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Discussion Starter · #1 ·
Hey Everyone:

Long time lurker, just decided to join to see if I could pick everyones brain here because this problem is driving me to drink. LOL

The vehicle in question is a 2018 Dodge Grand Caravan GT, it has the 3.6L PentaStar engine of course and the 62TE 6 speed. We have 2 others in the family. One is a 2012 with over 300K miles (500K KM) on it and still going, and a 2017 with just over 30K KM (18K miles).

Anyways I bought mine based on these vehicles and it was fantastic until it developed this intermittent idling problem that doesn't set the CEL, doesn't store any OEM or OBD-II codes, doesn't trigger the misfire counters on our scan tool (professional grade) even though it feel like it's missing.

We have had it to the dealer and get the usual "We can't duplicate the problem and it's not storing codes", well yeah, it's one of those weird things that I might need a mechanic with me for part of the day before it does it.

Anyway, I have monitored everything I can on the scanner when it's doing it and the only thing I can come up with to date is Knock Sensor #2 seems to fluctuate around in value like #1 does at idle but the value will fall substantially lower than Sensor #1 in it's bouncing around so either it's bad or it's detecting something on that bank of cylinders which is the miss type feeling I feel.

It's at it's worst when the engine is cold and moderately high idle and you put it in reverse and ease up on the brake. That's when it's the most pronounced and clearly feels like it's missing on one cylinder sort of thing.

Wanting to try the simple stuff first I pulled the Plennum off and checked all the plugs. They all looked like typical normal burn in colour. I replaced them all anyway with proper gapping on the new plugs and I swapped the coil packs from front to back to see if the problem moved from Sensor #2 to Sensor #1 with what I was reading on them and that didn't happen so I'm not sure what else to try.

Again we have 2 other vans in the family. One with the older head revision and one with the newer head and neither of them has this problem as I have driven both hot and cold and for extended periods of time on trips etc. with those family members.

Thoughts?

Thanks in advance.
 

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Discussion Starter · #2 ·
I should add, my dad is a retired mechanic in poor health so I have access to all of his old shop tools including a really good scan tool but not really access to his knowledge because he's not well enough to diagnose this with me.

Also this problem only exists at idle, above 1,000rpm it's as smooth as glass with no feeling or sound of anything out of the ordinary. Pulls along really well and it's a joy on the highway. Sometimes when it's acting properly the idle is so smooth you can't feel it running at the lights but then it's ugly cousin shows up and crashes the party.

Anyway, thanks again in advance... :)
 

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Always check all vacuum hoses when having idle problems. Intermittent problems could be a small crack in a hose that's not obvious, especially if it opens up as the engine rocks slightly in its mounts.
 

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Welcome to Allpar. Is it a low idle or a rough idle? If it feels like a misfire, is it random (multiple cylinders) or rhythmic specific cylinder)? Do you feel it in the vehicle body, like a metal-to-metal engine-to-body contact?
Are the engine/transaxle mounts adjusted into position correctly? Any bent or loose drive pulleys?
Can you perform a cylinder balance test? This may not be an actual measurable misfire, but some cylinders may be contributing more torque than others?
Have you tried disconnecting the knock sensors (it will set codes that can be later erased) to see if that affects idle quality?
 

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Discussion Starter · #6 ·
It's an intermittent slight stumble that occurs at idle. It comes and goes. The RPM never goes below 630rpm when it happens and when it isn't happening it's between 630-650. It honestly feels like a bad coil over / spark-plug but I know it's not this because we checked this.

I feel it through the but dyno. It's the most pronounced in reverse when easing up on the brake from the stopped position and only when it's acting up. It feels like a solid miss but no misfire counter so I think I'm just feeling the full effect when in reverse for the problem because of the way the engine/trans is torquing on the motor mounts. Doesn't feel like a bad engine mount. I have had that in the past on another vehicle and this isn't like that at all.

From what I can see the engine mounts look fine. Checked them all yesterday (including the one underneath at the rear of the engine)

I can't disconnect the knock sensors because they are burried under the plennum, intake, and oil to coolant heat exchanger on this engine. It would be like a 1.5 hours job just to get at them.
I can see that knock sensor #2's values as it bounces up and down with #1. #2 appears to go much lower on occasion than #1 so it's either picking up on the problem or it is the problem but I can't see this when the problem occurs as I'm usually driving and can't monitor the scan tool at the same time.
 

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Quote: "It's at it's worst when the engine is cold and moderately high idle and you put it in reverse and ease up on the brake. That's when it's the most pronounced and clearly feels like it's missing on one cylinder sort of thing."

Is the vehicle 'creeping' backwards in Reverse when it is most pronounced, or does it feel like a misfire when at a complete stop as well?

Can you check for proper motor mount adjustment? If the engine/transaxle rotates too far when in Reverse, it can 'bottom-out' and transmit roughness into the body.

If you 'power brake' it in Reverse, do you still feel the 'misfire' or does the higher engine speed tend to smooth it out? Could it possibly be a transaxle clutch 'shudder'?
 

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Welcome to Allpar! :cool:

The dealership may not have found anything but at least you have the problem on record.
 

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Discussion Starter · #9 ·
Quote: "It's at it's worst when the engine is cold and moderately high idle and you put it in reverse and ease up on the brake. That's when it's the most pronounced and clearly feels like it's missing on one cylinder sort of thing."

Is the vehicle 'creeping' backwards in Reverse when it is most pronounced, or does it feel like a misfire when at a complete stop as well?

Can you check for proper motor mount adjustment? If the engine/transaxle rotates too far when in Reverse, it can 'bottom-out' and transmit roughness into the body.

If you 'power brake' it in Reverse, do you still feel the 'misfire' or does the higher engine speed tend to smooth it out? Could it possibly be a transaxle clutch 'shudder'?
It only feels like light tremors in park or in gear at the light but inconsistent in its feeling. In reverse when it's cold in gear it's a minor tremor but when you ease up on the brake and let it start rolling backward with mild braking it feels like it's definately shaking like it's missing on one cylinder. It's just pronounced, the physical symptoms aren't actually changing, you can just feel it best in reverse.

We checked engine torquing in gear both in drive and reverse using one foot on the brake and the other on the accelerator slightly to see how much it torqued in each direction and it's minor and what looks like normal engine mounts. Definitely not rocking.

Does anyone know what the waveform on the Electronic Throttle Body on the PentaStar should look like? Mine seems to want to fluctuate between -10% to -20.5% even if I'm not touching the throttle and the RPM is consistent within 15-30rpm in either direction. I'm thinking this might be the source of my problem. Can anyone confirm their readings idling in gear and park fully warmed up looking at the ETC Duty Cycle?

Thanks,

Steve
 

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Discussion Starter · #10 ·
Oh and the other thing I noticed is about 900rpm and 1500rpm or so when in park and fully warmed up if I hold the idle there it will stumble. Only these two spots on the tachometer. Doesn't stumble like this at these RPM's while driving though.
 

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. . . .I can't disconnect the knock sensors because they are burried under the plennum, intake, and oil to coolant heat exchanger on this engine. It would be like a 1.5 hours job just to get at them. . . .
If you have access to a factory wiring diagram you could locate the invidiual pins in the connector(s) at the PCM which carry the knock sensor signal. Disconnect the battery. Remove the connector and very carefully unseat the knock sensor signal wire receptacle from the connector. Reattach connector to PCM. Reconnect battery and run the engine and monitor.
 
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