Rough idle at startup. what's the deal?

fiveoho · Sep 1, 2010

noticed the last few times ive driven the car that on occasion it will idle a little rough on startup for just a few seconds then smooth out. one particular time, it died. today i started it up leaving work and it idled terrible for like a minute. finally, it smoothed out. it never misses a beat while actually driving until today. the car was running at idle in a parking lot for several minutes and while cruising at low speeds, it started running rough again. lasted just a few seconds and then was fine.

what could be some possiblities?

thanks.

wmburns · Sep 1, 2010

The usual, check for vacuum leaks. Clean the MAF. Spark plugs and fuel filter just for good measure.

Consider IAC.

For an unexplained rough running shortly after starting, consider this could be an Evaporate purge. Esp if it runs rough and then magically improves.

Rejekt · Sep 1, 2010

Yeah... I'd clean the IAC. Mine did the same thing before.

fiveoho · Sep 2, 2010

new autolites and fuel filter about 4 months ago. whats the perferred product to clean the MAF and IAC? would it hurt to clean the throttle body as well?

please elaborate more on the evaporate purge.

thanks guys.

DOHC otto · Sep 2, 2010

Cleaners for the MAF and IAC should just be something that evaporates away clean, CRC makes a MAF cleaner that would work for both but isopropyl alcohol would work and be cheaper for the MAF and IAC, brake parts cleaner or carb cleaner for the t body.

Evaporative purge is part of the gas tank ventalition system that filters fuel vapors for the outside air, vapors go to a canister that traps the fuel vapors and the valve lets the vapor into the intake manifold on start up. If the evap purge valve isn't opening correctly it could be building up with vapor forcing the valve open causing surging in the system and your rough idle.

wmburns · Sep 2, 2010

How to troubleshoot IAC Idle problems

For the Evaporative purge there is nothing that can be done. The PCM runs the EVAP purge at random times usually between 30-300 seconds after cold soak motor start. However, an EVAP purge is not going to be noticable except on RARE circumstances.

Recommend that you clean the MAF. Use cleaner designed for the job. Monitor the long term fuel trim (LTFT) at idle and at say 2k RPM. Snap the throttle a couple of times. Post.

Monitor the IAC duty cycle. Post.

To confirm the TPS, monitor the TP_MODE. Move the throttle a few times and confirm the value changes from CLOSED to P/T (or how your tuner displays it). Post.

The usual causes for a slow idle are, a vacuum leak between the IAC and the intake tube, a bad IAC (usual cause), a bad TPS (less likely), or a a weak cylinder(s).

For a weak cylinder, concertrate on the usual things such as ignition, fuel pressure, and fuel system.

Finally, please review the following post 3

>
http://www.stangnet.com/mustang-forums/810729-starts-but-dies-idle-help.html

fiveoho · Sep 2, 2010

thanks again for the suggestions.

Will check for vac leaks and clean the MAF & TB. a gut feeling is the IAC, in which case i hope cleaning it will solve the problem ( i priced a motorcraft replacement ). will also consider the less likely PCV valve and TPS.

sounds like the evap issue would be the least likely. with two jobs, who knows when i will have time to work on this. not huge deal as it doesnt do it very often but quite annoying when it does.

DOHC - sorry i havent returned the pm. i keep forgetting the watch the video. stupid filter at work blocks it.

DOHC otto · Sep 2, 2010

fiveoho said:
DOHC - sorry i havent returned the pm. i keep forgetting the watch the video. stupid filter at work blocks it.

Its cool, check it out when you can its comedy gold !

Moostique · Jul 20, 2019

Rejekt said:
Yeah... I'd clean the IAC. Mine did the same thing before.

There is NO IAC on a 2006 4.0

RaggedGT · Jul 20, 2019

Moostique said:
There is NO IAC on a 2006 4.0

First things First-
Welcome to Stangnet

Secondly-This thread was started in the 4.6 V8 tech section, so I’m gonna assume they are not talking about the 4.0 V6

Third- You are the First person to post in this thread in 9 years

The forum takes a little bit to get used to, but you are on your way

Rough idle at startup. what's the deal?

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Been here over a DECADE and still no CT

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Rough idle at startup. what's the deal?

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