Hello, 2007 hummer h3 140k miles. Just finished rebuilding an engine. Car runs great , very smooth. One thing that I can’t seem to figure out is everytime I start the car doesn’t matter if it’s cold or warm. It runs rough for few seconds the become smooth. Why does it run rough ? What cause it? I checked and there are no codes. During the build I replaced the maf and map and all that almost everything is new. Fuel pressure is good, what could be the problem? Any ideas ?

 

I have no idea what you consider rough idle without hearing it? Cold startup can cause high idle clutch fan fully engaged yet never rough idle on my '07..
Post a video with sound on startup

 

I’ll try to post a video tomorrow it’s like you can feel it running rough for like 8-10 seconds then it smoothes out. I never had that happen before that’s why I was concerned . But see it happen regardless when it’s warm or cold. Even if the car is fully hot after a long ride, if I start it after like 5 mins it does the same thing

 

How smooth was your idle before? I have a 2009 i5 and it has a gurgly idle right at startup for a few seconds, but then clears, but general idle is slightly more pronounced than I'd like. I have new plugs, cleaned the MAF, checked vacuum leaks and grounds.

 

I attached a link since its too big to upload on here , but it doesn't matter if the car is cold or warm it does that for around 10 seconds like chug chug chug and harder vibrating and then it smooth like perfect. it wasn't doing that before so I am not sure what causes this, new spark plugs also and cleaned the throttle body

 

Did you check fuel pressure?

The Hummer H3 fuel pressure spec is typically
55–60 PSI (or specifically 57 PSI) with the engine running or key on, engine off (KOEO). A drop of 5 PSI or more within one minute after turning off the key indicates a potential issue. The system operates with the fuel pressure regulator built into the in-tank pump module.

• Normal Operating Pressure: 55–60 PSI.
• Key On, Engine Off (KOEO): ~57 PSI.
• Minimum Pressure: 58 PSI (when checking specific replacement pumps).
• Fuel Pump Max Pressure: Up to 63.8 PSI.

Testing Tips:

• Pressure Drop: If pressure drops 5 PSI or more within one minute of turning the engine off, the regulator or check valve may be failing.
• Symptoms of Low Pressure: Rough idle, poor acceleration, or hard starting.
• Location: The test port is on the fuel rail, and the regulator is in the tank.

 

this weird , but If I turned it off for 30 sec and start it again it does it again. Do you think it still may be fuel related? . Also if I primed the fuel pump like key on wait 5 seconds turn off the. Key on wait 5 seconds then start it does again, what else could it be besides fuel ??

 

What's the fuel pressure at startup & does it maintain 55-60psi at idle?

 

I can check , where on the fuel rail you check for pressure ? Is it that green cap?

 

EVAP purge valve possibly? Do you have a scanner that you can connect to and check live data?

Reason I say EVAP purge valve, I read they can become stuck open and essentially flood the engine with fuel vapor, car will run rough/rich until it clears out and then will be smooth again. Live data will show more than normal negative fuel trims during this time.