LQ4 Multiple misfire

[Billett]

So I've got a bit of a puzzle going on here. The shop I had my truck tuned at had some problems during the tuning process and I'm not 100% sure they nailed down the problem. Don't get me wrong, it runs pretty well, but I had my brother check the pcm on his tech 2 yesterday and we found some codes that my torque app doesn't pick up. Emissions codes were showing, which I knew about but still tick me off. They should have been turned off by the tuner. Besides that I had an oil pressure code. Fine. I pulled the original sender for the idiot light and replaced it with a sender for my gauge.

Big problem here is that with the torque app I was seeing multiple misfire problems, but couldn't narrow it down more than that. The tech 2 lets you see which cylinders are missing and it turns out it was all of them. Huh, weird. Brand new plugs, brand new wires, coils are old but I doubt they're all bad right?

Well it seemed like MAYBE cylinder 3 was having a few more misfires so we pulled the plug and swapped it. No change. Swapped plugs and wires a few times and still nothing.

Drove it around with the tech 2 counting misfires and noticed the weirdest thing. Accelerating doesn't cause any misfires. Idling doesn't cause any misfires. Constant throttle doesn't, and neither does WOT. However the counter SKYROCKETS on almost all cylinders when you gently decel. If I take my foot off of the pedal completely, no problem, but if I ease off say from 50% throttle gradually to 10% I get all kinds of misfires.

What the hell is going on?

I thought it might be vacuum, but wouldn't I get more misfires when accelerating in that case, because it would pull more air? I have no idea at this point, but I did take a vid to show the counter, just can't get it up until later. Let me know if any of you guys have thoughts.

[BR3W CITY]

Is your tcc/brake switch hooked up correctly and adjusted?

First thought Cam and crank sensors are out of phase, recording false knock and throw multiple miss codes.
or
If its happening on decel, what I imagine happens is one of two things; the above issue is preventing lockup (or unlock, depending when it happens). This would present itself as shuddering on decel, followed by miss codes. If the tcc switch isn't adjusted, and your decelerating while locked up, this can also throw codes. One begets the other, same symptoms.

Easy thing to do is verify the TCC/brake switch, then if its still an issue, there is a setting that can be changed in HPT. Basically, it will adjust the threshold for false knock to help ignore the cam. I forget the exact name, but I posted a thread about this a while back (and probably in my build) about how my tuner addressed the issue.

[Billett]

CPS was actually my next thought, but I haven't had a chance to get a new one, or look at the reluctor wheel.

TCC switch is a relay I connected to the brake light so that when brake is applied the switch is flipped. I used a diagram on here to wire it I think... I know the relay works because I can hear it clicking when I touch the pedal, but I haven't checked with the tech 2 about whether that sees the lockup occurring.

I think I tried a while back to get the truck up to speed, keep steady on the throttle and then hit the brake a bit to see if I could hear a change in the TC, but I don't remember if I heard anything or not. IIRC I didn't notice any difference, but I went with the RPM I was sitting at being pretty close to yours Brew and another member or two.

How is the TCC switch adjusted?

Don't have HPT still, but I can talk to my tuner if that seems to be it.

[BR3W CITY]

Well on mine, it uses the 90's style 4 post brake switch wired into the tcc wire, so if you hit the brakes it knows to unlock. I didn't use an additional relay that I recall, just the purple tcc wire and post #4 on the switch. The switch itself just has a threaded shaft and locknut like most of the stock ones did, just adjust it out until you don't have a ton of travel before it engages the pedal.

If you have the TechII, can't it show lockup? You should be able to get to about 47-49 and have o/d engage, and monitor if it actually is. On mine, lockup almost feels like another 1/2 gear.

[Billett]

Mine just has the original 1 post for the brake lights and I ran a relay from there. From what I remember the signal that the pcm is opposite what the switch puts out which is why the relay was needed. The brake switch is normally open and then closes and sends power to the lights when you depress it. The pcm wants to see power all the time and then none when the pedal is depressed. It seems a bit odd, but I didn't have any problems with the wiring itself.

I can use the tech 2 to see whether o/d is coming on or not, but it is my brother's from his dealership. He brought it over last night and took it back to work, but I think I can get him to bring it back again for this weekend. We didn't have much time last night so I was more interested in seeing if the misfires were from any particular cylinders.

[ls1nova71]

If you think its the crank/cam are out of sync, you can do a crank position relearn with the Tech2, that would be an easy test. Its a little unnerving because you have to hit the fuel cut off in neutral, bit it only takes a few minutes to do the whole thing. Pretty sure you need some kind of switch that lets the PCM know what gear its in, so if you deleted the NSS switch from the transmission, it may not work. In that case, it can be turned off in the tune.

[littlevictories]

if i was getting misfires from a carb'd rig i'd look in the intake for oil. lots of vacuum when you decel can pull oil thru a pcv. had that happen.

you mentioned emissions stuff not getting turned off, did the tuner mess with the DFCO at all?

just spitballin'

[BR3W CITY]

Originally Posted by Billett

Mine just has the original 1 post for the brake lights and I ran a relay from there. From what I remember the signal that the pcm is opposite what the switch puts out which is why the relay was needed. The brake switch is normally open and then closes and sends power to the lights when you depress it. The pcm wants to see power all the time and then none when the pedal is depressed. It seems a bit odd, but I didn't have any problems with the wiring itself.

I can use the tech 2 to see whether o/d is coming on or not, but it is my brother's from his dealership. He brought it over last night and took it back to work, but I think I can get him to bring it back again for this weekend. We didn't have much time last night so I was more interested in seeing if the misfires were from any particular cylinders.

Is this something particular to your switch? I've never run a relay for the tcc, always just swapped out the switch for a 4poster from like an 85, or something like the firebird 6pin and only use 4.

The pins on the 4post are correct afaik, with the rear set being the "make/brake" for the tcc. The front pair are N/C and the rear are N/O.
Its $6, and a lot less work and wiring.

[Billett]

Originally Posted by ls1nova71

If you think its the crank/cam are out of sync, you can do a crank position relearn with the Tech2, that would be an easy test. Its a little unnerving because you have to hit the fuel cut off in neutral, bit it only takes a few minutes to do the whole thing. Pretty sure you need some kind of switch that lets the PCM know what gear its in, so if you deleted the NSS switch from the transmission, it may not work. In that case, it can be turned off in the tune.

Still have the nss so this is definitely worth doing. Can't possibly do any harm right?

Originally Posted by littlevictories

if i was getting misfires from a carb'd rig i'd look in the intake for oil. lots of vacuum when you decel can pull oil thru a pcv. had that happen.

you mentioned emissions stuff not getting turned off, did the tuner mess with the DFCO at all?

just spitballin'

My brother has one of those handy camera-on-a-stick things as well so I'll ask him to bring it with him. It has a light on the end so maybe I can pop it in the intake and see if I find any oil. Don't know what DFCO is?

Originally Posted by BR3W CITY

Is this something particular to your switch? I've never run a relay for the tcc, always just swapped out the switch for a 4poster from like an 85, or something like the firebird 6pin and only use 4.

The pins on the 4post are correct afaik, with the rear set being the "make/brake" for the tcc. The front pair are N/C and the rear are N/O.
Its $6, and a lot less work and wiring.

No it isn't strictly necessary, but I have a ton of spare relays lying around and didn't have a 4 post switch so I just went ahead and did it. It didn't really take much time or effort and it certainly didn't cost anything, but I suppose a 4 post is cleaner. I'll have to check if its working first and if not I'll stop out and grab one.

[Andy4639]

Buy the 71 Chevy truck cruise switch it is exactly what is needed for the brake lights and computer lock up. It's what I have on mine.It has the correct 2 connections that the brake switch wiring will plug right up to on it and the other two connections you can run spade connectors to it.
here is the link I started a while back on the switch.

http://67-72chevytrucks.com/vboard/s...d.php?t=614263

[Billett]

Using the tech 2 I found out the brake switch is definitely working, but the TCC is not engaging. I'll have to see if it's a solenoid or the whole converter. The trans was bought on CL so you never know...

Didn't get a chance to do a crank relearn, but I'm going to try to take care of it this weekend. Still want to check on the intake as well.

[Hart_Rod]

Some guys use a relay for the TCC if they are running LED brake lights. Here's a thread that discusses that:

http://67-72chevytrucks.com/vboard/s...d.php?t=604032