1999 V10 rough idle after engine rebuild

[Shellkamp]

Is there a certain amount of starts that will cause the PCM to relearn to idle properly?

My mechanic claims that the camshaft (daily driver type, not high performance) was cut "big" so it will start idling better after it breaks-in. This feels identical to the misfire you'd get in a Ford when one of the coil packs went out, but as soon as you give it some gas it clears up.

Do I need to re-flash my PCM or is there another trick I can do?

Or should I just trust my mechanic that it will get better over time?

Just a little over 100 miles on the new engine.

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[bm02tj]

Did you have injectors cleaned and flow tested

 

[hoe...]

Sounds like an induction air leak.

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[Fast69Mopar]

Is there a certain amount of starts that will cause the PCM to relearn to idle properly?

My mechanic claims that the camshaft (daily driver type, not high performance) was cut "big" so it will start idling better after it breaks-in. This feels identical to the misfire you'd get in a Ford when one of the coil packs went out, but as soon as you give it some gas it clears up.

Do I need to re-flash my PCM or is there another trick I can do?

Or should I just trust my mechanic that it will get better over time?

Just a little over 100 miles on the new engine.

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Tell me how the engine is idling. Is it extremely rough and feels like it is misfiring?

Is the idle surging up and down?

Another thing I need to know is the complete cam specs.

If one of my mechanics ever told me that a cam was "cut big" and the engine would start idling better once it breaks in I would ask that mechanic to resign.

When choosing a camshaft there are a ton of variables that can come in to play. One of the biggest is valve overlap. Valve overlap is the amount of time an intake valve and exhaust valve are open at the same time. The more overlap the rougher the idle.

The reason I'd like to see your cam specs is to see how much overlap it has. You said it cleans up once the engine comes off of idle. This is exactly what a cam does with a larger amount of overlap.

If my memory serves me right the stock cam already had 31° of overlap. With the V10 being an odd-fire motor and a cylinder during every 75° it already feels a little rough. Installing a cam with more duration and overlap can make it feel worse even though it's running just like it is supposed to.

 

[Shellkamp]

I think the replacement thermostat my mechanic used is too low or malfunctioning. I remember it always running about 190+ degrees and since I got it back it's been more like 160-170. I confirmed the OEM called for 195 degrees.

To test my theory I through a tarp in front of the radiator to block airflow and allowed it to warm up to it's proper temperature. The idle behavior improves greatly. It was also throwing a code for "too low engine temperature" when I was having brake light codes checked out.

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[Fast69Mopar]

I think the replacement thermostat my mechanic used is too low or malfunctioning. I remember it always running about 190+ degrees and since I got it back it's been more like 160-170. I confirmed the OEM called for 195 degrees.

To test my theory I through a tarp in front of the radiator to block airflow and allowed it to warm up to it's proper temperature. The idle behavior improves greatly. It was also throwing a code for "too low engine temperature" when I was having brake light codes checked out.

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160°-170° is too low and those low temps will cause the engine to run rich. Get the correct thermostat in there and it will start to run better. It may not fix all of the problem but it will definitely help.

 

[Fitz-0518]

^^^^As stated, fully agree. My experience on a 5.7 rebuild. Put in lower temp thermostat. Ran lumpy. Computer "loop" could not close until sensor saw 195*. PCM kept trying to "adjust for condition" Replaced with 195* thermo,,,ran perfect. Fitz

 

[Shellkamp]

Truck idles much better now with the correct thermostat.

I'll post pictures of the truck soon, all done with the "restoration" on it.

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[Shellkamp]

https://www.ramforum.com/index.php?posts/2165805

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