[saracino]

My S4 sputters as I accelerate past 2000 RPM.It sort of pops and misses and it gave me the feeling that it was ready to conk out. I slowly made my way back home and at low revs the engine sounded fine. I've used the search option to find out if others have had this problem but can't seem to find any S4 threads.
I can list what it's not
My Cat is clean
The plugs are clean/wiring new
fuel pressure very good
Never had this problem before.
Any Ideas?
Thanks

 

[ghnl]

I'd suggest the 'usual suspects' - "false air" (leaks in the intake ducts or loose/cracked vacuum hoses) &/or faulty electrical grounds.

The link in my signature is about the S3 L-jet system. Your car has Motronic - essentially the same operation but differing in a few details (single crank sensor at the pulley vs two flywheel sensors, Idle Air Control vs Aux Air Valve, and the two L-jet computers are combined into the one Motronic computer). Thus, the basic concepts in the L-jet page will apply to your car, too.

 

[saracino]

Thanks GHNL
I'll go through the list and see what happens
Thanks again

 

[saracino]

ayy caramba, me thinks it's the AFM.

 

[Geezer]

I had the same symptoms with my '82. Found that the O2 sensor had come unplugged. Ran better but still not right. Then I discovered that the distributor has come loose and had rotated. Moved the dist back where it should be and all is right with the world. BTW the Dist. on my '82 does not affect timing it acts solely as a switch. When it rotated out of position it was allowing a crossfire to the wrong cylinder.

 

[Gubi]

ayy caramba, me thinks it's the AFM.

Because...?

 

[saracino]

We checked all the hoses for leaks or cracks or loose clips and all was good. The valve timing solonoid was checked as well as the vapor recovery cannister and they were both AOK. The Ignition coil is good, the intake runners are perfect,the throttle linkage fine. We checked the electrical grounds and even the fly wheel adjustment was spot on. The cat converter is clean as a whistle and the wiring and plugs are new. We even checked the valves and no problem . So based on the step[ by step approach suggested by BB member GHNL I'm comming to the conclusion that the Air Flow might be screwed up.

 

[ghnl]

Here's a thread I found that may apply: S4 flat spot.

The AFM is known to be quite sturdy and long-lived. So I wouldn't jump to any conclusions and condemn it too quickly.

Are you sure the oxygen sensor is functioning correctly? A typical lifespan is ~60K miles. The ECU (computer) needs a valid signal from the O2 sensor in order for it to be able to adjust the mixture. Without a valid signal the ECU goes 'open loop' (no feedback). Open loop is a slightly rich mixture. If the sensor is becoming iffy maybe the ECU is switching back & forth between open & closed loop modes.

Here's a thread with info about the AFM & O2 sensor: Further Adventures in Bosch Troubleshooting.

BTW, how is the idle speed? Steady or fluctuating? If the IAC (Idle Air Control) is malfunctioning the idle speed will become erratic. i wonder if a malfunctioning IAC would also affect medium speed running...? Here's some info about the IAC (aka ISA - Idle Speed Actuator).

Finally - have you checked for 'check engine' codes? (see reply #6 in that link)

 

[Gubi]

Agreed with Eric not to be too quick about condemning the AFM. Anyway, there's a procedure for testing it in the Cardisc.

Doesn't sound like an issue that the IAC would cause. Take Geezer's advice and check the cap/rotor for tightness. And yes, definitely see if there are any codes.

 

[saracino]

Thanks guys,will check it out in the morning.

 

[Art Vandelay]

Fuel filter?

 

[ddouglas77]

We checked the electrical grounds and even the fly wheel adjustment was spot on.

You have a 1991 with bosch motronic, there is no flywheel adjustment, your marks are on the crank pulley-not the flywheel. 10:1 I'd bet you have a broken crank pulley or your cams are off by several degrees. To determine broken crank pulley keyway, do either of the following-preferably both: Idle the car and watch the pulley for any sign of wobbling, have some one gun the engine a few times while watching it and also, loosen the tension on the belts and twist the pulley back and forth feeling for excessive play. Also, elminate the fuel filter (and Air flow meter)-as suggested by Art, by slowly reving the engine in neutral past your 2K mark, if it revs freely beyond 4K in nuetral, then you may have a fuel starvation problem (remember, the amount of fuel the engine needs under no load is almost nothing compared to being in gear under full load), this still will not eliminate bad cam timing or broken keyway on the crank pulley though-if those 2 things are messed up, you'll still not be able to rev high enough, you must eliminate those items first.

 

[alfaloco]

This sounds an awful lot like a crack in the flexible hose either between the AFM and the intake, or the short hose into the throttle body. When I had to replace the flex hose on my wife's S4 last year, the crack in the rubber would only open and pull false air when you revved the motor "con brio", otherwise if you accelerated gently, no miss or pops. Take another look at the intake hoses and clamps, I'm betting on a leak.

 

[Tifosi]

"con brio"...... Is that anything like "al dente", or quite a bit firmer?

 

[Bohica]

Check the flap inside the afm to make sure it isn't hanging up. Mine did and I used a little sandpaper on the edge of flap to remove a burr. Took me forever to find the problem, but it has run well so far.

 

[alfaloco]

Well, I do like my pasta "al dente", but the "con brio" I got from them thar Eyetalyun opera scores.

Loosely translated, it means "stand in it".

 

[Tifosi]

Hehe.......

Frequently playing your aria con brio for the peasants at large are we? (I'm not totally unfamiliar with opera, I just prefer metal )

 

[alfaloco]

Hehe.......

Frequently playing your aria con brio for the peasants at large are we? (I'm not totally unfamiliar with opera, I just prefer metal )

Nope, just the neighbors!

"Let them eat biscotti!"

 

[saracino]

This sounds an awful lot like a crack in the flexible hose either between the AFM and the intake, or the short hose into the throttle body. When I had to replace the flex hose on my wife's S4 last year, the crack in the rubber would only open and pull false air when you revved the motor "con brio", otherwise if you accelerated gently, no miss or pops. Take another look at the intake hoses and clamps, I'm betting on a leak.

GHNL suggested I check for air leaks since this is usually the reason for sputtering but after a thorough review it seems that all hoses and clamps are fine.The car has been sitting at my mechanics shop over the weekend so I haven't touched it in three days. I'll go over on wednesday and give it a once over based on all your valuable suggestions.

 

[saracino]

Check the flap inside the afm to make sure it isn't hanging up. Mine did and I used a little sandpaper on the edge of flap to remove a burr. Took me forever to find the problem, but it has run well so far.

On Wednesday we will remove the AFM and replace it with a borowed slightly used one thats in fine shape. If the problem persists I'll know it's not the AFM.
If on the other hand the sputtering disapears then I'll attempt removing the cap on this device and try some tinkering.
Thanks and with all your help I'm sure I'll get to the bottom of this .
Mille Grazie