VERY strange...

1999 original motor/trans/fuel pump.

New:
-TPS
-Pre-cat 02 sensor
-Distributor cap wires and plugs

Problem:
-After start up within the first 1/4 mile I get a backfire in the intake manifold with loss of power. After the backfire operation returns to normal.
-When going down hill, like a canyon, the problem is more persistant continuing until reaching level ground.
-Increase in fuel economy of around 2.5 MPG.
-Unplugging the upstream o2 sensor aliviates the problem but I hate getting 13 MPH

What I've done:
-New upstream o2 sensor
-traced out wiring pigtail from pre-cat o2 sensor to ground patching up all worn areas and cleaned grounds (at first I thought there might be a short).

What I've found from searching:
-Running lean
-Only able to find one post on a tread about running lean where the guy had the EXACT same problem as me and he replaced a fuse to fix it... Not quite sure how to check the fuel pump fuse being that it's incased in a black cover but im sure I can get it off with out damaging the insides.
-Pretty sure it isn't the fuel pump because it doesn't happen repeatedly in a driving session however it does happen each time I turn off the car and start it again (and because when the o2 sensor is unpluged it runs fine). If it were the pump or even a vac leak wouldn't it be a constant issue?

Pretty much Im stumped... Which sensors would affect the mixture? how would you even check them? (I do have a OHM meter and know how to use it)

Any help would be appreciated! I'm definately not a expert mechanic so if I said something that doesn't sound right to you please make a comment I would love to hear what you have to say, its all part of the trouble shooting process :cheers:!

Haven't a clue, so just long shots:

-With engine/ignition OFF,-step on throttle about 10-15 times thru FULL range. (working in new TPS)
-Search for tests and adjusting TPS (Do Analog test several times.)
-Check ALL electrical connections on/near throttle.
-Try OLD O2 a while. (could be flaky new one)
-(Really long-cycle connector on CKP off/on )

(- seems like thats about how long from start to the switch from open to closed loop?)

Good Luck,
Orange

Backfiring is the burning of fuel in the exhaust system. This occurs when oxygen is introduced into the exhaust.

You need to locate the leak that is allowing oxygen to enter your exhaust.

Do you have voltage at the O2 for the built-in heater? I'm just wondering if the engine computer is trying to use it before it's warmed up.

if youre getting a backfire thru the intake manifold that sounds more like a timing issue. have you checked camshaft position and crankshaft position sensors?

Thanks all for your suggestions!

I am getting voltage to the o2 sensor and the heater is working. I will also be getting a new manuel tonight so I can double check the readings on the o2. (Note: the check engin light is off since I patched up the wires a few days ago and has not retriped the CEL)

I haven't checked the cam positioning sensor, im in the process of getting a manual so I know what im looking for when I check it. Thanks for this one, its one I never thought of. Isn't a backfire in the intake also a symptom of running lean? I read that in several places, is it a myth? Also if it were the cam position sensor wouldn't it constantly back fire in the intake not just after start up?

I will be testing the fuel pressure and volume with in the next week (I still haven't ruled out the possiblity of the fuel pump or regulator)

I will report back when I have more info, thanks!

One more note, the 99 model year doesn't have the adjustable o2 sensors.