Strange Engine Starting Issue

[bearman68]

Implies it's not the fuel pump.

Did you say there were no fault codes?

Cheers

[bearman68]

Just had a really dangerous thought.

Remove the injector rail and see if it's spraying fuel under 'car no start' conditions.

There are some obvious risks in this, so do at your own risk

[glenmore3685]

Correct, there were no fault codes when I took it into the garage. I also haven't got a CEL since either.

Taking the injector rail off is probably a bit beyond my basic abilities, I guess the alternative would be to check a spark plug afterwards to see if its wet?

[bearman68]

Injector rail is only 2 bolts............

(Don't take my advice I'm in a bks mood tonight)

[Pumaracing]

Starting ok from cold but not hot can be due to overly rich mixture and failed coolant temperature sensor.

[glenmore3685]

Pumaracing said:

Starting ok from cold but not hot can be due to overly rich mixture and failed coolant temperature sensor.

Starting from hot is fine, its starting from a hot engine that cooled down for 10 minutes or more where the problem occurs. Once its completely cold again its fine.

[Pumaracing]

glenmore3685 said:

Starting from hot is fine, its starting from a hot engine that cooled down for 10 minutes or more where the problem occurs. Once its completely cold again its fine.

Ummmm duh! Of course a rich mixture will reignite immediately after shutoff when the combustion chamber is still stinking hot and able to ignite just about anything. If would stop running by itself in the first place if the mixture was so rich even a hot engine couldn't run on it!

Such an issue only "can" manifest when the combustion chamber has cooled down a bit for a few minutes but the ecu is still sending a full rich cold start A/F mixture down the ports because the temp sensor says the engine is cold.

[glenmore3685]

That makes sense, thanks for the explanation.

Will check the temp sensor, are there any obvious ways of testing if its faulty or is it just a case of swapping it for another one and seeing if that cures it?

Would cutting out due to a rich mixture normally generate a fault code on the ECU?

Edited by glenmore3685 on Tuesday 10th November 14:43

[Pumaracing]

You can check the temp sensor resistance at ambient and in hot water. A few thousand ohms cold, a few hundred hot.

The ecu doesn't know there's a problem if the temp sensor fails. It just thinks the engine is cold.

[sploosh]

Clocked over 200k in my Saab. They're great cars, but they do suffer from quite a few common problems.

My money is on the Crankshaft Position Sensor. If it is intermittent fault and you're not getting a warning light on the dash the diagnostics may not be showing the issue. DIY job if you're keen or I HIGHLY recommend Bagnall Saab if you're anywhere near Birmingham.

The other thing to do as a matter of course is to replace all the rubber vacuum hose with silicone. Easy DIY job and you'll save pain later on when you get odd faults as the rubber perishes..... possible your fault is down to vacuum leaks but in my experience leaks usually cause stalls when the engine is warming up ..... i.e. the first junction that you stop at in the morning.

Edited by sploosh on Tuesday 10th November 18:11

[glenmore3685]

sploosh said:

Clocked over 200k in my Saab. They're great cars, but they do suffer from quite a few common problems.

My money is on the Crankshaft Position Sensor. If it is intermittent fault and you're not getting a warning light on the dash the diagnostics may not be showing the issue. DIY job if you're keen or I HIGHLY recommend Bagnall Saab if you're anywhere near Birmingham.

The other thing to do as a matter of course is to replace all the rubber vacuum hose with silicone. Easy DIY job and you'll save pain later on when you get odd faults as the rubber perishes..... possible your fault is down to vacuum leaks but in my experience leaks usually cause stalls when the engine is warming up ..... i.e. the first junction that you stop at in the morning.

Edited by sploosh on Tuesday 10th November 18:11

Thanks for the advice, the garage didn't seem to think it was likely to be the CPS. They said if it was the rev counter wouldn't move when you go to start the car. Also I think a failing CPS would manifest itself in other ways rather than the specific scenario that my issue occurs in.

Edited by glenmore3685 on Tuesday 10th November 19:16

[glenmore3685]

Pumaracing said:

The ecu doesn't know there's a problem if the temp sensor fails. It just thinks the engine is cold.

Yes I understand that, what I meant was wouldn't a stall down to a rich mixture be picked up by the ECU?

[bearman68]

Pumaracing said:

You can check the temp sensor resistance at ambient and in hot water. A few thousand ohms cold, a few hundred hot.

The ecu doesn't know there's a problem if the temp sensor fails. It just thinks the engine is cold.

Hiya Puma Racing.
I had thought about the temperature sensor, but thought it would prob give a code as well. (Prob something like temp sensor range error).

[mighty kitten]

If we're talking about the Vauxhall engine then there is a Schrader valve on the fuel rail and the pressure regulator is on the end of the return line in the pump housing .