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Discussion Starter #1
Ive done quite a bit of searching on here and other forums, and cant quite find any threads that match the symptoms im having.

I installed a Vengeance VRX4 cam for a friend of mine in his 04 Z06 about 2 years ago. The car was tuned by Josh at Lashway Motorsports and ran great.

Fast forward to now.... I bought the car from my friend last week and noticed that on cold start, its undriveable. He said it just recently started acting like this, so Im assuming its not the tune causing it. It idles fine, but as soon as i even touch the throttle, itll rev and then stall. In neutral or in gear, doesnt matter, it stalls. It also smells extremely rich. The only code Im getting is for a rear 02 sensor that wasnt tuned out. Once the engine is warmed up, it runs fine.

So far, Ive cleaned the air filter, cleaned the MAF, cleaned the throttle body, replaced spark plugs and plug wires, and replaced the battery. None of that has improved the stalling issue.

Anyone seen anything similar?
 

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You really need to use a full OBD-II scanner/recorder and find the facts

1. bad 02 sensors command incorrect AFR, colder temps are, the richer AFR is and worse due to 02s that should have been replaced a long time ago
2. Possible then that would cause the CATs to clog up and AFR will be even worse
3. Using OBD-II scanner see what the fuel trims report on old start and then once warmed up

O2 sensors only last about 30,000 miles due to the high temps they operate in.
Using degraded 02s will cause bad AFR, loss of fuel mileage and performance

When using a good OBD-II scanner allows to look at the on-board smog tests, that helps determine if O2 and CATs tests pass help knowing quality of them
Yank one side of battery for a few seconds, that would flush all smog tests to incomplete state, hoof back up and do a drive and over couple of days see if
those smog tests go into a complete state or not

4. Powertrain changes over time and use and having a tune because of the CAM and other mods could have been on the limit and now requires retune after issues with smog, EVAP, O2s and CATs are corrected

5. also check that the smog pump and EVAP is working as it would effect cold starts.
 

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Discussion Starter #3
You really need to use a full OBD-II scanner/recorder and find the facts

1. bad 02 sensors command incorrect AFR, colder temps are, the richer AFR is and worse due to 02s that should have been replaced a long time ago
2. Possible then that would cause the CATs to clog up and AFR will be even worse
3. Using OBD-II scanner see what the fuel trims report on old start and then once warmed up

O2 sensors only last about 30,000 miles due to the high temps they operate in.
Using degraded 02s will cause bad AFR, loss of fuel mileage and performance

When using a good OBD-II scanner allows to look at the on-board smog tests, that helps determine if O2 and CATs tests pass help knowing quality of them
Yank one side of battery for a few seconds, that would flush all smog tests to incomplete state, hoof back up and do a drive and over couple of days see if
those smog tests go into a complete state or not

4. Powertrain changes over time and use and having a tune because of the CAM and other mods could have been on the limit and now requires retune after issues with smog, EVAP, O2s and CATs are corrected

5. also check that the smog pump and EVAP is working as it would effect cold starts.
I downloaded an app and got a bluetooth code reader to be able to see live data. By the time I got it paired,it was already into closed loop and running correctly. Ill have to take a look in the morning on cold start.

The car doest have cats (Hinson longtubes and x-pipe), and rear 02s are tuned out.

I replaced the battery today, so the long term fuel trims have been reset.

If the o2 sensors were the problem, wouldnt that cause the car to run poorly in closed loop, not open loop?

Ill upload a video of what its doing...
 

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I downloaded an app and got a bluetooth code reader to be able to see live data. By the time I got it paired,it was already into closed loop and running correctly. Ill have to take a look in the morning on cold start.

The car doest have cats (Hinson longtubes and x-pipe), and rear 02s are tuned out.

I replaced the battery today, so the long term fuel trims have been reset.

If the o2 sensors were the problem, wouldnt that cause the car to run poorly in closed loop, not open loop?

Ill upload a video of what its doing...
If in fact you installed a "code reader" app rather then a OBD-II scanner ( there are scanners for cell phones) you will not be able to see all the engine feedback sensors like O2s, timing, fuel trims, etc

Your first post stated : The only code Im getting is for a rear 02 sensor that wasnt tuned out, so that confused what was going on

You might want to check fuel rail pressure, engine cold, key on, engine off, using a fuel pressure gauge connected to fitting on left fuel rail
should report a solid 58 PSI.
Then someone watching that gauge try to start and see what the rail pressure is

All guessing and why a OBD-II scanner would report exactly what is going on, good scanners allow to record engine sensors and then later replay it would help
pinpoint what the cold start problem is including a tweak of tune needed.

Due to E10/E15 gas possible the fuel filter/regulator is weak on cold start ?

On cold start the PCM in open loop but within the first few seconds uses the O2s to determine what the average fuel injector pulse width ON time is
So if the O2s are degraded or an exhaust leak fooling them will effect what amount of fuel PCM commands to pulse width

Your video is too small for me to see anything of value.
Use OBD-II scanner and report what fuel trims (long and short) are, any current of history DTCs
 

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Discussion Starter #6
If in fact you installed a "code reader" app rather then a OBD-II scanner ( there are scanners for cell phones) you will not be able to see all the engine feedback sensors like O2s, timing, fuel trims, etc

Your first post stated : The only code Im getting is for a rear 02 sensor that wasnt tuned out, so that confused what was going on

You might want to check fuel rail pressure, engine cold, key on, engine off, using a fuel pressure gauge connected to fitting on left fuel rail
should report a solid 58 PSI.
Then someone watching that gauge try to start and see what the rail pressure is

All guessing and why a OBD-II scanner would report exactly what is going on, good scanners allow to record engine sensors and then later replay it would help
pinpoint what the cold start problem is including a tweak of tune needed.

Due to E10/E15 gas possible the fuel filter/regulator is weak on cold start ?

On cold start the PCM in open loop but within the first few seconds uses the O2s to determine what the average fuel injector pulse width ON time is
So if the O2s are degraded or an exhaust leak fooling them will effect what amount of fuel PCM commands to pulse width

Your video is too small for me to see anything of value.
Use OBD-II scanner and report what fuel trims (long and short) are, any current of history DTCs

Sorry, I meant scanner, not code reader. It does show fuel trims and o2 activity and such. Yesterday I wasnt able to get any good fuel trim info because I'd just replaced the battery, so the long term wasnt really telling me anything.

Since I changed the battery, the B1S2 O2 sensor code hasnt come back. Im assuming it will at some point because I believe the tuner forgot to tune that one out. The car ran fine with that code for a year after the tune. This problem didnt develop until later, so I cant imagine that code is causing this.

My scanner doesnt record anything, just shows live data, so its tough to look at everything before it warms up.
Readings at idle...
Coolant temp ~180F
Short term FT B1... 0 to -2.3%
Short term FT B2... 0.08 to -2.3%
Long term FT B1... 4.68%
Long term FT B2... 3.9%
Throttle position, at idle... 7.84%
MAP... 0.53atm
MAF rate... 9.85 g/sec
Timing advance... 21 to 26 degrees
O2 B1S1... 0 to -3.0%
O2 B2S1... 0 TO -2.3%



fuel pressure, key on, engine off -> 58psi, then drops to 52psi
fuel pressure, engine running -> 60psi
 

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Since coolant was already at 180 Deg. better to do this early AM when weather is cooler and cold start

Data shows B1 (left head) and B2 right head balanced as to fuel trims which is good
Long term reporting negative values so was a bit rich -2.3% but within working margin
MAF rate fine for idle but better to report MAF in Hz Freq
Look for O2 sensor as to voltages rather then %

As to MAP, not sure what afm means, better to see if values in KPA
if MAP value is in KPA then CAM design is causing lose of vacuum at low end, normal stock at idle like 35-40 Kpa

Did the 58 PSI with KOEO show right away or have to creep up ?
 

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Discussion Starter #8 (Edited)
Since coolant was already at 180 Deg. better to do this early AM when weather is cooler and cold start

Data shows B1 (left head) and B2 right head balanced as to fuel trims which is good
Long term reporting negative values so was a bit rich -2.3% but within working margin
MAF rate fine for idle but better to report MAF in Hz Freq
Look for O2 sensor as to voltages rather then %

As to MAP, not sure what afm means, better to see if values in KPA
if MAP value is in KPA then CAM design is causing lose of vacuum at low end, normal stock at idle like 35-40 Kpa

Did the 58 PSI with KOEO show right away or have to creep up ?
fuel pressure jumped to 58psi instantly, no hesitation or creep at all.


Just cleaned the grounds in the engine bay, hoping maybe that improves something, but not getting my hopes up.


I tried to get data this morning when it was cold, but of course it started up and ran perfectly fine. Ill try again tomorrow morning. On first startup, would short term fuel trims be the best data to be watching?


edit - looked back at the data i took earlier. I got B1S1 voltage at 0.9v, I missed B2S1 in voltage.
 

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fuel pressure jumped to 58psi instantly, no hesitation or creep at all.


Just cleaned the grounds in the engine bay, hoping maybe that improves something, but not getting my hopes up.


I tried to get data this morning when it was cold, but of course it started up and ran perfectly fine. Ill try again tomorrow morning. On first startup, would short term fuel trims be the best data to be watching?


edit - looked back at the data i took earlier. I got B1S1 voltage at 0.9v, I missed B2S1 in voltage.
Stoich would be about 0.500 mV so 0.9 is quite rich your really need to see B2S1 in mVolts to see if AFR is balanced

With engine in open loop the PCM ignores some feedback and uses the VE tables which is predicted air flow and adjusts fuel flow from that

Also

1. if heaters for O2s is not working then 02s would take too long to trip and become active, check the fuse for 02 heaters just to be sure it is not a issue
2. with scanner on with KOEO, then do a start, try and see anything odd in feedback such as timing and if injectors are being commanded, maybe about 3 mS ON time
3. After PCM has gone closed loop then all feedback sensors are used and see then what the fuel trims look like
4. If scanner supports GM enhanced and allows the scanner to request PCM to report all DTCs, history or current and see if anything shows up

5. You mentioned replacing ign switch and key but there also is a small wire in column that is know to have random VATS issues, Older car I'd turn VATS off in tune

6. colder temp might be enough if the clutch safety switch out of alignment, you could put a jumper wire across the switch to override it and see if that is a issue
7. fuel pressure looks good but sometimes the fuel filter/regulator allows pressure but less fuel flow

8. Just might be some change to engine, and tune is now too rich for cold start
9. On cold start the PCM commands the smog pump and EVAP on for 2 minutes, if the pump is not working then the AFR is too rich and fools the 02 sensors
10. any leak after the MAF would be unmetered airflow and confuse PCM as to command ON time of injectors
11. any exhaust leaks will confuse true AFR via the front O2s
 

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Discussion Starter #12 (Edited)
Started the car this morning, and of course it ran fine again and wouldn't stall.

I havent done anything with the ignition switch/key.

I did however, confirm that I have a failing clutch pedal position switch. The previous owner of the car, with not much technical credibility, told me that the car needed a starter. I had yet to experience anything that supported that. Today, leaving the store, I turned the key and nothing happened. While still holding the key in the start position, I pedaled the clutch a little bit and it started. Went to the dealership and got a new one for $15 and replaced it already.

Is there any way that failing switch could contribute to the engine stalling? I would think once the engine is started, that switch wouldnt really contribute to anything.


Data from this morning...
Coolant temp... 84F
O2 B1S1... on cold start, rose from .5v to .9v steadily. Once warm, up and down from 0.05 to 0.85v
O2 B2S1... same characteristics as B1S1
 

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Front O2s look like working OK

Only time will tell if you fixed the startup issue, for only $15 and the age of switch worth replacing it anyways
Just might be the switch contact was closing and opening at time of startup and confusing the PCM causing commanding injector pulse width on and off

Being you have a scanner now get used to using it and look to see what the average fuel trims look like and 02s for non WOT should average about 0.500 mVolts
There is a torque parameter if you scanner supports it which would allow you to see what flywheel torque is and from that calculate what the horsepower is

There are cheap scanners that allow scanning, recording, replay and exporting the data for spreadsheet so you can see what averages, maximum values to know health of engine
Buy a scanner that supports GM enhanced and not just generic

If it had been starter it would more apt to fail when hot, not cold and just would not crank which is not your problem
 

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I had trouble on cold start after setting overnight engine ran rough and would die. after warm up ran fine. Found boot from air intake to TB to be folded over and leaking. Fixed that ran like new. Hope this helps.
 

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Started the car this morning, and of course it ran fine again and wouldn't stall.

I havent done anything with the ignition switch/key.

I did however, confirm that I have a failing clutch pedal position switch. The previous owner of the car, with not much technical credibility, told me that the car needed a starter. I had yet to experience anything that supported that. Today, leaving the store, I turned the key and nothing happened. While still holding the key in the start position, I pedaled the clutch a little bit and it started. Went to the dealership and got a new one for $15 and replaced it already.

Is there any way that failing switch could contribute to the engine stalling? I would think once the engine is started, that switch wouldnt really contribute to anything.


So what was your problem because I’m having the same issue with my car.
 
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