Anyone Please HELP! Starting Issue
☠ CF Sheriff ☠
Joined: Nov 2009
Posts: 15,197
Likes: 18
From: Aberdeen, MD
Year: 1995
Model: Cherokee
Engine: 4.0L HO I-6, 703 Injectors, Brown Dog Super Engine Mounts
o2 sensor (the upstream one) controls the air-fuel mixture for the engine. If it is gunked up, not heating up enough, or the wiring is faulty, then the signal it sends is either ignored or wrong - causing the engine to either run rich or lean (usually rich).
There are two (sometimes more) o2 sensors. The one to be concerned about for a/f ratio management is the upstream sensor - located on the exhaust downpipe BEFORE the catalytic converter - right before the 90* bend. They just unscrew to replace.
There are two (sometimes more) o2 sensors. The one to be concerned about for a/f ratio management is the upstream sensor - located on the exhaust downpipe BEFORE the catalytic converter - right before the 90* bend. They just unscrew to replace.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
BUT SERIOUSLY shouldn't that throw a CODE? It's been doing this same bull crap for a year now, and I really think it would have thrown a code by now. That is throwing money at it, ugh. Thanks for the help, but help me understand why it would be an O2 sensor? They rarely don't throw a code....
CF Veteran
Joined: Nov 2010
Posts: 5,841
Likes: 117
From: In the middle of Minnesota!
Year: 1999
Model: Cherokee
Engine: 4.0
Not uncommon at all for an oxygen sensor to not set a code, but have symptoms. Happens more than you would think.
Oxygen sensors have a finite lifespan of usually between 100k to 150k, depending on how efficiently the engine has been running. Do you have this mileage on your engine? If so, consider replacing at least the upstream (pre-cat) sensor.
As I have mentioned before, 10 minutes on a scan tool will tell you EXACTLY how your oxygen sensors are working.
Don't beat yourself up here. If you are stuck, take it to a shop and pay them for this service. Knowing how your engine management system is working in REAL TIME, is invaluable here. Much better strategy than throwing parts at this problem and crossing your fingers.
Oxygen sensors have a finite lifespan of usually between 100k to 150k, depending on how efficiently the engine has been running. Do you have this mileage on your engine? If so, consider replacing at least the upstream (pre-cat) sensor.
As I have mentioned before, 10 minutes on a scan tool will tell you EXACTLY how your oxygen sensors are working.
Don't beat yourself up here. If you are stuck, take it to a shop and pay them for this service. Knowing how your engine management system is working in REAL TIME, is invaluable here. Much better strategy than throwing parts at this problem and crossing your fingers.
Last edited by tjwalker; Jan 9, 2011 at 08:26 AM.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
1. Have you tested the MAP SENSOR? Input signals from that do contribute to the fuel/air mixture. Easily tested and should be done.
2. Get a FRESH reading of codes and post the exact codes here for comment. We really need the exact code numbers in order to help you.
3. If you get stuck here and it appears that you might be at that point, you really should get this vehicle on a SCAN TOOL that can look at all of the engine management sensors real time. This is an extremely valuable tool that can save you a lot of heartache.
Other than your driveability symptoms, here are Some troubleshooting tips for extended crank times and to verify if "pressure drop" is in spec. (from factory service manual)
Good luck!
---------------------------------------------------
Hook up fuel pressure gauge on the schrader valve on the fuel rail
Start engine and bring to normal operating temperature.
Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)
Shut engine off.
Pressure should not fall below 30 psi for five minutes.
If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:
*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.
*If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - possibly the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.
You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely.
2. Get a FRESH reading of codes and post the exact codes here for comment. We really need the exact code numbers in order to help you.
3. If you get stuck here and it appears that you might be at that point, you really should get this vehicle on a SCAN TOOL that can look at all of the engine management sensors real time. This is an extremely valuable tool that can save you a lot of heartache.
Other than your driveability symptoms, here are Some troubleshooting tips for extended crank times and to verify if "pressure drop" is in spec. (from factory service manual)
Good luck!
---------------------------------------------------
Hook up fuel pressure gauge on the schrader valve on the fuel rail
Start engine and bring to normal operating temperature.
Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)
Shut engine off.
Pressure should not fall below 30 psi for five minutes.
If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:
*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.
*If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - possibly the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.
You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
No I haven't done this yet, I am at my dads mercy. Also pardon me, but what is a leak down test? Different from the fuel pressure test right?
CF Veteran
Joined: Apr 2008
Posts: 13,952
Likes: 17
Model: Cherokee
It will tell you if you're system is maintaining the correct amount of pressure.
A loss of fuel pressure can be caused by a bad check valve in the pump, a leak in the fuel system, bad/leaking injector(s).
A loss of fuel pressure can be caused by a bad check valve in the pump, a leak in the fuel system, bad/leaking injector(s).
CF Veteran
Joined: Apr 2008
Posts: 13,952
Likes: 17
Model: Cherokee
You cannot pinch off the hard pastic line on your rig. You'll ruin it.
Pulled you plugs yet to see if any are carbon fouled?
Timing is not adjustable on your motor, therefore have the timing checked is a waste of time and money at this point. Timing is maintained and adjusted by the PCM.
Your problem is fuel system related.
Pulled you plugs yet to see if any are carbon fouled?
Timing is not adjustable on your motor, therefore have the timing checked is a waste of time and money at this point. Timing is maintained and adjusted by the PCM.
Your problem is fuel system related.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
Not uncommon at all for an oxygen sensor to not set a code, but have symptoms. Happens more than you would think.
Oxygen sensors have a finite lifespan of usually between 100k to 150k, depending on how efficiently the engine has been running. Do you have this mileage on your engine? If so, consider replacing at least the upstream (pre-cat) sensor.
As I have mentioned before, 10 minutes on a scan tool will tell you EXACTLY how your oxygen sensors are working.
Don't beat yourself up here. If you are stuck, take it to a shop and pay them for this service. Knowing how your engine management system is working in REAL TIME, is invaluable here. Much better strategy than throwing parts at this problem and crossing your fingers.
Oxygen sensors have a finite lifespan of usually between 100k to 150k, depending on how efficiently the engine has been running. Do you have this mileage on your engine? If so, consider replacing at least the upstream (pre-cat) sensor.
As I have mentioned before, 10 minutes on a scan tool will tell you EXACTLY how your oxygen sensors are working.
Don't beat yourself up here. If you are stuck, take it to a shop and pay them for this service. Knowing how your engine management system is working in REAL TIME, is invaluable here. Much better strategy than throwing parts at this problem and crossing your fingers.
So I will see if I can put it on a scan tool, as long as it's no where close to the Jeep dealership.lol.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
You cannot pinch off the hard pastic line on your rig. You'll ruin it.
Pulled you plugs yet to see if any are carbon fouled?
Timing is not adjustable on your motor, therefore have the timing checked is a waste of time and money at this point. Timing is maintained and adjusted by the PCM.
Your problem is fuel system related.
Pulled you plugs yet to see if any are carbon fouled?
Timing is not adjustable on your motor, therefore have the timing checked is a waste of time and money at this point. Timing is maintained and adjusted by the PCM.
Your problem is fuel system related.
Any odds it's a bad PCM? or it needs updated?
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
CF Veteran
Joined: Apr 2008
Posts: 13,952
Likes: 17
Model: Cherokee
Doubtfull, but possible.
A carbon fouled plug indicates flooding. Time to start looking at the injectors.
You can try soaking them in Seafoam for 24 hours.
Pull the plugs again take pics of each, same goes for the injectors. Pay close attention the the discharge side.
A carbon fouled plug indicates flooding. Time to start looking at the injectors.
You can try soaking them in Seafoam for 24 hours.
Pull the plugs again take pics of each, same goes for the injectors. Pay close attention the the discharge side.
Thread Starter
Junior Member
Joined: Mar 2010
Posts: 35
Likes: 1
From: Ohio
Year: 2001
Model: Cherokee
Engine: 4.0 L
Is there any way to test if it's bad, both pumps were Airtexs, the autozone fuel pumps. what is recommended, factory bosch?
Last edited by Peeper; Jan 9, 2011 at 09:15 AM.