Hey all, kind of a newb to this, but here is what is going on: Car is a 94 with hotcam kit, lt4 module, exhaust, 3K stall, descreened MAF 3:23 gears. I bought a mailorder tune for the car and it had a real annoying cam surge at about 50MPH which is where I do most of my driving. I changed the TCC lockup points which helped, and took a few degrees of timing out in that area which didn't do much. EGR is also off. Data logging showed the BLMs were at 108 where I was seeing the surge...and were overall about 10 points under 128 across the board. The tuner told me "well, this cal should work great" and that is about it.

So, I changed the tune to not go into closed loop and the thing runs and drives awesome! I've decided that I want to go down the road of doing an open closed loop tune but I'm not exactly sure what to do. Here's what I have been able to find...please confirm if this is right:

Use open loop AFR vs coolant vs MAP for low/midrange
Use PE vs RPM for WOT
Use MAF calibration tables for fine tuning.

What I need to know:
Are the above assumptions correct?
What A/F should I shoot for (non wideband) for low/mid/WOT?
Am I possibly doing damage to my car by just running it in Open loop using stock tables?

BTW, I am using tunercats if that makes a difference.

Thanks for your help!

 

Anyone? Help!? Please??

 

What do yo mean when saying "doing an open closed loop tune"

 

If you mean an open loop tune, your assumptions are somewhat correct.

I would not recommend going open loop without a wideband. That is the only way to really know what your A/F ratio is. You should first make shure your fuel pressure is right all the way across the rpm range. You then calibrate your Maf and finally dial in the desired A/F ratio with the afr/coolant /map tables. As a last step you should fine tune the wot A/F with the power enrichment/rpm tables.

I run open loop all the time. My car drives great. I run an 847 cam.
I used to have a hotcam with a closed loop tune and it ran ok. You should not have problems with a hotcam and a closed loop tune. You might have other issues in your setup, probably fuel pressure or MAF calibration related.

 

Exactly. There is no reason the HOT cam can't be tuned to run well in closed loop. The mail order tuners can do it in their sleep.

 

I want to run a tune that never goes into closed loop. IE, no o2 feedback because the car runs great untill it goes into closed loop.
I have a mailorder tune from Madz28. It is OK, but most of my long term fuel trims are off, particularly cell 6 which is usually at 108 and has very annoying surge. If I could get rid of the surge, I'd be happy.

The MAF calibration tables match my stock tune. Since I have a descreened MAF, shouldn't these be somewhat different?

For those who are curious, datamaster file is here: http://www.extremepsi.org/misc/surge1.uni

Thanks for the help!

 

I understand what you want to do.

Point is, its not that hard to correctly tune the LT4 HOT cam to run well on a closed loop tune. What you apparently want to do is avoid finding the problem that is causing your engine to run poorly in closed loop, by tuning it in open loop. That's not necessarily the best strategy.

I run an open loop setup, but that tune was developed with endless hours on an engine dyno and lots of time on a chassis dyno. And if I change ANYTHING on the car that affects the engine, its back on the dyno to correct the tune. No big deal on a car that is mostly a "track car", but running open loop on a daily driver may result in poor gas mileage, fouled plugs, less than optimum performance, etc.

Cure the problem by finding out what it really is, not by covering it up with an open loop tune. JMHO.

 

I agree with you 100%. Problem is finding out what if anything wrong with the car. I will check fuel pressure, but considering how well it runs and drives in open loop, I find it hard to beleive there is anything wrong with it especially since the computer is trying to pull fuel out across the board. Vaccum is good, and I've already checked for leaks, so that somewhat rules that out. The only thing I can point as is the de-screened MAF and I'm not sure what kind of affect that really has.

I'm open to suggestions and ideas!

 

What happens when the PCM moves from open loop to closed loop? It uses the O2 sensors to alter the fuel injector pulse widths, and enables the short term fuel corrections. Faulty sensors, or anything else causing the PCM to see an incorrect rich or lean condition can cause the problem. Without using the short term corrections, the PCM is limited to how much fuel it can add (25%) or subtract (15%). But when the short terms kick in, there are no realistc limits. It sounds like its trying to overcorrect a "false" reading from the O2 sensors.

 

Injuneer, I have been having idle problems for a while with my car.. along with check engine lights from time to time.. If it was caused by faulty o2 sensors(which i get codes for from time to time) If I disconnected the sensors would it smooth out the problem? If it is the o2 sensors causing it? Also, will this cause the SES light to come on?

 

I agree with this, I just had this problem with my car. My passenger o2 would freak out and cause short term corrections to put my afr in the 10.2 range, which would cause it to immediately start cutting out.

Make sure also you don't have any exhaust leaks.

 

How big of a problem could a minor exhaust leak at the header flanges cause on a 3" x pipe system? I mean there is barely any restriction in this exhaust anyway right?

 

The problem is when the leak is before the o2 sensor giving it false readings.

 

ok cool i should have realized that i didnt know if it may have something to do with pressure or not though