Misfire on overrun

[NickCUK]

Car is a Peugeot 307 1.6 16v petrol, TU5JP4 engine, 90k miles.

Has a bit of hesitation/judder on the overrun. Happens on a warm day when trying to hold a lowish speed of say 50mph on a slight downhill. Pulls ok at normal low throttle openings. Doesn't feel like just one cylinder missing, feels more like two or perhaps the throttle valve fluttering about for some reason.

Have previously removed and decarbonised Throttle body & flap and replaced body to manifold seal.

Could this be related in some way to the EVAP Purge valve opening on the overrun? For the moment I have now disconnected this from the manifold and blanked-off the inlet and pipe which seems to have improved matters. Although not completely sure as misfire only happened under certain circumstances on a particular road. However also did other things at the same time so can't be sure which has made a difference: also disconnected the battery to reset the ECU and cleaned the plugs by soaking the tips in redex.

Is it possible that the petrol tank is letting air in somehow and on a hot day when driving downhill the ECU is opening the EVAP purge valve to allow the stored vapours get sucked into the maifold. This causing an air leak and running weak during this time?

[xantia_v6]

I would be checking the adjustment of the throttle position sensor. I think that it is a potentiometer mounted on the throttle spindle. If it is not adjusted correctly, or intermittent, then the over-run fuel cut-off may be operating at the wrong time.

[NickCUK]

I would be checking the adjustment of the throttle position sensor. I think that it is a potentiometer mounted on the throttle spindle. If it is not adjusted correctly, or intermittent, then the over-run fuel cut-off may be operating at the wrong time.

Should have mentioned that it also does the same when on cruise-control.

[xantia_v6]

Can you clarify the symptoms? I read that the problem is only felt at very light throttle or on the overrun? under what conditions does it occur when cruise is enabled? only going downhill?

[NickCUK]

Can you clarify the symptoms? I read that the problem is only felt at very light throttle or on the overrun? under what conditions does it occur when cruise is enabled? only going downhill?

Yep, only going downhill with cruise set at say 55mph. All other points of driving are fine.

[xantia_v6]

I am presuming that the cruise control on this engine is the Hella system with a vacuum pump, a vacuum diaphragm near the throttle body with a link that pulls the throttle open?
If so make sure that the link is adjusted to have only a minimal amount of slack before it sparts moving the throttle. Excess slack can cause "shunting" when the cruise reapplies the throttle after coasting.

[NickCUK]

I am presuming that the cruise control on this engine is the Hella system with a vacuum pump, a vacuum diaphragm near the throttle body with a link that pulls the throttle open?
If so make sure that the link is adjusted to have only a minimal amount of slack before it sparts moving the throttle. Excess slack can cause "shunting" when the cruise reapplies the throttle after coasting.

No it is all electronic, drives the motorised throttle body directly from the ECU.

[Rp0thejester]

From your original post, is it possible your over thinking things? Changing so many things at once is futile, one at a time. ECUs are very very very rarely the problem. The ECU gets the info from the car sensors and then works, if it's miss firing then it's not the ECU fault. First thing that springs to mind for me is over fuelling as petrol in a hot exhaust can make alot of noise. When you say fluttering, does it feel like a yes no to acceleration?

[ozvtr]

Generally speaking, the valves tend to fail open or closed. They don't typically "sort of work".
The valves are usually opened at high constant RPMs and closed at low RPMs.
Also vacuum leaks (an open purge valve would be considered a vacuum leak) only tend to effect idle, where the engine is only consuming small amounts of air. The other thing is the TU5JP4 only uses a MAP sensor and no MAF sensor. So it's not as badly effected by vacuum leaks. Idle revs would tend to go up though.

I'm suspicious that this is a firmware bug in the ECU. I have seen this before, but in TU3 engines running Sagem ECU's. The only thing common between the TU3 and TU5 is the way the ECU manages the engine. The TU5 uses a Bosch ECU. But both only use a MAP sensor to calculate the air mass. I have a 2004 C2 with the TU5 engine but it does not do this.
Sorry, but the "TU" engines are very simple which means there is little to go wrong with them.

What year is your 307 and how long have you owned the car?

[NickCUK]

Well tried it today with the EVAP disconnected and blanked-off on the stretch of road on which the problem is usually very noticable: virtually no hesitation, in fact what there was could have been me imagining it. Unfortunately not conclusive though as it was ten degrees colder and damp, need to wait for the next extremely hot day to be certain.

[NickCUK]

Don't know if this is correct or not, maybe they all do it, but I notice that sometimes when I remove the fuel cap there is a build-up of air pressure in the tank is this normal. SHould the EVAP canister vent that pressure to the outside air somehow?

By the way the purge valve is working, have activated it with Diagbox and it modulates correctly, I can then blow through the pipe.

[NickCUK]

...
What year is your 307 and how long have you owned the car?

Car is a late 2005 so is a 2006 facelift version. Had if for eighteen months now.

[ozvtr]

I ask these questions because it helps to get some context as to what is going on.
If the car "has only started doing this" and it ran fine before, then you know it can be restored. If it's done this since you bought it...well it might be terminal. Not saying that this is the case here!
Being late in the production I doubt it's a "bug" in the system. That should have been worked out by now. So you are more likely looking at a failure of some sort...but obviously not a catastrophic one.

Yes, quite often the fuel tank will build up pressure if the purge valve has not been cycled by the engine ECU. BUT THAT'S NOT UNUSUAL! As I said before the valve is typically opened at higher RPMs. "Around town" driving might not allow the engine ECU to open the valve. The charcoal canister, or purge canister, absorbs fuel vapor as the pressure rises and then releases the vapor when the pressure drops. So it keeps the pressure down when the purge valve is closed (but a small amount of pressure still builds up) then releases the stored vapor when the valve opens to the engine. The carbon in the canister "converts" the fuel vapor reducing its' volume. It's not like some sort of "overflow tank". It actually reduces the volume of fuel vapor as the pressure builds up. It's also very important that raw fuel not get into the canister or the charcoal will be destroyed! So don't over fill your fuel tank!
The whole point is not to let fuel vapors in the top of the fuel tank into the atmosphere, but that they be channeled into the engine and burnt.

Have you found any broken devices or broken green and yellow wires around the cylinder head? There are suppressors and earth shields for the ignition wiring that can get damaged but not drastically effect the engine performance. Could be something like that?

[NickCUK]

No broken suppressors that I have seen. Thanks for the explanation of how the EVAP works. It could be just by design and the hesitation is caused when the purge valve is modulating open and weakening the mixture; perhaps the fuel filler cap has a valve in it letting air into the tank.

Could look at fitting a restrictor in the pipe, like an old carburettor jet, but as it is a plastic moulded pipe, once it is in I won't be able to get ot out again.

[xantia_v6]

There is probably a procedure for calibration of the motorised throttle using a diagnostic tool. It would be worth trying that before making modifications to the system.