Surging only at startup

Oct 21, 2017
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I've been looking all over to find an answer but it seems this is a weird one. Whenever I start this car, doesn't matter if its warm or cold, it surges for a minute and then it smooths out. I drove for an hour and it ran perfectly fine then I stopped, turned off the engine, started it right back up and it did its little 1 minute hissy fit surge and smoothed out. It does not care what the conditions are if its starting, itll surge for a minute. Can't find anything on this and it does seem really weird. Sounds like I got a cam in it.
 
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jozsefsz

5 Year Member
Aug 11, 2013
1,127
261
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Cleveland OH Area
Well, the ECU commands extra fuel (via the injectors) and air (via the IAC) at startup - it'll rev higher for a minute. I'd guess timing out of adjustment, a dirty or malfunctioning IAC, a dirty or malfunctioning MAF, possibly a mis-reading intake or engine temperature sensor, or old O2's causing a rich or lean condition. Checking your spark plug condition might help pinpoint which one. All of this assumes you're completely stock - if you've had a tune and have other modifications, that's an adjustable parameter (startup enrichment) that needs adjustment. I always start with tune-up items, but there are a lot of possible causes. Even though this isn't a classical 'idle surge,' you'd benefit from running through that checklist posted on this site.
 
Oct 21, 2017
19
2
13
28
Well, the ECU commands extra fuel (via the injectors) and air (via the IAC) at startup - it'll rev higher for a minute. I'd guess timing out of adjustment, a dirty or malfunctioning IAC, a dirty or malfunctioning MAF, possibly a mis-reading intake or engine temperature sensor, or old O2's causing a rich or lean condition. Checking your spark plug condition might help pinpoint which one. All of this assumes you're completely stock - if you've had a tune and have other modifications, that's an adjustable parameter (startup enrichment) that needs adjustment. I always start with tune-up items, but there are a lot of possible causes. Even though this isn't a classical 'idle surge,' you'd benefit from running through that checklist posted on this site.
I'll try cleaning the IAC. Spark plugs are fairly new, motorcraft oem copper, all ignition components were replaced, verified, I cleaned my MAF and throttle body thorughly not too long ago cause I was having a lean issue, All my sensors have been replaced recently with NTKs. Car is completely stock. I've pretty much have done maintenance on all Mustang parts that commonly need attention except the IAC.. crossing my fingers for the IAC
 

jozsefsz

5 Year Member
Aug 11, 2013
1,127
261
114
49
Cleveland OH Area
Sounds like a good plan. If it's still acting up afterwards, you might consider checking fuel pressure which could pinpoint a failing fuel pressure regulator or pump. If they're still originals, they may be suspect. Cylinder balance test could also help pinpoint a failing injector.
 
Oct 21, 2017
19
2
13
28
Sounds like a good plan. If it's still acting up afterwards, you might consider checking fuel pressure which could pinpoint a failing fuel pressure regulator or pump. If they're still originals, they may be suspect. Cylinder balance test could also help pinpoint a failing injector.
But would any of that really be problems? Like I said after 1 minute the car idles and runs perfect like it's always have. 0-60 in its glorious 7 seconds like normal, I get up to 80 no problem on highway, sitting at a red light it doesn't even feel like its running its so smooth. Only at startup will it sputter like mad