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Discussion Starter · #1 ·
Is there a point where gas engines just need the heads redone? (I know I've spoken privately to a few of you, I just wanted to make this a broader discussion in case someone was interested).

Our family minivan just rolled 250,000 miles. It's an 03 Chrysler 3.8L pushrod V6, basically the Chrysler equivalent of the GM 3800 albeit with less power and economy. This is our trip car; we've put 136,000 miles on it in 8 years. It runs fine and still gets factory gas mileage but has repeating misfire codes on two cylinders, and idles rough. I've put new fuel injectors and two new ignition systems on it (no change in behavior). Compression test showed 180psi across all cylinders.

Just doing some math:
  • 250,000 miles
  • Assume average speed 50 mph.
  • Assume 2000 rpm cruise.

Given those assumptions:
  • This engine has run for 5000 hours.
  • The crankshaft has turned 600 million times.
  • The valvesprings have cycled 300 million times.
I have some tests to run on these heads before I do anything more, but just curious as to others' experience with high mileage heads.
 

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Before you go to all the trouble of taking the heads off, do all maintenance. You said you put two new "ignition systems" on the car - what constitutes a Chrysler "ignition system"? Not busting your balls, just wanting more details.

The good news is the compression suggests it's not the heads or the rings. So my guesses are the following:

1) Electrical fault either to the injectors or the individual coil packs of the two cyliders from the PCM - may be as simple as cleaning the contacts or you may need to have a mechanic test the systems for low voltage in the affected cylinders.

2) Valve springs may be getting weak - I don't think this is likely since you say it's also idling rough, which weak valve springs wouldn't normally cause. Weak valve springs would affect high rpm operation, not idling so much. Still might be worth visually inspecting the valve springs as a last resort in case one is actually broken, as unlikely as that is.

3) You missed a component of the ignition system. Plugs, plug wires, coil packs, something like that.

4) Possible vacuum leak near the affected cylinders. This would cause a rough idle but wouldn't affect high speed running as much.
 

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Maybe it's not an ignition issue?

#5? It may just have enough (carbon) buildup that, despite nothing wrong with the ignition system proper, it needs considerably thorough cleaning - MAF sensor, IAC motor, MAP sensor, PCV Valve & system ...

#6? The OE 18 year old fuel injectors are not getting any younger.

#7? I'd also check all the plastic & rubber hoses for vacuum leaks.
 

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Discussion Starter · #4 ·
@Marky Dissod Fuel injectors were replaced with OE replacements around 200k; no change in behavior.

@stonebreaker complete ignition is plugs, wires, coilpack. Basically everything high-voltage. Unlike GM's setup, the Chrysler coilpack is a one-piece 2x3 waste spark with the base built in, unlike the GM 3800 or Northstar with interchangeable 1x2 coils screwed to the base plate. Its a slightly cleaner design, but these coilpacks tend to develop cracks in the potting epoxy underneath, and then it's just a matter of time until a misfire.

It does have a two-piece intake manifold like the GM 3.1/3.4L V6s. I did remove the upper to replace the fuel injectors; I will check for vacuum leaks there.
 

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OK, if it turns out to not be a vacuum leak, go over all the electrical connectors, if you haven't already. Might be some corrosion on the connectors going to the fuel injectors or maybe the coil packs, or possibly a short.
 
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I would look at timing chain. kind of hard to check wear the traditional way with no distributor. you are "high milage" at this point, even if serviced regularly, good compression, etc. chains stretch some especially at this milage.
 
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