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Cannot Figure Out Why 4 Back Cylinders Won't Fire

13152 Views 77 Replies 31 Participants Last post by  drakej
About 3 weeks ago, while driving home from work, my 4 back cylinder ( 5, 6, 7, 8 ) abruptly stopped firing (found this out once I made it home to diagnose it).
I tested the coils for 12V and spark as well as tried switching the coils and plugs around, no change.
Limited on time to diagnose (I had to travel for work the next day) I left it at that point.
Fast forward to this afternoon and I go back at it. I first test the injectors to make sure they have both 12v and a neg pulse from the computer. Both being present, I pulled the injectors out and energized them with a battery to make sure they all sprayed/worked. I then swap the front 4 around with the back 4. No change, the back 4 cylinders still aren't combustible.
I read somewhere (I've done a bit of searching ) that it could possibly be the CKPS (crank position). I've only dealt with one of those on a Mercedes I owned. When it went bad, the car simply shut off and would not start at all.
So, I'm asking if anyone has seen this (or similar) and if not, any ideas on what I could possibly check out next?
I'm literally at a stopping point and out of ideas.
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You guys are fantastic

First off --- THANK YOU ALL for the replies!!! I don't think that is said often enough on forums (when questions are answered).
Okay, now ...
I think the CKP would throw a code if it was off.
Have you tried disconnecting and reconnecting the battery (I'd leave it off for 30+ minutes)
I did that (was hoping the computer had went "stupid" on me). I actually didn't get a code my whole ride home (about 30 minutes from when the cylinder combustion dropped). I have a CEL now but I'm sure it relates to the misfires.

CKP and CMP failures can ghost to other issues. but i doubt thats the case here.
But i dont have other ideas beyond what youve done. So check the CMP and CKP signals. you need an O-scope to do that.
they are hall-effects, so its a digital square wave.
Oh, and do a compression test. Its rare, but i have seen cams snap in half. Never on an LS though.
Don't have access to an Oscilloscope here in the office. Though it was my best friend out in the field while working in Woodward, Oklahoma the past few weeks. I'm a Wind Turbine Service Engineer for Siemens and we had a high fault rate on one of our RPM sensors (which is a Hall effect Square Wave as well). Denmark (wind division head quarters ... bunch of stubborn know it all's) refused to believe it was a manufacturing defect. Took readings from about 15 towers to prove my findings ... Totally off topic/subject I know BUT it's still a hot button with me (still fresh on my mind, LOL).
Funny you mention the snapped cam. A friend of mine thought the same thing as well. We were actually driving in his truck (with a worked 350 in it) about 5 years back and it snapped the cam (literally broke in two ). We both had ruled it out this time because like you, never heard of it happening to a LSx based motor.

Is fuel getting into the combustion chamber?
That'd probably be a quick indication if the cam is effed-up. It won't get in there if the valves are closed.
Yeah, I kind of suspected that (see original thread link at the bottom of this thread) since the plugs I pulled from the non firing cylinders weren't wet.

Good luck, and let us know what it is, I'd like to know just for the hell of it.
Yeah, you and I both. I "think" I'm going to go down as having the first (that I know of) broken cam'd LSx (and that Comp cam has been in there for close to 2 years and about 18K miles problem free).

Try pulling one valve cover and crank the motor by hand (crank pulley with big wrench). That'll give you a quick idea if anything is funny in the top end (like a broken cam)
I planned on pulling the valve cover, riggin' up the coils and running the motor with the covers off. I can't a more clear indication of valve events than that (I figure).

Are you sure it's getting spark? Pull the plugs, reconnect the plug wire, place the plug on a rag and ground it out with a screwdriver against the block. That'll rule out the spark thing.
I'm getting great spark. I have one of those in line Spark testers. It fires like a champ on all cylinders.

Are you sure it's getting fuel? You mentioned you jumped the injector but was it still hooked up to the factory rail?
I pulled each injector, place a bit of fuel in each one, power it up from a battery and blow thru them. They all squirt fuel with a very nice pattern.

What other things run in series to the back of the motor that if they get plugged, the rear 4 cylinders would stop firing?
Exactly what I was looking to figure out and to be honest... Nothing outside of the fuel rail. The fuel rail and injectors are both clog free.

My first thread concerning this issue
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You guys are STILL great! Much Appreciated Fellas!

He said he'd switched the injectors around, and 'noid lighted them.
Power wise, they are fed by bank, IIRC. Individual control (ground) circuits, and loosing 4 is just too weird.
Not any weirder than losing the back half of the cam, however.
Exactly ....
The coils & injectors for 2,4,6,8 are powered by Red > Pink for injectors ( S154 )
Red to coils ( S137 )
1,3,5,7 are powered by Lt. Grn > Pink for injectors (S155)
Lt. Grn to coils ( S138 )
I pulled the valve cover on yesterday to check the valve events while it was running. They are all fine so the cam is still in one piece. It also showed me that the reason for the plugs NOT being wet was not from the valve being in the way (from a fubar'd cam).
I even went so far as to change the fuel rails (I have a complete LS3 package waiting to go on). I figured why not, with it being literally a 10 minute job. No change to the situation.
With the injectors being supplied both 12v Pos and a Neg pulse from the computer ... fuel rail not clogged/blocked and injectors being swapped from working locations (1,2,3,4 >>>> 5,6,7,8 ) great spark (verified twice ) and still no combustion on the back 4 ...
Wonder what would cause the obvious lack of fuel???

Maybe I missed it, but how did you determine it is the back 4 cylinders that are dead?
Link to thread I started about this a few weeks back when it first happens covers all that. (long version, LOL)

Did you see if the header primaries have any heat?
Yep Yep, cold to the touch (until the radiant heat from the other 2 tubes reach them) and the fact that I can disconnect the coil or injector with no change to the motor on those cylinders.

The most common sense check is to pull one of the valve covers and turn the motor over. If the valves are actuating, then you narrow it down to fuel or spark.
Yep Yep, been there --- done that (so to speak).

If you have four dead cylinders and the plugs look normal, then it would seem at a glance that they are not getting fuel.
Yep Yep, I'm in agreement with you on that (at this point since I've deducted/eliminated all other possibilities). Now I just need to figure out what's causing the lack of fuel supply.
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At this point I would start diagnosing wiring issues or possibly a bad ECM. Its odd that its just half of two banks...usually wiring issues take out a whole bank.
At this point, I highly doubt it's a computer or wiring issue. The injectors aree receiving the 12V pos from the battery and the neg pulse from the computer. The injectors all work correctly (have been tested off the car and have been swapped around).
So the computer is getting the feedback it needs from other sensors to tell the injectors to fire.

Yes to above. Lots of good info here. If you have compression, the fuel and spark events are happening when they should combustion should occur.
Yeah, but now I'm quite sure the missing ingredient in the chain is the fuel.

Just throwing this out there but could there be a major vacuum leak leaning out those 4 cylinders.
Not a one ... One of the things i like about the LSx motor is the simplicity of it ... including the minimal amount of vacuum connections/hoses.

With the fact that it happened all at once, you would not think it would be injectors or coils.
Coils are fine (tested twice with my inline spark checker).

Another thing with a busted cam is it may still operate the valves if it broke crooked but it would be out of time enough to not fire those cylinders. I have seen an engine run on a busted crank. Usually it makes a knocking noise. I have never seen 4 components fail at the same time. Maybe I missed it but you did check compression on all cylinders right?
No knocking or any unusual noises. The car runs and drives (although not correct) especially once you get the RPMs up past 2300 (enough rpm to "mask" the dead cylinders) .
The only normal/logical check I haven't performed is that of a fuel pressure check. NEVER have I had a fuel pump slowly die that didn't moan and groan first. Nor have I ever heard of one being weak and causing half the motor supply side (injectors) to not fire or fire anemically. But ... you never know I guess.
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i lost several cilinders about a year ago ,but it moved and was different cilinders ... took the grounds off both heads sanded the head a little and put them back on...the left side ground is on the back of the head ,a ratchet wrench works best .
I did the grounds as well. That left side one is easy to do (with all things considered) using a box end 13 with a ratchet on one side.
Funny thing I noticed last night, after all the starting of the car, to check if what I had done fixed it ...I no longer have a check engine light. Whatever was causing the CEL, it must not be present in the past 3 starts (think that's how many the computer must see to clear a CEL) Car still only runs on the first 4 cylinders though, LOL.
Going to marinate on some ideas/thoughts for the rest of the week. If I'm still drawing a blank, I'm going to look into trading it in. Not something I relish doing but it's becoming too costly to keep sitting in my garage (damn rental car is costing me over $700 a month!).
Once I again I owe THANKS!

Yep, since you drive an 05, to swap the ECM you need a dealer with a Tech II to synch up all the modules (PIM?, etc). But id bet money thats what it is.
Hmm, never knew that about the computer swap (having to use the Tech II for more than CPS and CKPS programming). That kind of blows ... I've got a thing or 2 left to try (will look into powering the injectors as KTG suggested ... though I swear all the injectors are receiving a neg pulse from the computer).
Curious, does anyone know the going rate for an ECM in these cars and what's a TECHII re-write generally run? I'm guessing none of this can be done with HPT? I ask because my car currently has a "Tune By Chuck" (was done a few years back when I installed the cam, UDP, LT's and Svede intake) and I'm going to need to see him again as well I guess.

I dont have a wiring diagram in front of me, but its hard to imagine a wiring fault only effecting two half banks. Common grounds per bank and all that.
I have, for the most part, a complete service manual on file

I scoured that thing for hours while working in Woodward Ok. last month (not much to do when sitting in a hotel room). The injector/coil wiring is per bank (pretty simple layout). Nothing correlates on why only the back 4 won't fire.

Ive honestly never seen something like this. Ive seen plenty of stupid shit that shouldnt happen. but not this.
Wow, that makes me even less enthused about finding the issue. I respect the fact that you're an actual Technician (that diagnoses these things regularly I take it). Yet you take the time out to come chat about cars on here (that would be akin to me being on Wind Turbine sites discussing parameters ... which I never do, LOL).
I'm going to take some measurements to/from the computer to each injector plug > < computer pin. I highly doubt 4 just all of a sudden had an increase in resistance. But I do want to say I crossed all my "t's and dotted my i's.

Things that are new and different excite me. this gets a bonus for being really smurfing weird.
Well I propose a trade and sign with ya. You can have this one and I'll take your running one ... No questions asked :)

If the injectors fire when being powered at teh ECM connector, and the ECM powers and grounds are good, Its 99.999% certain its the ECM at fault. that missing .001% is from the fact that nothing is ever certain, and not doing the tests myself. Trust but verify and all that.
I've checked the injector wiring with two different sources. Once with a noid light. Secondly with a computer safe test light. It's simply a 12v powered test light with resistors and LED's. Attach it to positive it lights red. Attach it to negative it lights green. The fact that it has LED's allows it to be used like a noid light to read the negative pulse from the computer to the injectors. The non firing ones all have positive from battery and are getting a pulse from the computer.
For the coils, they are all firing which I verified with my spark tester that goes in line between the coil and plug.
Since i haven't actually measured ohm's between the injector clips and the computer plug, I'm going to do as you suggested (because it takes less than 3 volts to energize a LED so there could possibly be a high resistance. It would allow my Test Light to flash pulses but maybe the voltage is too low to open the injector correctly).
Oh yeah ... On buy a NEW car ... Not an emo purchase by me.
NEVER have i owned a "new" car (Dave Ramsey and Tom Martino fan here). My GTO was purchased by me August of 06 with 11K on the clock for $21.5K (financed out at 24.5) I owe next to nothing on the car at this point and it has less than 55K on the odometer.
If I am forced to trade it off, I'd purchase a Vette (07-09 is in my price and payment range) or a 330i Beemer (fun little twin turbo inline 6 and the 07-08 can be had for under 27k with low miles).
I'm really not trying to come off as a know it all or as being better then anyone at anything (though I am better then anyone that doesn't put forth efforts!). I realize how much I don't know every day I live (age does that to you, LOL). BUT I can do the basic math and if the repairs starting approaching the $3k mark (including rental car at this point) I'd be money ahead to trade it in. Would I get the full value of it, NOPE but I'd easily get 8K into the trade (have you seen what GTO's are selling for right now ... seems they went up in value for some reason). I was without a car note for 9 years before I bought the GTO and had kinda looked forward to being that way again for a while. But, the best laid plans of Men and Mice ...
Anyhow, no more doom and gloom thoughts (sell car/buy another one) ... I'm going to think positive and hope I discover something this weekend :D
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Well, I spent some time working on the car this weekend (along with R/R of my neighbors radiator).
Since I hadn't specifically checked compression, I removed all the plugs, fabbed up a remote starter switch (relay with a momentary switch) grabbed the compression tester out my specialty tool box and got at it.
I decided to get a baseline by checking my known good cylinders first. Install fitting and gauge into #1 ... hit the momentary a time or two, needle swings to 210. I decided to skip the drama of waiting to get at 5,6,7,8 and jump right to 5. Wouldn't you know it jumps right to 210 as well. Check all the rest and not a single one is lower than 205.
I still need ohm out the wires to/from the computer harness to the injector clips. If they all ohm similarly, I'll know it's not a resistance issue causing a voltage drop.
Following that, I'll be all out of ideas .... fml
The purpose of me testing resistance across the wires

BUT have you verified that they are actually spitting out fuel, when fed by your pump and your PCM?
I'm of the belief that the problem is in the fuel supply ... What I'm trying to discover is "why" the injectors are not firing (when being fed 12+ and Neg pulses from the ECM).
So I'll measure resistance across the wires to see if I have unusually high resistance (which I doubt would come along all of a sudden and only on the back four).
If nothing stands out, I can only surmise that my ecm may have shat the bed (something I haven't seen ... not an ecm dying in this fashion).
I agree

I believe there are discrete transistors driving the sink to ground for each injector. If it turns out that the problem does reside within the ECM, it may well be that those 4 transistors share a common circuit element or board trace that may have failed.
Makes perfect sense. I wonder if they do use transistors that share a trace on the circuit board. Seems low tech but I bet it's cost effective. I would think the whole thing were microprocessor controlled (but you'd still have trace on the circuit board for the mcp as well).

While it's low probablility that all 4 transistors crapped out at once, the failure of a common element would explain the problem and put a big bow on it.
Yep Yep, it would. I plan to get out of here (the office) early enough to get home and ohm test the injector wires. If they all are in balance with each other, the only other possible cause will have to be in the ECM. I'm not totally sold on that (again, I have never seen an ECM fail in this manner ... But this is the first GM product I've owned in over 15).
If the ECM is the cause, the new challenge becomes finding one as well as finding someone/place to Tech II it (or whatever needs to be done). I deal with CAN-Bus technology all day long at work (and I use the word technology in light regard ... really just a bunch of over priced cards that are forced to play nice and talk to each other LOL) and I'd much rather have a simplified platform. Oh well, I guess it's a job so I can't complain (too much).
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I can't disagree

I still bet money its the ECM. I have seen them go bad.
Well, I can't argue that point (as I have no experience in this area). Have you seen them fail in this or a similar manner?
I find this post most interesting because I am currently troubleshooting almost the same thing on my '04... same symptoms: rear 4 cylinders are cold to touch like they aren't firing, but I get spark and gas. It happened when I was cruising @ 50 or so, just like you.
I'll follow along on this post to see if anything turns up for you, and if I come up with anything new I'll post back as well.
Jeff I hate to say this but...It's actually rather refreshing to know I'm not the only to have this happen (as forked up as that may seem).
Since a computer hasn't alleviated your issue, I'm not going that direction (yet).

Nothing that archaic (I wouldn't put a carb on shyt anyway).
I was thinking more along the lines of

That's the complete harness (minus injector plugs) for my Mustang with the EEC-IV. Simple yet effective not archaic and barbaric (carbs!).
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Yeah, Mine as well

Sorry if I'm digging this one back up, but I finally got my issue straightened out and felt I should include this for the archives or if the OP is still having his troubles. I took it to a garage once winter cleared out and I had some time. They found the #6 cylinder exhaust valve sping was broke, and that's it. I guess that was enough to throw the rest of the cylinder's timing off because I was really dumbfounded how multiple cylinders just went dead. So yeah, new valve spring and it's good to go. It never threw a CEL code on any sensor pickups.
I never did update this but since it's at the top ...
Figured mine out back in January .... Had a small crack/line in the exhaust valve spring on #7 ... It didn't actually fall apart and break until I removed it. It snapped in half in my hand. I felt the same way as you, dumb founded on how multiple cylinders went completely dead because of one spring.
Interesting to note that your issues were the same along with the cause and cure.
I'm wondering if the beehive design of the springs I was running allowed it to sort of catch itself and not allow the valve to fall completely down to contact the piston.
I guess I'm "lucky" in the fact the valve didn't fall and meet the piston BUT ... I had the supposedly fixed Comp 918's on the heads. I put a stock spring on it and ran it like that for about 2 months until I installed my L92/LS3 combo on.
So anyone still running 918's ... you're on borrowed time at best.
Sounds like broke spring to me. Do what I did ... I stuck a stocker on there, for about 2K miles, until I pulled the heads for the L92 swap. Not overly impressed with PAC quality (seeing how many LSx guys have experienced broken springs).
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