Citroen C2 1.4i exhaust manifold crack

[OneRandomBX]

Hello,

I've written here about my issues with a cheap O2 sensor.

I've since replaced it and the car runs fine, apart from a sometimes lumpy idle and pops and bangs from the exhaust when engine breaking when the engine is cold (LONGFT% is always around 25)

I have noticed a crack at the O2 sensor hole, could it be the cause of the issues? I'm only wondering because the crack looks quite small

[mickthemaverick]

It could have a bearing. Try a temporary repair that won't last long but at least it might confirm the crack is your problem. Ideally get it welded but if that's not possible you could try a metal filler to see if it improves things, something like this:
https://www.ebay.co.uk/itm/314958082646

[Rp0thejester]

Good old 'chemical metal' works on everything, just don't put it anywhere you don't want it including threads

[OneRandomBX]

Just ordered some exhaust sealing paste, my only worry is that there may be more cracks where I can't see them

Good news is that I found a used manifold for only 30 euro, if it's crack-free I'll buy it and swap it over the weekend

I just hope my manifold isn't seized too bad...

[ozvtr]

(LONGFT% is always around 25)

The fuel trim is maxed out! That explains the popping and farting on deceleration. I'm not sure that crack is wholly responsible. Yes, it could suck air in through that hole and upset the O2 sensor. Of course replacing the manifold wont hurt. The paste will quickly tell you if that crack is your only problem.
The manifold should come off without any problems. However, all the nuts might not come off the studs. The studs and nuts may come off as one. Separating the nuts from the studs is not easy...but not impossible. Lock two spare nuts back-to-back onto the 'other' end of the stud. Remove the 'stuck' nut, then unlock the two nuts.

What engine ECU does the car have? What scan tool are you using? I can not get LEXIA to read the long term fuel trims on my TU3JP engine with a SAGEM ECU. Only the short term, which is not very helpful.

[OneRandomBX]

Not sure about the engine ECU

I've tried listening all around the intake manifold side using a bit of garden hose, couldn't find any suspicious hissing noises that would indicate an intake leak

I'm using a cheap OBD scanner I got from Temu for 10 euros

The exhaust putty is coming tomorrow, I'll have an update then

I have another concern though, when the oil filler cap is off while the engine is running it's blowing more air than I would expect
Not quite enough to blow the cap away but there is a significant amount of pressure and, I'm guessing, oil vapors
Anyone with experience with this engine that can tell me if this is normal?

[Stickyfinger]

Chemical metal WILL BURN OFF.....100% pointless IMHO.

That crack will grow, they nearly always do in cast iron. It needs it end drilling (to halt its growth), the crack "V" cut fully through then welded with the correct bronze filled rod (after pre-heating) on a pre heated manifold. Not complicated.

[mickthemaverick]

Chemical metal WILL BURN OFF.....100% pointless IMHO.

Indeed it will Alasdair which is why i said: "Try a temporary repair that won't last long". The point was to block the crack for a quick test run to see if the problem is cured, not a long term fix.