Backfire when accelerating
12-29-2018 | 12:56 PM
#1
Thread Starter
Newbie
Joined: Dec 2018
Posts: 3
Likes: 0
From: Longview tx
Year: 1996
Model: Cherokee (XJ)
Engine: 4.0 high output
Backfire when accelerating
i picked up a 1996 Jeep Cherokee sport 4wd Thursday and the person I bought it from said that it backfires through low rpms well I was driving it home and it was backfiring around 1500 rpm and was fine once I got up to speed and just coasted once I got on the gas a little bit it would start backfiring again and if I mash the gas to the floor it will backfire more it seems to run right if I don’t give it hardly any gas with the skinny pedal the lady replaced the crank sensor water pump and transfer case the Jeep has been sitting for about 3 years I pulled the plugs and the insulators have a brown tint to them and black stuff down at the bottom of the insulators plug wires look new and so does the distributor cap plan to change the plugs and see how it runs after that
Last edited by Jeepxjbeast93; 12-29-2018 at 03:08 PM.
12-29-2018 | 01:19 PM
#2
Senior Member
Joined: Mar 2014
Posts: 952
Likes: 55
From: Eastern Long Island, NY
Year: 1996
Model: Cherokee
Engine: I6 Cyl / 4L
I would do a general tune up first. Change spark plugs, wires, distributor cap and rotor.
Try to pull codes from the computer, see what it says.
Overall, I would change the oil, oil filter, brake fluid, and fuel filter, just for starters. Who knows how old they are. Differential fluids too (even if you don't change them, check them to see they are topped off - those seals leak over time). Then I would keep inspecting things all around.
Good luck.
Try to pull codes from the computer, see what it says.
Overall, I would change the oil, oil filter, brake fluid, and fuel filter, just for starters. Who knows how old they are. Differential fluids too (even if you don't change them, check them to see they are topped off - those seals leak over time). Then I would keep inspecting things all around.
Good luck.
12-29-2018 | 01:31 PM
#4
CF Veteran
Joined: Sep 2009
Posts: 8,567
Likes: 19
From: ☼ Blackhole Sun
Year: My Jeep is a GMC
12-29-2018 | 07:27 PM
#5
CF Veteran
Joined: Nov 2010
Posts: 5,817
Likes: 105
From: In the middle of Minnesota!
Year: 1999
Model: Cherokee
Engine: 4.0
This is how I would approach it....
1. Test FUEL PRESSURE. That would be on top of my list of things to check. As it would in most shops as backfiring is often due to a fuel/air imbalance. 1996 was a year of change for the fuel system. If you have the fuel pressure regulator under the hood, then you have the old style and your pressure should be 31 psi at idle. Then disconnect the vacuum line leading to the regulator and pressure should jump to approximately 39 psi. If you have the new style fuel system, then your fuel pressure spec is 49 psi, plus or minus 5 psi.
2. 1996 and you can check for any codes or pending codes with either a code reader or a scan tool. 5 minutes to check and I would do that.
3. Perform a full tuneup. Everything. Cap, rotor, Champion plugs gapped to .035, new wires, fuel filter if old style fuel system as they are easy to access along the frame rail and inexpensive to replace. A can of BG 44K fuel system cleaner may really be helpful for an engine that has sat for 3 years. This is a heavy duty cleaner which I have had great luck with. Search Ebay or Amazon. Spendy at around $20 but good stuff.
4. Test MAP sensor. A sensor that is important to fuel/air ratio.
5. A compression test is a good "snapshot in time" of the internal condition of the engine. The compression spec for the 4.0 is 120-150 psi with no more than a 30 psi variation between cylinders.
Good luck and be sure and keep us updated on how it goes!
1. Test FUEL PRESSURE. That would be on top of my list of things to check. As it would in most shops as backfiring is often due to a fuel/air imbalance. 1996 was a year of change for the fuel system. If you have the fuel pressure regulator under the hood, then you have the old style and your pressure should be 31 psi at idle. Then disconnect the vacuum line leading to the regulator and pressure should jump to approximately 39 psi. If you have the new style fuel system, then your fuel pressure spec is 49 psi, plus or minus 5 psi.
2. 1996 and you can check for any codes or pending codes with either a code reader or a scan tool. 5 minutes to check and I would do that.
3. Perform a full tuneup. Everything. Cap, rotor, Champion plugs gapped to .035, new wires, fuel filter if old style fuel system as they are easy to access along the frame rail and inexpensive to replace. A can of BG 44K fuel system cleaner may really be helpful for an engine that has sat for 3 years. This is a heavy duty cleaner which I have had great luck with. Search Ebay or Amazon. Spendy at around $20 but good stuff.
4. Test MAP sensor. A sensor that is important to fuel/air ratio.
5. A compression test is a good "snapshot in time" of the internal condition of the engine. The compression spec for the 4.0 is 120-150 psi with no more than a 30 psi variation between cylinders.
Good luck and be sure and keep us updated on how it goes!
Last edited by tjwalker; 12-29-2018 at 07:49 PM.
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