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Case study on misfiring 1998 Audi A8 Quattro


Old 02-07-2018, 02:38 PM
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Default Case study on misfiring 1998 Audi A8 Quattro

Here is a case study in case it helps those who do or did own a D2 A8, or are interested in owning one, one day. This thread does not intend to convey or imply that this car is for sale.

I bought this 1998 Audi D2 A8, WAUBG34D8WN007421, from someone in the Lake Tahoe area. He'd been driving it and it started misbehaving very badly, with a massive loss of power. He parked it and it's been parked for months, until I bought it last week for $550 with a clean title. I put in a fresh battery and tried to drive it onto the auto transporter trailer; it didn't have enough power. I took a running start with momentum and barely made it up the ramps. The ominous noises from the engine compartment were a concern for the seller and me too, plus for my automotive-genius tech girl who'd found me this car in the first place.

A few nights ago, this brilliant tech girl and I inspected and swapped out the coil packs by having the engine idle, then undoing the clip and observing if the engine speed fell (her idea, not mine). That worked well. We could identify non-firing cylinders. We swapped out the relevant coil packs, as in replaced them with known good used ones from my stash. And still, the car continued to misfire as before. The problem cylinders were 3,5 and 8. She found it interesting how these featured in the firing order.

We removed the spark plugs and did a compression test: results being 180,180,170,160,170,175,165,180 ... not the cause of the misfire. We smeared anti-seize on a new set of plugs, single-electrode NGK iridium ($7 each at AutoZone), left the gap unchanged and installed them, paying careful attention to the tightening torque. We also swapped out the ECU with a good one from my used parts stash. Still, the car continued to misfire as before.

We swapped out the fuel rail with injectors, from a good used one from my stash. Still, the car continued to misfire as before.

Then, she had a flash of insight but mischievously refused to say what it was. She rushed to my stash of parts, and removed something from the engine wiring harness, then swapped out that part on our project car (with the ignition off) and asked me to start the car. It instantly ran smoothly. "Ignition control module," she announced. She cleared the error codes and the check engine code went off and stayed off. The car runs great. I'm happy and proud to have such a brilliant friend ... and a well-running D2 A8.

Next, I plan to swap out the lower front valve body housing with one that's had its pressure regulator renewed by Eriksson Industries, the ZF distributor in Connecticut, to prevent the clutch A drum from failing in the classic way these ZF 5HP-24A transmissions fail.After that, probably a timing belt change and I'll have one more bullet-proof D2 A8. That brings the size of my D2 A8 fleet to six. I might well have the largest concentration of Audi D2 A8 cars in the county, possibly in the entire State of Nevada. :-)

And I certainly have a delightfully brilliant tech girl. Thank you, Miss K. You rock!

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Old 02-08-2018, 05:15 AM
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Yep, that is typical of a lot of cars with these ignition modules. The module was integrated into the coil packs with the 40V engine and for those I recommend carrying a spare coil around. For the 32v I recommend carrying a coil and a spare module.
Same situation in my 928S4s, many Volvos, 16V 944, etc. Fortunately these are cheap used so not a big investment to be carrying as a spare. The 928 has 2 modules and can actually catch fire due to the fuel/air from the dead cylinders pumped into the hot catalytic converter so it's major bad news to try to drive it on 4 cyls. I think the A8 is smart enough not to do that. The 928 converter gets hot enough to damage the car or even start a fire.

Even if someone does not want to change these parts themselves the spares can still save you if you get towed or limp to a service station where a tech will then have the parts to repair the car rather than having to order new parts and waiting for them to arrive. Likewise carry a spare serpentine belt (old one if need be) since it's not a common part but a quick fix, add a couple Harbor Freight wrenches (link two combination wrenches) if you are game to change the belt yourself in the Taco Bell parking lot.

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Old 02-13-2018, 07:18 PM
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Joel, you make several good points, thank you. :-)

I've ordered a renewed pressure regulator for this car from the ZF distributor, sort of the official version of the Transgo fix. After that's been installed, I look forward to driving it. It has, wonder of wonders, two armrests, neither of which is lying in the trunk which is unheard of for a pre-facelift A8.

The driver door latch is lazy so closing it sometimes causes a hard bounce-back until I use a screwdriver to push the latch back into a receptive position ... slamming it harder in such a situation would simply risk damaging the parts.

I also got it officially retitled. So far so good.

.. and that's the news for this particular car.

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