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Unread 03-21-2009, 09:44 PM   #1
AzBuckfever
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HIGH RPM backfire???

So I've been reading these threads and trying to diagnose an issue with my '95 XJ. I have an AX-15 (5 spd) tranny and when driving, I'll go to up shift and the Jeep backfires. I'm trying to explain it the best I can So for instance, I'll shift at 3,000ish RPMs from 1st into 2nd and when it drops, it back fires/pops. I think that makes enough sense.

A few things I have thought about are:

#1; I have a bad or some bad injectors (of which I intend on replacing my plugs and wires in the next couple days and wondering if I should include some new injectors as well).

#2; I'm getting a misfire/hesitation and not burning all the fuel which is being sent through the exhaust and detonating in the CV or Muffler (weird, I know)

#3; bad plugs/wires which is leading to a miss somehow, or the Cam sensor is bad.

Also, I'll say that recently, the distributor & O2 sensor was replaced from an off-roading misadventure The entire distributor assembly was replaced (shaft, cap, etc.)

One last thing as well....The engine does idle a little rougher than my previous '96 that I owned. I'm thinking that instead of diagnostics/preventative maintenance and singal repairs, I should just replace all of it; unless I am heading in the wrong direction?

Thanks for your help.

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Unread 03-22-2009, 06:19 AM   #2
tjwalker
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Any check engine light? Did you have these symptoms BEFORE you replaced the distributor?

I'd start by checking fuel pressure at the schrader valve on the fuel rail. Backfiring can be indicative of an improper fuel/air ratio. Gotta eliminate basic fuel issue as a cause and it only takes a gauge and 15 minutes of your time.

Pulling and inspecting a spark plug or two and closely examining them to see how they are burning may be a good idea.
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Unread 03-22-2009, 06:45 AM   #3
JoBo
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Check for exhause leaks. Is your manifold loose or cracked?
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Unread 03-22-2009, 10:44 AM   #4
AzBuckfever
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This was a problem prior to the distributor replacement; but it may be a little worse now...not sure....As for exhaust, I thought about checking that as well because I think the collector has been smashed/clamped. Maybe if I have time before work, I'll crawl under and check it out. Thanks for the ideas. Even bigger perk is I have a fuel pressure gauge from when I had my '96 and had some stalling issues.
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Unread 03-22-2009, 10:47 AM   #5
3shifty
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It just seemed a little odd to me how high you go with your rpms when you shift, I shift around 2.5k or so, and if i went up to 3k my engine would be screaming. maybe my tachometer is off..
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Unread 03-22-2009, 11:13 AM   #6
BrunoS
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funny...i shift my 5spd at many rpm's depending on power need. i can drive in traffic just past idle or scream it to 5000 rpm if desired. it's an engine, not a woman.
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Unread 03-22-2009, 11:17 AM   #7
BrunoS
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as for backfiring...weird...good possiblity ignition or fuel timing is off.

on the xj, distributor signal sets FUEL injection timing.

flywheel sensor set SPARK timing.

sounds really good that your fuel timing is off...the distributor is not supposed to be adjustable.
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Unread 03-22-2009, 11:44 AM   #8
AzBuckfever
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Well, the only thing I saw with the exhaust is a pinch (narrower area) right above the O2 Sensor and about where the collector is....checked the fuel pressure...34PSI at idle 40-42 at rev and recovers to 34 from 30 when throttle is released...

The other question I was gonna ask is when some of you shift (what RPM) I've gotten mixed answers on this and from my experience driving stick, I normally shift when it feels right I'll be replacing the wires/plugs, possibly injectors afterwards, and who knows what else in the next week or so....

So, what RPM do you cruise at and what RPM do you shift at? I've been cruising around 2500 and shifting around 3000....Been thinking maybe it's a little high but as said, got this info from some others on different boards.
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Unread 03-22-2009, 10:04 PM   #9
BrunoS
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pinched exhaust is normal. factory made and installed. yes, it is.

higher velocity exhaust flow assists in "scavenging" (sucks exhaust out of cylinders) and helps mgp. it's a discovery every XJ owner makes as in "hey my exhaust is pinched".

your fuel pressure is low. should be near 49 psi. I suspect a plugged $12 fuel filter. up to 96 year, very easy to change.

as for shifting rpm, i do it all: low rpm shift in traffic, 1 to 3 to 5, or if going downhill from a stop/slightly rolling in traffic, start in 2nd gear, etc.

my shifts are all over the place. as for rpm/speed, i can tell you it's 2k rpm at around 73 mph on the gps in 5th gear with 31" tires and 3.07 gears.

2nd gear tops out at redline 5200 rpm near 60 mph. it's fun. pulls like a banshee from just off idle, fun REALLY starts at around 1800 rpm.

your backfire i suspect is lack of fuel volume/pressure due to plugged filter, which shows up as extra fuel when you shift and rpm drops, the injectors catch up to flow as the rpm drops when you let off the throttle and bam you have your backfire. try changing the possibly original fuel filter and a few bottles of techron to your next full tank of gas. the techron ain't cheap, but holy cow it cleans injectors/heads/intake/exhaust/valves/piston tops/rings like you would not believe.
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Unread 03-23-2009, 10:29 AM   #10
AzBuckfever
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Not sure what any of you talk about when you say "Techron"...I bought a fuel filter and a couple cans of SeaFoam (O2 sensor safe). I saw some fuel injector cleaner and such but the closest thing to Seafoam was some Valvoline Synthetic cleaner. Another reason for the seafoam is that is the only thing they had a few cans in. Anyway, I'll dump a can in unless you don't think the 2 cans I bought will be too much?
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Unread 03-23-2009, 11:46 AM   #11
AzBuckfever
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So, I replaced the filter and still at 34PSI....That didn't help any Hmmmm, what to do, what to do....

UPDATE: WOW Bruno, your system must be FLAWLESS!!! So, replaced the filter and nothing. FInally, I found my Haynes manual and it says the pressure should be between 36 and 50 I think....If it's low up to 20PSI low, replace the regulator, if more than that, replace the pump. So I replaced the regulator and got 39 PSI...still not what I'd want but an improvement....I can't believe how much that little walnut size regulator costs though....$105.00 THAT'S INSANE!!!

Any other idears???

Last edited by AzBuckfever; 03-23-2009 at 02:13 PM..
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