2zz backfiring under load

[andibell]

Hi everyone,

Hoping someone may be able to help diagnose what's going on with my 2zz... 

At idle or under low/normal load there's no problems at all, it drives well and accelerates smoothly, however, if I try to apply load; increase speed from low revs, in any gear, it becomes hesitant, bogs down and either backfires, or possibly misfires.
Continued applied throttle will sometimes push it past the lumpiness, but usually it's a case of having to drop down a gear to compensate.  It's not happening at a specific rev range or point, it's more in line with trying to accelerate from low revs.  Seems to be around 3-4k in 2nd/3rd 2-3k in 4th/5th/6th if that makes any difference though.

So far I've tried cleaning and changing the MAF (no difference), replacing the short K&N induction kit with a full kit that goes to the original intake (under the rear left lights) and has the MAF the correct distance from the throttle body.
Also checked the O2 sensors (no codes and seem to be plugged to the correct points pre/post).  There's no apparent air leaks around the exhaust/CAT/manifold.

It feels like a fuel/air mix issue, given the backfire I'm inclined towards too much fuel or wrong mix, but I'm pretty much clueless mechanically so hoping someone on here may have some ideas or advice.

My next job is going to be check the coil packs & spark plugs to see how they're looking.

For reference, I'm running a Celica TRD 2zz with JDM Celica ECU and Corolla T-Sport 6 Speed box.

Thanks

[jonbill]

What did you change the MAF to? I had a bad experience with a cheap ebay one on my 1zz.
I think very broadly the options are failed sensor, air leak or mechanical (like cam chain slip)

I'd try to stop immediately after having the problem and whip the plugs out and see what they look like.

Also might be worth checking the cams are still timed correctly, that'd eliminate one of vaguely possible mechanical things and it's easy.
Also... I got elmscan Toyota recently and it shows if the vvti is working correctly - it wouldn't surprise me if it shows the lift timing as well on 2zz. Might be worth a look.

[andibell]

Thanks!

What did you change the MAF to? I had a bad experience with a cheap ebay one on my 1zz.

I changed it for a known good one from Mike K's 2zz, also swapped over most of the air intake pipework to eliminate any issues there as well.

I think very broadly the options are failed sensor, air leak or mechanical (like cam chain slip)

We came to pretty much the same conclusion yesterday, but couldn't find any obvious places it appeared to be leaking from

Also might be worth checking the cams are still timed correctly, that'd eliminate one of vaguely possible mechanical things and it's easy.

How easy?  I'm essentially a liability with a spanner 

[jonbill]

Thanks!

What did you change the MAF to? I had a bad experience with a cheap ebay one on my 1zz.

I changed it for a known good one from Mike K's 2zz, also swapped over most of the air intake pipework to eliminate any issues there as well.

I think very broadly the options are failed sensor, air leak or mechanical (like cam chain slip)

We came to pretty much the same conclusion yesterday, but couldn't find any obvious places it appeared to be leaking from

Also might be worth checking the cams are still timed correctly, that'd eliminate one of vaguely possible mechanical things and it's easy.

How easy?  I'm essentially a liability with a spanner 

Should be prettt easy - turn the crank on its pulley bolt clockwise until the timing marks on the timing cover line up with the mark on the pulley. Not sure which way round it'll be but expect it'll be obvious when you look. It is on 1zz.
https://goo.gl/images/4Yi9Yi

Then take the cam cover off.
The cam match marks should line up like this (I mean the dots on the sprockets that are facing each other and very nearly lined up, not the orange links that are circled in the pic!)

https://goo.gl/images/J4Bct4
That's it. If it looks like that it's OK.

[mikek]

Coil packs covered in oil and so are the spark plugs.

Valve cover gasket and o rings?

[jonbill]

..

[jonbill]

Could be, not sure it would cause misfire. Also...
Spark plugs  torqued up?

[mikek]

Not even if the coil pack has failed?

[jonbill]

Not even if the coil pack has failed?

I guess if one of the coils has failed it should show on the plug. But I meant I didn't think being all oily should make it fail, although AFAIK oil isn't a good conductor, so maybe!

[Carolyn]

I very much doubt that the valve timing has gone out.

Oil all over plugs and coil packs is the cam cover gasket - but that is not likely to cause the symptoms.  Worth sorting, though

My money is on dribbly injectors.  At least one leaker.