Engine Stumble at 297k Miles

 With 297,215 miles on the clock of our 2010 Expedition EL it was presumably time to change the spark plugs.  Having bought the vehicle used for $4,500 with 273k miles already on the clock there was a laundry list of things we could change preemptively but I figured I’d only change fluids preemptively and then tackle each service item as they presented the opportunity to be replaced. 

The Stumble Presents Itself

 A couple months back my wife and I had sporadically been noticing a stumble of the engine which typically presented itself when we would be slowing down while approaching a stop light or stop sign. Initially I would only notice it ever few weeks, but it got worse.  We took a trip to visit family in August and for the first time I felt it stumble while climbing a hill at highway speed.  It only did this once or twice on the trip up and then a couple times on the ride back home.  Odd thing is that the check engine light never came on as a result of any of the stumble incidents?

Upon arriving back home I got out my code reader, plugged it in and checked for pending codes… none?  Something was definitely off but what?  Based on the vehicle mileage I figured the spark plugs were due to be changed but there was a bit of an internal debate going on inside my head as to if I should also swap out the coils?  At $400 for a set of coils I soon decided I was content to start by just changing the plugs themselves first.  By the time I bought some new plugs and supplies it was already time to head out on another family trip; this time to the twin cities to visit family over Labor Day weekend.  Given the known concern of spark plug breakage during removal on the 3v 5.4 v8 engines I was not willing to change the plugs prior to our trip.  I opted to roll the dice and see how it would perform with the known progressive stumble.  Not so great.  Most of the larger hills on the highway induced the engine to stumble, which did not bread confidence to the passengers in the front seats.  The short of it is that the engine held out for the whole trip and we made it home just fine. 

Spark Plug Replacement

Once home though I knew those plugs were in desperate need of being yanked out and new ones installed.  Seeing as we’d been putting on quite the miles there was also the need to change the oil.  I gathered all the necessary supplies and recruited a little helper to… well… distracter more than helper, but they like to be a part of what’s going on.

I made sure to use the shop-vac to remove all the dirt and junk sitting around the coils and resting inside the plug bores.  I started with the passenger side front plug and worked my way back on the engine.  Those buggers were stuck in the cylinders.  I’ve never removed plugs with that kind of corrosion on the threads before!  Creak, snap, pop was the sound with the first few turns of each plug.  It was a disconcerting process but each plug came out completely intact without breakage.  I actually had to put a longer pipe onto the end of my 3/8” drive breaker bar to exert enough force to pop the plugs loose.  Given the amount of corrosion and junk on the plugs I also used the shop-vac to clean out each plug bore after removing the plugs also.  I measured the gap on each of the old plugs and each one was at about 0.050” gap so they were definitely worn and due to be changed.

With each new plug being installed I smeared anti-seize on the threads to prevent corrosion and dielectric grease under the cap of the coil.  While I presume this is the one and only time I will change the plugs on this engine, but if not, they’re next servicing should be much easier.  With all the plugs replaced and all the coils back in their proper locations I fired up the engine and it ran nice and smooth.  I let it idle in the driveway for a while and then took it out for a 10 minute drive, not a single hiccup.  Problem solved…

Throttle Body Service

The following weekend I felt a hiccup though, the stumble wasn’t completely gone.  If the coils were bad I was certain that an engine code would be thrown and the check engine light would be illuminated.  What else could it be?  Throttle body was the most likely culprit.  On my previous ’05 F-150 I had cleaned the throttle body and it did enable the engine to run smoother.  So again, with 297k miles on the engine of the Expedition I was guessing the throttle body would have plenty of filth on it that could be inducing the stumbles.  I removed the air intake and began working to disconnect the throttle body from the intake manifold.  Bingo! It was filthy!  There was enough residue built up on the inside diameter of the throttle body that I could hardly see daylight between the throttle plate and body; for which there should be a set small gap. 

  I pulled the can of throttle body cleaner out of the cabinet and began spraying and soaking the body.  I ended up getting out a nylon bristle brush and scrubbing the inside of the throttle body in order to get through all the buildup.  The air intake side of the body had a little residue but the manifold side of the body is where the bulk of the crud had been deposited.  It was a relatively simple process of spray, scrub, spray, scrub, spray to get the body and plate clean.  

  After letting the completely clean throttle body dry out I bolted it back in and connected all the sensors and installed the air intake ducting.

Big moment… how would it run? Better? Would the sporadic stumble be gone? Still present???                                   When I fired up the engine it revved up to 2,000 rpm and then dropped to 500rpm and then revved back up to 2,000 rpm and dropped to 500 rpm and then back up to 2,000 rpm and then settled out to a normal 1,200 rpm as it gradually warmed up.  I think what was happening is that there had been so much residue on the throttle plate and body that the engine computer had learned to open the throttle plate a little bit in order to get enough air.  With that residue now gone and the proper air gap enable again the engine computer was struggling to determine the proper throttle plate position for the air flow.  Within 20 seconds of running it was able to re-calibrate itself and run smooth but it was interesting to listen to it hunt for the correct plate position.  I again let it idle in the driveway for a while and then took it out for a test drive.  Both scenarios, driveway and test drive, the engine ran smooth and strong.  It has continued to perform well without issue since.

The Final Result

While the plugs were worn out, the sporadic stumble was in fact due to all the filth on the throttle body as we have not since experienced any stumble of the engine.  As I write this post the mileage has already increased to about 298,500 miles and has been running just fine.  300k miles here we come!