Hey guys, been trying to get the car sorted out for some time, it goes back and forth with fuel ratios and i can't figure out why.

I garaged the car for a couple months because it was so bad i was getting misfires, so i replaced all sorts of ignition parts and got my injectors sonic cleaned. Didn't really do too much to change it. The one time it did seem okay was when i took off the intake (TB forward) to help in checking for vacuum leaks. After i put it back on the BLMs looked good and it ran great (no misfires). The miss never came back, but the BLM split is back, i thought maybe it was a vacuum leak between the MAF and TB somewhere, so i put a new coupler on there, nice and tight, 98% sure there's no leakage there. So i take it out and still have a good split, though not getting clear up to my LTFT of 160 like before. When running at a decent RPM it seems to be pretty okay:



but when stopped and letting it drop down to idle it gets a small split:




which after a few seconds starts to shoot up on bank 2 and turn into this:






I'm not excessively knowledgeable about these engines, but from what i do know, 2 things stick out to me:

1. The IAC is way low, I've read about that causing splits sometimes (though my TB is stock and it's usually a problem with aftermarket from my understanding). I also have a very high idle for a while after startup, like 1600, i don't know if this would contribute to that as well?

2. My TPS 0 throttle voltage seems low, it's supposed to be around .65V from my understanding, does this mean someone closed my TB a bit with the stop screw maybe?

Just trying to get my head around this issue, I've had the exhaust looked over twice for leaks, searched pretty thoroughly for vacuum leaks (smoked the intake with no dice), and i'm stumped. Any insight is welcome!

 

It still could be the intake sucking air from the crank case. You might also want to check spark plugs for oil deposits as a clue. Did you check the intake bolts to see if they are tight? Also - does your O2 sensors get to switching back and forth? Can't tell from your snapshots if they are going high/low. You might also want to look at the throttle blades opening at closed throttle to see if that is making sense with the TPS voltage, and then maybe also try to reset the IAC, which is a keyed sequence, I believe.

 

I did check the intake bolts and they are good and tight (and were), the O2's are bouncing like they should, back and forth between .8xx or so down to .1xx or so.

If my intake was sucking air out of the crankcase, would that be a bad PCV valve or something? I'll be checking/fiddling with my stop screw to see if maybe my blades need adjusted. If my IAC is way down super low where it is, doesn't that indicated they need closed more so the IAC has to open more to keep idle?

thanks for the help

 

An old common SBC problem was sucking oil/air from the crankcase into the heads due to a bad intake gasket, so it is something to keep in mind when troubleshooting. You are correct - the IAC seems to indicate that the blades are open too far, so that is why I mentioned physically verifying them to see if it is obviously too far open, or is it just sensor not giving the right voltage in relation to the actual position. The reset I mentioned can re-home the stepper motor - and it looks like the cool GaryDoug 9495 scanning software you are using can do that for you with press of a button. Always try the easy stuff first.

 

Well, the intake gaskets are only a few months Old with under 1000 on them, so while possible I'm guessing not them, looks like some monkeying with the TB is in order tonight

Edit: I'm still a little sketchy on the tps voltage and iac counts, to me it seems like they're indicating the blades are closed too much, but they're open too far is what your saying? Guess I've got to reverse my thinking...

 

tps voltage means nothing really.

iac counts being that low at hot idle likely means air is entering somewhere other than the iac valve (throttle plates, vacuum leak, whatever).

lt1s have a special set of idle passages within the intake plenum that are fed by the IAC and the idle air bypass hole that distribute air more evenly at very low airflows than the throttle plates can. those passages can't work without the iac and bypass hole allowing most of the idle air.

by your trims, though, i'd guess vacuum leak or a dead cylinder.

that's a whole ton of trim. 150+ trims are dangerous.

perhaps it was ridiculously lean maybe due to a vac leak and dumped a ton of fuel in to fix it (kind of hard for that to be confined to one bank..). the super low IAC counts sometimes point to a vac leak.

or it was an exhaust leak and due to the inaccurate reading, it dumped a ton of fuel in that bank and it's running pig rich

or you have a non-firing cylinder on that bank pumping straight air into your exhaust... but usually at that point, the iac is struggling to keep it running, not closing to keep it down.

 

Yeah, i'm kind of at a loss on this. If i had a dead cylinder though, i wouldn't think it would all level back out and look great while under load, the trims only go crazy after idling for a few seconds. Would the cylinder strength test on your EE hack tool be a decent check for a dead cylinder?

 

cylinders can be dead at low rpm and come back to life as you rev out, in the case of an injector that's barely working, ridiculously low compression or whatever.

not a diagnosis, just throwin' it out there.

it's theoretically supposed to be, some people see accurate results and some don't. i'd recommend checking manually with the cylinder disable slider. you should be able to 'feel' a difference as each cylinder is disabled.