Possible to disgnose a modern petrol non starter at home ?
Discussion
To be honest I'm more used to fixing petrol engines with traditional electrics, distributors and carbs.
So when it comes to modern petrol engine is it possible to identify why a car won't start with simple tools and logic as a home mechanic even though it's got sensors all over the place or does it need the professional analysers that garages use ?
As a hypothetical example, lets assume I have a '51 plate Astra 1.6 petrol engine car that now refuses to start yet last week it ran fine. Lets assume the battery is fine & fully charged, all electrical connections are sound, there's fuel in the tank and it spins over but just won't fire up & run.
If it's possible, what would be the sequence of tests you would follow if you had this problem at home to try to find the fault - other than tow it to a garage or call the RAC ?
So when it comes to modern petrol engine is it possible to identify why a car won't start with simple tools and logic as a home mechanic even though it's got sensors all over the place or does it need the professional analysers that garages use ?
As a hypothetical example, lets assume I have a '51 plate Astra 1.6 petrol engine car that now refuses to start yet last week it ran fine. Lets assume the battery is fine & fully charged, all electrical connections are sound, there's fuel in the tank and it spins over but just won't fire up & run.
If it's possible, what would be the sequence of tests you would follow if you had this problem at home to try to find the fault - other than tow it to a garage or call the RAC ?
Krikkit - You're probably right, but it's a shame the home mechanic can't at least try to find simple faults like sensors going faulty without the need for expensive analysers.
My son did buy one off eBay a year or so ago to try to identify an ABS sensor fault on his Clio. It didn't help. So he asked a local garage to use theirs and nothing showed up on theirs either although of course the ABS light was still on. They said that not all codes can be picked up by and that it was likely it needed a main dealer analyser to find it.
My son did buy one off eBay a year or so ago to try to identify an ABS sensor fault on his Clio. It didn't help. So he asked a local garage to use theirs and nothing showed up on theirs either although of course the ABS light was still on. They said that not all codes can be picked up by and that it was likely it needed a main dealer analyser to find it.
Diagnostics is a skill in itself, somebody knowledgeable could probably diagnose most faults with a suitable electrical multimeter and some known sensor values.
Ive got hundreds of pounds worth of diagnostic tools but 9 times out of 10 they wont tell you the exact code or problem, so some further digging is required. Some are better than others but you still need the knowledge to be able to interpret the data.
Ive got hundreds of pounds worth of diagnostic tools but 9 times out of 10 they wont tell you the exact code or problem, so some further digging is required. Some are better than others but you still need the knowledge to be able to interpret the data.
Let's start with the basics
Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
Let's start with the basics
Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
PaulKemp said:
Let's start with the basics
Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
This.Is fuel getting to the combustion chamber, crank then whip out a plug and check its damp or smell of fuel
Is there a spark, plug out, lay on engine metal part crank and look for spark.
If ok check compression, should all be close to equal.
Any sensor or ECU problem would stop fuel or spark
Regardless of all the electronics on it, it is still a petrol engine and needs fuel, spark and compression to run.
Check it for fuel at the fuel rail, check for a spark and then check for compression. That will at least tell you if you are looking for a fuel, ignition or major mechanical fault.
All of the above should be possible with basic tools and borrowing a compression tester, or buying one, they are cheap enough.
Agree, that's the same as any petrol engine, so if there is fuel in the rail and compression but no spark from any plug where would you go from there ?
There are a variety of sensors and lets assume in this hypothetical discussion one of them has decided enough is enough.
Presumably to start an engine, there are some sensors that must be working while others are there to make the engine run better once it's running.
So what I'd like to know is what are the important sensors to check that could stop it from starting.
I assume the Crank Sensor is one of them, and if it's petrol supply related presumably there is a sensor to confirm the fuel pump is running.
Any others that would stop it sparking ?
As a home mechanic admittedly there is a limit to what you can do with a reasonable set of tools and a meter but it would be worth knowing which sensors to check before admitting defeat and asking the local garage to fix it.
There are a variety of sensors and lets assume in this hypothetical discussion one of them has decided enough is enough.
Presumably to start an engine, there are some sensors that must be working while others are there to make the engine run better once it's running.
So what I'd like to know is what are the important sensors to check that could stop it from starting.
I assume the Crank Sensor is one of them, and if it's petrol supply related presumably there is a sensor to confirm the fuel pump is running.
Any others that would stop it sparking ?
As a home mechanic admittedly there is a limit to what you can do with a reasonable set of tools and a meter but it would be worth knowing which sensors to check before admitting defeat and asking the local garage to fix it.
Edited by billm on Thursday 12th March 11:20
As I said above, with the correct manual & a meter you can do pretty much everything.
The problem is lots of manufacturers sensors/systems are calibrated differently.
Most temperature/pressure sensors are just resistances variable wrt the input, but without the info from the manual it becomes difficult to predict exactly what is right & what is wrong.
Most position/speed inputs are triggerred by keys/magnets giving pulsed inputs, but requiring power.
There are so many of them you really do need a proper manual, otherwise you just end up changing things until the problem is fixed/you run out of money/motivation/time.
The problem is lots of manufacturers sensors/systems are calibrated differently.
Most temperature/pressure sensors are just resistances variable wrt the input, but without the info from the manual it becomes difficult to predict exactly what is right & what is wrong.
Most position/speed inputs are triggerred by keys/magnets giving pulsed inputs, but requiring power.
There are so many of them you really do need a proper manual, otherwise you just end up changing things until the problem is fixed/you run out of money/motivation/time.
billm said:
Megaflow- Ah right thanks, I didn't know a failed crank sensor would do that. That's worth remembering.
That's what happened on my wife's Pug 307. Drove onto the drive one evening absolutely fine & wouldn't start the following morning. No spark & plugs dry. Towed it round the corner to the local garage who just happen to be an indy Pug specialist & their diagnostic tool showed it up straightaway.As far as manuals are concerned I buy the Haynes for the cars we've, only exception is my RRC & I've got the factory manual for that as well as the Haynes.
As has been said above, ignore the fact it has sensors its still basic air, fuel, spark.
You say it has fuel in the tank but do you know its getting through? Is there a spark? Without a code reader you could start by replacing the cheaper/easier parts i.e. plugs & fuel filter. Crank sensor failure could be an option (happened to my wifes Corsa in the outside lane of the M25
) but my initial money would be on a failed fuel relay.
You say it has fuel in the tank but do you know its getting through? Is there a spark? Without a code reader you could start by replacing the cheaper/easier parts i.e. plugs & fuel filter. Crank sensor failure could be an option (happened to my wifes Corsa in the outside lane of the M25
) but my initial money would be on a failed fuel relay. Haynes do a book on engine management that is ok.
It has a section on em theory. Photo section of a limited range of cars, some good sections on diagnosing/ testing.
Also how to test individual sensors etc.
A decent fault code reader is a very good idea too as it can help narrow down fault sources.
It has a section on em theory. Photo section of a limited range of cars, some good sections on diagnosing/ testing.
Also how to test individual sensors etc.
A decent fault code reader is a very good idea too as it can help narrow down fault sources.
If the car is a keeper get the Bentley manual for it, they are the dog's dangly bits.
Complete with FULL testing details for ALL sensors/electronics etc.
Yes, they are expensive, but Haynes are only worth buying for mechanical jobs, their coverage of electronics is almost non-existant.
Complete with FULL testing details for ALL sensors/electronics etc.
Yes, they are expensive, but Haynes are only worth buying for mechanical jobs, their coverage of electronics is almost non-existant.
billm said:
so if there is fuel in the rail and compression but no spark from any plug where would you go from there?
I'd start with the coil pack and work from there... You seem interested in delving into all sorts of sensors when all the signs shout a coil fault! Unless you've already been there and changed that, in which case... I'm out! I'm no good with electrics!
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