WARNING: Long read!

CUT TO THE CHASE:

• Replaced 3.7L engine with new/rebuilt, but still need answers.

HISTORY:

• Bought a 2004 2WD 3.7L V6 Durango back in November 2016 with 108,000 miles
• Super clean: the interior, undercarriage, and engine compartment
• Took it to my mechanic to get a once over and also cuz it vibrated on idle...I wanted this to last for years to come
• One previous owner, bought it from a dealership as-is with no warranty available.

INITIAL FIXES:

• differential gasket(it was leaking oil everywhere)
• transmission pan replaced (small leak)

STORY:

• I didn't drive it too much, as I was traveling a lot, so of course because the engine would vibrate moderately at idle I figured I might need a tune-up but hadn't gotten around to it til a few months later in January in the winter here in the midwest.
• No check engine light ever went on during the light driving I did for those few months. I put a bottle of SeaFoam into the gas tank when 1/4 full. And I was also putting 93 octane premium gas from Shell for the first couple of tanks...then switched to 87 for the couple tanks after.
• I couldn't deal with the vibration anymore so I had my mechanic replace the following:

MORE FIXES:

• all the spark plugs
• all the coil packs
• the latest revision hood cowl from the dealer(since I read about it causing issues on this forum)

STORY:

• It ran much much smoother with a very low vibration at idle. After maybe 10 miles of driving or so, the check engine light went on (it hadn't done that in the first 3 months at all). Took it back to mechanic. Showed up as misfires (cylinder 6 and general multiple misfires).
• Thought at worst it could be the injector(s) and that would have been a relatively cheap enough finish to repair the whole vibration issues. But he smoked it out to see if there are any leaks. There was a leak out of the oil cap so I got a new one from the dealer. I also got a new lockable gas cap from the dealer. I was hoping to just change those clear the code and see how it goes.
• Drove ok for I don't remember exactly but maybe the same amount of miles 10-15miles then the check engine light went on again. Took it back and figured yeah maybe one of the injectors, but he noticed a smell and white smoke coming out of the exhaust. I never noticed that before but it was literally billowing plumes of white smoke out of the tailpipe!
• How could it possibly be worse now after a tune up, basically? It should be at least the same...not worse, right? Possibilities is that now that I got a tune up it ran more efficiently and therefore there was greater pressure or some such thing....and underlying issue was able to present itself.
• It was thought that I had a head gasket blown(however minor) or maybe dumping fuel into the system for some reason.Mechanic checked the new plugs and noticed cylinder 6 plug was definitely wet. I didn't want to accept that it was a head gasket leak cuz the vibration was horrible now after the check engine light went on and it was white smoke for 2 minutes everytime I started the car. BUT the coolant temp never passed the halfway point.
• Decided to drive it and watch it carefully and see how it goes. Same thing...whether it was a cold cold day out...or an unseasonably warm day in February...whether I let it run for 20 minutes to warm up or just a few minutes...the smoke was there and so was the heavy vibration at idle.
• Out of nowhere, later one week, the check engine light disappeared on it's own and when that did so did the symptoms. NO smoke out the tailpipe at all...and it was super smooth at idle...even smoother than immediately after the tune-up. It was then I knew that this 2nd gen Durango could/should be running this super smooth...you couldn't even see the console, the seat, or anything vibrate one bit.
• I was happy....for 2 days...then the engine light came back on with the symptoms. Rinse and repeat this cycle of CEL on and off randomly a few more times and when it went off it would only be for a day or maybe 3 at most.
• Mechanic said he's never seen that where it would be intermittent, and presumably a head gasket leak...that it would still not overheat or even go above the halfway point. It may have been so minor but a leak is a leak right? He thought about maybe torquing down the head bolts but never tried that.

FAST FORWARD:

• Decided to just get the head gasket replaced and I didn't like the idea of spending $1,800 ish but I wanted it to last forever and I was lucky that the fact that it wasn't overheating at all was buying me time and now I was barely driving it, just in case.
• My mechanic was busy and really didn't honestly want to do the job cuz he said it would be a bitch, but if I can't find anyone, he'd do it when he had time
• I drove it sparingly for another month, as I begged him to fix it cuz I couldn't trust anyone else to do it right and be mindful of all the little things, until one day it made a crazy knocking noise!
• Yep, it was crazy enough that I only drove it 1 more block then called to have it towed to the shop. Turns out I threw a rod! Great! Now I was looking at a new engine replacement!
• Already committed and having no money to gamble on another car/truck. I decided to have him put in a new engine. He sourced out a few places. One of them offered a 3-year/100,000 mile warranty and pay for half the labor if it needed warranty...versus the 1 year warranty of the others.

MORE FIXES:

• Replaced with engine with the same 3.7L V6
• New oil pan from dealer(just cuz it was rusted out)
• new seals/gaskets
• new injector o-ring seals
• new thermostat
• new belts

NIGHTMARE:

• I went to pick it up (well sort of...just to check it out even though he installed it but was still testing it for a couple days)...
• It vibrated fairly bad AND for the first time ever(not even when the old engine was at it's worst did it do this) the coolant temp was past the halfway point and almost into the 3/4 point.
• no f'ing clue what was up, he said leave it to me for a couple days and let me figure it out. The fan seemed to work well and kick in when it should...but nothing. Could I have really gotten a lemon in a new engine?! Apparently, cuz he couldn't figure out why it was vibrating horribly...how rare for all this to happen.
• But the company had him run some tests and came to pick up the new engine and deliver yet another new engine. He said that they said something about the serial number of the old new engine (it's a long story he said)...and they didn't even think twice about swapping it out right away. Mechanic says it could have been a balancer shaft or something but no matter...swapping it out for another new engine.

2nd NEW ENGINE:

• Well my mechanic didn't charge me anything at all to remove or replace it (even though they paid him half the labor for it)....GOOD.
• He did a leak down and I think another test of the engine before he put it in the SUV. All seemed good

FINAL FIXES:

• 2nd new engine, obviously
• new harmonic balancer
• new water pump(actually he did that before)
• new radiator

FINAL STORY:

• ready to pick up...he said he did everything he could think of and I'm sure I'd be happy with it because now the coolant temp stays under the halfway point even if just a couple millimeters...no matter how long I run it or let it sit like he did for an hour.
• However on first start up for 30 seconds to a couple minutes...it still vibrates but very very low rumble. It's not super smooth like I know it can be...but noticable...I would equate it to the point after the tune up on the original engine.
• now paranoid because I didn't want this to be a beginning to another problem down the road...he said, look it's under warranty for 3 years...the engine company said to drive it for 800-1,000 miles and let the valves seat or some such thing...so I said ok that sounds fair enough
• The only other possible explanation, if this doesn't go away is the engine mounts but he said they looked ok, and were 100 a piece so he didn't want to just swap them out just because. I also don't have the money

BOTTOM LINE:

• Cost me $5,200 (engine I believe was 3,300) and he did me some favors...still a lot of money for me.
• I want this to last for years to come, but I want to ask anyone and everyone here...

QUESTION/COMMENTS?

• Is there anything...anything at all small or otherwise, that I can have him look at or pay attention to that could maybe make it super smooth like it briefly was during the "good times" of the bad original engine?
• Could this very very slight vibrating be symptomatic of something worse to come later? I can see that the harmonic balancer(new) wobbles a bit as the engine is running but told this is normal?
• Regular(break-in oil) was put in and recommended to drive it for 1,000 miles...would putting Mobil 1 fully synthetic be ok/better moving forward afterwards(Costco has a great deal on 6 quarts)?
• Do you think it's possible that it will in fact smooth out after driving it for several hundred miles?
• I literally just started driving it and it's the best it has ever been but still not super smooth...there is a very minor but noticable vibration only at idle(rpm needle does not fluctuate at all and stays put, fyi which is good i think)

Suggestions/recommendations? (I am committed and now have invested $10,000 with the price of the car and new engine so telling me to get rid of it is not an option). I just feel 'paranoid' something might go wrong although it all seems well enough now and does have this warranty still.

THANKS ahead of time. I want to make sure I have not overlooked things to think about and although my mechanic is really great...maybe another set of eyes or view may help in some way.

 

Do a cylinder balance test. (he'll know what I'm talking about) Find which cylinder is causing the miss, then swap injectors with another cylinder, see if the miss moves.

 

He said "injector balance test". Assuming it's the same thing? Also he said he can do it but if the Check Engine Light is not going on for a misfire/not misfiring, how can you see if the miss moves?

Also for anyone, I know I bought this as is/from the used card lot of a very big and reputable Honda dealer...but is there any way I can recoup anything at all from the dealer?

I reached out to the manager of the used car section of the dealer back in May and we never really talked at the moment because mechanic was still working on it and bad timing with the weekend. I also didn't know what to say/blow up inappropriately. He was informed I had a bad engine and that's why he called back initially.

Any recourse at all? I probably cannot get the $5200 for the new engine, nor the $4000 or so for the car, but I'm not losing out hope, and maybe they can compensate me in part, if not in whole. How should I approach?

Btw, so far so good...it still vibrates a little at first start (around 1,000-1,100rpm)...until it warms up in the engine(despite 90F outside temps)...then it's barely noticable.

EDIT TO ADD: Is this what my mechanic should do?