Engine management fault diagnosis.....
Discussion
Right. I have a wonderful little 306, TU series 1360cc, multipoint injection, coil on plug job. 1999 on a V, designation LX if it makes any difference.
Most of the time it is a blinding car. Round trip Gosport to Bristol in a smidge over 3 hrs the other week
Unfortunately, like many French cars of its age, it's starting to play up intermittently.
It'll happily cruise on the motorway, for an indeterminate period of time before just dropping away in power (as in there is no combustion). Keeping the accelerator at the same position results in an ignominious reduction in speed; I've never had the nerve to continue this below 60, so I'm not sure if the engine ever comes back in.
If I ease right off, and then back on to cruising speed, it's fine (for at least 2.3 seconds, but sometimes a whole journey). Then it's back down to its low-power state.
If, however, I boot it, the engine will cut back in violently at what seems to be the same throttle position. If I keep it nailed, it keeps on pulling. Now, the engine is entirely happy with this situation, but my licence isn't.
What is causing the engine to cut out?
My hypothesis is this: a false signal from either the MAP sensor or the throttle position sensor is confusing the ECU into thinking I've lifted my foot off the throttle, so it cuts fuel and/or sparks (like it is supposed to do on the over-run).
What is it most likely to be? Sensor or wiring?
How do I test the sensors (without taking it to a garage so they can tell me if there is a fault code, which could of course be either the sensor or the wiring....)?
Your thoughts please...
PS
Car history:
owned 14mths, 108k miles on the clock. I've done 14k. Changed water temp sensor (the one that talks to the ECU) about a yr ago, along with the Lambda sensor (which was f
ked after being doused in unburned fuel because of the coolant sensor...) Other than that, just normal servicing and fairly faultless runnning.
Most of the time it is a blinding car. Round trip Gosport to Bristol in a smidge over 3 hrs the other week
Unfortunately, like many French cars of its age, it's starting to play up intermittently.It'll happily cruise on the motorway, for an indeterminate period of time before just dropping away in power (as in there is no combustion). Keeping the accelerator at the same position results in an ignominious reduction in speed; I've never had the nerve to continue this below 60, so I'm not sure if the engine ever comes back in.
If I ease right off, and then back on to cruising speed, it's fine (for at least 2.3 seconds, but sometimes a whole journey). Then it's back down to its low-power state.
If, however, I boot it, the engine will cut back in violently at what seems to be the same throttle position. If I keep it nailed, it keeps on pulling. Now, the engine is entirely happy with this situation, but my licence isn't.
What is causing the engine to cut out?
My hypothesis is this: a false signal from either the MAP sensor or the throttle position sensor is confusing the ECU into thinking I've lifted my foot off the throttle, so it cuts fuel and/or sparks (like it is supposed to do on the over-run).
What is it most likely to be? Sensor or wiring?
How do I test the sensors (without taking it to a garage so they can tell me if there is a fault code, which could of course be either the sensor or the wiring....)?
Your thoughts please...
PS
Car history:
owned 14mths, 108k miles on the clock. I've done 14k. Changed water temp sensor (the one that talks to the ECU) about a yr ago, along with the Lambda sensor (which was f
ked after being doused in unburned fuel because of the coolant sensor...) Other than that, just normal servicing and fairly faultless runnning.If I were you I'd take the throttle body off and clean it first then give it another go. It's got the idle control valve in it which causes havoc when they get dirty as well as the throttle position valve. It's a simple and easy check as long as you have some T30 torx tools. Just take the air filter pipe off and look at the butterfly inside it. I'll almost guarantee it'll look like the insides of a chimney, especially as you've already had unburnt fuel ruining the lambda.
I take it that it was the pre-cat lambda you changed? If it was the post cat lambda it could be a contaminated cat which causes a back pressure through blockage after melting which would normally be irretrievable after failure so I doubt it.
If it's grubby inside just take out the 4x bolts and disconnect the wires and it comes off as a complete unit. (some of the later models had 2 water pipes too) It's really easy to do but make sure you clean it with an old toothbrush and some spare petrol. Carb cleaner has a habit of wrecking the insides. (seals in the idle valve etc).
Also check the air pipe at the back of the inlet plenum from the brake servo. They often crack and allow air in which of course upsets the MAP sensor and makes it over fuel, although it would normally hunt or over-rev at idle. MAP sensors normally effect the general running of the engine through the ECU and would trigger the management light as rule of thumb, but in 20+ years as a mechanic, I've never changed one.
And Lambda's don't burn out overnight either. It could be where the fault was before the lambda failed. If that don't work then maybe there is a throttle position sensor fault if it has one but from experience, I've never had such dramas and generally it's the 'potentiometer' (throttle body) which causes the most faults. If when you're 'cruising' the throttle body can get stuck then go nuts if the ECU is telling it to do something it can't do.
Although I hate working on french cars I've got to admit, the TU series engine was an amazingly cheap and cost effective lump and when it had problems it was always one or the other fixes. Nothing really goes wrong with them apart from the ancillaries around them. That's why they found they way into so many makes and models for so long. That should ease you a little I hope.
As a quick elimination trouble finder it's worth a try so if you have any mechanical knowledge give it a look and you might be surprised. I'd be surprised if it's much more serious than that.
Hope it helps.
I take it that it was the pre-cat lambda you changed? If it was the post cat lambda it could be a contaminated cat which causes a back pressure through blockage after melting which would normally be irretrievable after failure so I doubt it.
If it's grubby inside just take out the 4x bolts and disconnect the wires and it comes off as a complete unit. (some of the later models had 2 water pipes too) It's really easy to do but make sure you clean it with an old toothbrush and some spare petrol. Carb cleaner has a habit of wrecking the insides. (seals in the idle valve etc).
Also check the air pipe at the back of the inlet plenum from the brake servo. They often crack and allow air in which of course upsets the MAP sensor and makes it over fuel, although it would normally hunt or over-rev at idle. MAP sensors normally effect the general running of the engine through the ECU and would trigger the management light as rule of thumb, but in 20+ years as a mechanic, I've never changed one.
And Lambda's don't burn out overnight either. It could be where the fault was before the lambda failed. If that don't work then maybe there is a throttle position sensor fault if it has one but from experience, I've never had such dramas and generally it's the 'potentiometer' (throttle body) which causes the most faults. If when you're 'cruising' the throttle body can get stuck then go nuts if the ECU is telling it to do something it can't do.
Although I hate working on french cars I've got to admit, the TU series engine was an amazingly cheap and cost effective lump and when it had problems it was always one or the other fixes. Nothing really goes wrong with them apart from the ancillaries around them. That's why they found they way into so many makes and models for so long. That should ease you a little I hope.
As a quick elimination trouble finder it's worth a try so if you have any mechanical knowledge give it a look and you might be surprised. I'd be surprised if it's much more serious than that.
Hope it helps.
They are great engines. Had a 954 in my 106 at Uni. Good for 109mph apparently (down the hill past Wincanton on the A303 ). Drove it to Munich in a day, and back the next (long story involving a funeral), then back to Modena where I tooled around for 6 months covering 10k miles. Cracking little car. Only issue was ignition module with an intermittent fault. Got one of the nearest scrap 205 and away I went.
Back on topic. Hadn't thought there might actually be something physically wrong with the engine (like a cracked servo pipe). Will check that. Idle valve could be an issue. It doesn't settle to a just-off-idle position very well - i.e. touch the throttle, revs to 1500, back to 900, then settles at 1100 (unless I've stalled when it gets to 900) - so there could be a funny feedback loop in there somewhere.
Interesting what you say about MAP sensors; guess it won't be that one then. That in mind, my best guess is that the TPS has broken down a bit. Will try and find a second hand one somewhere. Guess it needs to be at 0V with the throttle closed?
Engine management light? What's that then? Not seen any warnings on the dash...
I'd like to buy a reader. It could be quite good fun. Only trouble is as soon as I do, I know it'll start behaving itself for months again, just like it did in the summer... If only it was a constant issue; then at least I could do some proper tests.
Cheers all.
). Drove it to Munich in a day, and back the next (long story involving a funeral), then back to Modena where I tooled around for 6 months covering 10k miles. Cracking little car. Only issue was ignition module with an intermittent fault. Got one of the nearest scrap 205 and away I went.Back on topic. Hadn't thought there might actually be something physically wrong with the engine (like a cracked servo pipe). Will check that. Idle valve could be an issue. It doesn't settle to a just-off-idle position very well - i.e. touch the throttle, revs to 1500, back to 900, then settles at 1100 (unless I've stalled when it gets to 900) - so there could be a funny feedback loop in there somewhere.
Interesting what you say about MAP sensors; guess it won't be that one then. That in mind, my best guess is that the TPS has broken down a bit. Will try and find a second hand one somewhere. Guess it needs to be at 0V with the throttle closed?
Engine management light? What's that then? Not seen any warnings on the dash...
I'd like to buy a reader. It could be quite good fun. Only trouble is as soon as I do, I know it'll start behaving itself for months again, just like it did in the summer... If only it was a constant issue; then at least I could do some proper tests.
Cheers all.
The TPS valve is part of the throttle body. If you take it off there's 2 bolt on components on it, 1 is the idle valve and the other is the throttle position sensor valve. Take them both off and clean the whole unit. I'll be very surprised if it's anything more than that. I've done so many of them I've lost count.
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