Latest News:

autotechnician - serving the independent workshop - logo

Engine management: diagnosing the basics

By autotech-nath on March 25, 2018

Autodata investigates a vehicle that has failed an emissions test but presents no fault codes or drivability issues.

A common misconception amongst motorists is that vehicle diagnostics is easy. Many think it’s as simple as connecting a tool that tells the technician exactly what’s wrong. Of course, it’s not that straightforward! Trouble codes derived from the onboard diagnostics (OBD) only indicate the problem area and further diagnosis must always be carried out to isolate the actual cause.

Despite this, diagnostic trouble codes remain an important starting point and are required by many tools to start the fault-finding process. But what happens if you have no trouble codes – where do you start then?

For example, a five-year-old vehicle that had covered approximately 40,000 miles failed the exhaust emissions test – the engine malfunction indicator lamp was not illuminated, and no driveability issues were evident.

With the vehicle in the workshop and connected to an exhaust emission analyser, the carbon monoxide (CO) and hydrocarbon (HC) readings were found to be higher than the specified pass rate allowed, indicating a rich mixture. As this commonly indicates an engine misfire, high fuel pressure or leaking injectors, the misfire counter data was checked first using diagnostic equipment, then the oxygen sensors.


The oxygen sensor signal upstream of the catalytic converter was fluctuating correctly within the 0.2-0.9V range and the oxygen sensor signal downstream of the catalytic converter was steady at approximately 0.4V. Values showed no signs for concern and were well within specified parameters. Next, the fuel pressure and fuel holding pressure were checked; again, all values were well inside specifications. While in the vicinity, and as a precautionary measure, the intake system was also checked to rule out any potential future issues.

After analysing all the data presented and having been reassured by the absence of trouble codes, the technician now needed to explore other potential mechanical issues. The spark plugs were removed and showed no obvious signs of wear. The engine compression was then checked as low cylinder compression should not be ruled out as  a source of incorrect emissions. This revealed that each cylinder was producing approximately 130 psi (8.9 bar) of pressure, but would this test be sufficient to rule out any internal engine problems?

The engine cylinder leakage test is an excellent option to see how well a cylinder is sealing and pinpoint any problems. It is performed on a static engine by first positioning the piston of the cylinder to be tested at Top Dead Centre (TDC) and then filling the cylinder with air to measure any potential air loss.

An acceptable air loss of five to ten percent indicates an engine in good working order, but the engine can still run fine with a reading of between ten and twenty percent. Anything above twenty percent of air loss, however, indicates that it’s time to investigate. The percentage of air loss should also be consistent across the cylinders, with any great differences indicating a problem in that cylinder.

As a guide:

  • Air leaking into the intake system indicates an intake valve fault.
  • Air leaking into the exhaust system indicates an exhaust valve fault.
  • A whistling noise or air leaking out of the Positive Crankcase Valve (PCV), oil filler cap hole or dipstick tube usually indicates air that is escaping past the piston rings; suspect piston ring or cylinder wall wear.
  • Bubbles in engine coolant or coolant being expelled out of the expansion tank could indicate air escaping into the coolant past the head gasket, cylinder head or cylinder walls. 

    The cylinder leakage test in this case showed one cylinder to have sixty percent air loss, whilst the remaining three showed readings well below twenty percent. When the cylinder head was removed and disassembled it was clear leakage had occurred past the valve seats and re-seating was implemented. As a safeguard, the cylinder piston was also removed; signs of partially seized piston rings were evident, so these were removed and replaced. Once reassembled, the emissions were re-checked and the vehicle passed the inspection with all readings well within the parameters.

    These simple tests identified a mechanical fault and ruled out components unnecessarily being replaced. In this case, the technician possessed the expertise to follow the correct path without fault codes to guide them – but access to an accurate, OE-manufacturer verified and trusted database of vehicle issues and known-fixes would have substantially reduced the time and cost spent on the repair.

    As always, the quality of a workshop’s diagnostic tools is crucial. In addition to verified trouble codes, Autodata’s diagnostics assistant, Assist Me, provides workshops with systematic guidance as they carry out daily diagnostics. If a solution to a specific problem has already been identified and tested, this will immediately be suggested to the technician. If there is no known solution for the problem, the technician can use Autodata’s technical information to test the possibilities, identify the fault and fix it.

    Autodata’s workshop application for professionals has thousands of verified diagnostic trouble codes and explanations. You can try Autodata’s diagnostic assistant for yourself, visit



About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
Aftermarket Media Solutions Ltd, The Joiners Shop, Historic Dockyard Chatham, Kent ME4 4TZ
T: 01634 816 165
Company no. 09625886
linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram