A4 Puzzles

Today's post covers the pitfalls of simply swapping out parts hoping you find the thing that doesn't work along the way.... by way of describing how we got T's A4 running again. You may remember my mentioning the purchase in this post from April.

First, I should explain how his A4 got it's name: Nemo. It is named after the orange fish cartoon character because the B5 A4 had the big driver outside mirror and the tiny passenger side mirror - just like Nemo and his little fin.

Coolant Leaks
It all started with a persistent issue with the Nemo's cooling system. This model, known as the "B5", has many of the same engineering ideas that the TDI engine in Hapy and Flash possess. There's the hard-to-reach plastic outlet flange with a coolant temperature sensor in it. It has the thermostat stashed under a plastic housing. Either or both of these can fail during an overheat. In Nemo's case, both failed. So, we replaced them. T had filled the engine with tap water (to save his wallet and the environment from leaking coolant) and took a test drive around the neighborhood. He drove until the thermostat opened up and then drove it back up onto the ramps to check for leaks and to drain and fill the cooling system when it cooled off... and then it wouldn't start again.

We discovered that the coolant temperature sensor wasn't good (Check Engine Light throwing a coolant temperature code), so we replaced it. That sensor was bad out of the box (thanks Napa!), discovered through a check engine light code and the temperature on the dashboard showing over 150*F first thing in the morning after sitting in the driveway all night. That needle shouldn't have moved.  So, we got another one from the only-on-the-east-side Discount Auto Parts. With that replacement, we no longer had a check engine light and the temperature needle was appropriately sitting on the peg when we first turn the key to run. Perfect. Except it still wouldn't start.

Could It Be Fuel?
First, T thought it was fuel-related, so he started with the fuel filter. These are half hidden around the fuel tank, so you need to partially lower the tank to get to the filter. Fun. Because they are so hard to get to, they're not replaced regularly enough, so this looked like a pretty likely issue. Even with the filter replaced, though, it wouldn't start. We did notice that the fuel pump wasn't as loud, so that filter probably needed a change. Next, we tried to check the fuel pressure with a tool borrowed from AutoZone. Unfortunately,the A4 1.8 Turbo engine fuel rail does not have a facility for testing the fuel pressure, but we were able to produce a stream of fuel at the injector rail once we disconnected the fuel line. So, the pump was pushing fuel, just we couldn't tell at what pressure. At this point, T elected to buy a new fuel pump as a precaution and replace the injectors. The pump hasn't been replaced yet at this point, but we're still not running. We could smell fuel during the start attempts, so we started thinking maybe it wasn't fuel... maybe it was spark.

Could it Be Spark?
We started with the basics: are any of the fuses related to engine control blown? No, none of them are blown, and they were all the right amperage for the fuse-box numbered spot based on the owners manual. How about the spark plugs? Ew, they looked gross, so we replaced them with a set of Bosch, the same manufacturer of the plugs we pulled. I know spark plugs are a religious thing. I tend to get plugs from manufacturers that are from the same country-of-origin as the car, so Bosch for German, NGK for Japanese, Champion for US... it's worked so far. I'm sure there's a "right" way, but with all the religious fighting, the "right" way has been lost in the noise. Anyway, new plugs didn't do it.

So, we tested the coil packs. At this point, one of T's friends was over, and he found a site on his phone that detailed the correct resistance range between the various pins on the coil packs. All packs were within the right range. If I learn the site we used, I'll post it on here.

Then, I found a mention on the Audizine site that the crank position sensor (CPS) can fail, but not trigger a check engine light. When this happens, the engine won't start because the computer doesn't know when to send a fire signal to the corresponding coil pack. Makes sense. So, I ordered an OEM one from AutohausAZ. The removal of one of these is a bear if you don't remove the oil filter first. With the oil filter out of the way, it's easy with a small Allen wrench. With it in the way, it can come off, but getting a wrist in there with an Allen key can test your patience. The new CPS didn't make a difference though. After our experience with the bad-in-box coolant temperature sensor, I was not convinced the CPS swap was a success.

We pressed on. The Audizine posts also described simply re-setting the ECU by removing a cable from the battery. Yeah, that didn't work, but I discovered that the ECU on this A4 is not the original, which implies that maybe the PO chipped the car and didn't share that. Or, maybe, he had a similar issue, fried his ECU and bought a junkyard ECU. That's not a bad thing, necessarily. It's just one more variable.

Ultimately, the issue was a bad ground for the bar that runs along the coil packs. In this particular A4, there was a separate ground wire run from the negative post to 2 bolts, one on each end of a metal bar. Over time, they had loosened up. Once they were tightened down, the car fired right up. To be fair, the coil packs are now throwing codes, so they will need to be replaced, but the car runs again.

That's today's adventure. Thanks, as always, for following along-