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I had some oh-so-wonderful hesitation and an exhaust backfire this morning. The car wasn't really up to temp - it had been running for about 15 minutes and it's ~68F this morning.

Some potential causes I've found are:

  1. Sticking exhaust valve
  2. Running way, way too rich, somehow getting ignition in the exhaust. Note that I have NOT installed a hey-look-at-me spark plug in the exhaust, though flames shooting from the tail pipe would be kind of cool.
  3. ??

I also have a ticking sound when the car is warmed up, but I don't think it's a valve since it doesn't sound like it's RPM-dependent. However, if it is a valve, it could potentially be related to the backfire.

When I rebuilt the engine about 4000 miles ago, I had the head serviced. Redecked, valve guides and springs. I had a sticking valve initially but it worked itself out over the first 50ish miles.

I've looked for exhaust leaks and haven't found any.

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  • Have you a possibility to compare the ignition pulses with the camshaft- and crankshaft position? – Martin Oct 3 '16 at 13:31
  • @Myself I'm not sure how to do that. I recently replaced both VVT cam actuator solenoids, might be related to that. – 3Dave Oct 3 '16 at 13:34
  • Running for 15 minutes and it wasn't up to temp? You sure about that? – cory Oct 3 '16 at 19:08
  • What is not clear is whether this was a one-off event or something persistent. Please clarify – Zaid Oct 3 '16 at 19:16
  • @DavidLively You'd have to have a 2 or 4 channel scope to time the pulses. You'd backprobe the CKP/CMP and the ignition coil signal from the PCM for whichever cylinder you're looking at. Once you have that you can overlay a 720 degree picture on top of your scope capture to determine when the coil is firing. You could also do an in cylinder pressure transducer deal plus the coil firing event and an overlay. – Ben Oct 3 '16 at 21:20
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The most common cause of backfiring in MAF driven cars is an air leak in the intake system. This causes the engine to misread how much air is entering the engine, and too much or too little fuel is injected as a result. If any fuel is unburned it will enter the exhaust system. Eventually this left over fuel can ignite.

But, you mentioned it had a sticking valve. I would be very certain this problem no longer exists via a compression and/or leakdown check before continuing. This could cause the same symptoms, rough running and fuel entering the exhaust.

The other alternative, hinted at in a comment, is that your engine is mistimed. This is much more involved to check. If this problem existed immediately after the rebuild I would be more prone to suspect mistiming, but for now I would focus on intake air leaks.

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