I recently took it upon myself to do a simple tune up to my 98 XJ. I recently purchased the vehicle with 145k miles and figured I would do simple things to refresh it. Today I began by replacing the air filter with a short ram style and paper filter. I then checked the plugs, which appeared very new and gapped them to .035.

I replaced the wires, cap, coil, and rotor. I started the engine and it ran very bad and back fired through the intake when revving. I checked the wiring on the cap over and over and could not see an issue there. I eventually put all the old stuff back on the engine and it is still running rough. I am at a loss. I checked all the vacuum lines I could find and they all appeared to be intact.

The diagrams that I have found for the distributor wiring order all show the Number 1 cylinder to be at the "5 o'clock" positon, which I had. I took a picture of the cap before I pulled the wires and put them back the same way.

 

Check and re-check that firing order. From DFlintstone:

"The book's wrong! #1 is at 5:00, viewed from the side. 153624 clockwise."

Cylinder numbers are Front of engine > 123456 > Rear of engine

 

Double and triple check the spark plug wires for proper installation.
1-5-3-6-2-4 is the firing order. As stated, cyl #1 is at the front, closest to the radiator.
Also check to make sure that you installed the cap with the correct orientation. It's possible to install it backwards.
You mentions changing the coil as well.
Make certain that all the leads have good connection, and the ground wires are clean and make good contact with clean metal.

 

This is the order I put the wires on the new cap.

I made sure to put the rounded tab on the left side. I took sand paper and cleaned up the four ground wires that are just beneath the coil and the base of the bolt and nut.

Could anything on the intake side have caused this issue? I took off an oil soaked K&N filter that was mounted directly on the TB and put a 90degree elbow on the for the short tube. I did not remove any hoses, accept for the CCV line. Could the freer flowing filter thrown off the MAP sensor of IAC? I read about resetting the ECM, but not sure if this is applicable here.

 

And I forgot to mention, the intake was very loud when the issues were present. Not just the normal sound from air getting pulled in, but concerningly loud, like a supercharger was pulling in a lot of air.

 

There's a few things intake wise that you can check.
Throttle body gasket.
IAC.
TPS.
Intake manifold itself. Check it for cracks.
Check the intake and exhaust manifold gaskets.
Double check every vacuum line and connection.
Worst case, check compression to see if you have a stuck intake or exhaust valve.

 

New wires are a b!$ch to snap on, even the old ones sometimes. I'd ensure all of your wires are pushed on both the cap and spark plugs completely.

 

It ran fine before you tuned it up, now runs poorly after you tuned it up. Double check your work.

 

Yep. Stop looking for OTHER problems. Most likely, it's the firing order or cap position.

 

I like to think i know a thing or two about a thing or two when it comes to engines.... but the truth is, that is the truth- i do know fractions and not the whole story.... which was proven to me this spring when i dropped a freshly rebuilt engine in my boat- a SBC 314 with all the goodies...

it was misfiring all to pieces- the intake was howling, and there were so many symptoms i simply didn't know where to start... this is in the boat... on the stand? it purred... I couldn't wrap my brain around it- and did what most do, and started throwing parts at it with a focus on the dizzy cap and rotor button... plugs... I even went to fuel- thinking i may be washing the cylinders with some sort of stuck choke...

then a dude i know rode his harley up in the driveway aside me and another bud studying on this thing.... listened to it a minute and asked about WIRES... i told him they were brand new MSD 8mm wires- best I could get my hands on and I had JUST installed them out of the box... he said "You have ONE BAD WIRE"... I was incredulous... now way.... i'd have a distinguishable single misfire if that was the case, amiright?

no... I'm not... ONE bad wire will create enough resistance to cause spark intended for that dizzy point to spray to others, and which causes backfiring, backfiring through the TB, all around rough (barely) running as it "sprays" spark inside the cap...

I recommend you systematically remove wires from the DIZZY side one by one... a single cylinder dropping off will still run... when you pull ONE wire and the engine runs better (with the exception of that one misfire) you've found your bad wire.

..... it's a dang shame that quality control departments are far less funded than marketing departments, no?

 

drewactual,

Even if that ends up not being his problem (though very could likely be). That is a very well written post. With excellent advice for the OP to try on his own.

Importantly, a bad cap, and a bad spark plug can cause the same problem. If the resistance is too high going down the "proper" path, it may very well jump to the next best path as you described.

 

So I got back out to the Jeep this morning. I traced the vacuum lines again, looked at the wire pattern again, and everything appeared to be in order. I decided to pull the plugs and let the cylinders and plugs air out. My methodology was that, the night before I remember checking the fitment of the new wires after it had run poorly. I found at least one, maybe, two wires on the cap had not been pressed on completely.

So today my thinking was that a couple of cylinders could have gotten flooded, causing the rough idle. I left the plugs out for 30 minutes or so, rechecked gap, and put them back. I started it again. I let the engine run for a longer period of time ~1 min, and the intake quieted down, the idle leveled out and no backfiring when blurping the throttle. It seemed to be back to it's normal self.

So I then put all of the new parts back on, starting it between each new part, to track any changes. I haven't driven it yet, but it is idling like before I had touched it.

Does this make sense to have been the culprit?


I appreciate all of the input!!

 

Hey. Whatever works. You did like Bustedback recommended and it worked.

 

It could be that something was not completely seated properly and the reason why it is Ok now is because on the second go around, you got it right.

I worked on a truck with an electrical issue and all symptoms told us to tear parts down and investigate. Finding no issues we reassembled and all was well. Best explanation in that case was a connector was loose or dirt and the disassemble/reassemble solved the issue.