finn_the_flying_fox

Hello all, I have an issue that has been plaguing me and I can't seem to sort it out. I have a freshly rebuilt lt4/lt1 bastard child for a 95 C4 that refused to idle in closed loop mode. Open loop is fine when its forced into it by EEHack. What I know about the engine:

Was rebuilt by me and has less than 25 miles on it.
LT4 heads
LT4 intake
1.6 rockers
BBK 58mm throttle body Upon measurment, its a BBK 52mm throttle body
Throttle body coolant lines bypassed
Some weird off brand adjustable FPR (no marking on it)
(presumed) LT4 hotcam (more on this later)
30 LBS injectors
Some weird "max air force" air filter and air tube and intake before the MAF sensor, after MAF the intake rubber bits are stock.
Long tube headers with 3 inch exhaust, magnaflow mufflers, no cats
AIR system removed and patched out of the bin
Modified tune done by some shop who's sticker has long sense worn away

The car came into my possession with these mods already on it, running decently well, other than high crankcase pressure (thought to be stuck rings from sitting, was actually cracked pistons), Which is why I don't know the specifics of some of them, and what prompted the rebuild.

What I've tried:
New O2 sensors
New Petris enterprises optispark (part of the rebuild)
New IAC servo
cleaning throttle body and IAC passageways
Vacuum leak check with a propane torch
Checked for disconnected connectors and hoses
Checked for stuck fuel injectors
Sniffed FPR vacuum line to check for gas
Checked if FPR vacuum line held vac

I also took a data log, which will be attached, and looked through it, I didn't see anything out of the ordinary in it, and all sensors seemed to be functioning correctly. I have also attached the BIN file for the ECM for those who wish to take a peek and see if there is anything odd going on in there.The engine seems to run and idle fine in open loop mode, but either idles very low (~400) or dies completely in closed loop mode.

Anyone have any insight into what could possibly be wrong?


EDIT: Meant to mention, seems to be running very rich as well, lots of pops and bangs on deceleration.

AmoriFati

Looking at the excel file, it is not always in closed loop. How long was this data run for? I know if the heated portion of the O2 doesnt work, and you have long tubes, it can go in and out of closed loop. But, it will idle if the tune is closed loop is tuned for it. In Open loop, its not using the O2s. I think it takes 90 seconds before it electronically shifts from open to closed and O2 sensors are up to operating temp via heater wires.
New O2 sensors doesnt mean they are working properly.

jmgtp

What does your idle table look like for desired rpm at operating temp? What about IAC counts are you seeing? BLMs?

Seems you are using eeHack which means you have the appropriate ALDL cable. You’re only stone throw away from doing some meaningful tuning. I’d change it to speed density mode and run some quick iterations of tunes through Trimalyzer to update the VE tables and see if that normalizes the idle. Then you’ll at least know if it is the tune that needs work or something else.

finn_the_flying_fox

During the trip, I had manually cycled it from closed loop to open loop and vice versa, to try to keep it from stalling, thus why you see it switching. As far as I know the heaters are working as the o2 sensors are brand new ngk units, but i'll manually check them in the am.

AmoriFati

Check the IAC to see if it changes if you disconnect it while running in Closed loop. In open loop, its not using quite a few sensors. Also, turn AC on and see if, in open loop, the idle changes at all. This will tell you if IAC is working. Or you need to put a meter on it. Data only tells you what the iac is being told to do.

finn_the_flying_fox

Idle table is normal, 850rpm as expected, idle counts are high, 100 when it doesnt die, shoots up to 160 when it starts to die, and looking at the BLMs says its adding fuel to the engine.
To answer your second half, the car wont even start without the MAF sensor connected. And not to get too stuck on things, but this tune worked fine before the rebuild.

finn_the_flying_fox

I'll give this a shot in the am, theres nothing promising the new IAC servo is actually working or not.

AmoriFati

Okay, years ago, on another forum for Camaros, some explained how the IAC data is purely data telling it what to do. You wont get the actual counts it is doing. Only what the ECM wants it to do.
I would give a test, turn AC see if it changes a bit, as it should change 50 rpm ( most tunes). You will hear it.
The engine in Open loop is premapped out.. It will run till it warms up with what GM put into. Closed loop, other items, sensors start telling ecm what they see so adjustments can be made.
Just a suggestions. All new sensors arent always good.

finn_the_flying_fox

I know all too well that modern sensors aren't always good out of the box, and sorry if I am coming across a bit combative, its been a long day.

AmoriFati

No worries. I am passionate about how the IAC was bad and I always thought it was due to datalogging. When I found out it was bad, via an ohm reading across points, I was upset, yet glad at the same time.
Something is pulling your idle down, IACs will do that if they dont move and adjust as needed.

finn_the_flying_fox

IAC servo is working fine, verified it by pulling it out and commanding it with EEHack, moves nicely.

For everyone, is there any chance the cam somehow got mistimed? We were careful on disassembly and reassembly to mark everything, but if its off by a tooth could it cause this issue and none others?

AmoriFati

If you marked everything and it was a GM cam and GM timing gear set, you should be fine.
I know EEhack showed it working, which is good.
Also, check AC on and off and see if idle changes.
I will research more when I get home. This is tricky without seeing it.

finn_the_flying_fox

Update, I raised the min idle screw about 1/4 of an inch out and now it seems to be idling good, not the solution I like, but it seems to be working.

AmoriFati

Sounds like you need to tune the idle up electronically.

MaxLean

Cracking the throttle on an LT1 isn't a good idea. You want all idle air to be coming from the intake manifolds internal idle circuit to avoid the dreaded split BLMs at idle. On my BBK 58mm, I found that as delivered, the idle circuit of the throttle body had insufficient air volume to allow IAC counts to be in the desired range, especially on a cammed engine. It's been decades since I did it, but I enlarged the passages in the throttle body as much as possible, and the drilled a small bypass, similar to the stock throttle body. I continued to enlarge the bypass until the hot IAC counts were around 20-30 with the butterfly's completely closed, of course.
Scott

AmoriFati

With a Hot Cam and a 95 Sequential port injection, it should be fine at idle, and IAC should be able to handle it. I know of the hole drilling and that does work on bigger cammed engines. Stock Throttle Body is pretty solid on idle with a hot cam.
I would put the factory Throttle Body back on. BBK has a terrible reputation. And a 58mm is huge for a 350 with hot cam.
My Speed Density 93 idled fine with a Hot Cam. But it was tuned via PCMforLess.

finn_the_flying_fox

Sadly, I don't have the original throttle body, as I bought the car with the enlarged one already installed.

MaxLean

The stock throttle body will likely idle better, since it's IAC capacity is better than the BBK. And it has the bypass hole that the BBK lacks, that I was suggesting you drill on the BBK. Here's a pic of the stock throttle body with the bypass hole that I'm referring to. It's between the throttles, in the IAC passage. (circled in red)

finn_the_flying_fox

Ok, I will reset it back to where it was, drilling may be a better choice, was just starting to get desperate, and that seemed to help lol. What do you mean split BLM? like split left to right?

MaxLean

Yes. It's common, especially on cammed LT1s, to have split fuel trims from bank to bank, especially at idle if the idle air is coming from the throttle blades. The air distribution in this manifold is pretty poor at idle. That's why GM made the LT1 manifold with a dedicated idle air circuit. Manifolds like the L98 tuned port, just dumps the IAC air into the plenum.