Power loss and backfire
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Joined: Dec 2011
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Power loss and backfire
so the other day I was driving to work and all the sudden my jeep starts to rub really rough, it felt like it was losing power and would backfire a lot. That was a week ago, today I put a new fuel pump on and driving it around the apartment complex I can feel a huge improvement but I’m still getting what feels like a dropped cylinder. It idols okay but you can tell something is up when you rev it. I’m wondering if my pump going out could have fouled my plugs? Or clogged an injector? If anyone can point me in a direction to check things that would be awesome. It’s a 1993 4.0 with 200K miles.
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Joined: Nov 2010
Posts: 5,841
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From: In the middle of Minnesota!
Year: 1999
Model: Cherokee
Engine: 4.0
Any check engine light? If so, you must pull codes from computer. In the absence of codes, there are a lot of possibilities here but this is where I would start. Simple stuff first, always.
1. FUEL: Even though you have a new fuel pump installed, you really gotta get some fuel pressure readings. It's not that unusual to get a bad pump. What BRAND of fuel pump did you install; hope it wasn't an Airtex! Testing must be done with a fuel pressure gauge which can be rented inexpensively from local parts stores; call around for availability. For a 93, you need 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator under the hood and the pressure should jump to approximately 39 psi. If there is raw fuel in that vacuum line, you have a ruptured diaphragm in the regulator and it must be replaced. Replace your fuel filter if it's been a while; they are inexpensive and very easy to replace on your 93.
2. SECONDARY IGNITION: Tuneup hardware. Always needs to be reviewed when symptomatic. Fresh Champion plugs gapped to .035, quality plug wires cap, rotor.
3. COMPRESSION: Never hurts to get a good snapshot in time on how the internals are doing. For the 4.0 the spec is 120-150 psi with no more than a 30 psi variation between cylinders.
1. FUEL: Even though you have a new fuel pump installed, you really gotta get some fuel pressure readings. It's not that unusual to get a bad pump. What BRAND of fuel pump did you install; hope it wasn't an Airtex! Testing must be done with a fuel pressure gauge which can be rented inexpensively from local parts stores; call around for availability. For a 93, you need 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator under the hood and the pressure should jump to approximately 39 psi. If there is raw fuel in that vacuum line, you have a ruptured diaphragm in the regulator and it must be replaced. Replace your fuel filter if it's been a while; they are inexpensive and very easy to replace on your 93.
2. SECONDARY IGNITION: Tuneup hardware. Always needs to be reviewed when symptomatic. Fresh Champion plugs gapped to .035, quality plug wires cap, rotor.
3. COMPRESSION: Never hurts to get a good snapshot in time on how the internals are doing. For the 4.0 the spec is 120-150 psi with no more than a 30 psi variation between cylinders.
Thread Starter
Senior Member
Joined: Dec 2011
Posts: 940
Likes: 2
Any check engine light? If so, you must pull codes from computer. In the absence of codes, there are a lot of possibilities here but this is where I would start. Simple stuff first, always.
1. FUEL: Even though you have a new fuel pump installed, you really gotta get some fuel pressure readings. It's not that unusual to get a bad pump. What BRAND of fuel pump did you install; hope it wasn't an Airtex! Testing must be done with a fuel pressure gauge which can be rented inexpensively from local parts stores; call around for availability. For a 93, you need 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator under the hood and the pressure should jump to approximately 39 psi. If there is raw fuel in that vacuum line, you have a ruptured diaphragm in the regulator and it must be replaced. Replace your fuel filter if it's been a while; they are inexpensive and very easy to replace on your 93.
2. SECONDARY IGNITION: Tuneup hardware. Always needs to be reviewed when symptomatic. Fresh Champion plugs gapped to .035, quality plug wires cap, rotor.
3. COMPRESSION: Never hurts to get a good snapshot in time on how the internals are doing. For the 4.0 the spec is 120-150 psi with no more than a 30 psi variation between cylinders.
1. FUEL: Even though you have a new fuel pump installed, you really gotta get some fuel pressure readings. It's not that unusual to get a bad pump. What BRAND of fuel pump did you install; hope it wasn't an Airtex! Testing must be done with a fuel pressure gauge which can be rented inexpensively from local parts stores; call around for availability. For a 93, you need 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator under the hood and the pressure should jump to approximately 39 psi. If there is raw fuel in that vacuum line, you have a ruptured diaphragm in the regulator and it must be replaced. Replace your fuel filter if it's been a while; they are inexpensive and very easy to replace on your 93.
2. SECONDARY IGNITION: Tuneup hardware. Always needs to be reviewed when symptomatic. Fresh Champion plugs gapped to .035, quality plug wires cap, rotor.
3. COMPRESSION: Never hurts to get a good snapshot in time on how the internals are doing. For the 4.0 the spec is 120-150 psi with no more than a 30 psi variation between cylinders.
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