NotLawReview

I've been battling some interesting issues ever since I got this car (replacing another '04z that was lost to the Memorial Day floods in Houston last year).

The mods the car has are as follows:
Heads/Cam package (NO IDEA on the specs)
FAST 90/90 manifold/TB
Callaway Honker intake
LG Long Tubes
Random Tech High Flow Cats and X Pipe
Callaway Double D Exhaust
Callaway Sway Bars

Ever since I got the car I have noticed that my fuel mileage has been way down as compared to my old car, (19-20mpg @80% highway driving) and I've always noticed a hydrocarbon smell that I chalked up to the cam/HF cats. However, of late the car has started to have some issues that have made me think that something isn't right, so I purchased a ScanTool OBDLink MX scanner to log what the car's computer was picking up. Here are the results:

https://drive.google.com/open?id=0B9...UxES3d4dV9oNDA


The symptoms that I was experiencing that prompted me to get this were:
Random rough idle that would bounce down to 500 or so RPMs and then up to 1200 RPMs, and would occasionally die. The idle would remain rough until I made it to the stop sign down the street where I would also make sure to get to about 2500-3k RPMs, and then everything would be fine. Also, sometimes paired with this would be an issue that was like the throttle was cutting out on me, as if the car was cutting spark or something. Very strange.

When this would happen, the error codes that I would get were:

P0522
P1546
U1016
U1096
U1255
U1040
U1000
(also have no comms to the RFA, and I have tested the wiring back there and it's good, so I think that receiver might be bad. The remote locks have never worked since I got the car, and I've tried all the usual suspects to fix them)

I have replaced the following:
-New AIR tube gaskets
-2 New ignition switches
-New valvesprings
-New spark plugs/wires with plug wire insulation boots installed.


I have inspected the wiring to the O2 sensors and everything appears fine, no melting from the headers.

I have inspected/cleaned the engine grounds from the battery to the chassis, block to chassis on both sides of engine, and checked/cleaned the connections at the starter.

I have checked the connection at the PCM and cleaned it up.

Fuses have been checked in both fuse boxes.

PCV line has been checked for leaks, as have the hoses connecting the MAF to the TB.

I have cleaned the MAF with MAF cleaner, and have also cleaned the TB butterfly.



Now, to the logs. Obviously the LTFTs are WAY high on both banks, so the engine thinks there's a lean condition. I don't currently think there's a vacuum leak, but I could be wrong. The next thing I'd do would be to replace the intake manifold gaskets, which I'm not super pumped about doing, if I don't have to.

Next, do the O2 sensors look like they're operating normally? It's my understanding that they should fluctuate in a wave-like pattern, which I don't see here as the numbers are pretty steady most of the time, and with spotty extreme changes. I'm hoping this is the issue as it would be the easiest thing for me to change out, but also don't want to just replace parts *****-nilly if not necessary.

Lastly, could this just be a bad tune?

I'm only now trying to gain an education on these reading with what I've found from the internet, so any help that anyone can provide would be great!

Thanks!
Pat


Edit: Here's a longer log that was just done last night, about 100 miles of driving done since the first log: https://drive.google.com/open?id=0B9...2V4ZDY0Rk9lX0U

Update: Here's a new log with the MAF recording in g/s, which keyed me in to the possibility that it's BAD, as the reading is at least 4x higher than it should be, which would explain the lean reading and rich reading O2 sensors:
https://drive.google.com/open?id=0B9...mRNMkxYejZFXzA


Here's a log with the MAF disconnected, which I know will only provide for so much in that the IAT is crucial for the PCM to figure out the fuel trims, and it's built into the MAF on my car since it's an '04.:
https://drive.google.com/open?id=0B9...2pkcmg1RG1ydm8


UPDATE 05/10/2016:

SURPRISE! BAD MAF and NO CODES.

The adapter (to convert a rear sensor plug to a front sensor plug) will be delivered today and the new o2 sensors will be installed tonight.

However, over the last few days it has really been bugging me that both the LTFTs were high and the o2s were reading rich, as they should be the opposite of each other, so I decided to look closer at the MAF readings. I changed my logger to g/s instead of CFM as in the original logs and noticed that I was getting 20+g/s at idle!

Bought a new MAF sensor and installed it last night, idle was IMMEDIATELY rock solid, and here's what I have logged:

https://drive.google.com/open?id=0B9...XVpRXV5MElIYjQ


As you can see, the LTFTs are now NEGATIVE and everything appears to have been learned as it should.

However, my fuel economy hasn't quite improved like I expected it to, and I think that's a result of the o2s reading lazy and rich. Once I swap them tonight I hope to see a significant change.

Also, once concern that I have is that the car is not pulling up high like it did with the really rich LTFTs and the bad MAF, and that surprised me as I was expecting a big change in the seat of the pants feel as I would think that the car would be making more horsepower with the A/F being closer to where it should be. The car gets to 5k RPM and just feels like it stops pulling and the power curve goes flat. I will note that it looks like the MAF is still providing a slightly higher reading than I would expect it to (10 g/s at idle), so I'm not quite sure what the cause of that is.

Hopefully the o2s make a big change in things and the trims get back to where they should ideally be, and my power/MPG will return.

Bill Curlee

PAT

I highly suspect that the O2 sensors are NOT functioning properly!!

They SHOULD,, at a fully warmed engine, at idle,, cycle / oscillate between 100 milliamps to 900 millivolts .. Real world are more like 150/200,- 850/875 millivolts. If you bump the throttle or accelerate, it will peg to 900 and when you de cell it will peg LEAN to 100 ma.

NOTE. You need to make PROF POSITIVE that you are reading full battery voltage on the OXY SEN Fuse for the O2 sensor heater circuits.

Also make sure that you have a 100% good ground at the sensor harness connector to battery ground.

The sensor heaters MUST function properly for the sensors to work properly and work for a long time..

IF,,, the car was tuned with ****** O2 Sensors, installing new ones will make it run better BUT, may NOT resolve your fuel economy issues. That is fixed by actually doing a dynamic on the road tuning to adjust your MAF Calibration, VE Table, and a few other minor tweak's.

If it were me, I would start with a CLEAN MAF and new O2 sensors.

You can spray around the intake manifold with CARB CLEANER. If the idle changes when you spray around the areas where the heads and manifold meet, there's a LEAK somewhere! Same for any other vacuum fitting area.


Please let me know what you think...

Bill

NotLawReview

Thanks Bill!

I forgot to mention (and have updated the original post) that I cleaned the MAF with MAF cleaner and also cleaned the TB butterfly.

I also tried spraying around the MAF with carb cleaner to find a vacuum leak and that did not result in any effect.

I'll check to make sure I'm getting full voltage on the 02 sensor fuse, and will also check for good grounds on the sensor harness connector.


I just ordered some connector harnesses from Racetronix so I can put some new rear O2 sensors in, and will be picking up some new sensors.

Worst case, the car has 67k miles, so they're due anyway.

Thanks again!

Bill Curlee

SPRAY directly around the intake manifold where it mates with the heads!

REAR O2 SENSORS?? They have ZERO to do with engine performance! In fact, they could be TUNED out so the PCM never see them. All they do is tell the PCM that the CATs are working correctly!

IMHO, if there not tuned out, get them tuned out. You only need new primary O2 sensors!

Why do you need new connectors??

" I just ordered some connector harnesses from Racetronix so I can put some new rear O2 sensors "

NotLawReview

I'm sorry, I should have been more clear. I ordered Rear sensors to put in the front locations, and the connectors are to convert from the rear plug type to the front plug type.

It is my understanding that in LT header applications the rear O2 sensors are better as they have more powerful heating elements, which is ideal for LTs given the location of the upstream sensors is a little bit further back, and the material of the headers does not hold heat as well as the factory units.

enoniam

O2s read in mV not mA but the values listed above are in the ballpark.

They will not peg at around 900 mV under most acceleration conditions or throttle bumps. They do that in Power Enrichment mode which if I recall correctly for a stock tune is only when the throttle position sensor is reading at 93% or higher throttle.

NotLawReview

So looking at the log of results, the fact that the majority of the time the O2s are at .89 -.9 it would seem to indicate that they're not oscillating as they should? It looks like they occasionally drop to a lower value, but it seems like it's when i'm coming off the throttle when cruising on the highway/at a steady RPM.

enoniam

The thing is that what drives your long term fuel trims high is the O2s reading low. The PCM dithers the short term fuel trims - when dithered up the O2 readings should go up and vice versa when dithered down. If it dithers up and the O2 doesn't go up it keeps pushing the short term trim higher and higher until it does get the o2 to go up. While pushing the short term trim in a direction it will also start pushing the long term that direction as well, but much more slowly.

NotLawReview

So in other words, it makes sense that my O2 readings are high given that my LTFTs are high (the O2s indicating too much fuel) but the STFTs aren't correcting?

What conditions could cause both LTFTs and O2s to read high concurrently? If there was a vacuum leak, I'd think that the O2s would be reading low, as the vacuum leak would be after the MAF, so the O2s would be requesting more fuel, right?

93Polo

1st 2 things that come 2 mind are make sure the O2s are switchng, and wired to the correct side. The second is vacuum leaks coming from cracked hoses, the intake manifold being warped at the TB mounting flange, or cracked on the bottom, be careful torquing the FAST intake in place.


https://www.corvetteforum.com/forums...old-crack.html
You may want to search some topics created by the member above on his problems.

Bill Curlee

You are ABSOLUTY CORRECT! I went back and fixed the error. I was trying to do two things at once and the millivolt lost out to milliamps in my cluttered mind state.. .

Sorry for the confusion and thanks for the back up.

Bill Curlee

If you have a FAST Manifold,, MAKE SURE that the bottom of the intake manifold isn't cracked. VERY common if the fast was installed improperly. That will for sire cause a bad leak..

BC

enoniam

Your LTFTs are starting at zero and the O2 low readings are pushing the LTFTs higher until the O2s are bouncing up and down. Seems strange that they are starting at zero. Normally they are remembered and aren't usually sitting for the most part right at zero. Unless you reset them via HPTuners or other programmer command, or had your battery disconnected for several minutes, they should be remembered, unless you've got a custom tune that doesn't remember them after engine shut down.

While higher than what tuners (and GM I believe) would want to see, given the mods made it is quite possible that the MAF simply needs to be recalibrated and you don't have a vacuum leak. It seems to me that the car is having to learn new LTFTs every start and until it starts to get things sorted you are having the issues you noted above. Having the MAF calibration off as far as it seems to be means that until it gets the relearning close it is pretty far off resulting in less than optimum AFRs.

I think I'd try to find a tuner and see what he makes of what it is doing.

NotLawReview

Well, I did disconnect the battery after installing the new gaskets on the AIR tube, and that log may not have been finished updating, so here's one that has about 75-100 miles on it after the first log:

https://drive.google.com/open?id=0B9...2V4ZDY0Rk9lX0U

I had the thought/worry of a crack in the FAST intake manifold, but the question that I have as to that is: wouldn't a vacuum leak cause me to have these symptoms of bad idle/etc. consistently? I wouldn't think they'd show up so sporadically.

After changing the O2 sensors, and then checking the manifold for a crack/leak, I'll take it to a tuner.

A MAF can only be recalibrated through a tune, correct? This wouldn't be a result of the MAF going bad, where replacement could take care of the issue, if that were the case?

Thanks again for everyone's help, I really do appreciate it.

-Pat

enoniam

First off I need to apologize - I've got my logic backwards. Low O2 readings will push the LTFTs higher. The fairly consistently higher O2 readings in your first data log should be indicating where the engine was at stoich equilibrium. I will go back and edit my previous posts.

In your second data log you seem to be at and/or near idle for the first 200 or so rows of data, your O2 sensors seem to be reading high and low as I would expect, and your LTFTs remain pretty steady in the positive 16/17 range. But look at what happens around row 225. RPM jumped above 2400 in the previous row and shortly thereafter the LTFTs drop down below 10 and the O2 readings indicate a pretty lean condition. Stock tunes switch fuel trim cells around 2400 rpm (as well as a couple of other switching points at lower rpms). You aren't above 2400 rpm for long and so you switch fuel trim cells again. The computer remembers the last LTFT it was in for each fuel trim cell so when it jumps back to the cell it returns to the last LTFTs it was using in that cell. (I've attached a poor image of the stock fuel cell ranges which are based upon engine speed and manifold pressure.) The computer learns each cell independently and it learns them at different rates because depending upon the driving you are doing it is usually spending a great deal of time in some cells and little time in others.

Long tube headers increase the amount of time it takes for the O2 sensors to begin to provide good data back to the computer. If the car begins to run better after a couple of minutes your problem may be fixable with just better calibration of the MAF and possibly the Volumetric Efficiency table and possibly tweaking the warm up parameters to increase the amount of time the engine is running open loop and not counting on the O2 data for closed loop operation. By going closed loop too soon it can be using bad O2 data to move the LTFTs when they shouldn't be adjusted until the O2s are able to give better data.

NotLawReview

I might have only had a few days of learning this, but that makes a lot of sense, especially as it relates to the way I can "kick" the problems after first startup by driving the revs up to 2500-3k on the way to the first stop sign down the street. Then everything runs fine as I'm driving, and only occasionally will have trouble back when down at idle.

I'll still swap the 02 sensors given it's probably time anyway, and then I'll go get it retuned.

Thank you so much for your help and explanations. I'll update this further as progress is made.

Pat

NotLawReview

Update: Here's a new log with the MAF recording in g/s, which keyed me in to the possibility that it's BAD, as the reading is at least 4x higher than it should be, which would explain the lean reading and rich reading O2 sensors:
https://drive.google.com/open?id=0B9...mRNMkxYejZFXzA


Here's a log with the MAF disconnected, which I know will only provide for so much in that the IAT is crucial for the PCM to figure out the fuel trims, and it's built into the MAF on my car since it's an '04.:
https://drive.google.com/open?id=0B9...2pkcmg1RG1ydm8

Thoughts? Does it make sense/look like i'm on the right track in thinking the MAF is bad?

I know that most expect there to be codes when the MAF goes bad, but the data seems to make it look like that's not always the case...

NotLawReview

SURPRISE! BAD MAF and NO CODES.

The adapter (to convert a rear sensor plug to a front sensor plug) will be delivered today and the new o2 sensors will be installed tonight.

However, over the last few days it has really been bugging me that both the LTFTs were high and the o2s were reading rich, as they should be the opposite of each other, so I decided to look closer at the MAF readings. I changed my logger to g/s instead of CFM as in the original logs and noticed that I was getting 20+g/s at idle!

Bought a new MAF sensor and installed it last night, idle was IMMEDIATELY rock solid, and here's what I have logged:

https://drive.google.com/open?id=0B9...XVpRXV5MElIYjQ


As you can see, the LTFTs are now NEGATIVE and everything appears to have been learned as it should.

However, my fuel economy hasn't quite improved like I expected it to, and I think that's a result of the o2s reading lazy and rich. Once I swap them tonight I hope to see a significant change.

Also, once concern that I have is that the car is not pulling up high like it did with the really rich LTFTs and the bad MAF, and that surprised me as I was expecting a big change in the seat of the pants feel as I would think that the car would be making more horsepower with the A/F being closer to where it should be. The car gets to 5k RPM and just feels like it stops pulling and the power curve goes flat. I will note that it looks like the MAF is still providing a slightly higher reading than I would expect it to (10 g/s at idle), so I'm not quite sure what the cause of that is.

Hopefully the o2s make a big change in things and the trims get back to where they should ideally be, and my power/MPG will return.