Diagnostic help please - acceleration flat spot when cold (09 4.6)

[WallyDog]

Hello! My 09 4.6 (110k mi) has had this problem for a couple of years now. When cold, the car will hit flat spots during steady, normal acceleration. Keeping pedal steady, car will lose engine power (not shot off, just lose power like gas pedal has been fully released). This flat spot lasts for about a full second, then it kicks back in to normal. Then in 1-5 seconds, the flat spot of no acceleration happens again. This whole problem can sometimes be avoided by accelerating very slowly. But driving normally, the problem happens every time.

It acts like fuel is getting cut off intermittently, like with a clogged fuel filter or something.

HOWEVER, as the car warms up, the problem gets less and less and then, eventually (like 20 minutes of driving), it will be ok. So I don't think it's a filter problem.

A couple of years ago, I had a garage do a diagnostic when the problem first started. The dyno showed the engine ran steady and the garage said to just let it go till it gets worse and triggers the CEL so they can get an indication of what is going on. Well, there is still no check engine light yet.

Another problem I have that may - or may not - be related is a shaky idle, but only when in gear. The idle isn't really shaky as the tach is steady. But the car vibrates roughly at idle. It goes smooth as soon as I put it in neutral, but rough when in gear. I think maybe the rough shake is a broken tranny mount?

So, smooth engine, no CEL, but intermittent, 1-2 second loss of acceleration when cold, getting better as car warms up.

Tranny problem? Low tranny fluid? An engine sensor that creates a lean fuel mix when cold? Gypsy curse? Bad blinker fluid?

If anyone has any suggestions on what to check, I'd REALLY appreciate it!

 

[EdP]

Th good news is, I know how to fix it. The bad new is, I only know how to fix it on a car with a carburetor. If you'd be willing to get rid of that fuel injections system and put on a nice Rochester 4 barrel, I can help.

Vibration at idle is often from being a bit too slow and taking it out of gear give you just a few more rpm to smooth it out. What is the tach reading?

You may be on the right path with fuel mixture leaning for some reason. No experience on a Gypsy curse.

 

[Gunkk]

Have you put an OBD2 reader on it and watched the short term and long term fuel trims?

 

[WallyDog]

Thanks, guys. RPM at idle is steady at ~1100 or so and ~600 in gear. (You're so right, Ed. I could fix the idle issue on my old Camaro's 4-barrel just fine!)

I haven't monitored the change in fuel trims. I'll give that a try and see what it shows.