I know this has been covered before but I'm hoping to get some thoughts on my never-ending issue with random rough starts. 2008 H3 3.7L with 116,000 miles. Same common symptoms that have been detailed many times: happens randomly, usually at cold starts (in the summer time more than winter), jumps up to ~1,000 RPM when I turn the key but very quickly drops to 2-300 RPM before jumping back to ~1,500 RPM and then finally stabling out. Been happening since 2014. I just swapped my stock exhaust for a Magnaflow cat back and wanted to capture the sound differences at start up but caught this on camera for the first time in a few years and figured it was worth asking some more knowledgeable people again. I documented this issue on camera for YouTube in 2017 and 36,000 miles ago. Throughout this entire time, I've had consistent P0420 and P0300 codes but no other noticeable performance decreases other than maybe not so great gas mileage.

Here's a list of things I've done both in an attempt to remedy to this issue as well as just basic, unrelated fixes and upgrades over the past few years:

- Clean TB and MAF numerous times (trust me, they are not dirty)
- Upgrade to Air Doc intake
- AFE Power air filter
- PCMofNC tune
- Stinger battery terminals
- Moved exclusively to Phillips 66 for Top Tier gas
- Only Mobil 1 full synthetic oil
- Techron every 3,000 miles
- Replace all spark plugs with OEM Iridiums
- Ignition coils replaced (few at different times)
- Replaced exhaust manifold
- Replaced vapor canister purge valve
- Replaced Camshaft and Crankshaft position sensors
- Remove/clean intake manifold and new gaskets
- Replace cylinder head gasket due to oil leak in one spark plug tube
- Professional induction cleaning at 115,430 miles

Still need to do a compression test though. I've had P0300 multiple cylinder misfire for years and years and P0420 due to burned cats. Still need to replace the main rear catalytic converter but that's the last issue I for sure know about. But I'm sure that was a result of the misfire issues, not the cause. I do get some white-ish smoke coming from the exhaust when I have this issue. The smoke you see here is exacerbated by the outdoor temperature being 8 degrees the day I took this video. Anything I have just overlooked? Tried to look through the intake manifold ports a few months back (before induction cleaning) to check the valves for soft valve seats but couldn't really see and don't know exactly what I was even looking for. As mentioned before, here's the video I got a few days ago before the Magnaflow went in:

 

You missed a couple or stopped short from the Book of Hoyle Thread on this topic...... Acts like it wants to stall/rough idle -No CEL -What To Do:

If the problem still persists:
6) Pull the camshaft actuator solenoid and look for crud on the sensor, on the screen in the motor, or a missing screen... replace if necessary, tighten mounting bolt to 10 N.m/89 lb in.
If the problem still persists:
7) Then do a compression test, a normal read would be appx 1482 kPa/215 PSI. You can follow that with a leak down test. According to Hummerz when a leak down test shows a valve not seating properly, he performs an induction cleaning first, worked on his rig, and many of his customer's vehicles.

 

Sounds sick, starving for fuel!
Did you verify ECT on the scanner? Check the vvt as Doc posted above? Check fuel pressure leak down?

 

VVT doesn't look too bad to me. Screens intact all the way around. Anything I'm not seeing? Also the ECT seems normal. Had it out yesterday (75 degrees in KS) for a while and scanner had the temp stable between 204-206 degrees for the last 5 minutes of my drive. I did replace the ECT sensor about a year ago along with both O2 sensors when I did the exhaust manifold change. If I end up changing the VVT solenoid, do I HAVE to change the oil with it as well? I know that's been documented but I just changed the oil 2,000 miles ago and don't want to have to do it again, especially if the VVT doesn't even need changing. Appreciate the help here. Next on my list was compression/leak down. Just took it to a local spot here for induction cleaning last month.

 

Engine oil change is needed when a screen is missing. You can try and clean, or get a cheap replacement with the updated part# 12615873

 

Cleaned up the VVT solenoid today and everything seemed okay to me. Screens are in good shape. I guess I'll just wait and see if/when it loses power again at start up. I still want to do a compression test so I'll try to get to that soon and report back with the results.

 

Quick update...engine completely stalled on the second start up today. Compression test coming up next. @Doc Olds or @hummerz assuming the compression is poor on one or more of the cylinders, what options do I have for correcting the issue? Is it just a new head at that point? Been doing all my own maintenance for a few years now and hoping there still might be something I can do without professional help but seems like I have made it to the end of the road.

 

Well, the CAS looks pretty good, but did you look in the hole it mounts into for crud in there?

Lets see what the Compression Test does and pleas also scan for codes. Let us know.

 

Did you check FUEL LEAK DOWN? It appears an Injector has gone bad.

 

Never had any consistent codes other than P0300 and P0420. So I was able to do the compression test this afternoon. The rental kit from my local O'Reilly's was garbage so I had my wife help read the results. She claims they all hit around the 140psi mark. I did a recheck on the #2 cylinder (second closest to fan shroud) and captured the result on camera and posted the video below. Seemed to max out at 150psi but had a very rapid drop off in pressure as soon as I turned the ignition off. Is this normal or should I expect to see the cylinder hold the pressure a bit longer? I expected a drop off but threw me off just how quickly it fell. Not sure if that's normal. I'll try to find a spot this weekend with a rental kit to check the fuel pressure and do a leak down.

Seems safe to report each cylinder was at approximately 140-150psi. How's that sound for 116,000 miles? And hummerz, thanks for the video. That dude is hilarious.