I enquired at both dealer and garage about diagnosing my car for being a bit sluggish and less peppy. Both said it usually takes one hour to find the problem\narrow it down for such a matter.

I'm just wondering what specific things typically get tested during this hour.

In my receipt it lists: Test drive, ignition, compression test I'm assuming ignition refers to spark, fuel injection and air intake.

I imagine they check air intake for any blockages, check spark plugs are delivering correct voltage spark, and fuel injectors are working. The compression test would be to check engine health. However can you break these steps down further? For example I imagine in addition to checking fuel injectors are working I imagine they would also check the fuel pump is working or if it is worn is this not possible? Timing is necessary for combustion so would they also check the state of the timing belt? With spark do they have a way to test the voltage be generated at each spark plug or do they individually test battery, ignition coil, plug wires and then inspect spark plug condition? seized brakes etc

Basically can you break down what specific things usually get tested in this hour?

  • No, we may guess, but to know exactly what was done you will have to ask them. The compression test for example should have given you the compression figures 1 for each cylinder - should be on the receipt... If they are, then it would have helped to tell us the values... – Solar Mike Dec 6 at 18:46
  • What car is this on? Unless someone broke out a scope I doubt all you suppose could of been done and likely it's exactly what the receipt says. They road test, test the secondary ignition system pull some plugs and do a cranking compression test. Maybe some other basics like checking the air filter or fuel pressure, codes, fuel trims etc... – Ben Dec 7 at 1:57

There are many things that can cause slugishness, including just 'feeling' and not actually being slugish.

Don't know exactly what THEY would do. Each place might check things a little different.

Here is where I would start -

  • Use a scanner. Check for codes. Check the sensor values to make sure they look correct. Check fuel correction values. Small corrections are fine, but larger corrections could point to something wrong.
  • Take a drive with the scanner and make sure the inputs and outputs look correct. Note anywhere things don't feel right. Does the transmission downshift correctly. Run to the red line without issues.
  • Check the air filter for blockage.
  • Check the plugs to see if they look dirty or worn.
  • Could run a compression test while they are out. Check that the values are over ~120 psi and all within 10%.
  • Check engine oil, transmission fluid, transfer case and differential fluid if equipped - level and how clean it is.
  • Check exhaust to make sure it is not damaged in a way that would restrict flow.
  • Check suspension, tire wear, and brakes. Could be something throwing off your alignment or brakes dragging.
  • Look for carbon deposits behind the throttle body, back of the intake valve (especially on direct injection engines), and on the pistons.
  • Might pull the valve cover to signs of sludge and to check the valve lash.

check spark plugs are delivering correct voltage spark, and fuel injectors are working

Can't really check coil output voltage. If there was a misfire code, they would swap coils to see if the problem follows the coil. If so, replace the coil. If not, they might swap injectors. An injector not firing should throw a code, or at least show up in the correction table.

I imagine they would also check the fuel pump is working or if it is worn is this not possible?

If there is a large fuel correction factor to add more fuel, they will check the fuel pressure.

Timing is necessary for combustion so would they also check the state of the timing belt?

Unlikely this would be the source of power loss. If they did check it, it would be under general maintenance.

A lot of this really depends on the vehicle, age, mileage, and previous maintenance. What you are really looking for is 'red flags' to work off of.

  • You forgot an important part. The MAF and/or the MAP sensors. Getting these air fuel ratio controlling (so malfunction=sluggish engine) things off and cleaning them with MAF sensor cleaner or residue free contact cleaner isn't hard and surely won't hurt. – Al_ Dec 6 at 19:59
  • 1
    Good point, If the MAP or MAF is having an issue, it should show up when running codes and looking at sensor values and correction factors. If the MAF is reading low, the ECU will need to correct by adding extra fuel. It might be reading slow when you rev up the engine. When you read the MAP values, it should show the calculated vacuum also. It should read around 20" at idle. This could also point to a vacuum leak if the MAP reading is off. – rpmerf Dec 7 at 13:13
  • Also, if it's a turbo engine, look for a boost pressure sensor and take care of it (cleaning or replacement) too. – Al_ Dec 7 at 17:51

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