Project Car Diaries: My $925 Audi S4 Is Side-Questing Me Into Little Fixes Everywhere

I was onto what I'll call phase two of the restoration process: a timing belt, water pump, thermostat service, automatic transmission fluid service, suspension refresh, and various other little jobs. The power steering and engine-stumbling side quests took priority, but it all resulted in an overall better and more reliably running performance German sedan.

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I can't say for certain, but I think the S4 was in a form of limp mode after I fixed its coolant leak. The car generally idled and drove well and didn't have any codes, but it didn't quite feel as powerful as it should've. Especially considering the fact that reviewers were able to launch the B5 S4 to 60 mph in under six seconds when it was new. Sure, mine's now 21 years old, but it still felt slightly underpowered.

After I put some fresh gas through its sad twin-turbo V6 and accumulated 100-150 street miles, the S4 started ever-so-lightly stumbling under acceleration above 3,000 rpm and exhibited a mouse-sized sneeze or two at idle at stop lights. Fair, the lil' fella was probably clearing its throat—I should continue driving it regularly and even give it the ol' Italian tuneup when possible.

However, this light bout of hay fever quickly turned into phlegm-filled flu. The car constantly misfired at idle once warm and gained revs in a similar stumbling fashion under load. Though, these symptoms never triggered any codes—I had some process of elimination to perform.

Internet research concluded that the most likely cause of this was a boost leak—the B5 S4 is infamous for having them at seemingly any mileage. After pulling off and inspecting various bits of intercooler and intake piping, as well as performing a proper, pressurized test, nothing stuck out besides a tiny leak in the passenger-side diverter valve.

Another culprit could be the mass airflow sensor. The easiest way to test this is to simply unplug it and see how the car runs—doing so didn't change anything besides finally getting a check engine light on the dash, so I ruled that out.

Up next, I replaced the N75 valve—the electronic solenoid that controls the turbos' wastegates—as well as the coolant temperature sensor. The latter sounds weird at first, but the B5 S4 community affirms that a failing factory unit will cause misfire-like symptoms. I also changed out the diverter valves for OE 710N units for good measure, a popular upgrade among S4 enthusiasts.

The car revved up slightly healthier and even made more audible boost noises! However, it still stumbled. None of this was for nothing, plus the parts were cheap and easy to swap. But I was still left scratching my head.

Up next was door number two in air, spark, and fuel: spark plugs, coil packs, and ignition control modules. I bought a pair of good-working-order ICMs from an eBay seller I trust and picked up six OE spark plugs from Pelican Parts for $30.

I could've tested for the ICMs with a simple in-line spark checker, in fact, the process of elimination here is quite painless. But for $6/pop, all I'd need to do is invest 30 seconds in swapping them over. Might as well have them on hand.

Plus, why not add them to my cart while I was already buying a gently used steering rack? More on that later.

I started off by changing the spark plugs. Even if they weren't the root cause, it'd still be wise maintenance on a car with no service history. The passenger-side bank was easy after removing the air box and loosening a nearby boost pipe, then, I moved onto the driver's side, which took a bit more time due to having less clearance, but it still went off without a hitch. The old plugs might've been the originals as they were quite cashed.

I also sprayed down the original ICMs and their accompanying connectors with contact cleaner for good measure.

The car fired up and there was no stumbling to speak of at idle! Not only that, but the brief amount of test driving revealed substantially reduced stumbling under load! Whether this was due to the plugs or cleaning off the ICMs, I'm relieved that the car's running much smoother.

Well, it is for now, at least.

A few weeks prior, I was peering underneath the S4 looking for the easiest and cleanest method for changing the power steering fluid.

But let me tell you, my S4 is a very proactive car. While out on a test drive it decided to spring a massive leak on the driver-side-end of the rack and spew out almost all of its fluid—thus saving me from having to undo any lines myself. What a nice fella. This kind of leak is only remedied by having the rack rebuilt, but I opted to buy a gently used, non-leaking unit from eBay instead.

When it showed up I got to work, and the job proved to be the most time-consuming I've ever undertaken.

First and foremost, there's barely any room to work. The rack sits inside its own little narrow portal against the firewall—that's right, behind the engine. It also must be disconnected, unbolted, and then tenderly pulled out from the driver's side wheel well. The incredibly tight space meant that undoing and later re-attaching the high and low-pressure lines, as well as the third mounting bolt, was an epic pain. I wrote up a quick how-to on what I figured out to be the easiest way to undo the third bolt and posted it to my ol' forum stomping grounds, VWVortex.com—a 90-degree driver was the hero of the day. No matter what I tried, I couldn't get to the bolt from underneath the car per other owners' instructions, possibly due to my car having a much larger automatic gearbox.

Word to the wise: there's no such thing as having too many new copper washers on-hand for the banjo bolts, and offset box-end wrenches were a lifesaver in retightening the lines. I wish I had them in the beginning. Finally, after several afternoons after work, lots of blood, sweat, tears, and more blood, I was done. If I ever need to do it again I'll be well-prepared and it'll probably go much, much faster.

Liqui Moly came through once again and provided a bottle of its Central Hydraulic System Fluid. After priming the system and running the engine to circulate the fresh green liquid, the car will hopefully be much happier having better-than-OE stuff coursing through its lines.

After taking to the street with a freshly restored power steering system, I finally got the opportunity to get the S4 thoroughly warmed up and put it through its paces and solve its stumbling once and for all.

I put the old diverter valves that came with it back in, though this time added hose clamps to their teeny vacuum connections for added peace of mind. The car seemed to accelerate slightly better but was still a little choppy.

So, were both of my brand-new OE 710N diverter valves junk? After playing musical chairs with both the originals and 710Ns, turns out that just one of the 710Ns was faulty. The car ran the best ever under my ownership with the good 710N and one original.

In fact, it ran so flawlessly that the charge pipe that connects the passenger side turbo and intercooler popped off—a great sign! To me, this means the car is boosting healthily. After quickly reconnecting it and taking it for another test drive, the car drove quite strongly. What a relief.

It's annoying that these issues popped up, but that's what happens when you take on a complex older German car with no service history and some clearly more-deferred-than-I-thought maintenance. As much of a head-scratcher (or sanity destroyer) that this car has been so far, I'm happy to be getting a lesson in all things hydraulic power steering and forced induction, as it's made me a more well-rounded, prepared, and generally knowledgeable wrench.

I'm hoping to drive my S4 pretty regularly after doing the timing belt job. Possibly even utilize it as a road trip steed this summer. Better it all happens now, rather than when I'm on the road far, far away.