I have an lumpy idle, figured out a pattern and I would like to hear your thoughts.

I posted a quirk with my car a while ago. Every few weeks I noticed that the idle became lumpy while I was at a red light waiting for the light to change. It was so infrequent that it confused me. Well, I just confirmed a pattern today! I realized that after I fill up with fuel, it happens at the first red light I come to. After putting a mile or so on the car it doesn't do it any more, until the next time I fill up again. It will always run smooth and strong when driving at any speed, this problem only happens when the car is idling and stopped.
Does anyone have any ideas of what may cause this behavior? Now that I know that it's consistent and I know when to expect it, it's not too big of a deal. However, I would really like to know why. Any thoughts are greatly appreciated.

 

I usually drive it at least 8 miles before I fill up. If its Leaking injectors, why would it be any different from stopping any other time, then restarting?
If it is leaky injectors, would a bottle of techron help?
Do you think it has something to do with the gas cap? Maybe a pressure issue when it's opened. It doesn't sound right to me, but I can't think of any reason why removing and reinstalling the cap would affect it.

 

Hmmm, that sounds like it makes sense. I do want to add that I never force fuel into my cars. After the pump shuts off, I stop pumping fuel in. I know it's bad for the charcoal canister to do that. Wouldn't all are cars do that if that was the issue? I'm sure I'm not the only one to fill up my car? I'm sure there is even those that force more fuel in after the pump stops the first time.
Curt, if that is the issue, what can I do to fix it? (Or at least confirm it?)
Other than filling it to 3/4 of course. Lol

 

Thanks guys, I'll try putting in a bottle of Techron. I'll also change the spark plugs just for good measure. If I change them, my car will have the new designed plug, plus I can also eliminate any possible incompetence from the dealer that I bought it from. They did a compression test when I bought it, so I don't trust that everything was torqued properly etc.
I'll get back with the verdict after I use up a full tank with Techron.

 

Thanks for the heads up Rob.

 

I didn't use up enough gas to put the Techron in yet, but the more I think about it, the more I fear that it's the intake manifold gasket. It has an inconsistent idle, and I just thought about something else now. When the car is idling and I stab the throttle quickly to about 4,000rpm, once the accelerator is released, the car drops to about 500 rpm's, then stutters and goes back up to about 1,050 RPM. I think it did this the whole time I've owned he car and it hasn't gotten worse.
What do you guys think?
By the way, here is a video of the car idling after a cold start. It is a little high, but it settles down after a minute or so. Look at the unsteady RPM's. If need be, I can also get a video of the way it drops down after I stab the throttle momentarily.

Cold Idle - YouTube

 

Yes Matt, that's a good point, I'm going to try and find it with some starter fluid. Not knowing if it has a problem is really bugging me.

3 questions:

1- Is the starter fluid trick a 100% indicator, or is it possible that it can be leaking internally and not sucking air from the outside, therefore the starting fluid may not make it rev when sprayed on there? If I spray around each runner and the idle doesn't change, can I be certain that the intake manifold gasket is fine?

2- What is the proper torque for the intake manifold bolts, I can't seem to find the spec in the shop manual. I would like to make sure that the bolts didn't loosen up a little.

3- I know propane in an unlit torch can be used to find the leak also. However, I don't have propane, I only have a Mapp gas torch. Can I use Mapp gas instead, or should I just get the starting fluid to be safe?

Thanks!

 

I just got back inside from spraying propane at all the gaskets areas on the intake manifold. I used a long tube on the torch so I can get right in there. There was no noticeable change in the idle. I guess that's good news.
I will try the Techron and new plugs, then report back. I'm thinking that the behavior that my car is showing is probably normal, and I'm just over reacting...who knows.
I attached two videos of my Tach. First one is with a cold engine. It shows how the RPM's drop to about 500 when the throttle is pressed and released. The second one was after approximately 5 minutes (after I checked the manifold). In that one, you will see that the RPM's no longer drop once the car was warmed up a little. Is that normal behavior for these cars?

[ame=http://www.youtube.com/watch?v=egob5AkPy1E]Ferrari 360 Rev with cold engine - YouTube[/ame]

[ame=http://www.youtube.com/watch?v=J4Y-Id7N3kc]Ferrari 360 Rev after running for 5 min - YouTube[/ame]

 

Hmm, if anything, that video seems to support that my car IS normal...
The Tachometer does sway exactly like mine does, which makes be feel better. Also, I would like to point out that if I build the revs gently, even with a cold engine, the RPM's will not drop like they did in my video. I'm pretty positive that if I did the same thing that you did, my car would have behaved exactly the same way.
If you get a chance, when the engine is first started cold. Just stab the throttle slightly, Maybe up to 3,000 rpm max, then release it abruptly. I wonder if yours will then be the same as my car?
If that is actually not normal, do you have any ideas of what it could be?
Hopefully Techron and a set of plugs will be the solution if in fact it is a problem.

 

Good point.

 

Thanks for the info. I'll mention it to the tech next time I take it in for an inspection.

I'm pretty sure that the gasket is not damaged, but I'm reaching for ideas because I'm not convinced that the car is running the way that it should. I don't feel that there is any huge issue as of now, but I would like to take care of it if there is in fact an issue, even if it's minor.
As I said above, my best approach as of now is to just let the tech know about my findings when I get the state inspection done.

 

Wow Joe.....OUCH....
I saved my reply to you so I can post it independently from the others.
Hmm lets see, I'm complaining about a "possible" problem here. One that shouldn't be a big deal once it's diagnosed properly (if there is even a legitimate problem). Not that big of a deal in the grand scheme of things if you think about it, right?
Now if I were to complain about a bad header, a cracked buttress, a headlight that doesn't go up, bad coils, backfiring issues and maybe some other things...THEN I may be able to see your point.

Touché!!!
See you at our next Cars & Coffee

Love you buddy, thanks to you I'm driving a fine piece of machinery! Now we just have to give my brother that nudge