Technical 1.2 8v 1000-1500 rpm jerking

[bluejohn125]

Well based on my experience with a long-standing ECU / lambda problem on my 2001 Brava 1.2 16V I'd say that one of your lambdas (probably the pre-cat) has failed and since this is the primary feedback to the ECU this could explain the original symptoms.

If you still have the lambdas swapped over then your ECU diagnostics readings will be swapped also (what you have written about bank 1 sensor 2 twice is confusing... presumably you mean bank 1 sensor 1 and bank 1 sensor 2...) but the sensor switching between 0.07 and 0.7 seems fine but the other one is dodgy. So make sure you connect them correctly (or carefully try different combinations) to see if you can actually identify a dodgy lambda...

If the pre-cat lambda has failed but you swap the leads over then you will get a useable reading from the post-cat lambda into the sensor 1 port of the ECU but it will have been altered by the cat. The failed pre-cat readings will go into the sensor 2 port but this is not so important so the engine should still run in some fashion.

On my car the actual ECU had a fault in the sensor 2 port (it was not connecting the ground wire) and this always reported as a slow-switching lambda fault, so everyone (incorrectly) blamed the lambda. But I had also mistakenly swapped the sensors over and the car seemed to run better like that, till eventually things got really bad with variable revving etc and it failed the MOT.

My faults were finally traced to:

1 Faulty ECU (repaired for £250 all in...)

2 Swapped over lambda sensors confusing the ECU

3 Crack around cat downpipe upstream of second lambda, allowing air in and messing up the sensor reading.

Once all that was sorted the car runs like a dream now... :slayer:

Happy hunting...

 

[plyons]

Note all that the lambda swap has been tried by KRaikkonenNo1 - not by myself! I've been watching KRaikkonenNo1's thread https://www.fiatforum.com/punto-ii/353928-jumpy-jerky-running-12.html related to the lambdas with interest though, particularly with the knowlegde of my post-cat's readings.

have you fixed it?

I'm afraid not. Rest assured I will not be keeping it a secret if I fix this damn thing - it's been annoying for years at this stage! The only thing I've really tried is the second-hand throttle body swap. It ran exactly the same afterwards. Now, maybe what I need is a brand new throttle body instead of a one that may have actually done more miles than mine?

My engine has only done ~40K of well serviced, motorway mostly, miles - so I'd imagine it's more likely to be susceptible to things being too old, rather than mechanical wear.

I had this idea about the throttle body (and indirectly changing the throttle position sensor) from this post:https://www.fiatforum.com/punto-ii/119399-punto-jerking-5.html?p=1293955.

This guy from the same thread seemed to describe exactly the same problems as I have: https://www.fiatforum.com/punto-ii/119399-punto-jerking-2.html?p=1262157. He got it working 'better' but not cured completely.

This other guy from the same thread traced his issues to the fuel pump, but he noted his idling was fine: https://www.fiatforum.com/punto-ii/119399-punto-jerking-8.html?p=2040722

what you have written about bank 1 sensor 2 twice is confusing

Sorry! Yes I meant bank 1 sensor 1 is going from 0.07 to 0.7!

I know I have a blow from the exhaust, put I believe it's near the flexible flange joint, in other words after the post-cat. This has lead me to believe that it wouldn't have much affect (unless less back pressure could affect things?).

My next plan was to:
1. Check plugs.
2. Swap post-cat lambda, since that's the one that's giving the bad readings, or exhaust manifold replacement (manifold, cat, lambdas), since it's rusted pretty bad, and it's 14 years old, and it' got the blow I was mentioning.

Two things about the lambda though:
1. The lambdas only come into play once they've warmed up, and if that's the cause my problem shouldn't exist when cold? My issues seem to exist warm or cold.
2. If I'm replacing the lambda, I guess I'm as well to get a new one, since these things are affected by age as much as use?

Also in relation to my idling revs. I've noted from the Fiat Manual that idling range is 750 +/- 50 rpm. So my question is: Is mine actually normal in how it behaves, or should it be pretty steady somewhere within the range 750 +/- 50? What do other people's 1.2 8vs do?

1 Faulty ECU

How does one begin to diagnose this as the issue?! As in what's the approach? I think I'll be suspecting the E.C.U. last, if only because it's that pricey.

 

[bluejohn125]

Remember that this is a closed loop system so any bad sensor readings (or bad burning / sparking etc) will give "false" inputs to the system and have it chasing it's tail... trying to sdjust something and checking for the response etc...

From my experience you wil only sort this if you can find ONE PLACE (e.g. garage or ECU specialist) that can go through everything on the system methodically with the right experience and equipment and connections in the trade, they can check it all ON THE CAR and diagnose any leaks (with smoke tests), ECU codes,emissions readings, wiring issues etc. I sent my ECU off for repair after having various diagnoses on the lambdas and ECUs but in general an auto-electricial will not do everything (like smoke / leak tests and emissions checking). My "rough-and-ready" autospark checked the lambda switching by taping into the wiring and ALWAYS blames leaks in the exhaust for problems in this area, and in the end he was partly right. But you need to find somewhere willing and able to focus solely on your car and you need to be prapared to pay for that if you are confident in them.

That said, if there are suspect items (like the cat /exhaust and lambda) that you are prepared to replace as "expendable" service items then go ahead, it shouldn't do any harm. I was very close to buying a new ManiCat for my Brava because it was original to the car, fairly rusty and cracked with 150k miles of service behind it, but I got my "rough-and-ready" mechanic to weld right around the cracked pipe joint and fit a (missing) bracket between the cat downpipe and the engine block (possibly the cause of the cracking...) and everything is fine so far (I think the Cat is still functioning OK, next MOT will tell).

If you get a new ManiCat they are expensive but try the CatMan, his price for a quality British replacement was very good. Note that depending on exactly when your car was registered different quality Cats may be required... mine was after the cutoff date so the emissions requirements were higher and a cheapo Cat (with less materials etc) would not have been good enough.

Good luck whatever you do. It can be expensive to fix but it's great when you do.

 

[plyons]

So, I swapped the post-cat lambda sensor for a second-hand one along with my original throttle body (now cleaned) and I think things are slightly improved, but not by much.

Also, the new lambda was reading flat zero for a period of time while driving - I'm guessing this isn't normal.

I'm going to try plugging the leak in my exhaust. At least the thing will be more quiet whilst I'm testing...

... then, I dunno. Can anyone at least confirm that the movement at idle is errant?

Here's my options as I see it:
1. Check plugs.
2. Checking the fuel filter/pump.
3. New lambda.
4. Bring it to an actual professional as bluejohn125 has suggested.

 

[fingers99]

Won't be the fuel pump if the issue is just at idle.
The fuel filter is not a service item.
Hunting idle is not normal -- should be 700 - 800 ish and steady.

Sort the exhaust.

That pipe to the airbox, from the rocker cover, if cracked, needs replacing.

If that doesn't do it, I'd think about a compression test. HG blow between cylinders will do all kinds of weird stuff, especially at low revs.

 

[fingers99]

Won't be the fuel pump if the issue is just at idle.
The fuel filter is not a service item.
Hunting idle is not normal -- should be 700 - 800 ish and steady.

Sort the exhaust.

That pipe to the airbox, from the rocker cover, if cracked, needs replacing.

If that doesn't do it, I'd think about a compression test. HG blow between cylinders will do all kinds of weird stuff, especially at low revs.

 

[plyons]

Thanks for the reply. You're right, I should just sort the exhaust as my next step. I'll also look into replacing the pipe to the airbox.

Really I have the feeling that the two identifiable issues, the hunting idle and the jerking at the slightly higher rev ranges are linked. It's also not completely smooth at other rev ranges, it's just not as blatantly obvious.

The jerking has the most affect in 1st and 2nd, but it can still be felt slightly for example, in 5th. I suspect the reduced visibility of the problem in higher gears is because the engine doesn't have the required power compared to the gearing to start jerking the car around.

I have a theory that the jerking at the 1000-1500 rpm range is because below this, the engine doesn't have enough power to cause the issue, and above this it has too much momentum.