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At least the parts are cheap.

Earlier this year I picked up a 1991 BMW 318i sedan to use as my track day and autocross beater for the summer. I fixed a few things and upgraded some parts before taking to its first track day at New Jersey Motorsport Park earlier this month, which went well. The car did a 600-mile weekend that included a drive to Pocono Raceway for a NASCAR race, which also went well. Then things started to go downhill. After Pocono, the car began to idle poorly. I thought I fixed the problem, but now it runs even worse.

It started after I got back from Pocono. The 2.5-hour long drive was seamless, the car happy to cruise at 80-plus mph at 4000 rpm the entire way. It was only when I got back to my home in New York when I realized the 1.8-liter’s idle was about 100 rpm lower than usual. It caused the engine to vibrate a bit more, occasionally causing the check engine light to illuminate. At times, the idle would become so uneven that I’d have to keep my foot slightly on the gas to keep it from dying.

The car could still run and drive with a bit of finesse, so I ignored the problem for a few weeks. Once I did decide to diagnose what was wrong, it wasn’t hard to find the issue: A massive vacuum leak caused by an old, crumbling hose. It connected the idle control valve to the intake manifold, and was cracking severely at each end. You could even hear the hiss of the vacuum leak coming from the hose as the car idled. This was clearly the problem.

While I didn’t have a direct replacement sitting around, there were a handful of very similar-looking hoses in my toolbox leftover from my S2000 project, so I decided to cut one to size and fit it into place. Hilariously, the car ran much worse after I installed my home-made solution. The car couldn’t even stay running, much less hold a steady idle.

Frustrated and convinced I had solved the hose problem, I began to assume it must’ve been the idle control valve that went bad, as they tend to start failing if not properly cleaned. I took the unit out of the car, drenched the inside with throttle body cleaner, and threw it back in. To no one’s surprise, that didn’t change a thing. The car would still struggle to idle, and often die on its own.

It was around this time my dad suggested I throw the old, crumbly hose back into place to see if that would improve things. So I did, and lo and behold, the car ran a lot better. It wasn’t perfect, but it was better than when I had the new hose on. So after about two hours, I was essentially back where I started. Sigh.

Upon further inspection of the hoses I found the new, home-made unit to have a much narrower inner tube versus the OEM unit. My guess is the new hose wasn’t allowing enough air to enter the engine at idle, causing it to run poorly. While the old hose isn’t in great shape, it allows enough air in... albeit with a bunch of leaks.

So I decided to leave the old hose on until a new OEM replacement arrives in the mail. In an attempt to keep the idle relatively normal, I taped both ends to eliminate at least some of the vacuum leaks. It should hold just fine until the new part goes in.

Stuff like this is why working on old cars is exciting. Not only do you learn about how a car functions, but you also get to hone your skills by learning how to craft parts and solve problems even when you might not have the right parts or tools at your disposal. Sure, situations like these might be stressful in the moment, but you come out of them more knowledgeable and better equipped to handle future issues.

In addition to futzing with the idle, I want to finally put in new rear shocks, which should be easy considering it’s just three bolts. After that, I want to figure out why my air conditioning isn’t working. Stay tuned.