Rough idle on startup

[LoganB]

Engine issues, how fun!

the other day I was running errands when my car started up and idled very rough, I was a bit scared but last time this had happened it was a loose connection on my fuel pump so my first thought was to put it in drive and do a circle round the parking lot, hoping it would kinda just go back to sitting in a normal position, problem goes away. Hooray!

Few days go by and we land on today, where I was hanging out with my friends and I noticed it again, rough idle after leaving a store in the parking lot, great, again my first thought is to drive it cause it fixed the issue the first time. I go home and the thing is driving just fine, no weird idle.

We take it to a car wash, (where it sits for 12 minutes as we wash it) and then we start it and it just dies. Clearly this issue isn’t going away and it was seeming to get worse cause it was happening More frequently (that made twice in one day), I start it up again with my foot on the throttle and it’s working alright, I let it idle at 2k for a few seconds before putting it in reverse and going home making sure not to let it idle on the way. Get home, no weird idle. Strange.

We didn’t really understand what was going on but we tried recreating the conditions of letting it sit after being up to temp, because so far, all 3 times it has happened, the engine was running fine before, was turned off, left for a few minutes, the started again. Behold, it ran super roughly, shaking like a dog and idling very low (shot motor mounts definitely didn’t help with the shaking)

Afterwards we couldn’t recreate the effect when the engine temp dropped enough. Every time it started normally, it started at ≈1100rpms then would drop to near 700, when it was idling rough, it was closer to 500 rpms.

we got to the conclusion it might be the idle air valve sticking while cooling down and not allowing the engine to breathe, then getting unstuck when the car has been running, does this sound about right to you guys? I think tommorow il try cleaning it out with some throttle body cleaner and see if it happens again, have any of you guys encountered this and does the idle air valve seem like the cause?

Thanks

 

[KevinVarnes]

Doesn't seem like an IAC would cause a rough idle. High, or low, or no idle yes, but it doesn't really have any control how the engine runs. Any idea on how long since the last tune up? So it only does it on a warm restart and never starts acting up if it starts fine?

 

[theterminator93]

Every time the car starts, it starts in open loop - fuel determined by the MAF sensor. After a few seconds it switches to closed loop - fuel fine tuned by what the O2 sensors are measuring.

It's possible the MAF sensor is acting up and delivering an incorrect amount of fuel when the engine is warm and in open loop, promptly fixed when the PCM switches to closed loop fueling after a little while.

Try cleaning it with electrical parts cleaner (see the tech article) - do not use carb or TB cleaner.

 

[1997ThunderbirdLXV6]

Do you have a code reader?

My crank shaft sensor issues had somewhat similar symptoms and regularly threw a pending code which would not trigger the check engine light.

I also at some point had rough idle just for several seconds when starting a hot engine in hot weather between multiple short trips. I attributed that to fuel vaporizing in hot fuel lines and a potentially failing fuel pressure regulator. Ultimately I was unable to ever measure fuel pressure because none of the rental testers fit, and the issue just never came back.

IAC, if faulty, would likely not allow you to start the engine in the first place. I've been there, too.

Cleaning the MAF sensor, combined with reseting the computer, is always a good idea. It'll make your car feel like new while it relearn its routines.

 

[LoganB]

So it only does it on a warm restart and never starts acting up if it starts fine?

So far yes, it’s never caused me issues mid drive or on first startup, it’s always been after I’ve driven it and turned it off and turned on on again a few

My crank shaft sensor issues had somewhat similar symptoms and regularly threw a pending code which would not trigger the check engine light.

I have a code reader, no codes, just an evap code I’ve had for ages but haven’t gotten to fixing

Cleaning the MAF sensor, combined with reseting the computer, is always a good idea. It'll make your car feel like new while it relearn its routines.

Try cleaning it with electrical parts cleaner (see the tech article) - do not use carb or TB cleaner.

Il try that first

Thanks!

 

[1997ThunderbirdLXV6]

When I had the apparent fuel vapor issue, I could hear whooshing through the lines.

Next time you restart a hot engine, try this: just turn on the ignition. You'll hear the fuel pump briefly prime itself for a few seconds. While it does so, do you  also hear whooshing in the lines? It would come mostly from the passenger foot area. If so, just prime the pump a few more times until the whooshing disappears.

As I said, I never found an ultimate fix; it just fixed itself. I did use some fuel injector cleaner though; nothing fancy, just Walmart stuff.

 

[LoganB]

When I had the apparent fuel vapor issue, I could hear whooshing through the lines.

Next time you restart a hot engine, try this: just turn on the ignition. You'll hear the fuel pump briefly prime itself for a few seconds. While it does so, do you  also hear whooshing in the lines? It would come mostly from the passenger foot area. If so, just prime the pump a few more times until the whooshing disappears.

As I said, I never found an ultimate fix; it just fixed itself. I did use some fuel injector cleaner though; nothing fancy, just Walmart stuff.

Funny that you mention this because I did find that there were weird gurgling sounds coming from the fuel lines/fuel pump area, you’d hear the regular sound of the fuel pump and a gurgling sound would follow. It could very well be that, I’m still gonna clean the maf sensor first tho since I did just buy all the stuff to do it just now.

 

[1997ThunderbirdLXV6]

Gurgling may be the more appropriate description, yes.