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Old 06-16-2019, 01:45 PM   #1
Mbeas96
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Default In theory....no spark = boom?

What happens when you lose spark during boost?

Cylinders that don't have spark get washed in fuel, then when spark comes back it hopefully burns it away? Can't be healthy...

But what dangers does this have? Any difference in losing spark at 1psi rather than 14psi? It can't knock if there is no spark right?
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Old 06-16-2019, 01:47 PM   #2
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Knock is not your only enemy.
A wet cylinder contaminates the oil.
Unburned fuel in the exhaust may lead to a muffler explosion.
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Old 06-16-2019, 02:09 PM   #3
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Generally, a gasoline engine is not going to knock if you suddenly lose spark. Just not enough compression to ignite the fuel without a plug. Now, if the tune is **** then yes, you could get knock without spark but let's assume everything works like it's supposed to.
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Old 06-16-2019, 03:04 PM   #4
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If the spark comes back after some fuel delivery you'll have an epic backfire that may damage all sorts things from the valves all the way to the exhaust.
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Old 06-16-2019, 09:21 PM   #5
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The fuel pump relay may shut off the pumps if there's no spark.
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Old 06-17-2019, 06:33 AM   #6
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Is that standard lh2.4?
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Old 06-17-2019, 10:46 AM   #7
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Originally Posted by lummert View Post
The fuel pump relay may shut off the pumps if there's no spark.
Only if the engine RPM falls to zero though. If you're coasting down the road in a manual trans car it would keep fueling as it doesn't know it's lost spark. More modern vehicles would shut off individual injectors if the per-cylinder missfire counter indicates incomplete/improper combustion.

Last edited by bmos; 06-17-2019 at 10:48 AM.. Reason: added clarification about modern vehicles
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Old 06-17-2019, 11:13 AM   #8
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A stock EMS would probably just result in some raw fuel getting pushed/sucked out of the exhaust port on the 4th stroke. If the cat is hot enough it may ignite downstream of the header. My 940's old EMS had some sort of weird boost cut that either retarded the timing so much that it was igniting on the 4th stroke or the turbo was hot enough to light that **** up because that car had some truly epic fireballs at boost cut around 6500 rpm. ****ty chinese turbo/wastegate...
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Old 06-17-2019, 03:00 PM   #9
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With Bosch Breakerless, as an example, with a manual car, if you lose spark, the tach drops to zero. Fuel pump relay shuts off. It will continue to run, as long as the transmission is coupled to the crank and the wheels are spinning. Just does full-on engine braking. Dealt with a random no-spark issue on an '82 242Ti. Ground wire for the ignition box was corroded to heck. The constant idle system did open the idle air valve a bit more when the spark dropped out, since that system wasn't receiving its signal.
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Old 06-17-2019, 08:30 PM   #10
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Quote:
Originally Posted by John242Ti View Post
With Bosch Breakerless, as an example, with a manual car, if you lose spark, the tach drops to zero. Fuel pump relay shuts off. It will continue to run, as long as the transmission is coupled to the crank and the wheels are spinning. Just does full-on engine braking. Dealt with a random no-spark issue on an '82 242Ti. Ground wire for the ignition box was corroded to heck. The constant idle system did open the idle air valve a bit more when the spark dropped out, since that system wasn't receiving its signal.
Forgot the tach ran off the coil rather than the crank sensor. Good call
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