lzep

I have a 1988 Audi model 80 Quattro (5 cylinder/5 speed manual trans) which suddenly began giving symptoms of misfiring or stuttering under acceleration. Once the engine rpm increases it runs smooth until I grab the next gear where the misfire/stuttering starts again. Once up to speed and at a constant speed it runs fine but any change in throttle setting will exhibit the symptom again until the rpm increases. So far I have changed plugs, wires, fuel filter. I have also been running seafoam/injector cleaner through the last few tanks of gas. I also checked to make sure the upper coolant temperature sensor is changing with the temperature(about 5.6Kohms at 65degF). I didn’t change the O2 sensor but I think this is a more severe problem than that sensor could correct. I also removed the CIS rubber boot to make certain the large butterfly was able to move freely. It seems that the idle speed control works, at least the idle becomes erratic when I unplug it. I also checked the best I could for vacuum leaks. I was wondering if the fuel pump is putting out sufficient pressure but it seems to work fine at higher rpms.???

SerbVdub

Run a scan on it and figure out which cylinder is misfiring under acceleration. That is the first thing I would do. If you figure out which cylinder has a misfire than I would go further and switch around some plugs, wires, (coils - if equiped), injector pulse test (under load), fuel pressure (under load). Ive had issues in the past with certain Audis where the crank sensor is cracked and under load it causes a misfire. Does it die at idle? Have erratic idle surge when warmed up?

lzep

It does not die while idling; it also does not run erratically at idle at any temperature. It just has a stuttering when accelerating from a stable speed. Once the engine RPM increases or stabilizes, it runs perfect.
I also have an 01 A6 with OBD2. I know I can monitor several engine functions while it is running on that one but I’m not sure how to do that on this old dog.

Loudgoldwing

The best lesson I've ever been taught for diagnosing the cause of a basic misfire (basic, meaning this was on a carbureted motorcycle engine):

High-RPM miss = fuel related. Runs well when needing less fuel, runs poorly when demand for fuel increases.

Low-RPM miss = spark related. Runs poorly when receiving low alternator power, improves when more alternator power is supplied.

Easiest way to figure this out:

Let the vehicle idle (poorly) for 1 minute. Lick your fingers, and BRIEFLY tap each of the five head-pipes, wiping off and re-moistening your fingers each time. Doing this, you will discover that one of your five heads isn't quite as hot as the others. That cylinder is your "problem child."

Once you've found the problem child, leave the car running and grab a small towel. Using the towel to keep hour hand from getting burned, gently wiggle the problem child's spark plug cap back and forth, SLOWLY removing it from the plug. If you notice the engine idle suddenly cleans itself up, it's a bit of rubber lodged in your new plug wire cap. Happens all the time. Turn off the car and clean that cap out really well.

If that doesn't work:

Pull the problem child's ignition wire all the way off the plug. Then disconnect it from its ignition coil. Do the same thing with another ignition wire, preferably the cylinder farthest from the problem child, but keeping in mind that each wire still needs to reach its proper coil.

Reconnect the (now swapped) ignition wires from plug to coil KEEPING IN MIND that you have switched BOTH the connection to the plugs AND the connection to the coils. DO NOT ONLY SWITCH ONE END WITHOUT THE OTHER!!! (example: Plug #1 connected to Coil #4 = DEATH TO YOUR ENGINE)

Once re-connected, start the car and let it run (likely still poorly) for one minute again. Test the heat of the head pipes. Is the "Problem Child" still the same cylinder? Or, did your issue move to the other cylinder (the new cylinder to which you moved the problem child's ignition wire)?

Obviously, if the issue moved, your problem is the ignition wire currently attached to whichever cylinder is not working right. If the issue didn't move, SWITCH THE WIRES BACK TO THEIR ORIGINAL POSITIONS, as there is no wire problem.

Once the wires are back where they need to be, do the same thing with the coils.

One thing I don't like, though. From what I've read, your idle is good. Opening throttle causes a miss, but the miss goes away when the RPMs get up higher in their range. And this only happens on one cylinder? I've never heard of this.

Loudgoldwing

{returns after an hour & a half}...

You know what? I take that back. I HAVE experienced this problem before. I used to own a 1983 Yamaha XV920 Virago. Air-cooled staggered V-twin. The back cylinder used to do EXACTLY what you're talking about. Idle was great, but as soon as you opened the throttle you would lose the rear cylinder until you built up the RPMs enough to get it firing again.

Turns out, the problem was an un-noticed crack on the underside of the rubber intake boot that connected the carburetor to the cylinder. At idle, the suction was weak enough that the crack was closed. So the bike ran fine. Opening the throttle increased the suction, opening the crack and starving the carburator of fuel-pull. But once you had everything really moving, the crack was small enough that the air rushing through hit a terminal point. It couldn't fit air through any faster, so the fuel would start to get sucked through the carb again. Not very well, granted. But enough that the rear cylinder would start hitting again.

Where could you have a vacuum leak where it would only effect one cylinder? I don't know. Is it worth it to switch 2 of your injector nozzles to see if the problem switches too? I don't know. But I can tell you that doing things like this is where I learned about 90% of what I know about machines in general.

Good luck bro. If my ideas didn't solve your issue, I hope that they might at least get somebody else thinking in the right direction.