Rough idle at start

[Zach Lynch]

Hi, I’m not sure if this is normal or not but anytime after driving for a couple minutes and letting my explorer warm up(1994, 4speed auto xlt) when I go back out to go somewhere and it is still hot, it will start very quickly and idle very rough sometimes it will drop to like 2-400 rpm, usually idles at 6-700 rpms... Any ideas?

 

[Roadrunner777]

Well, this won't be very helpful, more of a support reply... I had this and never solved it. It was worse in hot weather. It was worse when it was parked on pavement rather than dirt or gravel. I didn't get it that often because we don't have a lot of really hot weather in my region. I think it might be some variation on vapor lock.

 

[Zach Lynch]

Well, this won't be very helpful, more of a support reply... I had this and never solved it. It was worse in hot weather. It was worse when it was parked on pavement rather than dirt or gravel. I didn't get it that often because we don't have a lot of really hot weather in my region. I think it might be some variation on vapor lock.

It’s been 80-90 past little while and does it without a doubt every time, it’s really weird..

 

[Roadrunner777]

I think, if you wanted to tie it to fuel, you could do this: Next time you expect this to happen, turn the key from off to on, then off and on again, do that maybe 5 times. Then try to start it. If it starts more... normally, then you can tie the problem to vapor.

 

[Zach Lynch]

I think, if you wanted to tie it to fuel, you could do this: Next time you expect this to happen, turn the key from off to on, then off and on again, do that maybe 5 times. Then try to start it. If it starts more... normally, then you can tie the problem to vapor.

I might try That, when I start it I turn the key to on for a second and let my fuel pump prime when it shuts off I then start it. Still does it so I will definitely give that on and off 5 times tomorrow for sure!

 

[Roadrunner777]

I might try That, when I start it I turn the key to on for a second and let my fuel pump prime when it shuts off I then start it. Still does it so I will definitely give that on and off 5 times tomorrow for sure!

I'm just going to preemptively post this... if cycling the pump DOES help, there is some thought that this could ultimately be caused the supply fuel line depressurizing while parked. Pressurizing fluids increases boiling point, as you know from engine cooling systems or perhaps pressure cookers. So, if you wanted a diagnostic test for this, you would borrow, rent or buy a fuel pressure gauge and check for leakdown. Leakdown would ultimately be caused by a bad check-valve in the fuel pump, a bad pressure regulator, or even a slight leak somewhere in the system that could be so small it could be unnoticed. Fixing whatever it is would maintain pressure, which would eliminate vapor, and so on.

 

[kmack]

Another option to look at is a bad ECT sensor (Engine Coolant Temp). This tells the computer what the coolant temp is and the fuel trims are adjusted based on that. I had a similar issue, but it didn't happen all the time and didn't seem to be dependent upon outside temperature. After driving for a minute, it would clear up and idle fine. I just replaced my ECT sensor a month ago, but haven't put enough miles on it to verify the fix. But I had been told this was a common issue when the sensor starts to fail.