JrRifleCoach

Maybe a good experiment would be to swap the 02 sensors LtoR.
See if the trend follows them or not.

1993C4LT1

​​​​​​​I'll give a shot once the rain goes away.

pcolt94

FYI - Just saying

As a reference point using my Tech,1 I always show in closed loop a pulse width of ~3.3 - 3.4ms on both banks. Yours seems very high for idle.

My 94 (stock) is an automatic which gets lower mileage then manual. My dash shows about 25 m/g under 65 mph. around town and mixed expressway I might average 20. But my dash says when strictly around town is around 15 - 18 m/g and of course it varies with conditions. A lot of time the darn m/g is sitting at 16.


Replaced my O2s last year and think I pulled the originals. Using my Tech1 they swung fine and with the new ones everything is still good and no real improvement (172,000). I replaced them cause I was chasing another problem at the time.

The BLMs look OK but what are they while driving at a steady speed?

This is just me, had the car since 03 and not much has really changed in performance. Digital readout seems to pretty much agree when I do a real mileage using actual miles and fuel.

1993C4LT1

I've seen it go as high as 149 the blm, but it doesn't stay there long. On the highway, they are perfect. I went through the FSM yesterday, and looked for all the causes of injectors being pulsed too much. It said to check MAP, TPS, and throttle angle at idle. I did, and they are all withing range. Do you think if my throttle plates were open too much, it would cause them to pulse a lot? I know they say not to mess with the throttle screw. But it seems like that is the only thing that will fix my IAC at 0. I started a thread a while back, and the consensus was to mess with the throttle screw. Never did it due to FSM saying not to mess with it.

pcolt94

Is it all stock?

Those pulse widths are even high for a cold engine in open loop.

Any sensors feeding the ECM could impact the pulse widths.

I have hear LT1s have had intake manifold gasket leaks causing a lean condition. The ECM would then increase the fuel to get the A/F mixture correct.

Also check the EGR for a leak. May have to pull to get a good check on it.

It may be worth it go get a smoke test on the intake system just to make sure there is no leak.

I know on the 94 never to touch the throttle adjustment screw. The earlier Vetts had a procedure on how to adjust. But I would not mess with it if it says not to.

I assume you are reading long term fuel trim (BLM) rather than short.

I also put some new gaskets on the throttle body last year.

I would have to review some notes I have but I think if there is a vacuum leak, the IAC counts will be lowered and can be "0".

Does it ever fall out of closed loop once it goes into closed loop?

1993C4LT1

Would a leaking intake manifold gasket show up with a smoke test? I did several smoke tests on the car this past summer. And water tests. No leaks found. Car's BLM's are the same, making me think nothing has changed. I did do the intake gaskets July 2014.

I did have a code for the egr, May 2016. Now, with the new one, I can stall the engine by pushing on the EGR at idle. Makes me think EGR has no leaks?

I know that on a speed density engine, like mine, a vacuum leak would be a high idle. My desired rpm and actual rpm are at the most, plus or minus 100 rpm. To me, that indicates no vacuum leak. Even though the IAC is at zero. But, I will block off the throttle body and see if it stalls or not. Nor does the idle surge.

Yes correct, I am looking at long term fuel trim.

I'm not sure if it ever falls out of closed loop once it's already in closed loop. Will have to verify that.

1993C4LT1

And only mod the engine has is a coil pack conversion. Stock cam, heads and exhaust manifold.

1993C4LT1

A dumb question, at idle my TPS voltage is .57
And TPS percentage is 0, at idle. Since it's at 0% in idle, it's safe to say the throttle plates are not open too much?

FrankieD

You have to take a reading of your IAC and see where it is at idle. A good number for idle is between 40 - 60. Anything above that indicates that the throttle blades are too closed.

1993C4LT1

​​​​​​​It drops to 0 when the engine is warm.

FrankieD

That means that the blades are too open. The IAC is trying to compensate at idle to adjust idle down, but is out of room to adjust. I suggest you read the IAC at idle and adjust the blades until IAC reads between 40-60

1993C4LT1

​​​​​​​So even though my desired idle RPM vs actual RPM, are plus or minus 50 to 100 RPM at most. I would say it is adjusting it because the idle is not high? Not trying to argue, just trying to learn. There really are no vacuum leaks, so if I have to adjust the throttle blades I will. Will the blades being open too much cause a lean condition at idle only, or driving around as well? Or would it even cause it to run lean?

FrankieD

internet hearsay suggests that having your throttle blades open at idle such as in your situation causes split BLM at idle. The belief / reason is that at idle the majority of the air entering the engine should be feed from the IAC passage and not from the Main intake manifold plenum. Whether this totally corrects split BLM or not is still questionable..but it is noted that it does help to some degree.
I say adjust your blades to put your IAC counts at idle around 40 counts and tell us the results

1993C4LT1

Screw it, Ill do it tomorrow. I would do it now, but it's too rainy. Don't want the car idling in the garage till it warms up either.

pcolt94

Before I would adjust anything I would at least pull off the bellows and take a visual look in the throttle body to see if it is closing properly. Also check for excess dirt and carbon build up to see if it would affect blade operation and closure.

1993C4LT1

Got these pics yesterday,as I had thought of the same. Look good to me?

1993C4LT1

It did stop raining for a while yesterday. So I checked for vacuum leaks again by blocking off the throttle body. I used 5 microfiber towels and my hand. The car did stall. So I believe it's safe to say no vacuum leaks.

1993C4LT1

Any other ideas before messing with the idle screw? Cap on the screw is still intact, doesn't look like anyone touched it.

TorchTarga94

1993C4LT1

Car runs great, that's for sure. But the IAC at 0 I need to fix. Going to ohm the IAC valve to be sure. I guess if I mess with the TB screw, I can put it back in the factory position if it doesn't bring the counts up. Will mark it before I move the screw. Whenever I have time.