I have read several threads on this subject but decided to start a new thread to see if anyone has new information. 86 IROC 305 TPI. Engine completely overhauled about 25K miles ago. Ran great. Recently we started having the following problem. Cold morning start. Starts and idles great. Start down the road after a few minutes and seems OK. From a dead stop, still very cold you try to accelerate and the engine stumbles, almost dies, backfires through the intake, spits, stutters and you go nowhere. This happens a few times from dead stops until the engine starts to warm up. Still somewhat cold and I believe in open loop you have little power unless you really depress the throttle then she takes off. Finally, after warming up, probably closed loop, it runs great until the next morning. First question I have is what is the backfiring through the intake? I assume I am getting fuel ignition in the intake from improper spark timing when an intake valve is open or an intake valve is not closing completely. But why then does it run OK when warm? Some of the other threads talk about lean conditions, bad sensors, bad coil etc, but I can’t understand an intake fuel ignition unless a spark occurs when an intake valve is open. I do have a scanner and have checked the engine but only when warm. This computer has limited information. The O2 sensor reads millivolts not air/fuel ratio. What is the proper voltage for an air/fuel of 14.7% the ideal AFR? What should I look for? Ant thoughts would be greatly appreciated.

Hey, this is Jeff.

I'm not entirely familiar with the 3rd gen's, so someone else should answer the above questions...But for oxygen sensors, they are pretty much standardized for all older cars.

When up to operating temperature (which can take up to 10 minutes, depending on the sensor), the sensor will output a voltage ranging from 0 to 1.1 Volts. Low voltage meaning lean, high voltage meaning rich. 0.5 Volts is about the median of the sensor's output, and corresponds to a stoichiometric air/fuel ratio of 14.7:1.

Ideally, at idle, the oxygen sensor's signal should hover around this 0.5 Volt level. When monitoring the sensor, you will probably notice the voltage only changing every second or so, and fluctuating pretty dramatically. This is normal.

Traditional oxygen sensors (narrow-band sensors) are not at all accurate in reporting a specific air:fuel ratio. Instead, they merely tell the engine management whether the engine is running RICH or LEAN. If the sensor's output were a binary rich or lean, it would probably loose very little accuracy. Newer, and more expensive wide-band oxygen sensors, however, give you a much more accurate reading of the AFR's.

Someone else should repond to the other questions above. As I am not all that familiar with 3rd gen camaro engine management.

Jeff

I would absolutely love to get this problem out in the open and talk about it! I can not figure out what causes this! I hope someone smarter than me jumps in here and makes my day.

My car did not do this until about 3 years ago and now it does it evey time it starts up cold in the morning. Pops through the intake, sputters, no throttle response, everything described above. My car did it when the motor was stock and it still does it now. Obviously my car runs very well. It was being properly fueled when I had it on the dyno, and it gets great gas mileage. The engine runs fantastic once warm but until it gets warm it is a bear. Mine stumbles so bad some times that I'll let the clutch out and it will start stumbling and i will press on the accelerator, when the engine catches up with itself it will start spinning the tires!

Possibly the cold start injector is not working correctly. Maybe the switch turns if off too soon?

Surely someone else has this issue. Moderators, other smart guys, anyone? I think we are open to suggestions here.


Ploegi, I do not know how long the cold start injector is supposed to fire. I thought it was for start up only but i am likely wrong. I will do a search and see what I can find.... In the mean time anyone else got any ideas?

I believe I read in one of the other posts thst the cold start valve is only open for about 8 seconds. My car actually runs fine during that period.

I received this list of things to check from a mechanic. He told me that it could be any one of them and that this problem is a difficult one to crack. From all the other post on similar problems I did see many of these mentioned. I plan to record a session with my scanner and compare it to a reading when the car is running OK. I also plan to systematically go through many of these checks and will report if I find any thing.

Here is the list he suggested I check:

Vacuum leaks
Fuel pressure regulator
PCV valve
Oil consumption which may indicate a valve problem.
Bad plugs, sign of oil burning
Valve stem seals
Sticking valves
EFI, IGN grounds corroded or bad connections

The following three can cause a lean condition that a warm engine can compensate for:
• Warn cam lobes, sticking or burned valves
• Compression low
• Weak or broken valve spring

Ign. control module
Fuel injector controller
Temp sensor
Throttle position sensor or wiring
EGR sticking open or partially open before the engine is warmed up.

Check sensors for specified resistance and voltage. Checking at the ECU will eliminate wiring problems

I have been told a lean condition many times causes this problem but then again what is causing the lean condition?

I believe I have discovered why my car is stumbling and backfiring. I connect both a wide band oxygen sensor to the exhaust manifold and connected a scanner to monitor the computer. When the car was stumbling, backfiring and had no power the wide band said the car was running extremely lean. Ideal is 14.7:1. When I tried to accelerate during the stumbling period I was reading an average of 20:1. Some peaks as high as 22:1. According to my manual 20:1 is a free air reading. The average lean condition lasted for approximately 20 seconds.

I also captured several other sensor reading during that acceleration period. Everything else looked normal. The coolant temperature was rising, throttle position sensor looked OK, Injector pulse was increasing, and mass air flow increasing. One thing I question was the EGR duty cycle that opened to about 50% right when the car started to stumble. Not sure if that is normal or not.

The manual on my wide band meter says ignition misfires will cause lean conditions. Un-burnt oxygen flows into the exhaust manifold and the meter reads a lean condition.

At some point my computer stored two error codes. Code 44 oxygen sensor lean and code 42 electronic spark timing monitor error. I am going to focus on the code 42. I think my problem is ignition not fuel.

I would appreciate any comments or thoughts.

Interesting, The EGR is working mostly when the car is cold. Try this. Disconnect the vaccum line on the EGR valve and plug it. Then try a cold start and see if it has the same symptoms. This almost is starting to sound like a sticky EGR valve.

Both you of who mention having this problem try this trick and let us know the results.

Thanks,

I have no EGR function. My aluminum heads dont even have a heat riser for the EGR valve. My intake doesnt even get warm enough for the temp sensor at the base of the EGR valve to call for EGR function, hence I never get a check engine light for EGR anymore.

That measurement is definitely too lean (according to what the wideband says). While accelerating, the AFR's should be getting closer to 12.5:1 - 13.5:1, slightly richer on the scale under heavy loads.

Is the injector pulse-width roughly the same value when the car is stumbling vs. acting normally? Taking note of the Inj PW when the engine is stumbling vs. running normally, at the same engine speed, and engine load (MAF value), the pulse-widths should be relatively close to each other. If they are not, and the percentage difference in PW is roughly the same percentage difference in Air/Fuel ratios between the two running conditions (14.7:1 vs. 22:1), then you may be able to deduce that a variance in pulse-width is causing the engine to run lean, rather than a misfire causing the wide-band to read lean.

Just a thought, but is the alternator putting out the correct voltage immediately after the engine starts? A significant drop in system voltage can cause variances in injector pulse width, as well as differences in ignition spark energy. With the alternator in my Nissan I have to rev the engine past 2200 rpm immediately after startup or the alternator will not turn on. When it doesn't turn on, the ecu & electronics are only running off 12V or less, and the engine stumbles terribly...the same exact feeling as your camaro.

I had similar problems in two of my cars, replaced the EGR valves and it got a lot better, but still not quite perfect.

Guess that i'm the only one having this same identical problem at this time...these threads tell me that i will be referring back here a lot!