Back fire after adding K&N?

First off, vehicle is a 1989 Cherokee, 4.0L with the AW4.


I've had this problem for quite sometime, even before I added the K&N yesterday, but now it's happening more. This usually happens at lower RPM's, between 1500 and 2000. Generally, if I recall correctly, it's at 1/4 to 1/2 throttle and suddenly I'll get a pop out of the throttle body. This is only when the engine is under load, never when I am just revving it. I did a search here and there doesn't seem to be anything conclusive.

FYI, I removed the air box and everything associated with it to do the K&N. Replaced it with a FIPK style intake. I did plug the single vacuum line that had been running from the intake manifold to the box. Is this a no no? Last thing, it's winter so it's cold and has been in the 30's lately.

Dunno if this helps, but here's a pic of what I have now. Ignore the ricer breather on the valve cover please




EDIT: The motor has water everywhere cause I had washed it off...that's not the cause though I know!

Well for one thing, in cold climates you need the intake air heater you removed.
Your intake air temp sensor could be off also and without any heat things are going to be worse.

langer1 said:

Well for one thing, in cold climates you need the intake air heater you removed.
Your intake air temp sensor could be off also and without any heat things are going to be worse.

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Hmm.....that hasn't been on there since I bought the Jeep three years ago and it's not been a problem until recently. This does happen during summer as well, with 100+ degree weather, just not very often. I should also mention that this "back fire" happens when the motor is warmed all the way up.

Dunno if that helps at all.

Nothing wrong with the filter on the valve cover.....those plastic tubes break too easily anyhow. Already snapped two just touching them.

Blaine B. said:

Nothing wrong with the filter on the valve cover.....those plastic tubes break too easily anyhow. Already snapped two just touching them.

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Yeah, I was thinking about going and getting 20-30 feet of vacuum line and replacing all the plastic ones. I'm always worried I'll snap one in half.

EDIT: Man, I'm a lurker....check that post count out and reg. date!

Oh yeah, the vacuum line you plugged - I'd run it somewhere, like make a nipple on your K&N nexk or whatever. Usually the necks come with all necessary fittings. Was it custom?

I'd also make some sort of splash guard for the bottom of the K&N.....it's easy enough to get water on the bottom of the stock airbox bottom (just the bottom, I can see muddy dried up water on the bottom of mine). And that only has a hole on the front of the box, too.

You don't want that directly on the filter!

I've had an 87 pop (or buck) at about 1/3 throttle, it was a dead spot in the TPS (timing). Have an 88 that used to pop and buck at around 2000 RPM, usually under mild throttle while acclerating to cruising speed on the interstate. Throttle opened enough to lower the vacuum some. Turned out to be mostly the O2 sensor. Low vacuum conditions (could the cone filter be affecting vacuum?), mildy flacky O2 sensor and it would buck at right around 2000 RPM. Though I could accelerate through the problem RPM area without hardly noticing at all (under certain condiditons). Another thing was the engine would crap out well before redline, after the O2 sensor change it accelerated well past redline.
I drove around with a vacuum gage hooked up for a couple of weeks, I was trying to figure out if the buck was RPM related or vacuum related, turned out to be both. At a certain vacuum and RPM it would buck or pop.
A lot of guys around here replace the cone filter, with the stock box when the temeprature gets much below 40-50 degress (usually sometime around the middle of October). There mileage went way down in the winter with the cone filter. As expensive as gas is around here, it was well worth the effort to swap back to the stock box in winter.

Blaine B. said:

Oh yeah, the vacuum line you plugged - I'd run it somewhere, like make a nipple on your K&N nexk or whatever. Usually the necks come with all necessary fittings. Was it custom?

I'd also make some sort of splash guard for the bottom of the K&N.....it's easy enough to get water on the bottom of the stock airbox bottom (just the bottom, I can see muddy dried up water on the bottom of mine). And that only has a hole on the front of the box, too.

You don't want that directly on the filter!

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It's custom, and I will add something to it to support that vacuum line. No one seems to make anything for the non-HO motors like FIPK's. I'm also going to build something to protect it more and keep the heat out. Just have to go get some materials for walls around the filter.


Any ideas on the back firing? Would that vacuum line have anything to do with it?

dadilius said:

It's custom, and I will add something to it to support that vacuum line. No one seems to make anything for the non-HO motors like FIPK's. I'm also going to build something to protect it more and keep the heat out. Just have to go get some materials for walls around the filter.


Any ideas on the back firing? Would that vacuum line have anything to do with it?

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An open vacuum hookup on the intake might cause some trouble. Often a high idle.
The only vacuum line that really counts is the one from the bottom of the TB to the MAP sensor. The line has to be sealed and not broken or cracked and plugged into the proper opening on the TB (one is a blind plug the stopper can be plugged in upside down). Though if this line was cracked or broken or plugged in wrong, you'd likely have a really really bad idle.

8Mud said:

An open vacuum hookup on the intake might cause some trouble. Often a high idle.
The only vacuum line that really counts is the one from the bottom of the TB to the MAP sensor. The line has to be sealed and not broken or cracked and plugged into the proper opening on the TB (one is a blind plug the stopper can be plugged in upside down). Though if this line was cracked or broken or plugged in wrong, you'd likely have a really really bad idle.

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Pretty sure the one on the MAP is still fine. I forgot to plug that back in when I did my exhaust manifold....never seen so much black smoke coming out of the tail pipe! I had a diesel XJ for a bit