95 LT-1 Idle Cell Comparison - Humidity?

[spfautsch]

the maf isn't really good with air density

I'll have to dig up where I read this, but the word "mass" is used in the name because it's designed to measure air mass through the sensor. This implies it compensates for temperature and pressure by design. I'm going to have to dig but I believe this was the same source that mentioned humidity being the only thing a maf can't compensate for, only the O2s could do that. It might have been a Jeff Hartmann book or it might have been something I found online.

I'm going to work with the throttle blades to give it more air at startup and try to tune around that b/c it responded very well there. I suspect most of my other problems stem from moving the injector constant too far.

I'm also going to bump up the dwell in my controller to see if anything improves because I'm not sure where I came up with the dwell figure I used for these coils. I'm 99% confident there are no other issues with the ignition system. When the air was cooler and drier it was firing immediately every time and idling better than it ever did with the mechanical distributor. When I first saw 42-43kpa at idle I had to double-take on it because the last I recall seeing that good of idle vacuum was when it was stock.

Thanks for all the ideas guys.

I'd really like to know about the BLMs question two posts back. If no-one is sure I'll have to dig into the disassembly.

[Terminal_Crazy]

I’ve lowered my IAC values at startup around 40 i think.
Thereseems to be startup values in tables but it usualy starts as 160 ish.

I also reduced the air bleed size in the throttle body after reducing the idle timing as the iac valve couldn’t go lower.

[spfautsch]

Mitch am I mis-remembering, or did I read somewhere that you'd also drilled out your port orifices (the holes pointing at the injector tips)?

It sounds like there might be some misunderstanding and confusion on the subject - the how-to I've seen about setting a base bypass hole size and adjusting the throttle blades fails to mention that it should be done at maximum air density for the climate you drive in. For example if you're in a cold weather season and consequently high air density, you want lower IAC counts - say in the 20s at hot idle. This is the point where the engine needs the least amount of idle airflow.

I'm at the other extreme of the spectrum. In minimum air density situations you want enough bypass air to reach your idle target speed during a cold start without maxing out the IAC motor (160). This is when the engine needs the maximum amount of idle airflow.

Thereseems to be startup values in tables but it usualy starts as 160 ish.

If you're logging 160 IAC counts during cold starts it's an indication that the engine speed is below your idle target and the ecu has opened the IAC as far as it can trying to correct the situation with more airflow.

All this of course assumes your IAC position has been learned by the ecu. There's no feedback mechanism for the pintle position. The ecu has a relearn procedure it uses to extrapolate when the pintle is fully closed and then stores that position (not sure where or how) and tracks it whenever it commands a change in IAC position. But if you replace your IAC motor or the position is otherwise lost by the ecu, the counts aren't going to be very meaningful until the relearn routine runs. I other words, if your ecu thinks the IAC motor is at 50 and it's actually fully open, there's no way for it to know and it will continue to command it open in steps until the ecu's tracking variable is also at 160.

[Terminal_Crazy]

Hiya Scott,
Yes I drilled the manifold bleeds, no 2 i think is smaller of the top of my head. I drilled them as the drilling was crap, flash left in so needed deburring and they come out all over the sides of port rather than consistantly at the bottom like the chevy manifold.
Throttle body was drilled originally to get iac counts down. As timing was increased vac would improve and counts went down. As the tune and idle has improved (timing lowered) counts started dropping very low and ive fitted a restrictor back in. That’s why i suggested not drilling until your idle settles.

I’ll check logs tomorrow but i’m sure this always starts with counts around 160 and drops. Idle starts about 1100 rpm when cold. I’ve replaced iac and done relearns etc, iac vs temp tables all lowered and remembered values don’t seem to work.

Mitch

[spfautsch]

What are your ambient temps like this time of year?

[spfautsch]

Had to play used car salesman tonight so no progress on any front of interest. Thought I'd share this tidbit I found - not the source I recall finding years ago, but makes me feel better knowing I'm not hallucinating memories / facts / whatever.

From this page talking about some newer humidity sensor equipped MAFs being used on the newest Duramax engines.

https://gearsmagazine.com/magazine/a...rflow-sensors/

Humidity has a major impact on the density of the air and the amount of oxygen contained in the air column traveling into the intake manifold. Humidity takes up space, leaving less room for oxygen molecules.

Standard 5-pin MAF sensors are unable to compensate for the water vapor in the air, which leads to rich air fuel ratios when high water vapor conditions are present.

[Terminal_Crazy]

What are your ambient temps like this time of year?

Anywhere from 5C to 35C. Generally it’s very cloudy so in 20’s. 30 ish when the sun comes out.

[spfautsch]

We generally don't see lows near 5C from about mid May to mid October. I'm guessing your average monthly temp (i.e. several hundred samples per day averaged over 30 days) is a solid 3-6C lower. That doesn't sound like much but in terms of air density it is.

Whatever the case, I think a case of mistaken identity has been a major contributing factor. I won't be able to test until later tonight, possibly even tomorrow but I think the combination of all of the aforementioned factors have stacked up to cause me the severe cold start problems.

This, boys and girls is the part of the show where Scott eats his hat in front of you all. :-) Anyone have any ketchup?

After a bit of research it seems like this coil, which is what I have:



Is not one of the supposed "hot" or "truck" LS coils. Seems it's electrically similar to the common LS coils and needs a target dwell of 6ms.

Whereas this coil which looks quite similar:



Is a "hot" or "truck" LS coil and needs a target dwell of 4.5ms.

Anyone want to guess what target dwell I was using?