I need some help and ideas, guys. My daughter's jeep is suffering from some weird issues. Problem is that I am 2600 miles away (USMC in a course) right now and phone trouble shooting with a mechanically and electrically ... challenged, but enthusiastic young lady is difficult to say the least. She will be in an auto shop class next year in school, but that isn't helping now.

Vehicle info (not a jeep guy myself so please forgive me)
2000 Grand Cherokee Laredo
4.0L Straight 6
Automatic
200k miles

Prior work (I went through it before departing for CA)
Fluid changes- Brake, coolant, both diffs, transmission, oil, transfer
Tune up- Plugs, boots, coil, air filter, pvc, and filters associated with the above. Fuel filter is in the garage, but not done because the Haynes manual said only to do it if there was an issue with fueling.
Brakes were done as well with new pads as I flushed the old fluid out.

Known issues that I just couldn't get to before leaving:
Valve cover gasket leak Not major just makes a mess.
Needs shocks and maybe springs (lift kit opportunity for later?)

Symptoms (remember this is all by phone):
1. Multiple cylinder misfire 1, 3, 5. The plugs are new and have been pulled and gap checked just to be sure, but no dice.
4. Shifting erratically on occasion when check engine light is on. Resetting it fixes this and number (1) for a week or so. I figure it is caused by the miss.
3. Brake lights function normal before headlights are on. Turn them on and the brake lights appear on and then dim during braking.
4. One of the rear brake bulbs has failed twice, tail light filament still works in that bulb.

My first thought after doing some searching is grounds. I have her trying to find them and check them for breakage and corrosion right now. I don't know enough about the bugs Jeeps have (all vehicles have them so please don't take that wrong) to know if this is something with a common fix. Nor can I even tell if these are separate issues or related by an electrical issue. So I am seeking guidance. The local shop said head gasket for the miss, but there are no other signs of it (oil in gas or vice versa, over heating, steam in exhaust). They are used to dealing with military dependents while the service member is away, and love to make money off them.

My daughter loves her Jeep and wants to do many things to it and is not afraid to get dirty, but usually I am right there with her telling her the next step. I don't have that luxury for the next couple months. Worse when I do get back, I have VERY little time before I head out again. Oh and before anyone recommends a shop visit or expensive fix, this is a high school budget we are working on. She pays for her own vehicle stuff from a fast food job (house rules and her pride).

 

Okay brother. Old Navy vet here. In my opinion the easiest starting point for the cylinder misfire is to replace the coil pack. On high mileage jeeps, these tend to start shorting internally. There are 3 coils that fire 2 plugs at time. One on compression and one on exhaust. This is called "waste spark" and is designed to help that old tractor engine meet emission standards by burning the charge again! The most common culprit is Coil #1 which fires cyl #1 and #6 so there's a long connector inside the rail that starts the problem. How do I know? I've been there. rockauto.com has them for between $50-$70 depending one which brand you buy. See note below!

Another common problem with this model is melted plastic around the contacts in the tail light bulb socket holders, especially if you sit in traffic with your foot on the brake for long periods. There are 3 in each tail light and they are all the same as are the bulbs. The difference of function is in the circuitry in the lens assembly. To troubleshoot this, swap the reverse light bulbs and sockets with the brake light ones. If this cures the problem, go to the junkyard and grab a bunch of them from a couple of wrecks! What happens here is the bulb holder plastic gets soft and the contacts sink into them releasing the tension and breaking contact with the bulbs. The ground contact carries the most power and is the one most liable to break contact, like at night with the tail lights on as well as the brakes.

As you can see from my signature block, this is the same vehicle I've driven for 6 years and know best! Mine has 235K miles on it and it's getting those electrical gremlins more often lately. Right now, I'm chasing broken wires in the door connector harnesses. If my driver's door is open, none of the windows work. If it's shut, everything is fine.

I hope these suggestions work for you. You can Private Message me whenever you want, or we can hash it out here so everybody can share.

Oh! Wait! Rereading your post, I see you've already replaced the coil pack! You can try cleaning the connector plug contacts for it. One wire is power and the other 3 are grounded thru drivers in the computer. Also NGK plugs work best (ZFR5N). They seem to last longer and run cleaner than Champs. Just my opinion.

 

Replaced the sockets for the rear. They had to mess with the contact tabs a bit to get those to work. But apparently the brake lights are working properly now.

The coil pack was replaced right off when the miss started. Still the same issue. Is it common to get a bad one new? If so, it is easy to replace. If not I will need a new point to check. Still having them check grounds.

Thanks for your service, by the way. Though maybe I should say thanks for the rides? Lol

 

Wife corrected me, it is cyl 1 and 6 missing.

 

Since this is Dave's vehicle too I would give more weight to what he says, ...disclaimer.*
Multiple cylinders going out at once tells me you're looking for something more systemic, such as fuel pressure, vacuum leak, throttle position sensor, or crank or cam sensor. Leaning to cam and crank sensors, but I would put a fuel pressure gauge on it to start. Then put a scan tool and check the tps waveform Check fuel running and leak down tests.

 

Sorry it has been a while as I have been in the field. The wife and daughter returned the coil for a new one since the missing was in the two cylinders firing off the same coil. Now it is worse. They pulled the plugs and the rear plug was black and smelled of gas. So I know it isn't firing. New plugs in again (Autolite by their choice). Code is now simply 306 So it is the number 6 as expected.

I am wondering if this could be a stuck open injector (which may be beyond my family's ability without me there). I will have them swap out the filter since I have it already. However, the pressure is 51 during cycle down to 45 leak down with a constant 49 at idle so I have little hope that helps. Any next step ideas they could do?

Edit: now they have the dreaded blinking CEL. So I am now looking at a constant miss still 306 coded

 

Has anyone looked at the connector plug for the coil pack? It sounds like it's getting worst every time it's unplugged which makes me suspect a bad connection. There are 4 wires in that plug. One for 12v power and 3 wires that ground thru drivers in the PCM.

Looking at the harness plug with the lock up, the pins are numbered 1 thru 4 beginning on the right. Pin #1 is ground for coil #1 and is tan with red tracer. Pin #3 is ground for coil #2 and is tan with pink tracer. Pin #4 is ground for coil #3 and is tan with orange tracer. All 3 of these wires should show no or low resistance. Whatever the reading, they must all be the same.

Pin #2 is 12v power from the ASD relay, but won't show voltage unless the ASD relay is engaged.

The following is Advanced Digital Electronics 101 so may be hard to follow for your daughter, but whoever is working in it might understand. When the ASD relay powers up the engine, all 3 coils are energized to build magnetic fields. When each coil is commanded to fire, it is ungrounded by the PCM and the magnetic field collapses inducing the high voltage spark. There is a voltage spike in the primary wiring that is bled to ground thru a diode in the PCM to protect the driver. This diode may have been damaged from the first rail shorting out so it might require replacing the PCM. The only real way to tell is if the resistance reading is different on Pin #1.

 

The connector was a thought of mine as well. I am going to have her clean the connector in the next phase. Also she will swap the injectors around to see if the issue moves. They will be cleaned as she goes as well. #6 is a constant through out this whole ordeal as well. She has a valve cover gasket leak so I am wondering if I get walk her through spinning valves as well. Maybe next step.

 

So I had very similar issues on an 800$ 2003 I bought for the engine only. It was the coil pack. I had the misfiring, the weird shifting, the inability (almost) to climb any hills. I replaced plugs and coil pack and it started up right away. Did I miss the specific codes its pulling? Mine were 1 and 6 misfire, then the O2 and multiple cylinder codes. They all cleared with the coil pack. Also, another Navy vet here.

 

I've noticed a high percentage of garage mechanics replace the coil pack at the first sign of multicylinder misfire. Then they blame the 02 sensors. After that it's replace the computer. All high priced parts with easy labor costs. My mechanic uses his DRB scan tool first to determine where the problem is. He has a larger customer base because he usually fixes problems with minimum expenditure of time and money.

He also only charges me $65/hour because I'm a veteran and always pay cash!

 

 

They have pulled the injectors and in a fit of frustration the wife ordered all new ones. Don't know the brand, but I can promise they weren't OEM. I will post back when they get them back in. I guess swapping the #6 to a different cylinder resulted in leaks all over with 200k mile O-rings. She didn't know I had a pack of fuel O-rings from my days messing with turbo motors on OBDI cars (a lot of time you had to tune them with injector swaps).

Here is a recap:

We are chasing only a number 6 Cylinder miss now. It is bad enough to make the jeep hard to drive. Blinking CEL

We have had 3 (counting the original) coil packs in.
We have checked/swapped the plugs multiple times (now have NGK in)
Fuel pressure checked with the following readings: 51 psi at KOEO dropping to 45psi and running at 49psi constant at idle
The coil connector has been cleaned and "appears" to be in good condition. No testing since I can't explain over the phone has been done.
Injectors are now out and sea foam has been dropped in both the injector holes and the throttle body and the engine rotated to let it work through the upper half.
Throttle body has been "wiped out" as best they can reach without removal (don't know what that means, but...)
New injectors are ordered. I guess an "upgraded part" is the best I got for you all.
It was recently tuned up with the following items:
Diff fluids
Trans fluids
Oil/ filter
Air filter
Brakes (including fluid flush through)
Plug boots
Plugs
Fuel filter as well now (no effect on fuel readings above)
There are no codes other than the miss.

I appreciate all the help. This is my daughter's work car and we kind of need to get it up and running.

 

I've been thinking....can you smell the burning wood? LOL!

Seriously, has a compression test been done yet? Could it be possible it's got a burnt exhaust valve? An intake would be popping back thru the intake manifold, but an exhaust would bleed off the charge and misfire.

Seafoam is good stuff and I've used it many times, but I found another product that works even better for cleaning carbon out of an engine. It's called "Cataclean" and is made by Mr Gasket. My engine now has 235K miles on it and has some blowby that comes back thru the CCV system to foul things up if left to idle for long periods. I decided to see if I could clean the crap out of the converters to see if they were getting plugged. When I put this stuff in the tank according to the instructions, I noticed an improvement in 20 minutes! Smoother throttle response, snappier acceleration, and more stable idle. After burning 1/2 tank, I couldn't believe it had worked so well. Not only does it clean converters, but also fuel injectors, valves, and pistons. #5 cylinder seems to be the worst for getting fouled and that's right in line with the CCV vacuum hose. It used to be #4, but now has switched to #5. My attack plan is to install an oil catch can in the CCV line to trap that oil, then use the Cataclean again and see what happens.

This engine was running perfectly until the head cracked at 175K. With the new head and valves it started using oil, probably because the rings couldn't support the higher cylinder pressures. #4 used to foul quite regularly until I changed to NGK plugs. Now it's #5 that's giving me problems. It'll run fine until it gets really hot after a high speed run, then it noticeably misses. No completely dead, but just has a throb. It sounds like a V8 with dual exhaust. I'm thinking about trying another coil pack in case I've got a weak spark on that pair.

 

I tried talking them through a compression test. Didn't go so well. Wife started swearing and the daughter starting throwing tools from what I could hear. LOL. Ok not quite that bad, but you can guess. I guess the new injectors are in place now and it is running. Still waiting for a report on if the issue returns. It isn't for now I guess, but they have only had it out about 10 miles so far.

 

My brother paid a visit over the weekend. He owned a jeep that lasted over 400k so he took a look at it. He pulled the valve cover and found the piece of heat shielding wedged into a valve spring and the rest just floating around under it. I never heard any odd noises while I was home so I can't explain it. They say it is fixed though!