Why Are B1 and B2 Fuel Trims So Far Apart?

[bseitz]

Ever since a friend got his 02 Corvette back from a popular local Corvette shop after having a cam, AFR heads, and dyno tune done, he had overheating issues.
I used up 2 of my HPT credits to adjust his fans to what he wanted them set at just to see if we could get the temps down,....no help.
A week later we did a log and found that his LTFT's were running really lean on Bank 1 and Rich on Bank 2. They were like 5.8 B1 and -3.7 B2. I suggested he take the car back and have them fix it and retune....so he took it back.
I thought he may have had an air pocket, head gasket problem or poss. something blocking the coolant flow on bank #1???

The shop said they found nothing wrong with the car. They said they had the tuner back to take a look and the tune was spot on! My friend said that they told him my tuning software was bad! Well I say BS to that!

This week we installed a DeWitts rad. w/oil and trans cooler in his car to try to cool things down a bit. We left the battery disconnected for a while thinking this might help to reset the fuel trims.

After a hundred miles of driving yesterday and this morning I thought I would post up the log and tune for you experts to look at. Fuel trims are not as bad as they were but still off quite a bit.

Ignore the AFR as he has no Wideband/bung in his car.

Guys, I'm new to HPT and don't know enough about tuning so I'm asking for help in steering us in the right direction.

He is not very happy with that shop and doesn't want to take it back there ever again so I guess thats not an option. He's thinking of driving the car to a tuner in California but all of this is a whole other issue.

Personally I'm not sure that the car should be driven much until the fuel trims are safer looking. Could this be in the tune or a mechanical issue?

Any help with this would be much appreciated!
Thanks Guys, Bob

[EC_Tune]

I would look over the headers for leaks. If you can get some smoke around them before the fans kick on you should be able to tell if there is a leak somewhere. That's my first suspicion as the differential between the banks is most severe at low MAP and near idle engine speeds. That's generally where you find the issues.

The trims need a bit more attention as they are mostly positive but you need to leak check the headers first.

[bseitz]

Thanks EC. We'll check it out, probably replace the gasket.

[bseitz]

A Follow up:

Well here we are more than 2 months later after my friend replaced the Bank 1 header gasket. Inspection of the old gasket showed no exhaust leak. We went ahead and replaced the gasket anyway.

We did a scan on our breakfast run Sunday morning and the results showed no change. Bank 1 is still 4-6 points leaner than Bank 2. On top of that, at a few times, Bank 1 was as high (lean) as +12 and Bank 2 @ +7ish.

The only difference is that the IAT was 35-40 degrees cooler (65' vs 105' ish)than the last scan this Summer.

I would guess that the VE and MAF tables need work and probably put to use the PE vs IAT table which the previous tuner left stock @ zeroed out.

Still why are the trims so far apart?

EC any other ideas?

My friend sprayed the intake manifold and no idle changes but he didn't spray underneath it to see if a leak might be there. Would this even be safe to do?? Don't need to blow up his car! Not yet anyway!

Thanks, Bob

[bseitz]

Take a look at this scan. His complaint was that his car was running warmer than normal when the temps are cooler outside. Not only his trims between Bank 1 and 2 are apart the car is running way lean again. That accounts for why the car is rinning warmer. Again, I'm new at tuning so I would appreciate any advise on this.

Do you believe that the shop that did this got paid for this crap! On the brighter side, this gives me a chance to learn!

[EC_Tune]

+/- 3-6% between banks is not uncommon. I thought you had a much bigger spread.

What temp thermostat does he have in the car? Looks like it's at least a 180 stat maybe even the stock 192 stat in the car. Generally if you have a 160 stat the car will run in the mid 170's.

[bseitz]

He does have a 160 in there! On top of that I helped him install a DeWitts with an engine oil cooler back in September to try to get the temps under control. Ever since he got that car back from the shop that did the work it's been running real hot.
On the same run on Sunday my car, also with a 160 stat (milled heads and cam too), was running cool just like you said, 174-176. And I have the stock rad and no oil cooler! He now has all that extra cooling power and his car would still be on average 10-14 degrees higher ECT.

I feel bad for him. He really loves this car, spent way too much money having all this work done and now it just doesn't run the way it should. I told him that I would repost here asking for more help but warned that he might end up taking the car as far as Cali to find a shop that has more experience working on and tuning C5's.

[Bill@HPTuners]

Dunno about the ECT issue however the LTFT's look to be within 1% of each other from what I can see. You can make a custom histogram using the default #3 histogram as a guideline & the PID for bank 1 & bank 2 & compare. The trims avg is VERY close throughout the entire range.

I've included txt files of such so that you can see there isn't much of a difference.

[bseitz]

Thanks Bill.

Mine are so close to 0 all of the time now and now I think every car should look like that. I guess I should feel lucky.

Ok then, Aside from the warmer ECT there aren't any worries, at least with the tune.

[Bill@HPTuners]

Thanks Bill.

Mine are so close to 0 all of the time now and now I think every car should look like that. I guess I should feel lucky.

Ok then, Aside from the warmer ECT there aren't any worries, at least with the tune.

I wouldn't say all that, I agree with ECtune in that the trims really shouldn't be that far positive as they can affect wot fueling. I was just stating there isn't much of a difference bank to bank.

[EC_Tune]

Did the shop change the factory fan shroud on the car? If so the air isn't being forced through the radiator at speed (MPH). The fan shroud should seal pretty well to the radiator/AC condenser for maximum cooling.

[bseitz]

The fan shroud is still the stock one. It was sealed up pretty well, same as it was from the factory.

Even after we installed the DeWitts rad /trans cooler /engine oil cooler we made sure everything fit back nicely.

We are back to having warmer weather again so his car is running cooler and his LTFT's aren't as high. (His ECT still doesn't run as cool as mine though.) I believe the cooler weather was causing his car to run lean and in turn, higher cylinder head temps. (he has a Blackwing, no CAI if you were wondering)
One thing I don't know for sure is the CC's of his AFR 205's. He told the shop he wanted them milled for 11 to 1ish comp ratio using the GM MLS gaskets. 59cc's? 62cc's? All that would have an impact on ECT's.

That tune needs work. Would you suggest I start with STFT's or is the tune far enough out to start with LTFT's?

I purchased HPT just to play with my own tune as my car had drivability issues when I got it back from the same shop 2 years ago. No intention of messing with anyone else's car. Funny how when others find out you have tuning software that you get all these little requests and then it costs more work than you ever expected.

I guess that how one gains experience Huh

[EC_Tune]

Hehehe. One person on another board once had in his signature:
"Experience is directly related to the amount of broken parts accumulated."
Some have more broken parts than others...

As to the fueling. I would start out by disabling MAF (Engine Diagnostics) and LTFT's (don't forget to reset them in the scanner). Tune the VE table using the STFT's and wideband for WOT (if you have one) or get on a dyno that has a wideband.

Once you are happy with the fueling & spark, turn the MAF back on and tune it. Once you're done with that turn the LTFT's back on and recheck the whole mess with the MAF on.

Let me know if you need help.

[DSteck]

I purchased HPT just to play with my own tune as my car had drivability issues when I got it back from the same shop 2 years ago. No intention of messing with anyone else's car. Funny how when others find out you have tuning software that you get all these little requests and then it costs more work than you ever expected.

I guess that how one gains experience Huh

As a side comment... I've found this to be incredibly true. I get a lot of requests to scan a vehicle and trouble shoot, as well as requests for tweaks (which people want for free... and isn't happening because of credit cost!). Since I posted my success with taming my QM600 cam on CorvetteForum, I get asked at least twice a week to send my calibration file out! It's nutty.



FWIW, if you actually notice very different fuel trims between banks, it may be worth the time to yank the valve covers and check for a broken spring. I'm pretty sure I had a broken spring for a while because the delta between bank 1 and 2 for LTFT was in the 10-15% range. Eventually, they catastrophically went kaboom... but I'd be one was dead for a while.