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I've recently noticed when the engine's cold (not noticed when warm, but not really looked for it), if I come off the throttle and use engine braking in 1st or 2nd, I can hear a rattling/clanking coming from the exhaust in a series of irregular distinct pops. It could well be fuel igniting.

I've also noticed when going downhill in gear I'm getting about 50mpg instead of 99.9mpg so I'm wondering if it's something causing fuel to be pumped into the engine when there's no spark to ignite it in the cylinder...

I'm thinking it could be a dodgy throttle position sensor? That could cause the problem I guess, but it'd also mean the idle control valve wouldn't activate, so it'd stall all the time! Unless the ICV's stuck open?

Ideas?

Umm, if you can get the throttle position sensor off and rev the engine with the tps at idle, it should cut the injection off when it goes too far (I can't remember how far but my BX did this, say 2k?). I'm having to think back about 12 years but as far as I remember without the idle switch it hunted about loads.

Might rip the TPS out tomorrow and hook it up to the 'scope via a battery, see if it's ok

Could be worth checking the cable adjustment just to see that it really is OFF when you lift off.

on a L3 V6 when running open loop, the ecu fires both banks off injectors on the overrun to warm up the cat quicker, hence the popping, it should stop when on closed loop, but may start again after an overrun of more than 120 seconds (guess) to stop the pistons melting

James just made my post look like a girls

Not to worry, James is by way of being an expert, if we all knew as much about these cars as he did he'd be out of a job

jameslxdt wrote:on a L3 V6 when running open loop, the ecu fires both banks off injectors on the overrun to warm up the cat quicker, hence the popping, it should stop when on closed loop, but may start again after an overrun of more than 120 seconds (guess) to stop the pistons melting

open/closed loop? as in thermostat open/closed?

Hang on, it tries to warm up the cat by igniting petrol inside it, and just hoping it doesn't explode?

he talking about the engine control. When cold they use an open system (ignoring some sensors) when hot they use all the sensors.

I think thats about all i can remember from my control module(s) at uni, wasn't very good at it!

thats spot on, closed loop is basically with oxygen sensor monitoring, open loop is without the 02 sensor

as for the fuel burning in the cat, i dont think it does actually explode, if it did it would sound like a back fire, the popping is different style of combustion, not sure of the exact term

jameslxdt wrote:the popping is different style of combustion, not sure of the exact term

I think it's detonation if memory serves

That's pinking/pre-ignition.

steve_earwig wrote:That's pinking/pre-ignition.

spot on steve it used to be caused by the ignition timing being to far advanced and a simple fix of loosening the clamp bolt on the distrubuter and rotating slightly anti-clockwise would fix it totally irrelevant to todays engines but hey some of us remember older less complicated stuff

Or turning the knurled adjuster on the vacuum unit of a Lucas 25D4/6 (bet you don't remember that one tho )

I even remember setting the points !! ah the good old days no weirdo french
electrics and ecu's with more power than my first comp