e21 w/weber, have spark, have fuel, and it was running - then died. Help

Since it's heat-related, have you pointed an IR thermometer at the various ignition components when it's running and when it dies?

My money's on the coil, since you've eliminated the ignition module.

Jesse Ransom said:

I'm not ready to bet money on anything in particular.

It may be all dumb stuff, but if you've got fuel, and you've got spark, and it's still not running right, then... something has come up with extra dumb.

3lb of fuel pressure doesn't let the carb off the hook. (You've had float issues before, yes? Do we know everything's clean inside?)

Spark that varies from fat and blue to weak and yellow suggests something is unreliable there. (Got any other ignition modules? Coils? Ready to do something crazy like swap in a Pertronix? I'm not saying go parts cannon, but when there's a weak-ish spot that wouldn't mind an upgrade anyhow... If you happen to be inclined.)

There's always a reason. I want to be very clear that I'm not suggesting that this is your issue (it's not; it's really not) but I've had the spray tube in the primary venturi on a DGV come loose in its casting and rotate until it wasn't facing the right direction. Weird stuff can happen, but it's *still* a reason.

Maybe something very simple is behaving variably. Maybe something that shouldn't be an issue is having an esoteric meltdown (Esoteric Meltdown is now my Dr Teeth and the Electric Mayhem cover band). But there's a reason.

Electric fan causing a voltage drop as the car gets 20-minutes-hot? Not on an E21 I'm guessing, but it's been a while... Triple-check whether this car *should* have a ballast resistor? Sounds like there's some uncertainty there, and if it should have one but doesn't that might be making the coil toasty...

Yeah, float plunger had come loose, I could check it again, but I ran it pretty tight.    That was when it wouldn't even start, though - once I had fuel pressure at the carb that was the next look, fixed that, and that's when it fired up.  When it's running, it'll run to 7k through the gears, won't fall over at all.  I was going down the road at 2500 to 3k, and it just up and died.

Carb itself is basically new, and/but I ran carb cleaner through ever orifice as part of the mash up here, and it shoots out every opposite orifice, don't think anything is gummed up.

A week or so ago I had it idling for 20min, fan came on and off multiple times, never shut off.  That it was running so well last week is why I took it for a drive today, so if fan was going to pull something, it should've done it while it cycled on and off a half dozen times (or more) last week.  

Car itself does not run elevated temps at all.

The spare ignition module I plugged in didn't change anything, but, as noted before, it was from a non-running parts car anyway.

I've run through the wiring schematic, I don't see a ballast resistor noted anywhere, but as the injection was mechanical, not electronic, it wouldn't be there for that, and both the schematic and looking over a fairly open engine bay, nothing there.  I think I had a bad ballast resistor on one of my Z-cars years ago, it's what made me think of it here.  But, nada as far as I can determine.

Off to look at an exploded diagram of the distributor, and unless there's a doo-hickey in that, I don't think it can be anything but the ignition module.

My fear is I'll haul off and buy one and it won't fix it.  At that point, I'll think I'll bring the boy out on his vacation time and have him wire me up a megasquirt and all that.  I'd be sad to see the carb go, because carbs rule!  (Ok, boomer....).

In reply to 02Pilot :

Don't have an IR therm, I thought it was the coil, so I bought a new coil -  turns out I must've been smoking dope when I read the resistance on the old coil, because upon re-checking, both the new and the old coil had the same ohm reading (I think they read 2.5, or 5, or whatever a coil should run as far as ohms, don't remember the number, that was at least a couple weeks back... ancient history now, man....)

At this point, I'm inclined to think it IS the ignition module.

Since it had been sitting . . . something in the fuel tank working its way to the fuel line and blocking just enough to starve the engine?

Exploded view shows a pulse generator.  Google-fu finds this post:

https://www.bimmerforums.com/forum/showthread.php?1216303-Replacing-the-Ignition-Impulse-Sender

Long and short of it, sounds like the ignition module controls spark energy, which seems to be the problem.  I'll go check what I'm getting in the way of spark here.

EDIT: It's getting spark, seems a little inconsistent, can't tell if that's because I was having a hard time keeping it grounded against the strut tower bar (round surface, should've used the cowl... duh), but, it's getting spark.

In reply to OldGray320i :

Swap the new coil in just to make sure. 

In reply to tester (Forum Supporter) :

Left the new coil on it, looked prettier than the old one.  

I need food, so I'll see if it fires after dinner; if it doesn't, I'll swap the old module back on.

If it fires and runs on that, since it's run for 20 min at a time on that one (along with running it back and forth in the driveway a couple times as I jockey cars around to get one or the other out of the garage), that'll be a clue.  And/or get the test procedure down for the module and do that.

Getting closer, me thinks. 

And, as I think about it, the impulse generator would produce spark or it wouldn't (or so I think), it's the module that controls advance, and advance is a function of rpm, right?  Because it would start and idle a bit, but die as rpm climbed (then it decided it wouldn't even start, but I digress...).  But at some point in the process, more rpm definitely killed it.

Am I wrong?  

What does it use inside the distributor?  The pickup coil may be getting flaky.

Weird problem from RX-7 land.  Guy had (well, has) a nice GSL-SE model that he did a complete ignition system restoration.  New everything between wiring harness and plugs, and plugs too.  Also did a bunch of other stuff.  The car developed a habit of cutting out completely in right turns, like you'd shut the key off.

Three years later, he posted the fix.  He noticed oil on the coils.  Plural. See, there are two separate ignition systems, with their own timing profiles, so ignition SHOULD be redundant to a degree, but he replaced the coils and the cutting out problem disappeared.

The coils mount on their sides, and had leaked a lot of their oil out.  In right turns, the oil that was still in the coils would slosh to the "bottom" and the ignition would cut out.  Weirdest problem I ever heard of.

In your case, it's good to investigate fuel flow, but carburetors generally don't cut out all at once, ignition systems do.  And ignition systems are generally weaker at high RPM/load.

Another weird thought.  If you have a tach, disconnect it.  Have seen tachs fail to where they were pulling the coil negative to ground, causing spark to fail.  (This offer not applicable to current-sensing tachs, not sure what a BMW of this vintage would have)

02Pilot said:

Have you checked fuel in the carb and spark immediately after it dies? 

 So, since it died again went and rechecked all of this....

What's the old John Wayne quote?  "Life is hard.   It's harder when you're stupid"?  I'm a moron....

Went out, checked spark, good spark. 

Fuel pressure at the carb about 2.5-2.7lbs.

Only me here - and I'll never ask a neighbor for help, that would make sense... - so how am I going to see of the carb is still squirting fuel?

Aha!  Tape my phone you the strut bar, record while I crank it and gas it.  Check recording.

No fuel squirting out? 

Are you muther-berkleying kidding me?

Stupid float valve had come loose AGAIN.  Apparently it just walks it's way out bit by bit, letting the car run less and less, until zippo.

 Put a 10mm wrench on that stupid float valve and cranked it to the point of stripping threads. 

 That concludes this broadcast, you'll now be returned your regularly scheduled program. 

At least you figured out the problem. 

In reply to OldGray320i :

"Why is it every time I learn something, I feel dumber?"

Thanks for posting the fix!