Clicking sound could be due to piston slap

Hey there, time traveller!
This article was published 09/05/2003 (8201 days ago), so information in it may no longer be current.

QUESTION — I have a 1993 Ford Probe GT with a 2.5 litre V6. The car has 175,000 km’s on it, and it produces a loud “clicking noise” and runs really rough when I start the engine cold. However, the noise stops when the engine reaches normal operating temperature.

I have had the problem inspected by two mechanics, and one told me I needed a tune-up, and the other told me the clicking noise and rough running when cold was due to “piston slapping”. I had the car tuned-up (spark plugs, wires, PCV valve, air filter, fuel filter, distributor cap and rotor) and this noise and rough running persists when the engine is cold. What do you think the problem might be, and if it is piston slapping, what exactly is it?

ANSWER — Piston slap is a solid knocking sound heard when the piston is forced against the cylinder wall by the forces of combustion. Excessive clearance between the piston and the cylinder causes the slap to become much louder. After a minute or so, the piston expands enough to reduce the slap to a point we don’t hear it. Many vehicles will have a light piston slap when started cold but it wouldn’t cause rough running.

If your Probe has piston slap, the repair involves new pistons and rings and the engine block may need machining. Engine parts are expensive so the repair costs could be more than the value of the car.

Because of the work already done on your car and that two problems occur simultaneously, I suspect your noise may be caused by the valve train. When an engine is stopped, some of the engine valves are held open by the camshaft and lifters. The spring pressure that closes the valves can slowly force oil out of the hydraulic lifter so that when the engine is started again, there is excessive clearance in the valve mechanism.

Normally, this clearance is automatically taken up by the lifter but air trapped inside the lifter can take several minutes to bleed out. The lifter can’t take up the clearance and the valves don’t open as much. This can cause rough running.

There is an easy way to determine if the problem is piston slap or another noise. Have your mechanic short out each sparkplug wire at a time while the engine noise is occurring. This eliminates combustion loads on that piston. If the noise is caused by piston slap, it should stop or at least be reduced significantly. If the noise remains, then it is coming from elsewhere.

Another possible valve train problem is that one of the valves is sticking in its guide when the engine is turned off. With the valve stuck open, again there is excessive clearance in the valve train and a ticking sound. Compression on that cylinder would be low until expansion of parts allowed the valve to become free again, so the engine would run rough for a while. Replacing bad lifters would be worth repairing but fixing stuck valves could be as expensive as replacing pistons, so be sure to get more than one estimate before starting any repairs.

QUESTION — I have a 1989 Pontiac Firebird (2.8 litre fuel injected engine) with 185,000 km. On cold starts, the car starts fine, but after that, I have difficulty starting it. All fuel injectors have been replaced; fuel filter, PCV valve, spark plugs, rotor, distributor cap, wires and the engine timing set by a GM garage. Could it be the fuel pump?

ANSWER — The problem could be a fuel pump that binds when hot. Checking fuel pressure with a gauge would be a good first step. Another possible cause of the problem could be an engine temperature sensor that is out of calibration. Check the temperature readout with a scan tool to see if it is sending the correct values. If the temperature information is wrong, then fuel delivery is wrong and difficult starting can result.

A bad electrical ground between the engine block, the battery, and the body can cause hard starting when hot. Clean and tighten all ground connections on the engine and the body.

Finally, a faulty Mass Airflow sensor could be telling the computer to inject the wrong amount of fuel. Unplug the sensor and the computer will use default values. If it starts better, then the sensor may be bad.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca