jeep running rich, hard starting.
01-03-2010, 10:18 PM
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Model: Cherokee
jeep running rich, hard starting.
1998 cherokee, 4.0, bone stock.
Have done a ton of searching the last couple days, and found a couple problems that were close, but nothing with that I got.
About a month ago I discovered my vaccum leak in the line between the vaccum canister and the manifold, so i fixed it.
Not sure if its coincidence or what but now my mileage has tanked and is running very very rich. Black plugs and nasty smelling exhaust.
It now also starts funny. At first i thought fuel pump/check valve, but it starts fine when cold. I come out after it sits overnight and starts right up. BUT... If i dont let it come up to temp (atleast ~150F or so) and turn it off it will take forever to start back up, tons of cranking and gas pedal feathering. If it comes up to temp and I turn it off, it fires right back up. I have tried key on key off a couple times to prime and it make no difference.
I have already replaced, plugs, wires, distib cap, coil, CTS sensor, t stat, and seafoam through fuel system. EDIT: forgot to mention that replaced the O2 sensor about 10k ago
Any ideas?
Thanks
Ben
Have done a ton of searching the last couple days, and found a couple problems that were close, but nothing with that I got.
About a month ago I discovered my vaccum leak in the line between the vaccum canister and the manifold, so i fixed it.
Not sure if its coincidence or what but now my mileage has tanked and is running very very rich. Black plugs and nasty smelling exhaust.
It now also starts funny. At first i thought fuel pump/check valve, but it starts fine when cold. I come out after it sits overnight and starts right up. BUT... If i dont let it come up to temp (atleast ~150F or so) and turn it off it will take forever to start back up, tons of cranking and gas pedal feathering. If it comes up to temp and I turn it off, it fires right back up. I have tried key on key off a couple times to prime and it make no difference.
I have already replaced, plugs, wires, distib cap, coil, CTS sensor, t stat, and seafoam through fuel system. EDIT: forgot to mention that replaced the O2 sensor about 10k ago
Any ideas?
Thanks
Ben
Last edited by nichols735; 01-03-2010 at 11:04 PM.
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01-04-2010, 02:18 AM
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Year: 1992
Model: Cherokee
Engine: 6
Usually, when a FI motor is hard starting and showing signs of rich, one suspects a fuel system that is leaking fuel into the intake. This could be injectors not fully closing or a FPR that has a damaged diaphram, leaking fuel via the vacuum line back to the intake manifold.
The OP mentioned the need to feather the throttle when starting. Thus adding air to help light the engine.
I'd pull the vacuum line from the FPR as soon as the engine is shutdown to observe any fuel passing back. Or use a piece of clear tubing in place of the black rubber for observation purposes.
Good luck!
The OP mentioned the need to feather the throttle when starting. Thus adding air to help light the engine.
I'd pull the vacuum line from the FPR as soon as the engine is shutdown to observe any fuel passing back. Or use a piece of clear tubing in place of the black rubber for observation purposes.
Good luck!
01-04-2010, 09:58 AM
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replaced the pre-cat O2.
I have no codes other than the post-cat O2 fault, but that shouldn't effect any fuel trim (should it?).
I think MAP sensor is next to try, but that seems like a shot in the dark.
Also, I know the FP reg is in the tank, so how does it up the FP under load, or doesnt it?
I have no codes other than the post-cat O2 fault, but that shouldn't effect any fuel trim (should it?).
I think MAP sensor is next to try, but that seems like a shot in the dark.
Also, I know the FP reg is in the tank, so how does it up the FP under load, or doesnt it?
Last edited by nichols735; 01-04-2010 at 10:00 AM.
01-04-2010, 10:58 AM
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i was under the impression that the post cat O2 was used only to determine if the cat was functioning, and did not alter fuel trims in any way.
Thanks
Ben
Thanks
Ben
01-04-2010, 11:06 AM
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Year: 1992
Model: Cherokee
Engine: 6
Just a thought, is there any other location on this engine where fuel can be delivered other than the injectors? I'm refering back to the idea where a diaphram separates the vacuum from fuel.
01-04-2010, 12:02 PM
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Year: 1999
Model: Cherokee
Engine: 2.5 turbo diesel
Not sure about the 4.0 engine as I have a turbo diesel, but I build fuel injected race engines so do have quite a bit of experience with many types of manufacturer fuel injection systems.
The symptoms you have described sounds very much like you have a fault with the cold starting system.
Now virtually all injection systems have one of to types of cold start devices fitted.
Option 1. The 6 cylinder 4.0 engine you have may have a seventh injector fitted to the inlet manifold so it injects extra fuel and acts like the choke system on a carb. It gives more fuel until a certain temperature is achieved and then shuts off. This is often controlled by a very cheap engine block mounted temperature sender that sometimes has a dual function in that it supplies info to the guage on the instrument cluster. If you have noticed wierd temp readings on the dash, it may well be this little devil . It is usually this cheap little sender that is the culprit.
Option 2. The inlet manifold has a bypass system. Sometimes part of the manifold but more usually a seperate device made of moulded plastic or cast aluminium. It is a rotary valve in a tube.
This works by being in the open position when the engine is at normal temp so the engine is drawing say 95% of it's air through the inlet system fom the filter and the other 5% through the valve that is piped of after the filter and rejoins the inlet manifold near the throttle body. For a cold start, the valve is closed, so the engine gets an artificially rich mixture as it still injects the same amount of fuel but is receiving 5% less air. So it still acts like a choke on a carb. The valve is sometimes operated by a water feed and a bi-metallic spring and sometimes(more common) by a small stepper motor. The valve usually siezes and 99% of the time it does this in the shut position so the vehicle is always in cold start mode, thereby running rich and difficult to start when warm. It can be removed from the vehicle (noting which way round all the pipes go and take the multiplug wire off. Clean with an aerosol can of carb cleaner and a toothbrush. (use the wifes, not your own!). Replace and hey presto!
As I say I am not overly familiar with the 4.0 6cyl engine but I would bet it uses option 2.
I hope this helps.
Darren.
Feel free to ask me any other questions.
The symptoms you have described sounds very much like you have a fault with the cold starting system.
Now virtually all injection systems have one of to types of cold start devices fitted.
Option 1. The 6 cylinder 4.0 engine you have may have a seventh injector fitted to the inlet manifold so it injects extra fuel and acts like the choke system on a carb. It gives more fuel until a certain temperature is achieved and then shuts off. This is often controlled by a very cheap engine block mounted temperature sender that sometimes has a dual function in that it supplies info to the guage on the instrument cluster. If you have noticed wierd temp readings on the dash, it may well be this little devil . It is usually this cheap little sender that is the culprit.
Option 2. The inlet manifold has a bypass system. Sometimes part of the manifold but more usually a seperate device made of moulded plastic or cast aluminium. It is a rotary valve in a tube.
This works by being in the open position when the engine is at normal temp so the engine is drawing say 95% of it's air through the inlet system fom the filter and the other 5% through the valve that is piped of after the filter and rejoins the inlet manifold near the throttle body. For a cold start, the valve is closed, so the engine gets an artificially rich mixture as it still injects the same amount of fuel but is receiving 5% less air. So it still acts like a choke on a carb. The valve is sometimes operated by a water feed and a bi-metallic spring and sometimes(more common) by a small stepper motor. The valve usually siezes and 99% of the time it does this in the shut position so the vehicle is always in cold start mode, thereby running rich and difficult to start when warm. It can be removed from the vehicle (noting which way round all the pipes go and take the multiplug wire off. Clean with an aerosol can of carb cleaner and a toothbrush. (use the wifes, not your own!). Replace and hey presto!
As I say I am not overly familiar with the 4.0 6cyl engine but I would bet it uses option 2.
I hope this helps.
Darren.
Feel free to ask me any other questions.
