Hot starting problem

[Mark.B]

My 172 cup has just developed a hot starting issue where it'll either not start/take a lot of attempts to start it or fire up with the limp mode warning light on.

It starts fine first thing in the morning and if its left for an hour or so but if I stop quickly for anything it'll play up. I've got a basic fault code reader which isn't showing anything and I've got as far as checking the connections on the throttle body and pedal after reading something about it on here.

Is there anything obvious or common to try before I go any further?

 

[clio_ross]

Potentially crankshaft position sensor, they normally like to cause hot start issues.

Worth a check to see if you’re car has to old black plug sensor or the updated blue plug sensor, as the old black ones like to play up.

Regardless, it would be worth whipping the sensor out and giving it a good clean, as they do tend to get furred up with fillings/rubbish from the bell housing, as they’re basically a magnet and can attract all manner of crud.

 

[Mark.B]

Thanks, I'll have a look later. Probably makes sense as the likely culprit thinking about it, pretty sure its black as I cleaned it off when I replaced the engine recently.

 

[Louis]

Yeah itll be crankshaft sensor

 

[Martin_172]

As above, I usually fit the cheap ones from the motor factors and have never had any bother.

 

[Mark.B]

It wasn't the crank sensor 😉

Got the car upto my mates workshop and plugged into a proper diagnostics tester and it came up with throttle pedal and throttle body circuit faults. I'll try giving the throttle body a clean first as I would guess the flap might be sticking when its warm.

 

[Louis]

It wasn't the crank sensor 😉

Got the car upto my mates workshop and plugged into a proper diagnostics tester and it came up with throttle pedal and throttle body circuit faults. I'll try giving the throttle body a clean first as I would guess the flap might be sticking when its warm.

Throttle body errors are often thrown up for removed carbon canisters/acoustic if it's missing these?

 

[Mark.B]

Carbon canister is still on, acoustic valve is removed and vacuum line is blanked off.

Its been like that for the 3+ years I've owned the car with no issues, its fine when cold or left to cool down which makes me think it could be throttle flap related.

 

[clio_ross]

It wasn't the crank sensor 😉

Got the car upto my mates workshop and plugged into a proper diagnostics tester and it came up with throttle pedal and throttle body circuit faults. I'll try giving the throttle body a clean first as I would guess the flap might be sticking when its warm.

Could also be worth a check and clean of the big white multiplug under the fuse box as well, whilst you’re under the bonnet. I’ve seen a few others on here have throttle pedal fault codes come up and eventually trace it back to the age old ’corrosion in the white plug’. Worth a try before replacing the throttle body or pedal, should the throttle body clean not resolve anything.

 

[JamesBryan]

Could also be worth a check and clean of the big white multiplug under the fuse box as well, whilst you’re under the bonnet. I’ve seen a few others on here have throttle pedal fault codes come up and eventually trace it back to the age old ’corrosion in the white plug’. Worth a try before replacing the throttle body or pedal, should the throttle body clean not resolve anything.

Probably this.

Very rarely do the throttle bodies and pedals go wrong on these.

 

[Mark.B]

Thanks, might try that before I start messing with the throttle body 👍

I also read something while searching the problem on here about the wiring between the pedal, ecu, and throttle body being the problem and it only happening on cups. Awesome 😂

I'll give the easy things a go first!

 

[Mark.B]

Still got this problem and I've gone through quite a few things now.

Checked the white plug and the ecu plug and can't see any corrosion anywhere.

Swapped the throttle body, 2 sensors on the inlet manifold, ignition coil. Luckily I've got a load of spares!

Going to see if I can borrow the throttle pedal off my mates 172 to eliminate that. Still not convinced its a wiring issue as you can leave the car for 15 mins and it will start straight away again. Its almost like something needs to cool down slightly 😬

 

[Mark.B]

Another update incase it helps anybody.

Stopped off at my mates workshop on the way home earlier to check the codes again now I've changed a few things and was still throwing up the throttle body/pedal fault code. He thought it sounded like it was cranking a bit slowly so we put a jump pack on it and that extra boost made it fire up straight away 😮

Looks like all this hassle is down to either a dodgy battery earth lead or the battery itself 😭