My car hesitates (bucks) a bit at very light throttle. If I lay into the throttle or keep revs north of 2500 RPMs, all is fine.

Upon reviewing a log after driving around, sometimes, when TPS drops to 0% (along with MAF and IPW both dropping), MAP (kPA) spikes to about 100 and then drops. I've replaced MAP with no apparent improvement.

When this occurs, mostly when upshifting (M6) and < 2500 RPM, the motor races (RPM does not drop), which would seem to make sense to me, bases on the MAP reading.

It's like I have a MASSIVE vaccum leak(I've checked everywhere!) directly after lettng off the throttle, that then subsides. The throttle blades are definitely returning to the closed position...no apparent physical throttle linkage issues.

The problem seems more prominent with higher abmient temperatures. Any guesses as to what might be going on? Am I overlooking something obvious?

Motor has great power and behaves normally above 3000 RPMs.

 

I have the exact same thing happening in my A4. I also have a stumble (actually more like a flutter though) that happens. Ive been trouble shooting this for over a year now!

Heres what i've found out so far

-MAP is a good indication of how hard your engines trying. IF the MAP is at 100% then youhave a HUGE vaccume leak and the engines so lean it wont spin any faster (no vaccume generated) or for some reason the engines maxed out at that throttle in that computer Cell for another reason (Fuel/spark?). I have no idea what could be causing it. I had my injectors cleaned (Motovac) with zero improvment on this stumble.

-when I remove thed EGR vac line and plugged up the open end the car ran a bit better but it was still there.

-Replaced Coil, Plugs, Wires, engine block, valves, springs, Fuel Pump, filter, O2(err but got bosches back then :/). Did the ICM cooling MOD, I have a CAI but have cleaned the MAF sensor (this seemed to have helped a bit then it eventually reverted back).


Like you my car behaves great everywhere else and this happens only at light throttle...

 

http://shbox.com/1/EGR_flow.jpg
Check out this diagram from Shoeboxes library.

Looks like if the spring becomes worn over the years exhaust gasses could seep in even with the vaccume disconnected! I've always thought that the pintle was on the intake side, so I didnt think much of it.

I really think that this could be causing the high MAP readings. I've already ordered a new EGR and will have it installed tomorrow Over the weekend I did a lot of driving, half with the EGR hooked up and half with the Vac Line removed and plugged. The stumble was there in both situations BUT it got worse when the EGR was hooked up.

*Crosses fingers*

If this diagram is accurate, then Unplugging the Vaccume line from the EGR may not really "rule out" a bad EGR (which is what i've been led to believe). But rather just rules out a stuck open EGR...
I should also add that it would make sense that having high exhaust backpressure could cause the egr to bleed more air sooner as well.

 

Thanks for chiming in....I've wondered about EGR. I was thinking of blocking the exhaust supply line at the manifold, just to see. Toby360: what are you using for a scanner?

I just don't see that an EGR malfunction would account for a MAP reading as high as I'm seeing. If I'm at 15% TPS and 2200 RPMS and let off to 0%, MAP briefly hits ~ 100 kPA. I just dont get it.
IAC couldn't account for that either could it?

I've replaced my EGR, although that was 3 or 4 years ago.

I'm probably going to start plugging vacuum lines to the intake/throttle body and see where that gets me.

Keep us posted, as will I.

 

I use Freescan and have a few logs(and graphs) of it happening to me.

If the EGR is letting in enough exhaust, the A/F mixture would get too lean (the somewhat oxygen burnt up exhaust fumes) which could mean bad detonation, which in turn means less engine power when that happens. As soon as this would happen id imagine the intake would stop sucking air in as hard and the vaccume would go right down to almost nothing.

The MAP reading 1.0 isn't because of all the exhaust fumes entering the intake as if it were a vaccume leak...

This is just my theory of course.. but it seems logical

 

Wait a second... I have no EGR valve on my carm since there's a block off plate, but i still have the long metal line going from the pass. header to the manifold. The solenoid is still there, just plugged up on the EGR valve end. So does that mean that im basically getting nothing but constant exhaust gas in the manifold?

 

Your plate should keep the exhaust sealed off, as well as the port the valve uses to let the exhaust in.

 

Oh ok, just wondering. I'm still trying to chase down the demon thats slowing me down so I figured I'd ask. I just got a scanmaster though so I'll take some readings and post em up in the computer/diagnostic section and see what you guys think.

 

Looks like my stumble thats been plaguing me is gone with the new EGR! I had a chance to drive it a tad but was a bit congested. It DID feel much better from what it felt like before! Tonight i should know for sure, the old EGR I had, had a few problems #1 it didnt hold a vaccume for very long. #2 the springyness of the valve seemd a lot less than the shiny new one I got put in.

I'm crossing my fingers for when I drive it tonight -thats when its most noticable and theres little or no traffic...

 

Update: Well its partially fixed the problem. The stumble or flutters almost gone, but i can still feel a bit of a dead zone. Its not 100% fixed but definatly much much less noticable now. So, still there but not quite gone. Perhaps my crappy bosch o2's are part of the problem :/

 

you can drill a whole in the top of the intake and your car will not run lean the o2s will pick up the oxygen in the air and add fuel.

but your right map is a good indicator on how hard the engine is working.

 

I guess i should clarify What i meant by Lean and "Huge" Vaccume leak was that if the vac leak was big enough the car would run so lean it would be maxed out in the tables and unable to compensate any more (and throw a engine lean code as well) - And the map would also be maxed at that point.
Probably not something the EGR would cause though..

Just found out my coil is arcing a bit (popped the hood) doubt thats the problem though but who knows this problem aint ever gona go away....