Gtpguy

My car is backfiring at WOT before shift. However, this is why I posted in here.

I read this on here in another thread about my backfiring at WOT before shift: 5805 RPM (stock) then cuts back on at 5794 RPM. Well I know stock tire size is 245/50r16 and I have 315/35r17s now. Well, Sumitomos are shorter and narrower than true 315/35r17s and when I pass by a speed indicator it says Im going 3-4mph slower than my speedo reads.

Am I on the right path here? It makes sense, but would the car actually backfire repeatedly until I let off gas to have it upshift to the next gear? Otherwise the car accelerates nicely. Idle is a bit lumpy, but no hesitation is present whatsoever. Some are saying to change out the coil, but still the tire size program in the ECM is nagging at me now.

Would changing the tire size in the ECM help me out? And if so what should I change it to because the tire is shorter? I was thinking 315/ "30" r17.

Your thoughts?

Injuneer

The only thing that matters with regard to the tire size is the outside tire diameter. A 315/35-17 has the same outside diameter as a 245/50-16. That is not the source of your problem. Tire Rack shows the diameter of the Sumitomo HTR-Z as 25.7", which is identical to the stock tire size, if your car came with the Z-rated tire option.

Gtpguy

Right my car was originally equipped with Z rated tires and we verified it still has the 3.23 rear, why is my speedo off according to speed detectors, whereas my last car is was dead on with stock size tires with the same detectors. My speedo is 10% slower, thats a big margin of error don't you think. The detector says Im at 43mph, my speedo says 38mph. It seems that could play a role in shift points getting all screwy if the rpm and mph need to meet at a certain spot and they aren't meeting at the spot, the shift won't happen is the way I understand it.

Where do you think I should start looking to figure this out? Its annoying I cant do a full run because of hitting the rev limiter or fuel cutoff or whatever right at shift point.

chevy_dude97

You do not program the pcm for tire size to fix rpm but mhp. Unless your hitting your top speed limit there is another problem.

If your rev limiter is set too low and your shift points high then you could have a tunning problem.

Gtpguy

I obviously have a tuning problem. I just need to know where to look now. The speedo is off and the car is not shifting at WOT.

Here was the scaler tables we pulled from the ECM tonight: Forgive misspelling and terminology.

Current whatever its supposed to be in the tune:
Speedo Scaler - 517
scaler fraction - 4.38
scaler scan tool - 43.82
scaler pulses - 1.53

Ive searched and found this. Is this what it should be with 3.23?:
Speedo Scaler - 26
Scaler Fraction - 1.01
Scaler scan tool - 43.8
Scaler Pulses - 34.256

Injuneer

You indicated 2 different things:

First that the speed indicated by the traffic device was 3-4MPH "slower" than your speedo.

Then you indicated the speedo is 10% slower than actual speed.

To create a 10% speedo error (indicated speed greater than actual) your tires would have to be 10% smaller diameter than stock. No way the Sumi's are 25.7" x 0.9 = 23.1". IF your speedo is reading 10% slower that actual, the Sumi's would have to be over 28".

shoebox

That is what my stock bin had with 4L60E and 3.23s.

Gtpguy

Sorry Injuneer, it was a long weekend, my speedo is faster by 10% than actual speed.

And this was is in my PCM now:


Speedo Scaler - 517
scaler fraction - 4.38
scaler scan tool - 43.82
scaler pulses - 1.53

And there is a whole lot of other weird stuff too. Like someone just went in and changed numbers for fun.

Would having a stall and different gears necessitate a such changes? I need a bone stock 94 3.23 auto bin file for Tuner PRO. You guys have one?

bobdec

Gtpguy, I am not that familiar with TunerPro, but I know when using TunerCat that some earlier '94's have older revision 'B' PCM code that requires a different definition file ($EEB) to display the data so the editor sees everything correctly. When you say " there is a whole lot of other weird stuff too " is there other data in tables and constants that looks possibly scrambled with crazy or invalid numbers in the fields. That would be a symptom of the definition file not matching the data field offsets in the flash memory. Fix would be to load a later rec 'C' or higher into the PCM. OR see if TunerPro needs or requires a different definition file. I have a '94 A4 stock load .bin file at rev 'C' for a 2.43 rear, sorry no 3.23. I answer to your questions changing gearing requires a new tune for speedometer correction and for correct A4 shifting, a high RPM stall converter MAY require tuning to insure you don't hit limiter at WOT shifting because of converter slippage, basically tune the WOT shifts a little earlier or up fuel cutoff RPM. Depends on how radical the stall and your mods are..Data logging will tell if you hit that range.

Gtpguy

Exactly what you described, just weird wacky off the wall numbers. Injector flow rate is in 743.46!! And whatever my shift scalers are up there.

Anyway, I ordered a stock tune from MadZ28 ($25), hopefully things will look better. I wonder if mine is an early one, sept '93 was when it was first registered. Im sure MadZ28 would be able to send the version I need, right?

Gtpguy

Stock tune didn't fix my problem. But a new coil and coil wire did. The stock one had residue on it and the wire itself was rusted at the coil connection and wasn't even attached. What a load off.

Now to start playing with tuning

bobdec

Glad you got it running. Seems we get wrapped up in the computer a lot and it turns out to be an old school fix. Way back in '98 when I bought my car and never knew what a computer or tune was. It backfired, stalled and would not start. Towed it to a local shop (later found out they knew new crap about LT1's) , they put in a new PCM, did not fix, finally the mechanic just about electrocuted himself by laying his hand on the corroded terminal connection on the coil primary. Heard that story 1 month after I paid $225.00 (for a wire from the coil to OPTI). From then on I decided it was time to start reading and get into DIY for these cars or pay a lot of $$ to shotgun parts till it works. By the way to upgrade PCM code all you have to do is load a rev 'C' or later .bin file. After the tuner sends you a new file you will then be at the newer code level and TunerPro should then work w/o scrambling the data.

Gtpguy

No kidding, thanks Bob. Your right about the obvious fixes. Sometimes it seems to obvious so you overlook it because you are expecting it to be the hard fix. Thank goodness for the easy fixes.

chevy_dude97

i still don't see how tire size would have caused this problem... at least that is where I was going. Glad to see you got her back on the road. I know I hate the feeling of why won't my car work right...

LSWHO

Your PCM decides to shift when you hit a certain RPM and speed, then it tells it to shift. It actually shifts 400 rpm or so later, at least from what I've seen in the tunes.

If the tires are wrong, it will see the speed it needs to shift later and then shift later causing the rev limiter to be hit instead of actually shifting prior to it.

Basically it makes the RPM and mph not match anymore, both are inputs for the shift point.