty pump. Let us know what pressure readings you get.......
 
Since it sounds like a fuel issue have you gone outside the truck at the tank while someone inserts the key and cranks it. My understanding is that is cuts out after priming pressure when you insert the key and resumes under cranking. Have you pulled a spark plug and cranked while holding a connected plug to the engine (be careful don't shock yourself) to see if it is firing. I had the same problem w/ an old truck because the centrifugal advance weights would stick at advance position making it surge and the starting impossible until cranking jarred them. Don't think thats a problem w/ a modern jeep though. And yes a fuel filter doesn't restrict start pressure but fuel flow.
So yeah I'd say faulty pump or regulator. Also check the fuel pump relay if the pump is not engaging.
 
The fuel pump primes when I turn the key on. It runs for only one second or slightly less. I still haven't got a Fuel Pressure tester. Tomorrow is payday, so I will get one then. On a side note, I noticed today that my distributor was loose. I removed the bolt all the way and put a lock washer under it, as there wasn't one. After I tightened it, it didn't make a difference in performance.

I stuck a key in the schrader valve today, the first time (after turning the key to "on") it did not spray, fuel. The second time it did.
 
The rail should be pressurized after the first key on. It should actually hold pressure for some time after key off. Probably a fuel pressure reg. leaking down, or injectors.
 
UPDATE:
Alright, Fuel pressure did not even register the first time I keyed on, pump made no noise. Second time I keyed on, the pump made noise, but still nothing registered on the guage. A few more key on/off and the guage registers about 19 psi. Got it started and running and it stays at 19 PSI, no change when I remove the vacuum. Am I right in assuming that the regulator is at the head of the fuel rail where the fuel lines come in?

I also pinched the rubber fuel line between the pump and filter while it was running and had my lovely assistant watch the gauge, no change when pinched. 19 psi. Could that mean it is a bad regulator or just a bad fuel pump?
 
Last edited:
Unfortunately, it's likely to be a bad fuel pump. I think you're supposed to squeeze the return line though, to eliminate the FPR. If the pressure goes higher with the return line closed, then the FPR is faulty. If not, then it's the fuel pump.

Yes, the FPR is right up front on the rail end. Very easy to get to and change. The fuel pump's not too bad either, since you don't have to drop the tank, just block up the rear wheels to gain enough room to work.
 
Last edited:
Hickey said:
The fuel pump primes when I turn the key on. It runs for only one second or slightly less. I still haven't got a Fuel Pressure tester. Tomorrow is payday, so I will get one then. On a side note, I noticed today that my distributor was loose. I removed the bolt all the way and put a lock washer under it, as there wasn't one. After I tightened it, it didn't make a difference in performance.

I stuck a key in the schrader valve today, the first time (after turning the key to "on") it did not spray, fuel. The second time it did.
Click to expand...

If the distributor (whole shaft, not cap) was loose, and you tightened it, odds are the timing is still off right? Also it looks like you got a bad regulator or the injectors are leaking bad (which I doubt they could leak that much). Since its an earlier jeep I am suspecting regulator but yeah could be the pump also. My uneducated opinion is they should be replaced at the same time.
 
bajacalal said:
If the distributor (whole shaft, not cap) was loose, and you tightened it, odds are the timing is still off right? Also it looks like you got a bad regulator or the injectors are leaking bad (which I doubt they could leak that much). Since its an earlier jeep I am suspecting regulator but yeah could be the pump also. My uneducated opinion is they should be replaced at the same time.
Click to expand...
Timing is not adjustable. Unless you advance it or retard it by tooth engagement. Where is the regulator?
 
Thanks Badge. I may just change the regulator anyway, as it is quite easy to do so.

Edit: Holy crap! $65 bucks for that tiny little part. :rattle:
 
Last edited:
You must log in or register to reply here.

Similar threads

O
2000 XJ Failed Colorado Emissions high CO
2
Replies
27
Views
2K
Ken M
K
J
2001 4.0L issues
Replies
15
Views
261
Saudade
Saudade
G
Bosch o2 Sensor
Replies
7
Views
2K
GravyDavey
G
Z
1990 renix electrical issues
Replies
3
Views
85
cruiser54
C
my2monkeys
Bosch o2 sensor #12009 for Renix. Still all metal body?
Replies
1
Views
1K
Saudade
Saudade
Back
Top