How sure are you that the fuel batches are between driver and passenger? That doesn't make much sense from a firing order perspective. Assuming a 1,8,4,3,6,5,7,2 firing order, perhaps 4,3,6,and 5 are a batch and 1,8,7,and 2 are a batch, and your 4,3,6,and 5 aren't firing.
But the ECU has to interpret that signal then, yes?
I'd grab a multi-meter and find out what the ignition module is supposed to be sending out and test that (to ensure the ECU is getting the correct input), and then see what the ECU is sending back out. There's got to be some scrambled eggs between the pickup coil and injector harnesses...
Bob, tell 'em what your wonky tach is doing. It's got to be related.
Javelin wrote: But the ECU has to interpret that signal then, yes? I'd grab a multi-meter and find out what the ignition module is supposed to be sending out and test that (to ensure the ECU is getting the correct input), and then see what the ECU is sending back out. There's got to be some scrambled eggs between the pickup coil and injector harnesses...
I'm not doing ANYTHING anymore. I've done thrown in the towel. I'm letting someone else dick with it.
The tach bounced around on the 25 mile drive into my work on Sunday. That was a new symptom.
Tach drive on the coil in cap HEI is run by the ignition module right? GM ICM's are known to do strange things when they go bad..
it's not the large cap HEI anymore, but yes, it still derives it's signal from the pickup coil through the ignition module.
Bobzilla wrote: it's not the large cap HEI anymore, but yes, it still derives it's signal from the pickup coil through the ignition module.
A new module would be a cheap test.
that's the thing, I put a new module in to start with.
In reply to Bobzilla:
A new module doesn't mean that it's a working module. I had the exact same thing happen to the Javelin Bob...
I think about 1 in 5 ignition parts i buy have come to me bad...
I am a bit suspicious of the batch fire thing as well. I don't get how a module could do it if he's seeing spark when he should see it unless it's only missing at certain RPM ranges or something like that, but it's cheap enough to warrant 'throw parts at it'.
At this point I'd be buying a known good carb/intake just for giggles to verify the short block.
tuna55 wrote: At this point I'd be buying a known good carb/intake just for giggles to verify the short block.
Or use starter fluid....
tuna55 wrote: I am a bit suspicious of the batch fire thing as well. I don't get how a module could do it if he's seeing spark when he should see it unless it's only missing at certain RPM ranges or something like that, but it's cheap enough to warrant 'throw parts at it'. At this point I'd be buying a known good carb/intake just for giggles to verify the short block.
I agree. Batch fire weirds me out and I also believe there could be something very wrong in the block.. Crack.. bad cam to crank timing.. blah blah blah...
Fueled by Caffeine wrote:tuna55 wrote: I am a bit suspicious of the batch fire thing as well. I don't get how a module could do it if he's seeing spark when he should see it unless it's only missing at certain RPM ranges or something like that, but it's cheap enough to warrant 'throw parts at it'. At this point I'd be buying a known good carb/intake just for giggles to verify the short block.I agree. Batch fire weirds me out and I also believe there could be something very wrong in the block.. Crack.. bad cam to crank timing.. blah blah blah...
I know you're just getting back and all, but you know we're not supposed to agree, right?
Good to see you. Stop changing your name so much, it's impossible to tell people apart anymore!
Fueled by Caffeine wrote: I agree. Batch fire weirds me out and I also believe there could be something very wrong in the block.. Crack.. bad cam to crank timing.. blah blah blah...
But that doesn't jive with his compression numbers....
any one around your town have a good old fashion scope that would show your primary and secondary ignition
Haven't read through the entire thread but stick with it, I had an '87 Z51 4+3 car and the handling and torque were both incredible. Its one of the few cars I've sold that I really wish I had kept!
it's currently sitting here in out shop at work. Firing order, spark and fuel have all been confirmed (like I already told everyone, but this is a fresh set of eyes looking at it), he was going to run a new compression test and leak down either last night or today.
and compression was up a little, average was 160psi across the board. Leakdown was less than 3% leak. So... he's on to other things. Good to know this car has stumped more than myself.
I've NEVER had a car "beat" me before like this. It's a small victory to me that it's beating someone else as well.
Bob what compression is this motor supposed to be?
My 7.8:1 motor pulled 150psi across the board.
wet or dry Ben? Remember, these rings haven't even had time to seat in yet. GM calls for anything above 100psi "good" for a 9.5:1 engine. 160psi dry is a decent reading. What's more important is the variance between cylinders is even smaller than before.
Given the fact that this one has stumped so many folks, I'd guess you're looking at a less common cause such as a failing (not failed) part. As others have suggested, I'd start swapping out ignition parts with known-good (not necessarily new) parts. In addition, maybe check the wiring harness just to make sure it's not grounding out somewhere.
I'm sure that's what he's to at this point. I am not working on it anymore.
Has he made any progress yet?
hopefully know a little more this afternoon.
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