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I am working on a 99 Grand Cherokee with about 175k miles. The code is p3001 (misfire on cylinder 1). When I do the power balance test the is no change in the RPMs when I unplug cylinder 1s injector. When I put a stethoscope to the injector I can tell it is working. There is also a lot of “fish biting” especially if I go over a bump with tells me that there is an electrical problem. Where do you think I should go next with my diagnosis?
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Check the spark plug and the condition. Also ensure you are getting spark to that plug. Switch the spark plug then the wire/coil with another cylinder and see if the error moves to the other cylinder… you can do same for fuel injector… last case do a compression test on that cylinder.
Sometimes fuel injectors can stick…a few light taps with a screwdriver may help…. they might need a cleaning as well…
Karl
Thanks. I am working on this again. I pulled the plugs and didn’t see anything suspicious. I replaced them anyway. I also put a stethoscope to the injector and it sounded to be working fine. A compression test showed the first cylinder to be 95psi and 2&3 to be about 115. No change on wet test. The misfire happens around 1800rpm. So now I am thinking coil or fuel delivery.
[quote=”JgS” post=87010]Thanks. I am working on this again. I pulled the plugs and didn’t see anything suspicious. I replaced them anyway. I also put a stethoscope to the injector and it sounded to be working fine. A compression test showed the first cylinder to be 95psi and 2&3 to be about 115. No change on wet test. The misfire happens around 1800rpm. So now I am thinking coil or fuel delivery.[/quote]
95 PSI seems pretty low…(did you do this compression test when the engine was recently at operating temperature with a Wide Open Throttle?) I believe the PSI should be around 175 (don’t quote me on that). The service manual should also have a bottom limit… If you did this on a cold engine, the compression will be lower…
I would find out what the PSI should be for your engine… Regardless of all of this, since the compression did not increase on a wet test then the valves are suspect or there may be a head/head gasket issue.
Get a reading for each cylinder and post it here… Sometimes it can show a bad head gasket..
For example on an inline 6 if the results are:
1- 150 2-145 3-75 4-80 5-150 6-147
This shows a problem with Cylinder 3 and 4 which may be a bad headgasket.. Not guaranteed though..
Low compression can be caused by leaking/worn/or burned valves.. Also jumped belt/timing chain issue/timing is off can cause this as well… (all cylinders will show low compression)
A leak down test may provide further answers…
If your compression is within specs, then yes, a fuel pressure test would be a good choice… You may also have a dirty/clogged fuel injector… Fuel injector cleaning on the rail is the only way IMHO… Pour in tank treatments are Snake oil IMHO as well.
-Karl
From the results you posted, it looks like you have low compression on cyl #1. Specification according to the FSM is 120-150 psi with a maximum variation of 30 psi between any two cylinders. Try the other cylinders and then a leak down test next. Head gaskets are very common on these engines, usually from the coolant passages to the head becoming blocked and causing overheating (sometimes not evident on the coolant temp gauge).
Thanks for the advice. I am going to run the compression test again this weekend and maybe a leak down test. The reason I am leaning towards the coil is because the head was machined and the valves were serviced within the that 12 months, also the timing chain, head gasket, and injector were replaced.
If you have a compression issue, don’t worry about the coil or the secondary ignition system. The engine needs to be mechanically sound in order to run properly. If you have low compression as you have posted about, it can cause a rough running condition. More information on diagnosing performance issues can be found here.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Good luck and keep us posted.
If its a 4.0, which is what it sounds like, it has pushrods not a timing chain. Also ignition problems are pretty uncommon with the coil rail setup unless something is damaged. If the underlying issue prompting the head work was not resolved, it is likely that is the problem again. Also, look under the oil cap for the head casting identification. The 0331 heads (the stock head in the 99 4.0 WJ) are known to be prone to cracking; the newer TUPY casting is much more reliable.
If it is by chance a 4.7 V8, then you can swap coil packs around and see if the misfire follows it.
Thank you all for the help so far. It is a 4.0. I ran the leak down test today. I couldn’t get a clear reading on the percent lost but it’s around 40%. It was a cheep tester. What I did find out is that I was leaking pressure through the intake. I pulled the rocker arms off and was still loosing pressure through the intake. I’m pretty confindent it is a bent valve or cracked head. But since I am a armature at this I thought I would ask if there is anything else I should look for before I pull the head off.
Sounds like you are on the right track. The only other possibility that comes to mind is carbon build-up not allowing the valve(s) to seat properly, but I would still pull the head so you can inspect them. After all, the best part about the 4.0 is the head gasket is only like a $22 part! Good luck and keep us posted.
Last week I had the carbon professionally cleaned from the car. It didn’t help so this week I pulled the head. I was expecting to see obvious signs of a burnt valve but it looks fine. I think I will still replace the valve and spring but I thought I would show you all what it looks like in case I am missing something.
Actually that valve doesn’t look like it’s seating well in the pics. You can check the sealing of valves by putting solvent in the combustion chamber (put the plug back in) and see if it leaks out. If it can’t hold solvent, it can’t hold compression pressure. If it holds solvent, then it’s not leaking and you can move on to the next cylinder.
Keep us posted.
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