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Unread 11-11-2009, 12:33 PM   #1
RebuildRookie
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Backfiring 2.5L

1st: my muffler is blown

2nd: I can feel good flow coming out of the cat. (not by hand, dust and dirt
sturring up real good when running in the garage)

3rd : Scared to put a new muffler on, with the thoughts of possibly
blowing the new muffler, been down that road before.

4th: Only backfires after being driven for a while, engine is good and warm
usually about 7-10 miles. The harder I drive it the louder the backfire
If I am easy on it, it is more like a "poof". sounds like its right at the
front of the muffler, which is the blown end.

Any ideas?

Also, I could make it backfire on command before (I really know my Jeep), but in the last couple of months I have made it my DD. I had so many bad vacuum leaks before, I was only getting 8-9 MPG and now I am getting 15-16 after fixing those.

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Unread 11-11-2009, 12:37 PM   #2
Jamesbbh
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How do you blow a muffler? I'm not being a smart a** in any way I'm just wondering lol. Have you tried changing out plugs/wires for the backfiring issue? It might be unburnt fuel ending up in the exhaust.
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Unread 11-11-2009, 12:54 PM   #3
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The vacuum leaks were so bad that it ran extremely rich. Runs great now that I have those fixed (performance wise) almost doubled the MPG. Most of the time its a poof or a really loud poof. Not the normal fire out the tailpipe "bang" that you normally see. Of course its on the cat end of the muffler where it is blown, so no exhaust is going out the tailpipe.
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Unread 11-11-2009, 01:08 PM   #4
5.7YJ
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Does it bacfire/poof right after you shut it down or does it take a couple minutes?
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Unread 11-11-2009, 01:09 PM   #5
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Within about 10 seconds. I turn the key off and before I can open the door. So yeah right after I shut it down.
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Unread 11-11-2009, 01:18 PM   #6
5.7YJ
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It sounds like your running a bit rich and your cat is not up to burning all the excess fuel.
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Unread 11-11-2009, 01:19 PM   #7
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Quote:
Originally Posted by RebuildRookie View Post
The vacuum leaks were so bad that it ran extremely rich. Runs great now that I have those fixed (performance wise) almost doubled the MPG. Most of the time its a poof or a really loud poof. Not the normal fire out the tailpipe "bang" that you normally see. Of course its on the cat end of the muffler where it is blown, so no exhaust is going out the tailpipe.
If this only happens when the motor has warmed up, then I would first suspect the CTS (Coolant Temp Sensor). The Vacuum leaks could also make this problem worse causing the O2 sensor to think you are running too lean and then in-turn richening the mixture. Although it sounds like the vac leaks were repaired and the problem now persists. (hopefully the O2 sensor wasn't fouled by the rich mixture due to the vac leaks) It could also just need more time to adjust to the "new" conditions without the vac leaks. If the repairs where very recent, then at least three full warm-up and cool down runs have to be made before the ecm can adjust.
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Unread 11-11-2009, 01:36 PM   #8
5.7YJ
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^^Yes, it's most likely your CTS or O2 sensor causing the over rich problem. The cat might have also been ruined from the over rich condition and can't burn up all the unburnt fuel, yet it's hot enough to ignite the fuel vapors left after the cat which causes your POOF!
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Unread 11-11-2009, 02:46 PM   #9
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Quote:
Originally Posted by 5.7YJ View Post
^^Yes, it's most likely your CTS or O2 sensor causing the over rich problem. The cat might have also been ruined from the over rich condition and can't burn up all the unburnt fuel, yet it's hot enough to ignite the fuel vapors left after the cat which causes your POOF!

I agree with the ruined cat.

Quote:
Originally Posted by idaholtby View Post
If this only happens when the motor has warmed up, then I would first suspect the CTS (Coolant Temp Sensor). The Vacuum leaks could also make this problem worse causing the O2 sensor to think you are running too lean and then in-turn richening the mixture. Although it sounds like the vac leaks were repaired and the problem now persists.

The repairs were 2-3 months ago.

(hopefully the O2 sensor wasn't fouled by the rich mixture due to the vac leaks)

New(er) O2 sensor, came on the used exhaust manifold I bought to replace my broken exhaust manifold. I did inspect it and it look alot better than my old one.


It could also just need more time to adjust to the "new" conditions without the vac leaks. If the repairs where very recent, then at least three full warm-up and cool down runs have to be made before the ecm can adjust.

I have been driving it everyday for the last month.
Now a question to both, my temp gauage bounces around. I have to tap it to get it to read correctly (the ribbon circuit board on the back of the cluster is cracked and broken). Going to get another one soon. But the temp usually reads between 190-210. New thermostat installed during other repairs 2-3 months ago.

How would a bad CTS make it run rich? Is there a signal that goes from it to the ECU?

I was not aware that the CAT burns exhaust. I thought it only pulled C02 from the emissions. Am I all wrong on that.
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Unread 11-11-2009, 02:49 PM   #10
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I know its a touchy subject. But in mississippi there is no emissions testing. If I cut the cat out until I can (afford to)get a new one, would that help.
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Unread 11-11-2009, 05:36 PM   #11
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Quote:
Originally Posted by RebuildRookie View Post
I know its a touchy subject. But in mississippi there is no emissions testing. If I cut the cat out until I can (afford to)get a new one, would that help.
As far as the CTS, it is different from the gauge sender. The gauge sender does nothing other than drive the gauge. It is located on the top of the head near the firewall on the driver's side and has a single wire. The CTS is located on the thermostat housing (has two wires) and it's sole job in life is to feed info to the ECM. The ECM uses this to determine whether or no to be running in closed loop or run controlled. In closed loop operation during engine warm up, the ecm runs the fuel mixture rich, once temp is achieved, it is leaned back out.
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Unread 11-12-2009, 06:30 AM   #12
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Quote:
Originally Posted by idaholtby View Post
As far as the CTS, it is different from the gauge sender. The gauge sender does nothing other than drive the gauge. It is located on the top of the head near the firewall on the driver's side and has a single wire. The CTS is located on the thermostat housing (has two wires) and it's sole job in life is to feed info to the ECM. The ECM uses this to determine whether or no to be running in closed loop or run controlled. In closed loop operation during engine warm up, the ecm runs the fuel mixture rich, once temp is achieved, it is leaned back out.
Alright so I know about the gauge sender on the head.

I however do not have a CTS on my thermostat housing on my 88 2.5 TBI. No wires around it, Must be why I didnt know about the CTS That must be on the MPI equiped models.
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Unread 11-12-2009, 06:49 AM   #13
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I havn't had my 2.5 in a while but I believe the CTS is on the intake manifold. It has a water passage in the intake that it goes into.
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Unread 11-12-2009, 07:11 AM   #14
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I was thinking that was the MAT sensor. I need to look at my chilton book.
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Unread 11-12-2009, 07:20 AM   #15
5.7YJ
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The MAT sensor is on the top side of the intake from what I remember, the CTS is on the side. (I think) Here's a pic of the sensor.

Last edited by 5.7YJ; 11-12-2009 at 07:37 AM..
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