Surging only at startup

Snazzysaucedude · Feb 6, 2020

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.

jozsefsz · Feb 7, 2020

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.

Snazzysaucedude · Feb 7, 2020

jozsefsz said:
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 · Feb 8, 2020

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.

Snazzysaucedude · Feb 8, 2020

jozsefsz said:
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

ben42 · Nov 20, 2023

Ever have any luck snazzysa

Snazzysaucedude said:
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

Ever have any luck bud ? I'm having same issues

Matt12eagles · Nov 28, 2023

Go swap the IAC, it has done my stang engine an amazing amount of good honestly. Make sure to “set” the idle once done. (Not sure if this is required, but who knows who tinkered in mine before)

1 install with IAC unplugged.
2. Plug in new battery.
3. Set throttle body stop screw to hit tps voltage of ~1V + or - .1 v.
4. set throttle body idle speed to ~850 (it’s a rubber cover on top of throttle body, remove cover and use Allen key to set)
5. Turn off car and disconnect battery. Let sit for 20min.
6. lug in IAC (no accessories like lights or AC on), reconnect battery let idle for 10min
7turn off car. Disconnect battery for another 10-20min
8 start engine with all accessories on. Drive around for 20 miles at your leisure and enjoy how smooth your idle is.

All of these steps may or may not be necessary but MAN did they help out my previous stalls, bogs and wandering idle. Only thing that worked for me was the above.

Surging only at startup

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