View Full Version : AFM Problems and Knock Response

08-03-2010, 11:31 PM
2nd gen 3sgte and everything is stock setup except for the 2.75 inch exhaust from the cat back. I have done a complete tune up as well. The coil tested good. The cam and ignition timing is correct. Compression is great. I am using 93 octane with NOS octane boost.

I still can't figure out why i am having such a pita problem with knock response. I have TVSV LEDs wired up so i can see that i am getting knock response. Everytime i boost i get knock response. Most of the time it is when i get into the higher rpms. I see the red LED light up and the ECU pulls timing. After a couple minutes i can usually boost again for several seconds until i get a knock response again.

The only thing i can figure is that the TMIC is heatsoaking like crazy. I also have the auto transmission that is generating extra heat as well. So maybe the charge air is just too hot and the TMIC cannot compete with the engine bay temps. The outside temperature has been 70 degrees when i drive at night and it makes no difference at all.

All of my test driving has been on the highway with no stop and go traffic.

Another problem i have started having is that sometimes at low throttle my engine will bog down and spit and sputter a little. i have also experienced it at full throttle. I got trouble codes 22, 24, 32, 35, and 41 when this happens. This leads me to believe my AFM is acting up. If this is the case i am wondering if a faulty AFM could be causing my detonation problem. I'm wondering if it is causing a lean misfire or something. I clocked the AFM a few days ago and that is when the problem started. I positioned the AFM back the way it is supposed to be but with no luck.

When i inspected my spark plugs i seen no sing of detonation. Also, when i had the head off of the motor the pistons looked great and there were no signs of detonation or hot spots. There was barely any carbon build up on the pistons. So i know that this has to be a post swap problem and a direct cause of something i am doing or not doing.

08-04-2010, 05:20 AM
I tested the AFM. My multimeter only reads 20k ohms internal.. The readings i got are as follows.

VS - E2 was .47 ohms
VC - E2 was .28 ohms
THA - E2 @ 70 degrees Fahrenheit read at 2.28 ohms

The diagram says that it should read between 200 and 600. I don't know if that is just the way my multimeter reads or what. i'm guessing .47 = 470?


I opened the AFM and it was very clean inside. The wiper arm was making good contact with the conducting strip and there was no grooving at all.

I'm going to check the wiring harness in the morning.

08-04-2010, 10:29 AM
Your harness to ecu connections are the most likely culprit here.
Trace your wires, make sure you have really solid connections and see where that leaves you.

Ask Luni or Joel, I had similar issues in my AllTrac and it was wiring related.

Hopefully yours is as well.

As to it being heat soak, I will venture not. If it was it wouldn't let you boost again after a few seconds.
It would just stay in boost limited (safe) mode.

Do you have any severe vibrations like an exhaust pipe hitting the chassis?
Is your knock sensor wiring intact still?

Just listing possibilities. :)

08-04-2010, 02:37 PM
I haven't heard anything rattling. I looked for things that might would rattle and set the knock sensor off. I couldn't find any. I would have had to cut the plastic insulation from the knock sensor wire to visually inspect it. Would a crack in the wire shield create faulty knock response? I know it is a coaxial cable.

I forgot to add this bit of information to my 1st thread.

I do remember hearing a sound of something like valve clatter or coins rattling in a can right before getting knock response. I only heard it a couple times and was brief. I am guessing that could be knock.

If i am getting detonation then i have to figure out what's causing it. I need to drive the car and pay attention to my A/F meter to see if i am running lean under boost. I forgot that i had it hooked up now. I know that i noticed it was running a lean idle yesterday before i stopped working on the car.

08-04-2010, 06:38 PM
sorry cant help you buddy, i got rid of my AFM and went speed density with an AEM

08-04-2010, 11:36 PM
I guess the AFM is toast. I did find a loose connection on the AFM ground wire. The bad connection was at the back of the ECU harness. Everytime i moved the wire the engine would shut off. I fixed it but it did not make any difference. The engine is misfiring and bogging down. I still have the same check engine codes.

08-05-2010, 03:51 AM
I'm going to pull the board out of the AFM in the morning and check for a bad resistor.

joe's gt
08-05-2010, 05:22 AM
When these 2nd gens sit for a while, I think it affects the AFM. My afm was bad from the very first start up when I did my swap. The resistances, however, showed that it should have been fully functional. Definitely could be the culprit.

08-05-2010, 10:42 PM
Ok, these things have no resistors. I soldered up a couple of loose connections where the plug terminal make contact with the board. I'll try it Saturday morning but i don't have high hopes.

08-10-2010, 10:05 PM
Well i've managed to fix a couple of loose connections with the AFM but i am unable to rev past 4000rpm without missing.. All of my check engine codes have been cleared. I still suspect the AFM is the problem. My idle is a tad erratic. I'm going to check the spark plugs in the morning. I may have fuel fouled one of them during the troubleshooting process.

08-11-2010, 01:25 AM
Check your ignitor and coil.

joe's gt
08-11-2010, 03:58 AM
Well i've managed to fix a couple of loose connections with the AFM but i am unable to rev past 4000rpm without missing.. All of my check engine codes have been cleared. I still suspect the AFM is the problem. My idle is a tad erratic. I'm going to check the spark plugs in the morning. I may have fuel fouled one of them during the troubleshooting process.

Sounds exactly like what was happening when my AFM was bad. If one of the board members will lend you one, that would be awesome.

08-11-2010, 06:49 AM
Yeah, i'm going to get Rob to lend me one.

08-20-2010, 05:07 PM
wow, ive been having almost the same exact problems. when i press the gas it bogs down and and lags. i cant floor the car without it not pulling like it should. all i get is a series of popping and a massive loss of horsepower. a possible culprit is that i have my AFM clocked. its rotated 45 degrees. taking off the top of it and soldering the wires sounds like a good start for me.

hows the situation going with your car? any luck?

08-23-2010, 06:19 PM
Ive been a slackass and I made bad promises. But it IS going out tomorrow.

Just for shits Bruce, the one thing ALL of those codes share in common is the E2 ground wire.

You should check continunity of the E2 wire on all those sensors though. A bad ground to E2 on the electrical system anywhere could affect ALL of them. Cause the E2 wire just junctions together between them all.

Something worth looking at. To be honest I dont think its your AFM. I think it is wiring. Im gonna send you mine tomorrow tho. I promise babe.

08-23-2010, 08:04 PM
you just want me to send him the one you sent me luni? i still have it and would take 1 day to get to him and be cheaper.

08-23-2010, 08:08 PM
Yes. I forgot you have my AFM for testing.

Yeah, damn, I wish Id have remembered that.

Bugs, Ill forward you his addy.

08-23-2010, 08:58 PM
a couple of suggestions: You can check the AFM at the ECU end of the harness. Meaning, just as you check the resistence at the AFM itself, do the same right at the ECU using the ECU pinout diagram for your model (BGB).

Do you still have the stock Cat in place? If its never been punched out, or replaced, its entirely possible its causing a restriction that leads to backpressure and in turn creates extremely high combustion chamber temps, and lack of power over a certain RPM.

What spark plugs are you running? (brand/type/heat range)
what boost level?

08-24-2010, 05:17 AM
I actually did have a poor connection at the E2 ground. I fixed that a couple of weeks ago. I have gotten all of the codes to clear but motor still runs like poop.

I'm going to gut the cat. that's the only thing i haven't done yet.

Thanks Loon. No worries man. I know you're busy as of late.

08-24-2010, 03:57 PM
Have you totally reset the ECU so the codes are gone now? I mean, if you pull codes, it doesnt read anything back right?

08-24-2010, 04:56 PM
Yeah, I reset the ecu and did not get any codes afterward. I pulled apart the afm in an attempt to discover what was wrong with it. I'm getting funky resistance readings. So i probably just made things worse.

I'm going to double check the connections this afternoon.

08-24-2010, 06:13 PM
Bugs shipped that AFM out today. You should PM him and find out what you owe him for shipping :)

08-24-2010, 06:59 PM
Bruce, just thinking here. Those error codes point to a few things, primarily the AFM but with a few extras like the TPS and the MAP. These are all +5V powered sensors.

Can you check voltage at the VC pin @ ECU and at those sensors to verify that you are seeing +5V? Should be a pink/blue wire. VC is the power supply for the +5V sensors...

I've got a suspicion maybe somethings up with that wire.

08-25-2010, 04:26 AM
Yeah, they all do share VC and E2.

VC should always have 5 volts to it with the ignition set to ON.

08-25-2010, 04:41 AM
I'll check that in the morning.

08-28-2010, 02:42 AM
Hey sucka. You ever try my AFM? How did it do for you?

Dude, I think I figured out your shifting button problem.

You know how you got shift up and down buttons wired to your center console? You should find yourself a 7th gen GTS steering wheel with the manual shifter buttons on the back of it. And wire those bitches in!

08-28-2010, 05:35 AM
I got the afm yesterday but haven't been able to test it out. I'm going fool with it in the morning. I also have to adjust the tps sensor being that i swapped it out for another and never set it.

That's a good idea on the steering wheel. I'm definitely going to find me one. I hope it'll fit.

08-28-2010, 07:02 PM
I had a little time to troubleshoot this morning.

I hooked up the AFM and it made no difference. The engine still misses and i can smell it over fueling.

My main problem is the fact the E2 is not grounded. When I took voltage readings at the ECU i back pinned E2 and VC and got no reading. I then probed a different ground wire and got a 5v reading on VC. I do have continuity between the E2 wire at the AFM harness and the E2 wire at the ECU harness

I then got me a length or wire, attached it to a good grounding point, and spliced it into the E2 wire on the ECU harness to give it a ground. I was able to get a voltage reading between E2 and VC then. It made no difference to the way the engine runs.

I'm still getting codes for the AFM, intake air temp, and TPS sensor. I did not have a chance to check the voltage for VC at the AFM and TPS but my guess is that i will not get a 5v reading when I do. I think I have a wire break somewhere in the loom for E2. I did take the E2 wire coming from the AFM harness and directly grounded it to the chasis which still made no difference.

I know for a fact that I'm not getting ground to E2 on the ecu. So i attached the E2 wire to a good ground at the ecu and AFM and the problem persists. I checked where all of the ecu grounds bolt to the strut tower and the connections are clean.

08-29-2010, 04:55 PM
I fixed the problem. The E2 and VC wires both had a bad connections inside the harness for the ECU. I pulled both wires out of the harness and soldered the wires to the connectors. Placed everything back together and and now the motor runs great.

I have the AFM Rob sent me hooked up. I haven't hooked mine back up to see if it will work. It was raining so I only test drove the car for a few miles before going back home.

I'll recheck the timing and adjust the idle Tuesday when I'm off work. Then I'll drive the car and see if I'm still getting knock response. I hoping I fixed two problems at once today. If not i'm going my next step is to pull the catalytic converter off and gut it.

The rattling that i had been hearing and thought was audible knock turning out to be the cover for the TMIC. I did not have it bolted down and noticed it was rattling when I revved the engine.

08-30-2010, 11:58 AM
Yay! glad you got it running right. Now luni, remember bruce has the loaner afm next time you go lookig for one. lol

08-30-2010, 06:58 PM
Awesome!!! That is good news.

09-05-2010, 10:55 PM

So even after i installed the AFM and fixed the wiring gremlins, I was still having a major problem with knock response. The timing was being pulled any time i got into the throttle.
For the life of me i could not figure the problem out. So I decided to pull the converter off this morning and check it out. It was in great shape and had no melted spots. I then decided to remove the catalyst and was able to push it out in one piece. I put the exhaust back together and took it for a drive. I still had knock response and once again the timing was retarded.

Then i went to the parts store and bought some lucas Octane Boost and filled the gas tank with 93 octane. Still got knock response. By this time i have had enough and was going to park the car for the day. I pulled into my shop and left the call idling as i walked outside to piss. I came back inside and heard a vibration coming from the hood. I pushed down i little on the hood and the vibration stopped. I figured out that the vibration was from the aluminum shroud that i had built around the TMIC cover. There was a portion of the shroud that was making contact with the hood and rattling like hell. i trimmed 1/4 inch off of the shroud to take care of the vibration.

I then took the car for a good 30 minute drive down HWY 49 South and boosted as much and as hard as i could. I had the boost turned up to 13psi and even switched over to high boost and ran 15 psi for a few pulls. Not one time did i get knock response.

So problem solved as far as i can tell. That was the first time i was able to drive the car and feel its full potential.

Stupid intercooler shroud was causing my knock response the whole time.

09-06-2010, 03:16 AM
I'm really glad to hear that you've got it worked out finally. If there wasn't enough air going through the intercooler, that would do it as the air charge would probably be pretty warm. Enjoy the boost now! It's very addictive!!:smile1:

09-06-2010, 03:30 AM
glad you figured that one out, those weird problems are the worst

09-06-2010, 03:35 AM
I guess i know now that the knock sensor is set off by other vibrations outside of the motor. I guess the sound of the rattling shroud was within the same frequency that the knock sensor reads.