I have a 97 Z28 that I use for road racing alone. Was running great, had to rebuild the motor due to blown head and now immediately on start up after the rebuild its backfiring on any throttle tip in and doesnt want to idle until its warm, if I go up on the throttle very slowly it doesnt do it. its basically a stock motor with a big cam, custom tune, AIR removed, headers with factory O2 sensors in basically the factory locations on an aftermarket y-pipe. No changes to anything with the build except I did my best to fix some air leaks at the old air pump fittings on headers and a leaky header collector. The voltages at the MAF wire are in spec and its showing voltage on the scanner, no blown fuses, and I drained the tank of the old gas put in fresh with some octane boost for the heck of it. unplugging the MAF made no difference. Im tempted to try a new MAF but it was working fine before and it just doesnt seem likely. It seems like its got to be something simple but I dont know what. Heres a table of data from a scanner and some numbers from in the past when it was running well. you can see I had a large air leak in bank 2 that I since fixed but it never caused it to backfire or throw a code. Now it seems it is dumping fuel into bank 1. this would seem to indicate a bad injector or a bad exhaust leak on the drivers side but I know the exhaust on that side is tight as a drum, if an injector is bad and its dumping fuel into the other cylinders could this cause a backfire like that? Then again, this wouldnt throw a p100 code... (though I think the sound is coming from the passenger side). Thoughts? Thanks!

 

So on further reflection:
1. Is is safe to assume the O2 sensors on bank one are working OK since they are both giving the same result?
2. Is it safe to say the MAF code has nothing to do with STFT1 being high? I figure if the MAF was telling the computer it was getting way more air than it actually was then STFT would be high on both banks right? I really dont know what Im talking about if you cant tell.
3.I understand that a MAF can be malfunctioning even if voltages at the harness are normal, but if it is bad, is there any way in hell it could be causing the P100 or the backfiring or is this just a red herring?
4. I am wondering if there is an injector not firing on bank one since I am very confident there is no air leak causing the lean condition. I checked the injectors on that side and all have continuity and all have power. Ill check if I can get them to click tomorrow. But even if I find a bad one, is there any way in hell this could be causing the backfiring?
5. I know that a bad plug or plug wire can cause misfires and hesitation but can it cause backfiring? Again the same wires and plugs ( I cleaned them, they had hardly any time on them) were working fine on this same motor before the rebuild).
6. Is taking the temp of the primary tubes and looking for a cooler one a reasonable way to look for a bad injector?
7. Would the computer report a misfire if an injector isnt firing?

 

A lot of questions that require involved answers. And, the data log is of very limited value without time stamps. What is the unit of measure for the MAF reading - grams/second or pounds/minute? The “running normally” portion of the chart is cut off. Also some misconceptions, since the MAF signal is not a voltage, it's a variable frequency that rides on a 5 volt bias voltage. The P0100 code is for “insufficient signal activity” but the log shows activity (guessing it’s pounds/minute). Did you unplug the MAF sensor at any point with the engine running? That would store that code in memory, and that won’t clear until the vehicle is driven a specific number of “drive cycles”, or until you clear it with a bidirectional scanner, or pull the PCM BAT fuse for 30 seconds.

I'll have more time to respond in the morning, and access to my laptop for amount of typing required.... a lot easier than the iPad I'm staring at now. But one more basic question - when you use the word “backfire” do you mean fuel and air igniting in the exhaust? Some people also use that term for an open valve or faulty timing causing the combustion to “backfire” into the intake manifold.

 

Did you make any changes at all to the engine during the rebuild? How big is the cam? Who did the tune?

I'm a bit confused on the nomenclature for the O2 sensors. It would appear:

O2S11 = Bank 1 - Pre-Cat
O2S12 = Bank 1 - After-Cat
O2S21 = Bank 2 - Pre-Cat
O2S22 = Bank 2 - After-Cat

If those are the two sensors you are referencing, the pre-cat and the after-cat, they are not giving the same results and they should not be. If you are saying that the results are the same for "STFT1" and "STFT11", those are short term fuel trims, not O2 sensors. Why they show it twice, and attempt to associate the STFT with the Pre-Cat sensor is odd. The fuel trims are the same because Bank 1 can only have a single STFT for each operating Cell (which is not shown in the log). What is confusing is why the STFT is steady, almost constantly adding 10% extra fuel (except for one frame). The STFT's should be moving around rapidly, as the PCM toggles the A/F ratio repeatedly back and forth from slightly rich to slightly lean, to help the cat function.

This is where lack of time stamps becomes a problem.... how many frames per second is the log recording? The PCM updates almost 10 times per second. If the data log frame rate isn't getting close to that rate, you aren't getting all the info. Also note that the PCM sends the data in a sequence, so the data is not all taken simultaneously. It steps through the data sequence so each individual piece of data is slightly offset in time from the previous.

What scanner/software are you using?

Correct, if the MAF numbers are "off" it would affect both banks the same way. See comments in post above regarding units of measurement, and the issue with the code even being there, when the sensor appears to be working. That is based o the assumption that the units are pound/minute, and you were just revving the engine under no load. On the other hand, if the units of measure are grams/second, the numbers are too low, and that could be setting P0100.

See comment in earlier post regarding MAF "voltage" There is no "voltage reading available for the MAF in the PCM. The MAF measures the electric power required to heat the sensor wires to a specific delta temperature above the inlet air temperature. It converts the current to a variable frequency about 2,200 Hz (idle flow), and ranges much higher at WOT/max RPM.

A problem with the injector electrical circuit will set a code. You can listen to the injector with a stethoscope to see if it is clicking. A mechanically failed injector will not set an injector code, but might display the click. But in OBD-2 the PCM counts misfires, and that data can be retrieved with a scanner that can access GM enhanced parameters. Not all scanners can do that.

A leaking injector can supply excess fuel that will burn with any excess oxygen in the exhaust, A leaker can often be detected by checking the fuel pressure when the PCM primes the pump, and shuts off (without starting the engine). A rapid loss of pressure MAY indicate a leaking injector. But there are other causes of rapid pressure loss, so you have to pull the fuel rails up and pressure the system, looking for drips..... do only outdoors, cold engine, fire extinguisher handy, containers under the injectors..


Cross-fire between two wires when a valve is open, and you can get a backfire through the exhaust or the intake manifold.


Might work, but only if its a severe misfire or misfueling.. In the long run, most of the issues you have raised to this point can be detected with a more complete data log. I review these for people all the time. Sometimes the problem is obvious, other times it isn't. GaryDoug's free Scan9495 is the best data logging software available. Doesn't help you with your OBD-2, but he has also done OBD-2 scanning software. Might want to take a look at that, if whatever system you are using is not able to produce a more detailed and complete log.

https://www.firebirdnation.com/forum...nner-software/

As noted above, the OBD-2 misfire detection system would pick up ALL misfires, no matter what causes them. The PCM looks at the crank position data from the CKP sensor, and compares it to other data to identify irregularities in the rotational velocity off the crankshaft, caused by misfire. P0300 covers random misfires. All scanners seem to be able to read that one. Then there are P0301, P0302, etc. for individual cylinders. And with on OTC/GM Tech-2 you can pull up the misfire counts by individual cylinders.

 

Thanks so much for the reply, and no hurry, seems race season wont be starting any time soon The following are responses to your post #3

1. Yes, I mean backfire in the exhaust is what it sounds like. The exhaust is y-piped to the passenger side so not easy to tell which side its coming from and I have to be sitting in the drivers seat to give it gas to make it happen.
2. The MAF was not unplugged while running. It was unplugged and plugged back in while off.
3. The MAF reading is in lb/min.
4. I cleared the P100 code after the first fire up and 20 min run. It did not recur on start up #2, but did on start up #3, Also of note it doesnt want to start easily, on the first try it fires quickly then immediately dies. on the second try it cranks and cranks and sputters slowly to finally run. on 4 builds since 2007 it has always fired just barely touching the key. Would it throw that code simply from starting and dying quickly like that?
5. The "running normally" part means the last two columns on the right are from 2018 when it was behaving and running completely normally, I recorded them at the time because it was running lean during a dyno run and I was trying to figure out why (it was an air leak).
6. Also it has a new few filter and fuel pressure was checked recently as was fine.

Again it seems like its something dumb, like I forgot to plug something in but Ive checked everything.

Also, Is is also safe to say this isnt a vacuum leak given its only on one bank?

 

Responses to post #4.

First, you are so awesome to take the time to help me, thanks you!

1. Your interpretation of those designations are the same as mine. Sorry yes what I wrote was confusing, what I meant was that both the STFT on bank one are up at 10, so can I assume that the O2 sensors driving that are both working? I mean it seems there is no point is taking them out and switching them to the other side to make sure they are working since they seem to agree with each other with what is happening on bank one? I had asked the tuner if there was any way to cut back the severe rich gas smell coming from the exhaust, I think due to the tremendous overlap of my cam and I think he set limits on the fuel trim for that, I could be making that up. It was done by Dynotune motorsports in Columbus Ohio on the dyno, I have gone to him for years but mostly just for certification for racing. its just a scantron scanner, for what its worth it took maybe 10 second to record those 9 frames.
2&3. Maf recording is lb/sec. Regarding voltage at the MAF I was referring to the the voltages there is supposed to be on the harness (4-6 v on pin A yellow wire , +12 Ignition between Pin C pink and pin B black and also as a double check +12 V Ignition between pin B pink and chassis ground). I realize these dont tell the whole story.
4 . Ill put votage to them with the motor off and listen for clicking. Wouldnt a leaky injector make the exhaust rich, driving fuel trims down rather than up? I was thinking more like a dead injector causing the lean condition >> high fuel trims.
5. Got it.
6. My scanner wont connect to any computer past Windows 8, I bought an OBD II scan tool and will try to use his OBD2 software to get better reports, thanks for the advice.
7. Yea, my scanner has something called misfire detection and I think its said "no" or something similar.Anyway it doesn't sound like a misfire, its a backfire for sure.

So - When Im off work again Ill check temps on the primaries and pull all the plugs on bank one and check them for fowling and see if the injectors are clicking on that side. I doubt its plug wires again because it wasnt doing it before and it ONLY occurs with throttle tip in but Ill look at them also. Besides getting more logging info what would you recommend I do next?

 

I need to go through your responses in detail.... but I remembered something odd about the 1997 MAF sensor wiring. For some reason, the 97 is particularly prone to a failure of the pink 12 volt wire. The break occurs somewhere in the harness bundle, and the cure is often to run a new 12 volt wire to the MAF sensor from the correct fuse. Maybe with all the work being done, engine pulled, etc, the wire has failed and is making intermittent contact.

Also to clear your info on testing the MAF wiring, having 5 volts on the yellow signal wire does not mean the MAF sensor is OK. That 5 volts is a reference signal supplied by the PCM. If you want to test the signal, you have to have the MAF sensor connected, and the engine running. You need a multi-meter with a frequency capability. Probe between the yellow wire and a chassis ground to see if it reads approx. 2,200 Hz at idle.

When the MAF sensor fails, the PCM defaults to the speed-density method of calculating the mass air flow. That calculation involve RPM, IAT (absolute), MAP, and volumetric efficiency. The RPM, air temp and manifold pressure come directly from those sensors. But volumetric efficiency (VE) comes from a table that is part of the programmed "tune". If you have a large cam, the VE tables had to be updated to match the changes in VE resulting from that cam. If it wasn't done, or wasn't done correctly, the engine may stall or run poorly when the MAF sensor fails.

You mentioned you have checked the fuel pressure and it;s OK. Did you check for pressure bleeddown after the pump primes, and then shuts off? Rapid loss of pressure can be caused by two conditions that have opposite effects. If the check valve in the fuel pump has failed, rapid pressure bleedoff leaves the engine short on fuel for the enriched A/F ratio cold start = slow start/extended cranking. Test this by cycling the key back and forth from RUN to OFF several times before turning the key to TART. On the opposite side, if the leak is an injector, the engine can be flooded and delay starting. To test this, put the accel pedal on the floor before turning the key to START, and hold it there while the starter is cranking the engine. This puts the PCM in "clear flood" mode, where it reduces or even shuts off fuel flow to clear the flood. A flooded cylinder dumping fuel into the exhaust could produce combustion, at least if the exhaust is hot enough.

 

Never mind. After reading through like 30 threads about p100 I found a single response from a guy who mentioned his buddy swapped out his opti and got p100 and backfiring and they found they had some plug wires swapped. Low and behold so did I, 5 and 7. It is absolutely unbelievable how normal this engine sounded with a giant cam and 5 and 7 swapped. Sorry for wasting your time, thanks again -

 

Spoke too soon. it fixed the backfire but when I went to start it again today it went back to stumbling on start and would not idle at all. out of frustration I turned it off, unplugged the MAF, twisted it around 360 degrees and plugged it back in. fires right up and idled! It seems that would support a bad pink wire. I have ordered a new pigtail.

 

If you are referencing my comment about the common failure of the pink wire, it is generally inside the harness, not the pigtail.

 

I tried bypassing the pink wire and it went back to stumbling, throws p100 and wont idle no matter what with it bypassed. but the pink wire is literally twisted into a "pigtail" going into the plug, always has been like that (the black and yellow are not), and when I stripped the wire going into it to bypass it I could see two broken threads of wire. So thinking the wire may be broken so close to the plug I cant bypass it I replaced the whole pigtail with wire going one foot back up into the harness. started it back up and its back to starting and idling (sitting still) fine, no p100, dont know if the new plug had anything to do with it. Ill have to start it and run it a few times to see if this intermittent problem is gone. Now all its doing is wanting to stall whenever i let off the gas (only while driving), goes down to about 650rpm for 4-5 seconds of stumbling then recovers. Ill spray some carb cleaner around and look for a vacuum leak. I know it has done this to me before after a rebuild and I cant remember what I did if anything to make it better, but it got better. does the IAC need time to re-lean?. I bought a bluetooth OBDII reader and downloaded Gary Dougs OBDII software but they wont talk to each other, Im thinking gary dougs software wasnt intended for bluetooth so now Im going to get the OBD link USB one he recs on his site. maybe then I can get some decent data for you to see. thanks again -

 

Shoebox has the IAC reset procedure. I don't know how often it's needed. I never needed to do it.

4th Gen LT1 F-body Tech Articles

 

I've had it through several starts and heat cycles and the no idle/p100 has not returned after the rewire! Still stumbling for about 5 sec to 600rpm when let off the gas while driving but doesnt die. I can rev it all I want while sitting still and no stumble. Await the obdii cable!

 

Well the stumbling for about 5 sec when let off the gas while driving decreased then stopped happening altogether after about 20 min of open road, 1am driving where I was able to vary the throttle up to full (not just 20mph in the neighborhood). That same thing has happened to me before after rebuilds, not sure why. Anyway it looks like things are settling out. Thanks for your feedback as always. I'm still waiting on that other data cable if I can get it to work and it shows something out of whack ill post it here.