Hi Guys,

My engine won't idle when its cold. Once it heats up it runs great and idles around 750 in park and about 550 in gear. I have already reset for minimum air for other reasons. My IAC and CTS are working properly. I have already worked with my Main VE Table. It seemed to help a little. I'm a little lost when I look at the Choke and Cold Engine AFR. I havn't been able to find any good threads that explain them. Could anyone give me some ideas where to go next? Or if someone is willing to have a look at my .bin that would be awesome!
Its a 355 and I'm running a 7747 ECM.

Thanks!

 

Nick

No two engines are alike (especially modified). Take a look into your current BIN file using 7747-mine.XDF. When engine is just started and cold it is running in OL. A very cold engine needs rich AFR to idle and this AFR is set by the following two tables. Under Tables/Functions locate the following two tables:

Cold AFR vs. VAC kPa (Added to AFR vs. Temp)
O/L AFR vs Temp.

The OL AFR is calculated from your VE tables and that is why you saw some improvements in cold idle. VE tables are a baseline - 14.7:1 AFR.

You may have to lean out (if you are too rich) or decrease AFR if it is too lean at low temperatures. The only way to know if you have access to WBO. At a minimum, if you have a heated NBO monitor its voltage during start up (you can do that with a DVM), but let NBO heat up before starting engine. NBO can be used as a crude indicator during cold start. If NBO voltage stays above 850mV at all times you are rich, and below 200 mV it is lean. Unfortunately, NBO is only accurate around 14.7:1 AFR and its indicative accuracy is poor else where.

Choke table is only used during start up and its AFR effect is decayed quickly. Leave them alone since engine starts OK.

I do not get very cold temps here in SoCal so my tables are of little help to you.

//RF

 

Send me an e-mail at ultm8z@yahoo.com. I developed a commanded AFR calculator for open loop mode. You plug in your values and it'll tell you the commanded AFR for each temperature as a function of MAP.

Using this tool, I iterated to good cold start operation. For example, out here in SoCal, our morning's are not bitterly cold, so I was able to lean out my cold start AFR to about 13.6:1 @ 8°C as opposed to the factory's 13:1. May not seem like much, but it did make a non-trivial difference.

In fact, I don't know why I didn't think of this before... I have a bunch of Excel tools for tuning- AE, PE, open loop, etc. I'll create a page on my website for downloads of these tools.

EDIT: Website is updated. The open loop tool is there for download.

 

Hi guys, Thanks for the advice. I looked at ULTM8Z OL AFR Tool. I didn't really understand how to use it. I played with the Cold AFR by hand and saw next to no improvement. I went +/- up to about 2.0 from where my table it at now... Should I have to go much further than that? Should I try like a AFR of like 8.0 at -4C or woud that be way too rich? ULTM8Z could you possibly expand on the instructions you gave on your website for your tool?

Thanks Again!

 

What tuning software are you using?

 

Without looking at your OL AFR spread sheet (ULTM8Z), I would say that it isn't the same as required for the '7747 code. In $8D the AFR is a percentage from the stoich value. In $42 ('7747) the AFR is actual AFR, or steps thereof.

In the sense that if the open loop AFR table has 114 (raw) in it, it is 11.4:1 AFR (divide by 10). Then the choke AFR is an AFR value that is subtracted from the open loop AFR. So it could be 1.5, which makes the current AFR 9.9:1 (11.4 - 1.5 = 9.9).

Nick, don't forget that the SA also plays a role in cold running. And, a weak fuel pump can also cause problems as a cold engine requires more fuel.

A bad EGR (if used) will also cause issues with idle.

RBob.

 

Thanks for the info

 

Hey Guys,

ULTM8Z, I'm using TunerProRT. I think RBob is right, Im used to the 7747 which is probably why I couldn't understand how I was supposed to use your tool. But thanks for the input!

RBob, I actually hadn't even thought about the SA being different for cold start... Duh! haha. I'm guessing that is my problem. Ill be playing with that tonight. I think my EGR is okay but Ill check that if the SA doesn't fix it up.

Thanks Guys, you've all been a lot of help!

 

Nick

I went back through some old notes and search old Diy-EFI posts (there is a lot of 7747 related info there). Assuming that you have good FP and EGR is not acting up (disable by increasing EGR enable temperature to maximum). So let see if my logic makes any sense.
Cold ambient temperature (around 0 to 5C), engine starts ok (commanded cranking AFR is 3.00) it is decayed out to -> (O/L) AFR 10.4 + 2.5 (Cold AFR vs VAC kPA) = 12.9:1 for the first couple of minutes while coolant is still cold. In addition, IAC is moved to 110 (IAC position vs Temp). Engine should be idling at 1100 RPM! Coolant Comp Spark Advance table should provide around 25 deg timing (40 kPa). (All values are based on stock ASDU calibration)

IFRC you have a very mellow cam, but engine may like to to be a touch richer when cold due to overlap - it is worth a look (drop O/L AFR vs. Temp @6.5C 10.4 to 10 and see how it likes it)??? Try to datalog your cold start.

//RF

 

10:1? IMHO, that's waaaayyy to rich. The engine would be drowning in fuel. I wouldn't think you'd have to be any richer than 12.5:1 as a minimum. My cam is more radical than factory, but is still relatively mild and I'm running just fine at 13.6!

 

Yeah, I built it around $8D and have no idea how it differs from other masks.

 

Hey RF, so I data logged 2 cold starts this morning. The first, I did what you suggested, no change really. The second, I played with the Cold SA like RBob brought up. I added 2 degrees throughout the table. That one didn't really do anything either. I data logged both... I don't have a WB and I have a single wire O2. I'll post up my data logs but they probably won't tell us much from the single wire NB right? I uploaded my bin if your wondering what my SA tables and Cold AFR look like.

Thanks!

 

Sorry guys, Just looked at that second log... Don't bother looking at it... its short and looks like I got nothing but bad data.... I've been logging with A+B jumpered. Would this cause my IAC to stay closed at all times? From the log looks like my IAC is staying at 145.... Ill go verify that my IAC is actually moving...

 

Hey Everyone. Well I definitely can use all the info you guys gave me to get my Cold AFR and Idle speed better... Unfortunately... I've figured it out... I should have caught this way sooner... It was my IAC. A little while ago I took it out to check that it was working and the pintle came out. Well I guess I didn't get it back in right because the stepper motor wasn't moving the pintle... replaced it and started right up perfectly...

Sorry for not catching that sooner... I thought it was fine since I had just checked it. Anyways thanks for the info because I'll definitely be using it.

 

It is good that you discovered the IAC was bad. Please note that with the '7747 ECM you should not use anything across the A&B terminals. Short, 10K, 3.9K: nothing. Leave them open while either driving or data logging.

RBob.

 

Yea, but that is not a final value used by ECM for commanded AFR. Cold AFR vs MAP is added to that table as well. 7747 tables can be tough to figure out sometimes since the final value is addition of several variables. What's important he found IAC problem.

Hey Nick let us know if replacement IAC got that cold start issue solved.

//RF

 

Cool. Glad you got it figured.

 

Hey Guys, Well the IAC is definitely working and fixed me not being able to idle when its cold. Now I'm idling way too high when the engine is in open loop. I brought my commanded IAC steps vs temp way down. My idle came down with it but then stops around 1000 RPM and 39 IAC steps. I can watch it come down perfectly according to my steps vs temp table then just stops at 39. Any Ideas what would make it do this? Ive read over lots of IAC threads and can't figure out what would cause it to stay open like this...

RF I see you were working with someone else earlier this month on sort of the same issue. His IAC steps hit 39 and wouldn't go any lower? Any thoughts?

RBob, reason I've been jumping AB is due to bad data... without it I get tons! of bad samples. Then when I jump it 99% of them go away. Ive even switched ECMs and the problem is still there. Thoughts?

Thanks Again

 

Hey Nick

The other gentleman IFRC had a different ECM (427 running $0D code, which way more sophisticated than early C3 7747 ECM). Take a look at at IAC Position vs. Temp (ASDU calibration)
CTS CTS IAC
*F | *C | Steps
Cold| Cold | 145
15.8 | -9 | 120
42.8 |6 | 110
60.8 16 100
75.2 24 90
87.8 31 80
98.6 37 65
109.4 43 50
120.2 49 40
132.8 56 24
145.4 63 24
159.8 71 24
176 80 24
195.8 91 24
224.6 107 24
276.8 136 24
Hot Hot 24

First things first - reset IAC - re-establish fully closed position. Second did you adjusted your min idle screw before???? If this is the case go through a minimum idle speed adjustment procedure (i.e your engine should be able to idle at 450-500 RPM). If the throttle plates are cracked open just a bit by the minimum idle screw IAC will be ineffective since more air will flow past partially cracked open blades. Somewhere in the past I saw a comment about IAC logic - if it does not see RPM drop for a commanded step it stops adjusting down.

It should go below 39 counts - 24, but I would edit that table and drop to about 10-15 counts.

//RF

 

With A&B shorted the ECM will use a higher then normal desired idle speed. Which in most calibrations is set to 1,000 RPM. This is just one of the side affects of shorting A&B together. There are others.

As for getting more errors with A&B open, shouldn't be that way. I would look into grounding issues with the ECM to the engine block, engine block to battery, battery to frame, and of course the ALDL cable to the laptop.

RBob.

 

Forgot to mention, in the Tuning Guide Book sticky, chapter 5, '8746/'7747/'8063 section: IAC Logic or some such title. Everything you ever wanted to know about the IAC but were afraid to ask.

RBob.

 

Ahhhh - now I remember where I saw it before - Post #9!

 

Hey Guys,

Ive read that thread a number of times and it all makes sense.

I reset for minimum air again. It corrected the problem. I was Idling in open loop right around 800RPM. Then I put it in drive, then back into park and it went right back up to 1000. My IAC steps still won't go below 39...

Anything else that could be causing this?

Just when I think I'm getting somewhere I get hung up on something else...Thanks for all your help guys!