Morning Sickness – Why Is There No Assistance When Starting From Cold?
Symptom: This is the term given when the power steering doesn’t work first thing in the morning but after operating the heavy steering a few times power assistance is restored and the power steering will operate perfectly until the vehicle is again started from cold. Vehicles that suffer this problem often are Jaguar XJ40 and Saab 900 but the problem has been seen on Fiesta, Ka, BMW E36 and Rover 200/400.
Cause: The spool valve that directs the pressurised fluid is situated in the pinion assembly of the power steering rack. This valve body consists of a number of chambers separated by glass loaded PTFE square section rings. These rings act as seals between the different chambers. As the steering is turned these rings rub against the valve body/rack pinion housing. On a microscopic scale the tiny glass particles, which give the PTFE strength, wear into the material of the pinion housing.
On some racks the pinion housing is manufactured from cast iron and the wear that occurs is not noticeable however many racks are manufactured from aluminium alloys which is much less wear-resistant. Over time the glass-loaded PTFE rings wear groves into the pinion housing. When the power steering system is warm/hot the glass loaded PTFE rings are in an expanded state and create a seal despite the groves. When the power steering system is cold the rings contract and do not provide a seal. The pressurised fluid leaks between the chambers of the spool valve rather than being directed to the body of the power steering rack. The result is no power assistance until the rings re-establish a seal.
Solution: The power steering rack needs to be replaced. In order to make the power steering rack serviceable again during the remanufacturing process the pinion housing is bored over-sized and a precision ground steel sleeve is pressed and locked into the valve body to bring it back to the original size. The steel sleeve is much more wear–resistant than the original aluminium alloy and therefore the re-engineered power steering rack will not fail due to morning sickness in the future.