vette_tweak

I've worked on tuning two stroker vettes and noticed a similar problem in each. The engine runs very lean at idle but close to stoich elsewhere. The problem does not change with temperature. The block learn values are 155 to 160 at idle. As soon as I even just barely touch the throttle, the block learn goes back to near 128. I've added lots of fuel via the EPROM and fuel pressure, but I can seem to fix the problem or even affect the A/F ratio at idle. One of the cars has new plugs, a new TB and a new O2 but the problem still exists. Any input is much appreciated!!

Aaron's 87

MAF or MAP cars (the ones in your sig or others)? It's near 128 everywhere else? That's weird. Barring mechanical problems with fuel or vacuum leaks, you'll have to play with the MAF scalars to richen it up (if MAF). Your O2 is switching in closed loop, right?

vette_tweak

Thanks for your reply Aaron. Both cars are MAF and this problem occurs when the system is in closed loop mode. One is a 383, the other a 420CID. The engine vacuum is a fairly steady 19 on both. If I touch the throttle or it's open up to 70%, the BLK learn values are between 126-130. Looking at the EPROM data, I see no reason for this anomaly. The MAF scalars have not been adjusted to compensate for the larger CID. If the MAF scalar at 700rpm was off even by 25%, I would think I should still be able to adjust the fuel and see the result? Regardless, I'm going to try it! Thanks for the advice.

Chris@SD

I am taking a guess that both of these cars have big cams in them. If so, what you are experiencing at idle is reversion with the large pulses of the cam. Try incresing the injector constant a little and see if that gets rid of it or helps.
Good Luck,

Jimmy69

Since I'm using the slower SVO injectors, I adjusted my injector offset vs. battery voltage table to fix this problem. Now idle BLM's are just below 128, and cruise BLM's range from 115-125.

Jimmy

drive it

And here's another way to skin a cat..... in tunercat the tables-open loop f/a ratio % change vs. load and also vs. temp. actually also effects closed loop. I had the same prob. of getting blm's real close to 128 everywhere with just changing inj. constant except at idle. First I adjusted the maf table at 8gm/sec. by 8%, but noticed that I could have 8gm/sec with different lv8's and different blm's. After using the above tables-now its there. Hope this helps.

Aaron's 87

What rpm are you idling at? I assume your inj constants are close, or BLMs elsewhere would be way off. I didn't think of using the open loop tables for closed loop, but it's worth a try. Are your MAFs modified? I'd still play with the scalars at the value where you're lean. Also, post this question on the thirdgen.org prom board for more ideas.

vette_tweak

I tried burning a richer chip last night. I increased the open loop AF vs coolant temp and the open loop AF vs load. The BLK LRN was still pegged at 160 (the max # possible ) during idle ( 700-900 RPM ). I increased the fuel pressure from 43 to 63 PSI and the BLK LRN only went down to 154!!! I'd try playing with the scalars but I'm using WINBIN and the $6E program MAF low end scalar mapping address is incorrect. I wish there was an open loop AF vs rpm map!! The 420CID is using SVO 30 lbs injectors. If they are slower, I'd think the problem would be still be there at higher RPMS.

I do realize that the open loop AF vs temp/load does affect the closed loop operation because the BLK LRN starts at 128 using the #'s from the open loop mode. The problem is that I can't change the AF ratio during idle only. It doesn't really make sense. If the motor was running so lean, it's idle should be erratic and racing. It idles just fine in open loop and actually idles worse when it goes into closed loop mode.

The 420CID is using Hooker headers that locate the O2 sensor in the secondary tube of the 7th cylinder about 5 inches from the port. That seems like a bad place!! Perhaps the O2 is not measuring the AF properly at idle???

Thanks for all your advice. I'm going to really crank up the open loop AF vs load at around 32-64 gm/sec and hopefully that'll fix my problem ( but only if the O2 is measuring the AF correctly!! )

-=Jeff=-

Here is what I did.. I had the SAME problem on my 89 with a 355 Miniram.. it was at 128 everywhere except idle..

you need to find what the Idle RPM are for that condition and follow this formula
This is changing the MAF Scalar tables.. it worked perfectly for me.. in my PROM. At that time I was running SVO 24# injectors. I set my PROM Constant to 26.. and worked from there.. I can send you a copy of my latest BIN tonight when I get home, if you would like.

89vette

Hi. Jeff is right on with his explanation. Your MAF is most likely ported and will give false airflow readings at idle. However, a slight vacuum leak will have the same effect at idle. There will be air entering the motor that the MAF does not know about. The MAF is not measuring all the air and as a result will not inject enough fuel. The O2 picks this up as a lean condition and adds fuel to compensate hence the high BLM. I would actually look there first using starting fluid to eliminate any vacuum leaks prior to fooling with the MAF tables.

Jason

ZylaRace

I think the propane idea is much safer. If it does flame, it won't burn up your harness. i.e. It doesn't stick to anything like a fluid will.

89vette

I don't have any luck with the propane. I agree though that propane is safer than starting fluid. I just use quick squirts.

Jason

drive it

Thanks for all your advice. I'm going to really crank up the open loop AF vs load at around 32-64 gm/sec and hopefully that'll fix my problem ( but only if the O2 is measuring the AF correctly!! )[/QUOTE] Does this mean you're idling at 32-64gm/sec maf reading???????? It should be 8-10gm/sec. or thereabouts idling at 800-900rpm even with a cam. At most 14gm/sec. Or did you mean load-lv8?? Lv8 should be in the ballpark of 50-60 at idle. If it's gm/sec. then there is a problem with the maf readings. As far as a modified maf-mine is screens and heat sinks out without a problem..... As for the O2 sensor-thats where mine is too; so far it's worked OK, but I'm planning on moving it further down so it will look at all 4 cyl. on that side. Of course I'll have to change it to a heated sensor then. The only other thought is if the IAC is not actually closing when commanded it can sure throw things off-does the idle rpm stay the same as you turn on the AC and put it in gear? If not take a look at the IAC. Good luck!

Aaron's 87

I still say it's in the Scalar tables. I believe my WinBin file works fine. The IAC is a controlled vacuum leak so check that. Check for other vacuum leaks. Check for exhaust leaks. Check O2 operation. Ported MAFs are problems.

vette_tweak

Yes, I meant the LV8 is around 45-55 at idle. The MAF reads around 8-13 gm/sec at idle. I now have an updated definition file for WinBin and I'm going to change the MAF scalars tonight.

About the ported MAF: The cross sectional area of my ported MAF is larger vs. stock. Therefore the air velocity is slower at a given rpm vs stock and the ECM thinks there's less air going into the motor. Should this condition not exist across the MAF entire operating range? Why would the low flow MAF calibration be off but the mid and high flow calibration be okay? Perhaps porting the MAF causes non-linear behavior of the system?

I don't doubt modifying the MAF scalars will correct the problem. But is it the cause of the problem or a change that is compensating for the actual problem? Being the analytical type, I can't help but ask these questions in an effort to actually understand what's going on in there!

Scott 87 IROC-Z

I have been trying to fix this on my new motor for 2 months now. I will be idling with BLM's at 128, then after a minute they start to go to 160 unless I blip the throttle. I believe that having the screens removed from the MAF and the large cam is causing the problem. I pull 17 - 18 gm/sec idling at 950 rpm and changing the MAF scalar tables had very little effect on the problem. My solution; convert to Speed Density. I got my new ECM yesterday and will be performing the swap this winter. Sorry for not being much help. Just wanted to let you know your not the only one with the problem.



[Modified by Scott 87 IROC-Z, 9:58 AM 10/4/2001]

88L98

one thing i havent seen mentioned here yet, do you have an egr on there? i was recently seeing some problems with a lean at idle, turned out the vacuum diaphram in my egr wasnt working and i was pumping exaust into my intake during idle and all that, which will give you a lean reading.

-=Jeff=-

Yeah the EGR could cause the same problem

I also had a problem with my Vapor Canister.. it was bad and also would cause a lean condition

ChrisB

Are you running heated O2s? If not, you probably should consider a conversion (just relay an 12v source vs. key on to the O2 sensors (or I guess run would be better)).

*any* exhaust leak will be especially prominent at idle, and can wreak havock with your o2's. I have had problems with the strength of hooker flanges. They seem to love to warp (I use 2 gaskets and long-bolts).


I can give you a definite workaround (at least this works on an LT1, not sure if you have all the same tables).

Find your %TPS to enable WOT/PE. For say 0-1600 or so (or 1200, depending on your idle). Set this to 0%. This will effectively keep you in power enrichment mode at idle (learning disabled). Your idle A/F ratio will now not skew the rest of your L-trims, and using normal open loop afr tables (as well as the PE table now - don't forget to check that for your idle rpm's) you can set your a/f ratio exactly where you want it.

Watch out, in my experience the "stink" that people always seem to think is rich is generally lean. A lean car will smell much worse - it will burn your nostrils, etc. A slightly rich idle is also much steadier/more stable than a lean idle. Make sure you adjust it at full load (a/c, etc.) and don't forget to consider timing also.

vette_tweak

I tried modifying the MAF scalars at the low end and it worked!!! At idle and at low load/RPM, my BLK LRN values are around 126-132! Great! Thanks for all your help!!