I'm usually pretty good with diagnostics but I can't seem to figure this one out. This car is starting to wear on me and now that I have a LS1 car also I am about damn ready to yank out all the LT1 stuff and put in a LSX or a BBC as I've been having various problems with this engine management system over the last several years.


About the car...
97 WS6 A4
Stock bottom end
LPE CNC heads
AI 226/234 cam
OBDI PCM
All the bolt ons and supporting mods

A run down of the problem...
The car runs fine for the most part with a good AFR, but randomly during idle and part throttle driving it will lean out. It will actually lean out bad enough that the wideband won't read it which means 18+, sometimes it is even bad enough the car will start missing and cutting out. The couple times I have tried very short WOT pulls it is completely lean every time.

I was running the same tune up and had not changed any parts from before when this started happening. It used to run a little rich at roughly 12.1 AFR with the same tune up at WOT.

I haven't been driving the car because I don't want to burn up the motor running this lean but I'm tired of seeing it sit too so I mess with it when I have the time.

I drove around doing some datalogging and had my mechanical fuel pressure gauge hooked up as well. There are no DTCs and nothing out of the ordinary in the datalog from when the car does this. The fuel pressure is good and never drops.

Things I have done so far:

1. Fuel. Good pressure. Also unhooked lines and ran jumper wire to fuel pump to pump fuel into a container and see if it is pulling any air into the line. No air, everything's cool.

2. ICM. I know the ICM does have something to do with the firing of the injectors, so I did swap in a good spare ICM I have just to try it. No change

3. OPTI. As I understand it the opti also ties into the firing of the injectors. I removed my MSD opti from the car and swapped in a good OEM spare I had. No change.

4. PCM. I swapped in a spare PCM I have that I know is in good working order, no change.

5. Tune up. Like I said, I am running the same tune up as I was when the thing used to run a bit rich at WOT. I even tried several other bin files for different setups just for the heck of it and I cannot get anything to give this thing fuel. No change so I am pretty confident that there is a problem with the car.

6. MAF. I unplugged the MAF and ran it in speed density mode. Still the same problem persists.

The car starts and idles and runs fine, except for when it leans out. I can't figure this one out. Anyone have any ideas?

I tried to give a good description, but by all means ask me some questions.

 

A missfire will show up as a lean condition on the wideband. Take a good hard look at your plugs wires and ignition coil. If its not that it may be the optispark effing up and losing signal momentarily. I would look at global things like the coil and opti first though.

It makes perfect sense that the car would lean out and act up under wot if the ignition is weak.

I would have said check the maf readings under wot but with your speed density test you shoulda been able to make it rich.

If you want to send me a log I will take a look.

 

You helped me before with the knock retard ordeal. I never did figure that out LOL. That last year now this. I think the car is actually running lean then starting to misfire because of lack of fuel, not vice versa. I do have a good spare coil I can swap on though just for the heck of it. I am 100% positive that the opti on it is in good working order. I'll go ahead and try swapping to the other coil.

Would a FreeScan log do you any good? I don't have Datamaster anymore.

 

Datamaster is easier but freescan works. If you would leave some notes on whats going on where etc just insert a row.

How have you determined that it missfires as a result of going lean, rather than missfiring making it show lean.

 

I'm not really sure now. I went out and swapped that coil and it wasn't going lean any just sitting in the driveway idling. The idle was good and smooth. What's odd is if you stab the gas quick you can get the thing to backfire through the intake. I was able to do this with the MSD opti also.

I haven't had my plugs out in a while, but my Moroso plug wires were new last year. They still look like new and I am very good about keeping them routed and tied away from hot or moving parts. I see no arching going on.

Where should I go with it now?

 

Just take it for a drive and see if it starts acting up again with the different coil.

The backfire in the intake manifold is the slow response of the maf sensor to a sudden dramatic change in the airflow. Most lt1's with any power do it that i have seen running mafs. It wont do it if you come up on footbrake and nail it but from a dead idle out of gear it will. No way to adjust 'pump shot' in the lt1 pcm that I have seen. If you switch to speed density and you're good at tuning it you can make the backfire go away and pick up a little throttle response (although very little track performance will actually be gained if any). Whole lot quicker to lookup tables than wait for a wire temp to change and then look up values and interpolate for g/sec. It could also be something in your ignition timing from idle to stabbing the gas doing it.

 

I took it out again and it still does the same thing.

Could the o2 sensors have anything to do with this? From what I understand they are not used at WOT but I wanted to look into it anyway. It has Bosch ones on it that are a couple years old. I know people have been known to have issues with them.
It's odd, when I slam the throttle to the floor, it goes lean INSTANTLY. If I just leave the gear selector in first or second gear and ease into the throttle very slowly and gradually and get the motor up to 5,000rpm or so the wideband goes lean

 

It could be the o2 sensors but is the car pulling alot of fuel out? do the mv from the o2 sensors oscillate properly or are they flatlined up over 450mv?

No matter what your blms do if you are using the blm locker wot should work as usual. What type of grams/sec are you seeing Dan, do they fit with where you were back when you were tuning it or are they showing way less?

 

Oh now what the eff, you're pulling the same stunt I am (saw your for sale thread). I have a front mounted turbo setup ls3 in my garage waiting for me to trip over enough cash to finish it. Good luck with the swap.

 

I don't know man, **** it LOL. It's getting to be such a pain in the ***. Not having Datamaster makes it a pain too. FreeScan sucks. I didn't want to spend the money to buy Datamaster with how close I was to parting it out.

I like the LT1. They can definitely run. Mine has just been too troublesome and it's time to move on to something else so I can hopefully regain my enthusiasm in this car. I appreciate your help man. The LS3 turbo setup should be awesome.

 

Go boosted if you can at all afford it. Stop thinking about 4 or 500rwhp and step into where you are a loser if you don't make 650. Lt1 guys could learn the boost lesson too, 99% of the h/c stuff out there will get smoked by a pretty much stock boosted motor.

 

Can't afford boost right now. Believe me I'd like to though. It's probably in the future.

 

I have had a bad MAF problem that is similar to your symptom. I replaced my aftermarket MAF with the stock MAF (1997 SS), and it solved my problem. The MAF was not sending correct frequency to the PCM. It appeared to be filter oil fowling. I have also changed the MAP sensor. Also, I have had backfire with a bad opti (roto and cap, or fowled). Most of the backfire was @ high rpm. Hope this helps. B. (97ss 383ci D1 MM6).