Well, a couple months ago I embarked on a project to take care of a "knock" in my 98 XJ's 4.0 engine (253,000 mi). Let me stop right there and say, I have very little business doing something like this. I mean, seriously, I work in IT. I fix computers, not engines. Regardless, money was\is tight so I thought, what's the big deal? I'm relatively mechanically inclined, lets do it! Sounds like a bad lifter (ended up being that plus a broken piston skirt). Fast Forward ->->-> 11 weeks. This is where it stands.

I have replaced\installed the following(bear with me on this one):
- Lifters
- Valves\springs (Valve job done by a professional engine machinist)
- Planed the head (Also done by a professional engine machinist)
- Pistons\rings (All 6)(Connecting rods\ends checked = OK)
- All Bearings in the engine (Cam bearings done by professional engine machinist)
- Camshaft
- Crankshaft was polished and checked by aforementioned "professional engine machinist"
- All gaskets\seals.
- Water pump
- o2 Sensors (Both)
- Exhaust Manifold (Banks Stainless Steel)
- Intake (K&N Cold Air Intake)
- Front Pipe (exhaust pipe that meets the manifold)
- Distributor Cap, Rotor, Wires, Plugs (Distributor is ~20 months old)
- Crankshaft Position Sensor
- Camshaft Position Sensor (plastic plate on the distributor)
- Serpentine Belt
- Dual Oil Filter relocation kit.
- Engine Mounts.
- Entire block cleaned, painted. Cylinder bore's checked.
- Timing Chain\Sprockets (and the little rubber tensioner that was in pieces)
- Alternator
- Ignition Coil
- Thermostat
- Upper\Lower Radiator hoses
- CCV valve\port\adapter (whatever the correct term is) (toward rear on Valve Cover)

Still here?...Cool! Thanks!

So right now:

- The vehicle starts hard. It starts a little easier if we have the upstream o2 sensor unplugged and the hose going from the brake canister to the intake off. (Almost like it needs more air)
- It idles smooth but with it in park and the rpm brought up 1500+ it starts to sputter, pop, misfire, (and it sounds like it backfires but I'm not sure if it's into the intake or exhaust (I can check again tonight)).
- With the vehicle in Drive, the rpm can be increased until the rear tires spin and it purrs like a walrus the whole time.
- With everything plugged in, all the hoses on, and engine idling the vacuum at the intake is at 14 inches (which is way too low).

- During this "saga" we disassembled things down to the timing chain again last weekend and verified that the timing marks were perfect and the distributor was seated correctly.
- We checked the Intake Air Temperature sensor with a meter and based on the air temperature it tested functional.
- Idle Air Control valve\module was removed, cleaned & re-installed. But we don't' know of a way to test to see if it's functional.

So far I have stretched and exhausted my knowledge base on this vehicle. Never thought I would be comfortable changing a valve cover gasket, let alone pulling the entire engine. I would feel better about it if it was running properly afterwards, but hopefully someone here will have some input.

Hopefully I didn't forget anything, I've been working on it so much I may have forgotten to mention something we tested. Any help\guidance\reality-checks would be appreciated. Thanks in advance!

 

I know you said the timing marks line up and the distributor is seated correctly, but the symptoms you are describing sure sounds to me like the timing is off. If you have a Dodge dealer closer by I would trailer it up there (if you have the means) and have them take a look. I helped a buddy put a rebuilt 4.0 in his this summer and he had similar issues. He took it to the dealer and they adjusted the timing and now it runs great. I'm no timing expert, so the details of that they did I'm not too sure. Maybe someone else can chime in.

 

We thought the same thing, that it sounds like a timing issue. So we took it back down to the timing chain to triple check it. I have a Jeep dealership about 10 minutes down the road from me, but they always charge a lot. I'll keep that in mind though if it comes to it.

 

The dealership should look at it and diagnose it for free, at least the local one here did for him.

 

Forget the dealer. They don't know their **** is drilled or punched. Besides, they will charge you at least 1 hour diagnostics fee - $85-$100.

First check to see if your spark plug wires are installed correctly.



The #1 is at around the 5 o'clock position, as you can see in the pic. The distributor turns clockwise. The firing order is 153624.

If the wires are installed correctly we can move on to actual distributor installation.

 

Thanks for the response! Yes, the wires are installed in the correct order .

 

We followed the distributor installation instructions noted in the service manual. Are there any sensors that could cause these symptoms? Or is a timing issue the probable cause based on the info I've presented?

 

Just in case, I would loosen the distributor and partially pull it out of the block, then reseat it one notch clockwise. Button it up and try restarting it. You may have to turn the distributor a little forward and back to tweak it. If it is no better or gets worse, repeat the procedure, but this time move it two notches counter clockwise. Repeat the tweaking.

Verify that it is not 180 degrees off. It probably isn't, because it does try to run.

You can also make absolutely certain that if you are at verified top dead center, #1 piston, on the compression stroke, the rotor is pointing at the #1 terminal on the distributor cap.

 

A way to check if the distributor is installed correctly:

Find TDC compression on #1 cylinder.

Align the timing mark on the vibration damper.

Remove distributor cap, rotor, and Cam Sensor.

Insert a 3/16" punch or drill through the alignment hole in the plastic ring. The punch/drill should pass through the alignment hole in the distributor body.

If it doesn't., the distributor is installed incorrectly.

Pics:

Note postion of the rotor. It is just past the #1 cylinder post.

 

We tried lifting the distributor and moving it clockwise one tooth. Reinstalled everything and bolted it tight. It now starts up instantly and runs like a champ. No misfiring, backfiring, or popping in the exhaust. Now, we checked vacuum at the intake and it's still very low, at 14 inches. The gauge has a notation at this level that says "Late Ignition Timing". So obviously there is still a problem.

Unless I'm mistaken, isn't advancing the distributor technically just a band-aid on a potentially bigger problem? The timing should have been perfect with the rotor at the #1 post and the #1 cylinder at TDC on the compression stroke. Any thoughts or input are greatly appreciated.

@CCKen I didn't see your post before we advanced the distributor clockwise 1 tooth. I will double check the installation with the alignment hole and get back to you. I honestly didn't even notice anything there the last time we had it out.

Thanks for all the input guys!

 

@CCKen: With the distributor advanced 1 tooth from where we had it originally the alignment hole is now lined up. The jeep runs really smooth, the only issue that we're having now is low vacuum at the manifold (14-15 inches).

Any thoughts as to what could cause low vacuum (and a slight loss of power)? We checked for leaks but didn't find anything noticeable around the intake. I'm not very familiar with vacuum issues so if anyone has ideas about what we can try I'm all ears.

Thanks for the input guys, it's a real relief to finally have it running smooth. I took it out for a drive for the first time in almost 12 weeks last night. Runs like a champ (even with a slight loss in power).

 

14-15 "/Hg vacuum at idle is not that low (sorta). Spec minimum is 13".

Edit: Low vacuum can be attributed to the fact that your rings haven't seated yet, resulting in a low compression in each cylinder. Run it for about 500 miles and check again.

The CCV fixed orifice fitting you installed on the valve cover must be the one designed for your model year XJ. Earlier versions had a different size orifice. Double check this.

Did you check the lifter preloads? A milled head and, probably block deck, will change lifter preload. I'm not saying this is your problem - I just didn't see in your list of things accomplished.

 

Thanks for the info!

We checked compression after the rebuild and the lowest compression we had was 148psi and the highest was 160. A little on the high side according to the service manual I have. Hopefully that's not a problem.

The machinist said that he planed the head when he had it. The block was not touched at all aside from a good cleaning. I can check lifter preload when I'm home later. Is there a specific procedure that I should follow? I didn't see one in the service manual I have. I did see conflicting procedures online, wasn't sure if anyone has any recommendations.

I will double check the CCV fitting.

 

Crane Cams has two procedures, one using a capscrew turn method (.020-.060 preload=1/2 to 1 turn of the capscrew). The other is taking measurements on pushrods. I prefer the capscrew turn method. I will post a pic later.