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A quick fix is nice. Just tell me what part to replace, sure that's pretty painless. Sometimes this is possible for a routine and painfully obvious problem like say the brake pad indicators are screeching on pads with 50,000 miles on them and you replaced the same pads at 45,000 miles previously, the rotors are smooth and undamaged, and there is no excessive heat at the wheel with the noise, and the steering is not pulling and a quick look reveals no meat visible on the pads. Order up some of your favorite brake pads and set Saturday morning aside for a brake job. You can still be wrong, it could be the hub/bearing, sticking caliper, glazed pads, etc. but ordering and throwing this part at those circumstances probably makes sense.
On more complicated problems with many possible causes with expensive parts and a lot of labor involved, Trynostics and the parts cannon can be very painful, time consuming and expensive. I see so many threads here where the OP starts off with a list of everything that has already been replaced and no joy. Sometimes this can actually make things worse if working AC Delco parts are replaced with inferior after market replacements or the installation is sub-par. . I recently helped someone with a system function problem. Before we started, all the parts involved were replaced and the problem persisted, no joy. After a diagnostic process we found the problem, an open wire between two of those replaced components so in this case the repair did not involve any parts but rather a wiring repair. This could have been discovered at the onset with proper diagnosis. There was no signal into the relay but there was a proper signal out of the previous component, a classic wiring fault. Then you test for continuity and short to gnd (and power if stuck high). With a wiring diagram, a meter and a few skills this could have been determined in under an hour even with some dash bezel removal. Now I believe that the OP has a better appreciation for this targeted approach. I spent time helping another OP recently with a complex problem who was ready to give up because their truck has been unreliable for 3 years. That's a long time to be living with a truck that runs bad and can break down at any time. The toughest problems are intermittent electrical issues and unspecific engine problems like rough idle. There are so many possible causes and if it does not do it all the time then additionally, you have to create a strategy to catch it in the act. Travel with a fuel pressure gauge and spark tester so when it won't start in the Walmart parking lot you can find out if its spark or fuel pressure and now you have cut the possibilities in half.
For something like lack of power or rough idle the cause can be one of many systems, such as:
You start with trouble codes and a full list of symptoms. Then do some research to find out every possible cause for that problem. Shop manuals, the Google, or come here and ask. Do any of the clues suggest where to start such as misfire cylinders or fuel trims. Then keep an open mind and work your way through each area with a top level check first. Don't get stuck on a bad assumption, spending weeks trying to find/prove that assumption. Let the testing results guide you. Do you have spark at a few plugs, both banks? Is it strong and have consistent timing? If so then check ignition off of the list for now. Sure if you get off into the weeds with a rare or intermittent problem we may have to come back to this one but most likely we can rule this out. That just eliminated a whole bunch of possibilities. Ignition switch, PCM, ICM, coil, distr, wiring, crank sensor, ignition cables, ... If you had made an assumption that the coil must be bad without testing, now you may have a cheap aftermarket coil and the same problem. A fuel pressure test, spark check and compression test on at least any problem cylinders and you will most likely have your faulty system.
When a top level area does not test correctly then drill down through the individual possibilities until you find the problem. So, if there was no spark, is there spark out of the coil? If so then its between the distr and plugs. If not then check the power and grounds to the ICM and coil and then the ignition pulses to the ICM and out to the coil. If good power and ground and no pulses from the PCM then check the crank sensor, ... Then repair or replace the part or wiring that is confirmed to be faulty.
Same thing with fuel. Don't just assume a bad fuel pump, that job sucks and a good pump is expensive. Do the full fuel pressure tests in the sticky and check power and ground at the pump. If you have insufficient pressure at the fuel filter connector and good power and ground at the pump then a fuel pump replacement makes sense.
On electrical issues, get a wiring diagram and follow the signal or power flow from one end to the other (or the middle) until you find where the power, ground or signal is improper and then test to find out if its wiring or the part. So if your starter wont crank don't just replace the starter, or even take it out to be tested - yet. Trace the signal flow and test. Is there 12V out of the ignition switch and into the crank fuse, then the relay and the trans switch and on to the purple wire on the starter. As soon as you find the place where you don't have 12V as expected find out if its the component or the wiring and perform a targeted fix. If you have a call for starter on the purple wire at the starter solenoid and good clean power to the starter motor then take it out and have it tested.
The last and very important step is to verify the fix. Retest that function and see if the values are as expected after the repair. There was no power on the purple wire at the starter and your testing narrowed it down to the trans neutral switch. Replace the part and then remeasure at the the starter purple wire. If you replaced the fuel pump, retest fuel pressure tp confirm.
This is a much better way to work for complicated problems. Its almost always cheaper and faster unless of course you get lucky on a Trynostics guess but that's not likely to be your outcome. You don't work on 50 cars a month so your educated guess is not very educated. All of the best mechanics work this way. The GM shop manuals work this way. Fancy scanners with trouble shooting guidance work this way. I work this way. A great example is South Main Auto on you tube. Watch Eric O. chase down something vague like a no start condition with a logical flow - scan for codes, what was the customers specific complaint, go for a road test if it runs, is there spark, is there fuel pressure, compression, etc then drill down when a top level system is not working. Scanner (Paul) Danner is another good example. Both are excellent, Eric is more of a mechanics mechanic. Paul is a teacher and a little more academic.
If all else fails, here is your diagnostic flowchart:
Just happens that we got another car to work on with no symptoms but smoking from the right bank (grey/blueish - rich idle).
Proper diagnostics started:
- Fuel, fuel pressure and regulator - checked o.k.
- spark at every sparkplug - checked o.k.
- exhaust gas tester says - left is o.k. right is odd.
checked with dedicated tester, somebody has turned down the idle trim on the right bank to excessive low levels to pass emissions testing and why the readings on the exhaust gas tester were odd.
The previous mechanic never really diagnosed and addressed the problem. They just worked around it.
Turns out 1 injector is leaking at cylinder no. 2. Properly diagnosed in 2 hours (had to remove the plenum to get at that - PITA).
Essentially exactly the way you suggest to solve a problem.
10-05-2021, 01:58 PM
Diagnostic and repair work flow
