Replaced a cracked header & now it backfires

Airjunky

Airjunky

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Dec 13, 2018
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Liberty Lake WA
I recently replaced a cracked exhaust header with the stock 2 pc exhaust manifolds. This involves removing the EFI, intake manifold, exhaust, unplugging all the O2 sensors and swapping 2 O2 sensors from the old header to the manifolds.

After installing the manifolds I drove the Jeep about a hundred miles. It ran great, no codes, no exhaust leaks.

Then on Saturday, cruising on the freeway at like 70 mph, I started feeling a knock. I pulled over & went to check the engine. I'll attach a video of what it sounded like. Initially I thought it was a horrible knock. But then realized it was backfiring repeatedly through the intake. I checked the vacuum lines on the intake, they're fine.

I thought maybe I had gotten a couple of the O2 sensors installed incorrectly.... Looked up a diagram of where everything went and they appear to be fine. Plus the plugs are all keyed differently so it would be tough to get them mixed up.
I don't have any CEL or codes being thrown.

I took my time getting the intake and exhaust manifolds in place, knowing the intake goes down over the alignment pins. Plus I can see the edges of the intake and exhaust pretty good.

Any suggestions as to what the problem is?

 
Buddy I’m not sure what’s going on but a friend of mine did his exhaust and has something similar I’ve got him tracking this keep us posted.

I really don’t want to think it’s about to blow.
 
AndyG said:
Buddy I’m not sure what’s going on but a friend of mine did his exhaust and has something similar I’ve got him tracking this keep us posted.

I really don’t want to think it’s about to blow.
Click to expand...
So can you tell me what it was that he did?

I removed the EFI, intake manifold and the header. I installed a new Felpro gasket & new O2 sensors in stock used exhaust manifolds (the 2 pc). I tried to be really careful getting the exhaust and intake manifolds in place and properly aligned. I read about a few guys messing this up & having huge vacuum leaks, problems with the engine revving really high, etc.

After the job was done I drove the Jeep for 2 days, maybe 100 miles. It ran great, no leaks, no codes, etc. Then cruising about 70 on the freeway, I felt a knock in the gas pedal & realized the Jeep was slowing down. I turned down the tunes & I could hear it knocking. I pulled over and ended up flat towing it home. After I got it home, I realized it wasn't knocking, but backfiring repeatedly.
 
Have you tried resetting the PCM and if so did it help? I heard its a good thing to reset the PCM after installing new sensors like you said above.

How to reset PCM
  1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds to discharge the PCM capacitors, which maintain the Adaptive Memory.
  2. Reconnect the Battery Cable.
  3. Turn Ignition Switch to the “On” position but DO NOT start the engine.
  4. Turn Headlight On.
  5. Turn Headlights Off.
  6. Turn Ignition Key Off.
PCM is reset
 
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Generally the same ...His knocks though, we think his is piston slap.

Yours seems like a timing issue, or you may have had a cam position sensor go bad coincidentally. Have you checked for codes?

He’s @SamwiseGamJeep so you guys can track each other.
 
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AndyG said:
Sensors need to be NTK brand. Really need to be . Trust us.
Click to expand...
Yep. I bought NTKs
DylanM345 said:
Have you tried resetting the PCM and if so did it help? I heard its a good thing to reset the PCM after installing new sensors like you said above.

How to reset PCM
  1. Disconnect the POSITIVE battery Terminal and touch it to ground for 30 seconds to discharge the PCM capacitors, which maintain the Adaptive Memory.
  2. Reconnect the Battery Cable.
  3. Turn Ignition Switch to the “On” position but DO NOT start the engine.
  4. Turn Headlight On.
  5. Turn Headlights Off.
  6. Turn Ignition Key Off.
PCM is reset
Click to expand...
I thought I was resetting the ECU by just pulling the negative. Guess I'll try this method instead.
Thanks
 
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Backfires.....
Ignition troubles

Don't rule out coincidence.
Last time you did a routine tune up? Condition of your coil rail?

Last 3 questions...
Fuel pressure and fuel trim are?

Lastly what happens when you floor it while it's misbehaving.

Bog down? Cough cough and take off?

I lied...

1 mpre...

Did you get all sensors hooked back up and or undisturbed?
 
Thanks for the input Rubicon88. But this engine only has 10k miles on it. It ran great Saturday morning, all day Friday and Thursday after I did the work.

I thought about ignition problems, but with no distributor, what can I do? The coil rail is in good physical condition. Wiring harness looks good, nothing broken or melted.

I haven't measured fuel pressure yet. But I have read a few things that mention backfiring thru the intake being a lean condition.

As for fuel trim, I believe the ECU reset procedure mentioned above doing just that? I did this procedure and it doesn't appear to have changed anything.

As for flooring the gas, I haven't tried that yet.
 
It sounds more like an exhaust leak to me than anything else. Start the engine and then run you hand as close as you can around the exhaust and see if you feel any air pulse with the sound.
 
If you can't get your hand close enough the other option is to try a small strip of paper and hold it so that if there is a leak it will make the paper flutter.
Other option is to spray a mist of water over the exhaust next to the head and see if it blows the mist back towards you.
 
It's not an exhaust leak. I ran my hand all around the exhaust and intake manifold at the head and am not feeling anything. I had a buddy come over and look at it. It's very clearly coming from the intake tube....the air cleaner.
Swapped out the crank position sensor and there was no change.
 
Wildman said:
Okay look at your IAC as if that isn't working properly it can cause issues.
Click to expand...
Pulled the IAC and cleaned it with some brake cleaner. It really wasn't bad, not a ton of carbon. Although it was made in Mexico so maybe it was replaced with a cheapie at some point.
I haven't seen a CEL since before the job. And I don't think it's ever stalled.
The idle is down 100 or so RPMs. And once the RPMs started going up.....I cut it off at maybe 3k RPMs.
 
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Man you are making this HARD to figure out. I don't know where else to point you towards as I've listed everything I could link of.
Back firing thru the intake in my experiance usually means a stuck valve or a timing issue. And how changing your exhaust header from a cracked one to a new one I would say look in the area of the things that you touched or changed. I'm stumped.
 
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