Constant pinging

Blackmetal2088 said:

Hello, I have a 1999 Mercury Grand Marquis... it pings especially under load. ... It is ... Getting 27 mpg on the highway ... the pinging is driving me nuts. ... So, what gives any ideas on what it could be?

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I re-read your first post, Black, and something jumped out at me — your mileage! 27 mpg while an attractive performance may well be because the engine is running lean. If that is true, then your problem either lies in the fueling or in the tune. If the car has the original OEM tune and the engine is unmodified stock, then the origins of the problem are fueling-related. The lean mixture gives you both mileage but also detonation sensitivity.

I would begin chasing down fuel system sensors and check your pump's in-tank plumbing for possible cracks in the plastic tubing or PPRV that could permit a pressure leak. A quick and dirty test of the plumbing would be to monitor your idle fuel pressure with a pressure gauge screwed in where your Schrader valve is located. Let the engine come down to idle for a few moments and then shut it off. Watch the pressure gauge on the rails. It is supposed to maintain that idle fuel pressure at the injectors for easy and quick starts. If the pressure bleeds down from the idle pressure, that is telling you there is a potential leak in the fuel system that you need to find.

Under normal driving conditions, a leak will drop Ford's base fuel pressure below the 39.15 psi differential pressure across the injectors, that the ECU looks for to properly fuel the engine. The OEM fuel system uses a manifold vacuum-referenced Fuel Rail Pressure Sensor (FRPS) to maintain the 39.15 psi Ford uses as their base fuel system pressure. The FRPS senses manifold vacuum so the ECU can calculate the fuel pump speed required to maintain a continuous differential pressure at the injector. That pressure in an unmodified fuel system is always the 39.15 psi Ford uses as their base fuel system pressure.

For example, if your engine pulls 15 inches of vacuum at idle, the FRPS will see this from the vacuum reference signal it picks up off the manifold. When it senses the high manifold vacuum (idle conditions), it will signal the ECU, and the ECU will reduce the speed of the variable speed fuel pump(s) that Ford uses to maintain the 39.15 psi differential pressure across the injector nozzle. The net effect is the engine will idle with perhaps 30 psi or so of fuel pressure, not the full 39.15 psi. The differential pressure will still calculate out at 39.15 psi. By reducing the fuel rail pressure, the ECU will maintain the 39.15 psi differential pressure across the injectors. However, as soon as you open the throttle and intake manifold vacuum drops (pressure rises towards atmospheric), the FRPS will signal the ECU for more fuel system pressure to again return to the target 39.15 psi differential at the injector.

If there is a compromised (leaky) section of the soft fuel line in your fuel system or a leaky PPRV, then the pumps will not be able to meet the 39.15 psi base fuel system operating pressure the ECU is attempting to maintain. At that time, your mileage will begin to increase because the engine is running lean. The other thing that will occur is the engine will become knock-sensitive to even slight increases in load from throttle position or road conditions. The engine operating temperature will also increase, and if you are going up an incline, you will begin to notice increased engine temperatures and eventually knock.