2004 Jeep Grand Cherokee 4.7 problem

Okay guys this one is taking me for ever to diagnose. It runs bad! No codes but seems to be misfiring bad. The back 02 sensors are fixed rich. But no O2 codes. I did a relative compression test and was using a coil to mark my cylinders and the coil don’t have spikes some times. It shows it being pulled to ground but no spike. On the relative compression test it shows one cylinder that I’m using the coil on is better then the others. At first I thought 02 sensors but it runs bad as soon as it starts and it’s hard to start. Any ideas? I will try to send pics of my relative compression test in a min. 

Here you go

so the min you said no codes always makes me think of vacuum leak. I have had 2 cars this week running rough, Misfire counts like crazy but no codes. and all turned out to be vacuum leak.

Yeah that’s what I thought at first but no vacuum leaks pored water all over it and everything. Put a vac gauge on it and it was bouncing pretty good so I told them it seems to be a valve problem but I will have to do further diagnostics to know for sure. They started look at changing the motor. I told them I would like to know for sure. They said I could do further diagnostics. So I’m going to pull it back in the shop Monday if we get the other jobs finished up. I was thinking start with amp clamp in the fuse box on the injectors. Look at the coils again with the probe and pull the valve covers. Any ideas on other tests I should do? I might do compression test too. I did a relative compression but couldn’t really see anything.

Hey Mike, I'm T.J. and am interested in offering any help that I can to assist you with this diag, but I need a biiiiit more(read: a lot more) info about what you have observed so far. First thing's first, initially you said the rear O2s are fixed rich; is that rich as in always pegged over 800-900 mv, or just staying above 450 mv? Then on the same token, what are the front O2s doing? Along with the sensor voltages, what are the short and long term trims doing for both banks, and can you force the sensor voltages, and in turn, the trims rich or lean? Misreporting O2 sensors can cause awful engine performance, so with the rear sensors doing something odd, the fuel trim feedback system should be high on the list as a possible culprit. On to your relative compression test, did the engine crank evenly or did it sound uneven? The answer to that will determine the next procedure. Even or uneven, doing cylinder drop tests will help narrow down which cylinders to go after, so that would be a good next step. I would wait to pull the valve covers until more testing is done, and there is a lot more to test before that is necessary. Hope to hear back from you with some more details and look forward to trying to figure this one out with you.
Travis T.J. Harrison

I only had one rear o2 fixed rich. I can’t remember but think it was 800 +. One time on a dodge nitro I had a front o2 that was cutting out cylinders. Will a rear o2 do that? I checked a couple injectors that didn’t seem to be firing right by looking at coils with my probe the injectors looked fine but I didn’t do a amp test. The other o2’s had movement. I can’t remember if I took the time to make sure the where 200-800. So I need to look at them again I guess lol.
On the crank it did sound uneven not really bad but it did make me wonder. I put a compression gauge in the cylinder that didn’t show the spike on the coil and the compression showed over 100 so I moved on.
I did hear some ticking pretty loud a few times under the valve cover. So that’s why I put a vacuum gauge on it and noticed the bouncing vacuum. Why would the coil not show the magnet Collapse only sometimes?
Turns out that all of this started when they put in a now radiator….. they finally told me it did get over heated. That made me wonder if the vacuum gauge was telling me head gasket.
But still thinking about the loud tapping some time under valve cover… At first I was told it was running rough so the put new spark plugs but it didn’t fix it.

And fuel trim was ST 0 and LT -25

Cylinder drop test… I did unplug coils on the cylinders I thought was not right. And a few didn’t change anything that’s why I’m thinking fuse box amp test for injectors then valve cover.

I have another question it’s off topic kinda. My probe doesn’t do very good at watching the coils fire I have to unplug the ground some times for it to work. It might be my (homemade) cord but I don’t think so I’m scared it might be my Zeus I did purchase it used. Do you ever have issues with yours like that?

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I have another question it’s off topic kinda. My probe doesn’t do very good at watching the coils fire I have to unplug the ground some times for it to work. It might be my (homemade) cord but I don’t think so I’m scared it might be my Zeus I did purchase it used. Do you ever have issues with yours like that?

Without seeing the waveform, it's tough to say for sure. But I've taken secondary waveforms with several different Snap-On products, and none have had an issue. Can you take a screenshot with your Zeus and post it?

If you still can't make it work reliably, you might be better off scoping the coil primary voltage instead. The Zeus can handle the inductive kick without issue, and the primary still has tons to tell you about the spark and cylinder conditions.

You've got a tapping under a valve cover? AND it's been overheated?   Red flag. Do a head gasket and relative compression before proceeding. There's no point in chasing anything else until you've done those, IMO.

Man on my phone it won’t let me put pictures on here for some reason. There is just no wave form at all until I unplug the ground then I can see the wave form and have even found secondary ignition problems with it. I just don’t understand why it won’t work with the ground plugged in.
I plan on a compression test. Not a relative compression because I did a relative compression and didn’t see the problem. But I already did a compression test on one cylinder. The cylinder I unplugged the coil on as a cylinder drop test and it didn’t change the idle. So that’s why I’m leaning to amps on injectors.

How would you test for a head gasket?

Okay I guess I don't know how to count   I was looking back over my relative compression test pictures I don't know how I missed this. It looks like I only have 7 vvvvvvV and one is big. I'm using this tablet so I think I can post a picture. Man this one was driving me crazy! Im pretty sure that is also the cylinder I did a compression test on and it showed over 100psi. So what could make it draw down like that and show good compression?????stuck closed exhaust valve? I'm going to try to send a picture that shows the spike missing on the coil too

Here is the other pic

Okay I think I have bugged you enough for tonight tell me what the pictures tell you tomorrow. It's late I better stop thinking about this and get some sleep lol

wrote:

Okay I think I have bugged you enough for tonight tell me what the pictures tell you tomorrow. It's late I better stop thinking about this and get some sleep lol

Sorry I missed you last night. If you're around for the next group chat, stop by! We'd have a good time doing some live diag.

I grabbed your first pic and inverted it for clarity:

 

Definitely a low cylinder. What were your scope settings for this capture? Which ignition coil were you connected to?

In your second shot, is this the missing spike you're referring to?

 

I think this comes down to your scope settings. Snap-On products will need Peak Detect turned on to catch detail on ignition events like that. It looks like you're on a two second screen? That should be just fine. But don't be afraid to make the Zoom function work for you, if you aren't already.  Try one second or 500ms.

How would I test for a head gasket? I've started doing pressure pulse and manometer testing almost exclusively. I've got some examples posted here if you're interested in learning how:

www.scannerdanner.com/forum/diagnostic-t...s-blown.html?start=0

Blue water/block check is another option. Leak down testing each cylinder and looking for bubbles in the radiator would be another. Last would be to get the engine hot, shut it off and pull all the plugs, then put a pressure tester on the rad. Come back in the morning and look for coolant in the cylinders.

The blue water test I have seen lie to me. 2 different times with 2 different types. So I don’t trust it. If the radiator starts steaming it will even turn the water back blue lol. I do look for bubble and pressure bleed down. But yes I’m going to look at the link you gave me for sure!
I’m pretty sure it was the #2 cylinder I had marking on the relative compression test.
I didn’t think nothing about peak detention! I was going down the rabbit hole lol thinking it only don’t have compression sometimes that was why the spike was only missing sometimes. It’s really good to have this forum to be able to talk to others guys about scope testing! I haven’t heard about anyone else in town using a scope. I might be the only shop in town that uses one lol I know a few people that have them but don’t know how to us them lol I really appreciate your help!

No problem at all!

If your ignition trigger was on #2, then I think you've got enough evidence to revisit compression and leak down on that cylinder. Let us know what you find!

I should have the other job out of the way and be back on this one tomorrow. If I have time I’m going to play with the scope and pressure transducer on my compression test. And I still might do a injectors amp test. It depends on what other jobs pop up if I will have time to play or not.
I have found the main problem with this diagnostic! It was me thinking I could run thru these test real fast! If I would have took a little more time on the relative compression I would have seen it! And not had to do everything else I did. That was the first test I did LOL!!!! That is one way to get humbled! LOL worked probably 3 hours before I found it was probably a valve issue or head gasket when I could have known it in 15min and probably found the cylinder if I would have took the time to look at it better LOL well we live and learn. I will do my best to remember to update this and say what I find. Thanks a lot!

So with the one fixed rich, if the upstreams were switching, then that sensor is most likely lying/is faulty. Rear O2 sensors can have some authority over the fuel control and if it was stuck and the computer was trying to adjust fuel negatively to compensate, then it could cause too lean of a mixture, creating misfires on that bank. That is not too likely though. As for the compression test, it depends on how far above 100 that cylinder was, and I would check more than one before I base anything off the one cylinder having over 100 PSI. That ticking is not a good sign, so checking the compression on all cylinders is probably a good idea, especially with the info that the engine overheated. With the trim numbers you posted, it sounds like the vehicle was not in fuel control at the time you saw those values, with a 0 on ST but negative 25 on LT. May not go into control due to rear sensor issue. I'm with Tyler on this one about your missing spike. Scope settings on the missing spikes, and the leak down test/pressure test and inspect for coolant in cylinders to check for head gasket failure. Lol he's a little beyond me with the manometer testing, so I may have to try some of that myself! That cylinder is definitely low on the relative compression test, so that would be the suspect cylinder to attack, for sure. Sounds to me like you have a lot of direction now, but yeah, definitely post the update on what you find!