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S4Motronic- sluggish acceleration, fuel starving

1022 Views 6 Replies 4 Participants Last post by  alfaloco
I just went through an extended search for these symptoms, and came across brim88's episode with the 87 Quad, which sounded very much like what I'm struggling with on my wife's '94 S4 Spider. I also read and saved the tech reference from Gubi on the Bosch airflow meter repairs (thanks to both of you).

brim88-- did the replacement of the airflow meter resolve the issue on your Spider?

Our car idles perfectly. It revs just fine. No other symptoms except that when you put the foot down and ask for acceleration, you hear this low roar as the throttle plate opens, but no response. It bogs, just barely speeds up, and you have to actually let up on the gas just to make some headway. No power at all on hills.

The fuel and air filters are new or nearly new, the airflow meter is clean, and the flapper seems to move without binding. I locate no air leaks nor false air in the intake system (new convoluted hose, secure clamps). The battery's new last spring and grounds are clean and tight. The idle air valve is secure and does not leak at the seal ring. The throttle position switch is correctly adjusted for full open/off idle positions and clamped securely. Fuel has always been Chevron 93 octane for the past 3 years we've had the car. No strange noise from the fuel pump nor tank negative pressure.

At this point I am suspecting the AFM, then secondly the fuel pressure regulator, then the pump inlet sock strainer, then the pump itself. What says the Alfa BB?

All feedback is VERY much appreciated. Keep the shiny side up, folks!
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Is this something that just started happening, or has it been getting progressively worse over X amount of time?

Presuming you went through the AFM info in regard to checking the carbon tracking and such, I'd hazard fuel pressure or volume is the likely culprit.

If at all possible, you'll want to temporarily mount a fuel pressure gauge in a location where you can see it while driving to deterimine what's going on at the exact time it's doing the bad thing. (pressure should jump up to around 38-42psi when up on the throttle hard and around 32-36 when at a light cruise. In any event, it should not drop into the 20's unless you are doing some serious engine braking, and then it might not even drop that low)

Once you get pressure determined you can do volume checks to help decide if it's the pumps or the regulator. (good volume w/low pressure = regulator, poor volume w/low pressure = pumps and/or filters)

BTW, does this occur only when at half a tank and less of gas or regardless of the tank being completely full?

If it's of the 1/2 tank or less variety, it may be the stepped tube that connects the in-tank pump to the sending unit has failed letting the main pump suck air instead of fuel.

Another potential culprit is an internally roached soft fuel line partially blocking flow between the pumps or from the hardline to the rail. (blocked regulator return lines usually results in higher than normal pressure which wouldn't affect the engine the way you describe)
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Thanks much for your post. This began several months back, and has gotten progressively worse. It began as a sort of "fluffy" throttle feel, if you know what I mean. It doesn't seem to respond to how full the tank is, though.

Your points on the regulator are spot-on, and I have a pressure gauge setup for injection system pressures which I will likely be using next. I'm beginning to suspect either the pump, or filter sock on the inlet. But the fuel pressure test is quicker to do and may reveal the problem.

Somewhere I have the resistance reading min to max for the AFM potentiometer, IIRC can be read through terminals 1 and 4. I'm going to look at that and remove the cover so I can visually inspect the scale itself. I'll do the bench test too with the 9 volt battery, which may tell me more than ohms. Heck, I may just substitute the AFM off my '91 Spider and see what happens!

I appreciate your help! I will post the results here when the dust settles on this problem.
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Is the Throttle Position Sensor ruled out? I believe there is a thread several months ago about an S4 with a similar problem. Basically the car wouldn't go above 2000 RPM without choking.
Hey Dave:

Don't have a clue as to what is causing the issue. I suggest that you give the car to Bud and we'll chop it up for scrap:D:D:D Who likes yellow anyway?:D

Maybe a dying boost pump in the tank. Or as noted sock filter. How is it running at idle? Does it act like its developing a miss? How do the spark plugs look? Are they showing lean? The AFM swap sounds like a good idea. Did the car recently throw any check engine lights? Perhaps we can have a quick tech session over in your garage one evening this week. Get in touch with Bud and George and get an evening set up. Tuesday night I am gone. But will be around Wednesday and Thursday evening. We need to get it figured out quick if you plan on taking her to the concours.

Best Regards,
John M
Thanks for the replies, guys. The throttle position switch has been set, and opens/closes at the correct points. It is clamped down securely.

Forgot to mention too, the car gives no codes, nothing. I know the self-diagnosis works because I checked it when I first looked it over prior to purchase.

John, I'm just going to switch the AFM's between the two cars and see what happens. I got a funny feeling that might just reveal the gremlin. It won't take long, anyhow.

Scrap the yellow CE? Don't let Irene hear you say that! She'll give you one her infamous laser stares that'll go right through ya. Besides, Budsy's been trying to get her to sell him that Spider ever since he first saw it. Close, but NO CIGAR, mister!

Thanks again.
Well, I switched AFM's between the Spiders and the problem remains. Sluggish acceleration, poor response to the pedal. So I'm moving on to fuel pressure, verifying the pump delivery, and filtration.

Again, many thanks to those of you who did respond. I appreciate the advice.
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