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04 3.4 Impala hard start 02 sensor question

Impala sometimes starts on first try. Otherwise it will take several tries.

Done so far:

Fuel pressure regulator
Gas cap
Battery

Put OBD on it. On closed loop:
O2 s1 B1 fluctuates between .1 and .9 volts.
O2 s2 B1 bounces around a little bit but stays constant which I don’t think is normal. I enclosed a photo on s1 and s2. There are no codes or check engine lights.
Any ideas?

Both O2s normal and would not cause your starting problem.

More likely a leaking backflow valve in the fuel pump or a leaky injector. Verify by doing the key dance. Key but not start. Key off, repeat 2 more times. Start normally. Does it fire right up? Try a bottle of Techron fuel injector cleaner. If it doesn’t clesr up in a tank or 2 of gas, replace the fuel pump.

Mustangman, I did the key dance and it fired up on three consecutive occasions. You thinking fuel pump on this?

I did put a bottle of fuel injector cleaner in about 200 miles ago, not noticing a difference yet.

Sure sounds like a fuel pump. It is an 04, a GM car, and if its up over 100K miles without a pump change, its due.

You can live with the hard start for a bit if you need to. The pump is running, it just won’t hold pressure overnight.

Good chance replacing the fuel pump will fix this. A shop would first make a fuel pressure hold test. If the fuel pressure isn’t holding to spec after turning the engine off, they’d do add’l tests to determine if it was the pump or a leaking injector etc. But if you don’t want to do all that and just want the problem fixed, if you are lucky a replacement fuel pump may well do the trick. As mentioned above, the O2 sensor signals are normal. The first one is before the cat and registers every too-lean/too-rich transition as the computer tries to maintain the correct air/fuel ratio; the second one is the post-cat sensor, which doesn’t register the immediate too-rich, too-lean conditions the first one sees. The engine has to run well even with the O2 sensors not working b/c they don’t work until they heat up, so the first 5 minutes or so after starting the car the O2 sensors aren’t used. If you want an assessment of the fuel mixture function, that’s called a fuel-trim measurement. Any good shop can do that for you.

I retract my earlier statement. The key dance didn’t work today. Similar symptoms as before. I had to give it a little gas to get it started. I had to feather the gas to keep it going as well for quite awhile until it stayed running. Based on this are you thinking injectors?

No, not injectors. I am still thinking fuel pump but I wonder; Is the throttle body clean? Are air intake hoses from the MAF to the throttle body in good shape? No cracks?

I’ll take a look a little later today and let you know. Anything on my scanner that would pick this up? Fuel pump looks like it might be fairly straightforward to replace as it is accessed in trunk?

If the car has a fuel pressure sensor, it could be scanned. I don’t know if this car has one… I doubt it.

Mustangman, got around to changing fuel pump and filter. Seems to start a bit easier but there are still occasions where I have to do key dance or if it is really not starting I sometimes have to give it a little gas. Any other ideas?