2010 Mustang GT - randon, intermittent, fresh hard starts with rough idle.


New Member
Jan 29, 2020
Leander, TX
I have a 2010 Mustang GT
110k miles
5 speed
New fuel pump
New fuel filter
New MAF sensor
New Spark Plugs
New ignition coils
New crank position sensor
New cam sensors
No check engine light

Roush Intake
Roush exhaust
SCT progranner

Not every time, and for the most part the problem arrises after the car has been sitting uncranked (for example over night), it will have a hard time starting (not from lack of power). It will crank, fire up briefly, then quickly stumble back down and shut off. Sometimes it will stumble and hover/bounce aroubd 300-400 rpms a few seconds and then putter out. Then it takes a few seconds of cranking to get it to pop back off. Sometimes you can play with acceletator, at this stage, and get it to iron itself out with a some effort but sometimes it seems it seems as if it just chokes itself out, bogging down and kills again. After 4 or 5 rounds of cranking it will usually figure out what to do on its own and run. Every few, intermittent, hard starts it will give you more of a fit but always gets to a point of running. Once you get it running and warmed up its fine. It seems like the rpms have a hard time maintaining a stable level. You can rev the engine up to 3000 - 4000 rpms, let off petal, the engine quicky idles all the way back down 600rpms, bounces back to 1100rpms, then finds its way back to 800rpm and will sit there. Seems like it fluctuates just a little bit, like maybe 780-850rpms if you just watch the needle. I don't know how steady the rpms should hold at idle. The idle rpm is set at 840. Seems to act the best there. Ive set it at 640rpm before and the car seemed like it didnt like it. Had a harder time managing that value it seemed like with how low the rpms would dip and try to recover. Ive only had a few instances in the last couple hundred miles where it would die while out driving, when coming to a stop at stop light or something. The rpms would fluctuate a bit to low and it couldn't recover and die. The few times it has killed like this while driving, it seemed to be a pain to start back up.. not every time though. Almost like the engine flooded.

Little back story:
Bought the car not running. Sat for months (maybe even a year) Installed rebuilt engine. (When pulled old engine discovered timing had been set wrong, why I don't know) Got it running. Didnt have a hard start issue until after driving the car a few days/maybe 50 miles of road time at this point (coincidence im sure as it doesnt always have a hard start as it is). So it safe to say that this problem existed from the get go. The engine itself is solid. Has good compression. Has all new timing components. Put new crank sensor, new cam sensors, new ignition coils, spark plugs, fuel pump, new filter, MAF sensor, cleaned the throttle body. I have access to a really nice Snap On diagnostic tool and have checked fuel rail pressure for leak down and it hasnt failed the test. Did evap system test to check vaccum leaks and it passed. Ive done the Key On Engine Running test and it passed. The only issue ive run into other than this hard start is after initially getting the car running i would get occasional random misfire codes from debris clogging iniectors and have had to pull the injectos a few times and clean them of the debris (assuming from the car sitting so long there was build up in fuel lines or something). But once the injectors were cleaned and reinstalled no issues with misfire. Put new injector o-rings as well.

What could be the possible issue with the hard start