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Old 06-12-2010, 04:34 AM   #1
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Default My turn to have a dead engine..I think..

Well coming home tonight I was crusing down a hill and downshifted to engine brake. When I got back on the throttle the RPMs dithered for a moment and my car let out a HUGE backfire. Was at about 5,500rpm. Since I was in gear, I tapped the throttle again... BAM BAM.....BAM. I could even see the road lighting up behind me from the fire out the exhaust.

So I pull over and put the clutch in and rpms go to zero. Try cranking the car and it turns but no fire. Every few rpms the engine "chugs" but no fire...not even a 'almost start' shudder. It also does that hard momentary stop like timing is off every now and then.

So I get the car towed home and start pulling everything off thinking that the timing belt had skipped or broken. As soon as I pull the VC off I can see the t-belt is intact. OK so maybe it jumped. So I continue to pull off the belts, some of the timing covers and whatnot. I get just far enough in an hour that I can see the crank pulley for the t-belt.

This is where it gets interesting.

I turn the crank enough to get the TDC mark at 12 o' clock and check the cam timing. It's all dead nuts perfect.

Now it's panic time and I can't figure out what is wrong.




Cliffs-
RPMS act a little funny while engine braking
Throttle applied- (car moving and in gear) car shoots fire. This tells me that no spark is happening but fuel is.
Car will not start.
Engine turns and chugs out the exhaust, but no fire..no sputter of a bad bad idle.
Pull front of engine off enough to check timing marks and all are spot on.
I panic.
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Old 06-12-2010, 09:31 AM   #2
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Same thing as the other guy. CAS or coils.
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Old 06-12-2010, 10:10 AM   #3
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Take it easy and follow some steps

1.Check if your coils ignite.
2. Check plugs
3. Check that you are squirting fuel in the cylinders
4. Take a compression test.
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Old 06-12-2010, 11:19 AM   #4
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Check grounds...
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Old 06-12-2010, 11:31 AM   #5
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Ren't you having coil troubles on track when they superheat?

Could be they've just finally died? I'd certainly start there.

Do you get spark when cranking?
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Old 06-12-2010, 12:45 PM   #6
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Sounds like a no spark situation to me as well. If you really want to, spark checkers are cheap at autozone, if your afraid to burn your girley fingers.
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Old 06-12-2010, 01:29 PM   #7
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Well, I put everything back together and changed out the samn sensor while I was at it. Car almost started. I had to hold the pedal at WOT while cranking and the car would occasionally run VERY hard for about 3 seconds before quitting with a backfire through the intake. Would not start at all with no throttle input.



Checked crank signal via my laptop and it counts RPM when trying to start.
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Old 06-12-2010, 03:54 PM   #8
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Sounds like you flooded the crap out of it on the decel. Backfires are a good indicator of unburnt fuel.

I'm not too familiar with what type of fuel management you have but high rpm with no throttle input may have dumped a bunch of gas in there?


Pull plugs!
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Old 06-12-2010, 05:12 PM   #9
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replace the plugs and try again.
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Old 06-12-2010, 05:20 PM   #10
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What do the plugs look like? Wet, dry, cracked.......melted?
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Old 06-12-2010, 05:39 PM   #11
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I'm hoping its nothing serious. In for spark test.
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Old 06-12-2010, 06:15 PM   #12
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Not sure if someone already suggested it, but: compression test? this way you'll know if there's a hole in one of the pistons at least
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Old 06-12-2010, 06:34 PM   #13
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Quote:
Originally Posted by 18psi View Post
this way you'll know if there's a hole in one of the pistons at least
Or a valve or six.....

I burned 6 exh valves at once and had similar symptoms, though it happened on accel. It ran on 1 cyl until I let it out of gear for a red light (non miata bp).
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Old 06-12-2010, 08:57 PM   #14
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Quote:
Originally Posted by flounder View Post
Sounds like you flooded the crap out of it on the decel. Backfires are a good indicator of unburnt fuel.

I'm not too familiar with what type of fuel management you have but high rpm with no throttle input may have dumped a bunch of gas in there?


Pull plugs!
Flooding is the first thing I thought of too.
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Old 06-12-2010, 09:04 PM   #15
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He's running a Hydra
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Old 06-12-2010, 09:31 PM   #16
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Quote:
Originally Posted by Doppelgänger View Post
Throttle applied- (car moving and in gear) car shoots fire. This tells me that no spark is happening but fuel is.
Car will not start.

How do you get fire without spark?
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Old 06-12-2010, 10:00 PM   #17
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well regardless of whats wrong the plugs will almost always give you a good hint. pull em out and check them. almost sounds like burnt up valves, or maybe streched valves.
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Old 06-12-2010, 10:08 PM   #18
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Quote:
Originally Posted by Savington View Post
replace the plugs and try again.
Quote:
Originally Posted by miata2fast View Post
What do the plugs look like? Wet, dry, cracked.......melted?

Plugs were in good physical shape...other than a fuckton of unburned fuel on them. Took them out, cleans and re-installed. Car turned over, almost started (idled at >500rpm while holding throttle WOT) and then died.
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Old 06-13-2010, 12:02 AM   #19
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Pull the pump fuse and crank the engine over with all four plugs out a few times and put some new plugs in it.
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Old 06-13-2010, 01:10 AM   #20
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Quote:
Originally Posted by flounder View Post
How do you get fire without spark?
Ignition of unburnt fuel in a hot exhaust pipe.
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