Cannot Figure Out Why 4 Back Cylinders Won't Fire
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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
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
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BUT have you verified that they are actually spitting out fuel, when fed by your pump and your PCM?
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The purpose of me testing resistance across the wires
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'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).
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I may be wrong, but 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. 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.
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I agree
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).
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).
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 still bet money its the ECM. I have seen them go bad.
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I can't disagree
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?
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Not with this particular symptom, but given how complex the ECM is its not surprising.
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I find this post most interesting because I am currently troubleshooting almost the same thing on my '04. I think the only difference is that my plugs are wet with gas (and smell raw gas in the exhaust). Anyway, 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've swapped coils, plugs, plug wires with known good ones (or from cylinders that are working).
- Fuel pressure is good
- injectors all tick with non firing cylinders wet with gas
- pcm has been swapped with a stock program (no codes other than crank angle position not learned yet)
At least on the '04 I did not need a tech 2 to sync the pcm. I bought it from a vendor on ebay who VIN programmed one and I followed the key method to sync the module passwords (it took a couple iterations, but finally worked).
My obd2 software does not show any misfires in realtime monitoring while running, even though the rear 4 are clearing not firing (cold to touch). So I wonder about a comment made earlier about the crank angle sensor. Could this be defective? Has this been ruled out? It was mentioned early in the post but I don't recall it being a non-issue. I'm just as perplexed on this... though I haven't checked the condition on my cam (cartec 2x cam). 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.
Thanks,
Jeff
- I've swapped coils, plugs, plug wires with known good ones (or from cylinders that are working).
- Fuel pressure is good
- injectors all tick with non firing cylinders wet with gas
- pcm has been swapped with a stock program (no codes other than crank angle position not learned yet)
At least on the '04 I did not need a tech 2 to sync the pcm. I bought it from a vendor on ebay who VIN programmed one and I followed the key method to sync the module passwords (it took a couple iterations, but finally worked).
My obd2 software does not show any misfires in realtime monitoring while running, even though the rear 4 are clearing not firing (cold to touch). So I wonder about a comment made earlier about the crank angle sensor. Could this be defective? Has this been ruled out? It was mentioned early in the post but I don't recall it being a non-issue. I'm just as perplexed on this... though I haven't checked the condition on my cam (cartec 2x cam). 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.
Thanks,
Jeff
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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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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.
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Yeah, Mine as well
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 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.
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Here I go digging this one up yet again.... ;-)
(just so busy these days)
MM Alexander> Glad to hear you got this figured out! And also nice to know I wasn't the only one baffled by such a strange behavior. But I count myself (and you alike) quite lucky, because there was a chance that the piston could have smashed the valve since I learned the LSx series engine is no long a non-clearance engine. I'm pretty sure all the LT1 and earlier SBC engines wouldn't "run" into this problem... so to speak. 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.
Mik1587> From what I can imagine, here's what I think occurred. Since the valve spring is busted, it can't hold the valve closed. When air & fuel enter the intake valve , it wants to leak out the open exhaust valve. Compression will be low and combustion may never take place (if it does, it blows right out the still open exhaust valve). Because of this lack of combustion, the engine will not spin as fast... because of this momentary slow down in rotation, this throws the ignition and/or fuel delivery off slightly. Because the LS1 uses the crank position (it's rotation) to time these 2 events, it cannot compensate for the slow down. That would explain why the engine ran better when I revved it to 2K - 3K rpm. The rotational slowdown was compensated by the other cylinders pushing the crank around and therefore it put fuel/ignition timing back on track. At low rpm, it is more sensitive to loss of rotational momentum and so that's why it affected multiple cylinders after it. That's my best guess anyway.
(just so busy these days)
MM Alexander> Glad to hear you got this figured out! And also nice to know I wasn't the only one baffled by such a strange behavior. But I count myself (and you alike) quite lucky, because there was a chance that the piston could have smashed the valve since I learned the LSx series engine is no long a non-clearance engine. I'm pretty sure all the LT1 and earlier SBC engines wouldn't "run" into this problem... so to speak. 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.
Mik1587> From what I can imagine, here's what I think occurred. Since the valve spring is busted, it can't hold the valve closed. When air & fuel enter the intake valve , it wants to leak out the open exhaust valve. Compression will be low and combustion may never take place (if it does, it blows right out the still open exhaust valve). Because of this lack of combustion, the engine will not spin as fast... because of this momentary slow down in rotation, this throws the ignition and/or fuel delivery off slightly. Because the LS1 uses the crank position (it's rotation) to time these 2 events, it cannot compensate for the slow down. That would explain why the engine ran better when I revved it to 2K - 3K rpm. The rotational slowdown was compensated by the other cylinders pushing the crank around and therefore it put fuel/ignition timing back on track. At low rpm, it is more sensitive to loss of rotational momentum and so that's why it affected multiple cylinders after it. That's my best guess anyway.
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Damn good read, epic tech lounge troubleshooting in here. 
I NEVER would have thought of a valve spring with those symptoms. Excellent analysis of failure mode, zakaron.
I NEVER would have thought of a valve spring with those symptoms. Excellent analysis of failure mode, zakaron.
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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.
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