Craig Shippy explains how to quickly diagnose fuel pressure fault codes. Things aren't always what they seem.
Vehicle: 2009 Audi S4 4.2 V8
Symptoms: Poor performance, low power
- P0087; low fuel pressure
- P1573; left side engine mount solenoid valve open circuit
Checked fuel pressure. Fuel pressure low on high fuel pump. Replaced both high pressure fuel pumps at a tune of 500 bucks each. Still had the same fault codes for low fuel pressure and that useless engine mount fault code. Cleared the fault code and fuel pressure came up to spec, then dropped off with check engine light on. Got tired of seeing the engine mount fault code, so replaced it. Only 400 bucks. Did not want to see that fault code any more. Installed the engine mount and went to lunch at 10 AM due to frustration. Came back from lunch, cleared fault codes. Mind now clear after half a pack of cigarettes. Started the car while looking at the fuel gauge. Fuel pressure spot on even after 10 minutes of running. Started scratching my head, jumped in the car and went on a road test as the beefy V8 bellowed down Main Street. Fuel pressure did not budge. Solid as a rock.
Drove back to the shop puzzled. What fixed the problem? Did the mice in the shop feel bad that I had worked on the car for a week and got nowhere?
Finally, it hit me. I put the car back on the lift and unplugged the engine mount. BOOM the fuel pressure dropped. It was the engine mount all along.
Do not overlook a bad engine mount fault code. Engine mounts are monitored by the engine control module (ECM). If the ECM detects a bad mount, it cuts fuel pressure to prevent that big V8 from jumping out the engine bay. The mounts react to throttle angle and hard cornering differently. Now that’s a great way to keep everything bolted down in the corners.