Outback 2.5 Heistating/Backfires While Acclerating

[WeezWagon]

Hi All,

I have a question about a buddies 2004 Outback I am doing some work on, it seems to be hesitating, and backfiring? when you get on it. I notice it mostly after 1st gear (it's an auto). Also cruising along at 40-50 it will act like its cutting out but pick right back up again. To me it feels like a precat O2 sensor has gone bad. I pulled the plugs and they are pretty white, like it is maybe running lean? They are brand new, so a little hard to tell yet. No check engine light has come on.

 

A little history on the car (this is where it gets interesting).

Someone had driven it into a bog and it sucked in a bunch of water, they sent the motor to a fella for a "rebuild" which looks like he just did the heads on. They put the motor back in, ran it for a few months and it got terrible rod knock, and wouldn't stay running clouds of blue smoke etc. Enter me. We were able to squeeze a junkyard motor out of the guy who did the "rebuild" so we swapped that motor in, and hooked it all up, runs great at idle after a new idle air control valve, crankshaft position sensor, and some tinkering.

 

I have a few theories, the first being that the previous motor spewed so much oil down into the exhaust that the O2 sensor is fouled up and is giving bad readings, causing the motor to cut out/backfire when loaded.

 

The second is that the TPS is bad, I did the voltage test on it and was reading around 5v instead of .5v. Also if I move the TPS all the way to either end of the slots, I can hear a solenoid in the transmission start to buzz. Which makes me wonder if the issue could be in the transmission.

 

Car has 38,000 on it. But it lives up in a rural Alaskan village, and sees -40 in the winter and dirt roads all year. My general rule on calculating wear up here is times mileage by 3.

 

Any suggestions or comments are welcome. Thanks for the time.

Edited November 18, 2014 by WeezWagon

[lmdew]

Check the Grounds

Could be a MAF sensor

 

TPS causing a buzzing sound is normal, from what I've experienced.

[1 Lucky Texan]

vacuum gauge testing might help rule out clogged exhaust and vacuum leak.

 

getting freezeframe or live data would be nice, fuel trim numbers can tell you a lot.

[WeezWagon]

Good to know the TPS related noise is not out of the norm. Pretty sure there is not a vacuum leak, but testing further is always good. Unfortunately I don't really have access to the tools needed to get freeze frame data.

 

If the MAP is bad is there a way to test voltage on it, trims would help a lot here but again I don't have the tool to get those. Just trying to take the best guess here.

 

Thanks for the help.

Edited November 18, 2014 by WeezWagon

[1 Lucky Texan]

what year model car? even with low miles - there is a TIME side to the schedule and the car could have skipped a coupla teeth on the timing belt - that could lead to a backfire. (my wrx had only 53K miles and the tensioner seemed weak and 2 idlers were very loose - the toothed one was noisy and wobbled slightly)

 

on some cars, a bad MAF or O2 can be found by unplugging one and noticing if the car idles better. That would mean the stock ECU a:f map is doing a better job than the ECU does with 'bad' data from one of the 2 main a:f sensors.

 

but I'm not sure it's a good test for all year models? - maybe someone else can explain if I have it wrong.

Edited November 18, 2014 by 1 Lucky Texan

[WeezWagon]

Forgot to put that in the original post. Its a 2004. Also it idles great. Just like it should, the only time I notice the noises and hesitation are when driving it. Unplugging sensors does not seem to help....

[1 Lucky Texan]

if the issue seems to always be at a specific throttle position - could be a TPS problem.

[WeezWagon]

Seems like it's an "under load" situation that causes the issue that's why I am leaning toward A/F sensors. If I catch it doing it while driving around and downshift the transmission into 3rd it goes away. Throttle position stays the same, but RPM increases because of the shift.

 

Timing is dead on, checked and tripple checked that, I really don't think it's a mechanical problem. Valves are adjusted correctly.

 

If I understand the car correctly the MAF takes over once the TPS tells the idle air control solenoid that the car is no longer idling? And the TPS should 'learn' idle position after a few drive cycles.

 

In park it revs through the range just fine, but does a little studder at the beginnning if you jam on the gas really fast.

[msteel]

Just fixed these exact symptoms on my 2002 Outback.  It was hesitating and then as the problem got worse, I started having small explosions in the exhaust.  One cylinder was not firing and unburned fuel was getting into the hot exhaust.  I replaced the plugs and wires (and the valve cover gaskets since the seals around the plugs were leaking oil onto them) and all was well again.

[WeezWagon]

I have heard of the valve cover gaskets leaking to make misfires, but from what I have seen these gaskets are not that bad yet. These plugs are brand new, maybe I will check the wires on them. Thanks for the info. Were you getting any CEL when this was happening?

[WeezWagon]

Cylinder #2 plug wire was bad. Replaced and no more hesitation.