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Backfiring...

13K views 231 replies 15 participants last post by  mike_breaker_5 
#1 ·
Ok, so I've always wanted a wrangler and I finally got one. It's a 93 2.5 5spd. It's not perfect but I always like a project! I already plan on swapping a 4.0 in it and have already bought one. I'm sure ill be asking tons of questions when I get around to doing that:)

But my post for today is about it running bad. It runs fine until its warmed up and then it starts bogging down and backfiring bad! I'm talking downshifting to 2nd on the highway to get it to move. Never overheated or got close to being hot. I've already changed the plugs, wires, dist cap, rotor, oil and filter, and air filter. It was pretty nasty underwood so I cleaned her up a bit.

Any ideas?
 
#3 ·
Ok, so I've always wanted a wrangler and I finally got one. It's a 93 2.5 5spd. It's not perfect but I always like a project! I already plan on swapping a 4.0 in it and have already bought one. I'm sure ill be asking tons of questions when I get around to doing that:)

But my post for today is about it running bad. It runs fine until its warmed up and then it starts bogging down and backfiring bad! I'm talking downshifting to 2nd on the highway to get it to move. Never overheated or got close to being hot. I've already changed the plugs, wires, dist cap, rotor, oil and filter, and air filter. It was pretty nasty underwood so I cleaned her up a bit.

Any ideas?
Your emissions/fuel/timing etc. are controlled by the ECM. When the engine is cold are running in open loop mode and on the factory presets. Once the engine gets up to temp you go into closed loop mode and run on learned memory BUT you sensors are always checking those numbers. You have something that is out of the factory parameters.

idaholtby mentions checking for any stored codes. That is step #1. Does your Check Engine Light go on when you start your engine? If not someone pulled the bulb. You have to check for stored codes even though the CEL may not be lit up. If it does light up check your DTC's (trouble code by doing the following:

Post #3;
http://www.jeepforum.com/forum/f12/engine-code-reading-311539/
 
#4 ·
Does it backfire under load or at idle after its warm?
What size are your tires? stock? My engine is different than yours, but I find that when sometimes im on the highway and im trying to accelerate, sometimes i backfire because i put my jeep under too much load, especially with stock 3.07 gears lol.

Have you checked your vacuum lines? most importantly your vacuum advance on your distributer. You can use some carb cleaner and spray around the vacuum lines when your engine is running, if your RPM changes you have a vacuum leak.

Good luck with your new Jeep and welcome! :cheers2:
 
#5 ·
plugged cat or muffler.
 
#7 ·
Congratulations on your first Wrangler!

What you are experiencing is called Testeronic Deflation. You aren't actually hearing a backfire but the sound of the afterburners failing to ignite which is pretty common with a 4-banger Wrangler. You have the downshifting part correct but I'd bet the PO didn't remember to tell you the last 2 crucial elements that will enable your Jeep to perform correctly... giving you that white-knuckled, nut-tingling euphoric rush of adrenaline that is called JEEP! First, when entering the highway, you MUST have Kenny Loggins' "Danger Zone" playing on the stereo as loudly as possible w/o distorting the sound. Secondly, when you downshift into 2nd gear you must quote (verbatum and in an excited manly voice), "It's time to kick the tires and light the fires!" The second you finish the quote, downshift into 1st, throttle to the floor and hang on! :cool:
 
#10 ·
Congratulations on your first Wrangler!

What you are experiencing is called Testeronic Deflation. You aren't actually hearing a backfire but the sound of the afterburners failing to ignite which is pretty common with a 4-banger Wrangler. You have the downshifting part correct but I'd bet the PO didn't remember to tell you the last 2 crucial elements that will enable your Jeep to perform correctly... giving you that white-knuckled, nut-tingling euphoric rush of adrenaline that is called JEEP! First, when entering the highway, you MUST have Kenny Loggins' "Danger Zone" playing on the stereo as loudly as possible w/o distorting the sound. Secondly, when you downshift into 2nd gear you must quote (verbatum and in an excited manly voice), "It's time to kick the tires and light the fires!" The second you finish the quote, downshift into 1st, throttle to the floor and hang on! :cool:
Naw, you only get that when you get married...or is that testeronic separation...I forget already.....:laugh:
 
#11 ·
Thanks for all the reply's guys. I've been busy at work and haven't had a chance to reply.

1. The check engine light does NOT come on when I start it up...so I guess I'll have to check the bulb.
2. I'm about 30 miles from the closest auto parts store so I'm kinda hesitant to drive it that far to check the codes. Unless there is a trick that doesn't require a code reader.
3. The exhaust sounds like ****. It rattles and is pretty rusted. Can't see and obvious holes in it but I wouldn't be surprised if the car or muffler where clogged. If so would it not affect performance while engine is cold?

I don't really want to redo the exhaust now before I swap the motor in a few months...unless I can use the same exhaust with the I6.

Oh...and ain't no lightin' tires on this thing. It sees a hill and slows down!
 
#13 ·
Thanks for all the reply's guys. I've been busy at work and haven't had a chance to reply.

1. The check engine light does NOT come on when I start it up...so I guess I'll have to check the bulb.
2. I'm about 30 miles from the closest auto parts store so I'm kinda hesitant to drive it that far to check the codes. Unless there is a trick that doesn't require a code reader.
3. The exhaust sounds like ****. It rattles and is pretty rusted. Can't see and obvious holes in it but I wouldn't be surprised if the car or muffler where clogged. If so would it not affect performance while engine is cold?

I don't really want to redo the exhaust now before I swap the motor in a few months...unless I can use the same exhaust with the I6.

Oh...and ain't no lightin' tires on this thing. It sees a hill and slows down!
If the CEL had a good bulb that did not light up when you hit the ignition your Jeep will not start UNLESS all the emission's equipment has been disabled.

Check the CEL bulb. I'll bet it is pulled. Steal another bulb out of the dash for the time being and stick it in the CEL bulb socket. It never blows since it is quite heavy duty. Once it is replaced go to this thread post #3 I think it explains how you can read the codes;

http://www.jeepforum.com/forum/f12/engine-code-reading-311539/

Then for the codes go to this thread post #2

http://www.jeepforum.com/forum/f12/reading-my-engine-light-code-question-723082/
 
#12 ·
if the exhaust is rattling i say it's definitely a plugged cat. the ceramic cake comes loose and gets lodged in there sideways. i had exactly this happen to me on my way to work one day.
 
#15 ·
The cat converter can often be the problem.

But the first issue is the OP's YJ starts but it show no CEL during startup.....I smell 'cover up' by the PO. You know the drill....no CEL..... nooooooooo proooooblems!!

And bet is there is a DTC that will show up when the OP gets the CEL bulb to light up. From there maybe we can figure out what is going on without throwing a bunch of parts at this YJ.
 
#16 ·
Ok...I lied. The check engine light comes on at start-up. And then goes off. So there doesn't seem to be a code unless there is one stored.

I pulled off the air cleaner and the throttle body was NASTY! Cleaned it up the best I could without taking it off right now. I revved it up and it has a pretty bad hesitation on the throttle response. I think it's actually backfiring from both the tailpipe AND the throttle body. I wouldn't mind doing a full exhaust as long as it will convert over to the I6 when I finally swap it over. I'm sure the cat is trashed.
 
#17 ·
Ok...I lied. The check engine light comes on at start-up. And then goes off. So there doesn't seem to be a code unless there is one stored.

I pulled off the air cleaner and the throttle body was NASTY! Cleaned it up the best I could without taking it off right now. I revved it up and it has a pretty bad hesitation on the throttle response. I think it's actually backfiring from both the tailpipe AND the throttle body. I wouldn't mind doing a full exhaust as long as it will convert over to the I6 when I finally swap it over. I'm sure the cat is trashed.
Pull the codes if any are stored. Ignore 12, 33 (espically if you do not AC), 55.

Run it after you clean the crap out of the TB with carb cleaner while you have it running. How does it run now once it is warmed up? Still running crappy?

If you want to determine if the cat converter or muffler is the problem, cut the exhaust after the O2 sensor but before the cat/muffler somewhere on a straight section like where it crosses over from the drivers side to the passengers side behind the base pan If it runs good now the cat and/or muffler is NG. If nothing changes it probably not the cat or muffler. You can fix it with a 2" OD x 2" ID adapter and two muffler clamps if you have no welder.

Bottom line is it gets real expensive throwing parts at an unknown problem.

If there is anything 'off' with any part off the emissions system you should get a lit CEL telling you that a DTC has been set. That is the first thing you need to find out.

If any problem exists after the emissions sensors, the O2 being the last one then no DTC gets set and no CEL lights up.

Then you can safely assume that if the fuel is good and you have decent compression the cat is clogged.
 
#20 ·
Ok. So the wife went out with the girls, and the kids wanted to go to the movies and wanted to take the jeep. The movies are about 20 miles away so we took off. She ran fine until about half way there and then started hesitating while at highway speeds. After the movie on the way home, it started running horrible after about 5 miles. It literally sounded like popcorn popping under the hood with absolutely no power at all. Going up small hills on the highway I think I was doing 30-35 mph.

If it was timing it would run bad way before 5-10 miles on the highway right? Also looking into setting the timing on the forum, everyone is saying that it is computer controlled and that advancing the dist won't do any good...so,if it IS the timing...how do I set it??
 
#21 ·
change the god dammmm cat!

i'll put $10 in your membership account if it's not the problem!
 
#22 ·
change the god dammmm cat!

i'll put $10 in your membership account if it's not the problem!
At this point I'm inclined to agree with mike_breaker_5, especially since it took less time to stumble coming back since the cat was already hot from the trip to the movies......but I ain't bettin' no stinkin' 10 bucks!
 
#24 ·
if it was timing it would be running terrible all the time. he said it ran fine for a short time in b/n fits.
 
#25 ·
mike_breaker_5 said:
if it was timing it would be running terrible all the time. he said it ran fine for a short time in b/n fits.
No it would not neccasarily. The chain can stretch more once it heats up. I would check the timing. Back fires are caused by a timing issue or raw fuel being dumped into the cat from a cylinder not firing. He said its backfiring through the throttle body and exhaust. Sounds like timing it's firing to soon and then dumping off the fuel that's left into the exhaust. Timing is the only explanation I see that would cause both his symptoms.

ECM sets ignition timing. Mechanical time is still set by the chain and sprockets.

Sent from my iPad using JeepForum
 
#26 ·
Ok. Thanks for all the reply's. I'll check the timing tomorrow after work. I'm also gonna call and get a price on a full exhaust system. Anyone have any recommendations?? Does both the 4 and 6 have the same exhaust as I am going to swap??

Thanks again guys. And even tho she isn't running right...she's still fun as hell to drive!
 
#27 ·
The exhaust systems are different. For a temporary test only because defeating a cat converter hurts not helps you, get the exhaust system apart and punch a large hole through matrix of the cat converter. Stick it back together with a muffler pipe adapter and the run it.

If it doesn't bog on you once it is warmed up you probably have a bad cat converter. Replace the exhaust system once you do the engine swap.

Warning: Those loose pieces of matrix in the cat you created can bounce around and get stuck in the outlet of the cat converter shutting you down permanently.
 
#28 ·
Exhaust is only 300-350 bucks from manifold back at a reputable shop in my town so I'm go an go ahead and replace it. He said that he'd run it 2.5" so that when I swap the motor most of it will still work. Said he'd have to redo from the manifold/header (haven't decided yet) to the converter once the swap is done...so no biggie there. Still gonna throw the timing light on it tonight and see what I can find out. At this point, hopefully it's just the cat!
 
#30 ·
Yes. It does exactly that. It's also worse going up hill or any slight grade. Had to stop 3-4 times the other day just to get home. I'll smack it when I get home and see. Like I said. I'm sure it's trashed.

Hopefully I'll get a little more power once it's fixed cause this thing is a dog! I had a friend that had a TJ with a 4 banger and I don't remember it being this bad.

Again, thanks for all the help guys!
 
#31 ·
Ok...so I just got home. Crawled under the jeep and guess what...no cat. So now I'm really stumped. My o2 sensor is directly behind the manifold, it zig zags over to the passenger side, straight thru to the muffler. Which is kinda funny seeing that it just passed inspection. You can see where the car was taken out and a straight pipe welded in place of it.

Thoughts? I'm about to check the timing. I'll let you know what I find out.

Guess mike_breaker_5 owes me 10 bucks! Haha
 
#36 ·
Ok...so I just got home. Crawled under the jeep and guess what...no cat. So now I'm really stumped. My o2 sensor is directly behind the manifold, it zig zags over to the passenger side, straight thru to the muffler. Which is kinda funny seeing that it just passed inspection. You can see where the car was taken out and a straight pipe welded in place of it.

Thoughts? I'm about to check the timing. I'll let you know what I find out.

Guess mike_breaker_5 owes me 10 bucks! Haha
What do you mean the O2 goes straight thru to the muffler?????? It should be in your exhaust downpipe

If the O2 sensor wires are being subjected to too much heat that might cause your problem also. Insulate or move the wiring to see if that makes your YJ run better.
 
#34 ·
i'll have to get the cash in your account friday on payday. i would still check the muffler for a plug. find out what is rattling in the exhaust. the cat may have let loose and plugged the muffler.
 
#37 ·
the o2 sensor could just be all around fried. that could cause backfiring and a loss of all power.
 
#38 ·
Agreed! He can check it using a volt meter. They are 60.00 or more so it is not a cheap part to test by replacing it.

Buy the good one with the correct plug not the cheap Chinese knock offs that aren't worth spit.
 
#39 ·
i use the bosch ones, i have always had good luck with them.
 
#40 ·
I forgot to mention the best brand! Thanks for filling in the blanks for this foggy brained old fart!!!! :laugh:
 
#41 ·
No no. I meant the manifold. O2 sensor. Then pipe all the way to the muffler. Sorry for the confusion.

The po of this jeep didn't drive it much. Actually got it from a friend. Could it be some kind of clog in the muffler? Since it was sitting could it be a wasp nest or something in there? Just going out in a limb here. Gonna replace the exhaust anyways. Should I put a cat on or leave it off?

If o2 sensor is bad wouldn't my CEL be on?
 
#43 ·
No no. I meant the manifold. O2 sensor. Then pipe all the way to the muffler. Sorry for the confusion.

The po of this jeep didn't drive it much. Actually got it from a friend. Could it be some kind of clog in the muffler? Since it was sitting could it be a wasp nest or something in there? Just going out in a limb here. Gonna replace the exhaust anyways. Should I put a cat on or leave it off?

If o2 sensor is bad wouldn't my CEL be on?
Listen to me real good! I have 65 candles going on my next stinkin' birthday cake AND I have a wife that thinks I am an idiot....I don't need the extra stress........ :laugh: :laugh: :laugh: :laugh: :laugh: :laugh:
 
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