Nothing super crazy here, but a fun one nonetheless. So let’s jump right into it!
The subject: 2007 E63 AMG
The complaint: engine surges while idling, but only when hot, CEL not illuminated, runs well otherwise
Not actually the one pictured above, but you get the idea. This one was towed in to me last fall. It had hydrolocked due to leaking injectors. But it’s tuned and has long-tube headers. It sounds glorious. Well, at that time I had the engine out, replaced the head bolts, made sure the bottom end was good, all new plugs, injectors, cams, lifters, and a bunch of other things. Basically the engine should be bulletproof now. Every common problem was taken care of, and then some.
So why is it running poorly now? Well it’s got a thermostat fault code, and a bunch of oxygen sensor fault codes. But none of those will cause a surge. The thermostat will be covered under parts warranty by the way. The oxygen sensor faults could be caused by the surge and/or the tune/headers. So fault codes are unhelpful.
Mixture adaptation data looks slightly rich on one bank and slightly lean on the other. However both sides are almost neutral, well within normal. Again, not helpful.
So, what’s the issue? Well it sounds and acts like a large vacuum leak. Mixture adaptation data should show this, unless it’s such a recent problem that it hasn’t adapted yet. So quick, easy check; spray brake clean around the engine at idle to see if it surges! Cheap, easy, fast!
Fine. Break out an actual tool. The smoke tester! Plug the intake, inject smoke through a vacuum hose. Still nothing. Check the vacuum reservoir, still nothing. Even tested the engine for a solid twenty minutes. I didn’t find smoke, but the gauge on the smoke tester didn’t drop to 0, which it should if there truly were no leak. On the other hand, it didn’t show full loss either.
I had to be missing something...
The brake booster! I’ve honestly never seen one fail on one of these. Brake booster failures are quite rare. Notice how the gauge on the tester is showing full loss (little red ball at the top, no leak and the ball would be at the bottom). There was so much smoke pouring into the cabin it looked like something was on fire!
It always feels good to find a smoking gun when diagnosing a problem.