Intermittent Engine Stumble
 

I have a 2002 XG350 with 140K. It has begun an intermittent engine stumble. At first happened only when coming to idle from highway speeds (eg: tollway exits). Would go away when I shifted into neutral and hit the accelerator. Mechanic said "EGR valve". Replaced it but no go. Symptoms becoming progressively worse, now happens occasionally while at speed. Also slower to respond when revved. Then it goes away and drives like a dream again. Check engine light has come on a few times. Mechanic says trouble code reference is to "ignition malfunctions" with no additional details. He is suggesting new Electronic Ignition Coils but no guarantees. He's an honest dude, but $500 guess on car with $2000 trade value is a tough bet. Any help?

XG is totally stupid to work with unless you know exactly what is going on..

MIL is illuminated... WHAT ARE the exact codes as read from the PCM ??

I can tell you the EGR valve was a waste of money, has nothing to do with accell, EGR only operates when commanded by PCM during cruise and decell to a duty cycled solenoid the closes off the line to allow vacuum to pull the valve, jiggle the intake vacuum that MAP sensor sees disturbance and the Ox sensors see change of exhaust content.... so we not going to toss a 0401 EGR FLOW code..

Go get the codes read, and post them here... then we'll talk shop..

Coils, they're good for burning through the body, see that large grey/white spot on the winding.... look at the wires.... are they looking like they played in white talc, and plug life is horrible in them --> bank 1 will burn the center electrode to the porcelin,,, good for misfire as gaps toooo wide, then it takes toll on wires and coils..

Had another XG today, $1200 worth of pricing, hope the fella comes back and buy it all..

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If anyone else cares to take the poor man's temporary fix approach, this cost me approx $5 and one day to do. The repair is still holding up well after three months.

Two of three of my 04 XG350's coils were cracked where the coil body meets the lower boot, causing block arcs. (I found this by running the engine at low light. You will see sparking either at the coil to block mating points or you may hear and see sparking between the main crank pulley and frame) I didn't have the $$ to replace all of them at $80 each, so my next best bet was to somehow further insulate the coil pack from the block.
Enter rubberized undercoating spray and black RTV high-temp gasket maker. If you're willing to try this fix, it's actually pretty simple, but can get really messy, so wear rubber gloves start to finish. You have been warned!
Remove the coils and visually inspect for structural damage. If they look intact, then read on for a cheap but effective repair.

Step 1. Clean everything with a brush. Dry thoroughly.
Step 2. Separate the lower boot from the coil pack. One of mine snapped the brittle plastic, but once apart, goop with black RTV high temp gasket maker. Re-assemble.
Step 3. Tape off wire and boot connectors. Suspend coil pack in open area with coat-hanger through one of the bolt holes.
Step 4. Spray entire coil (try not to get too much over-spray on boot portion) with rubberized undercoating. This stuff bubbles up when applied too heavily, so get a light coat, then wait until it's tacky and use a wet finger to smooth out bubbles.
Step 5. Wait for first coat to appear dry (will go from wet looking finish to matte). Apply second coat a bit heavier then follow step 4 again for smoothing.
Step 6. Dry overnight. If your boot connectors cracked or broke when disassembling, you can use 3M vinyl electric tape to wrap boot to coil. A zip tie will keep it from unraveling.
Step 7. Re-assemble and test under very low light. The primary objective here is to ensure there is no more visible arching or engine hesitation.

If fix was good for the coils and you haven't replaced your wires or rear spark plugs, then you may still hear and see sparking between the main crank pulley and frame about every five seconds. At this point, you've fixed the front, but somewhere between the coils and rear spark plugs you're still leaking spark. Inspect as much of the wires as possible for the tell-tale white powder. If any is present, consider doing a full spark tune-up as the problem will require you to remove the upper manifold.

Auto-zone sells a set of wires for ~40. I did mine for good measure as I was changing plugs out anyway. Two of my coils were arching pretty consistently and there was a noticeable hesitation in the idle. After realizing how much these coils were, I tried this repair. Engine now idles smooth as silk. Now if I could just figure out what's wrong with the Air Bag, ABS and TCS lights, and random Limp-mode triggering over pot-holes and railroad tracks, this would be a great car!