Running Rich - Exhaust Investigation

As part of the investigation into the apparent running rich mixture and higher fuel consumption I have started working on looking at the last piece of the "jigsaw" The exhaust.

I have been thinking that the exhaust system could be partially blocked or  restricted either by the CAT or the two exhaust boxes.

This would lead to back pressure where CO etc in the exhaust was coming back towards the lambda sensor causing the typical 0.8 to 0.9 readings. Also this would also cause the exhaust analyser to try to suck air during the MOT sampling causing the over Lambda (o2) reading

So I managed to find a second hand CAT from e-bay and some new gaskets. Plan was to remove CAT inspect and run diagnostics to see what Lambda sensor reads without CAT or any exhaust restriction. Then try "new" CAT to see if anything changes.


1. Removal of the CAT involved soaking the bolts in penetrating fluid over night.
2. Bolts where not that bad to remove. CAT came out - noticed that a gasket was missing from the CAT out side.

3. Inspected both ends - Both looked like running a rich mixture. Lots of black soot on both ends.

4. There is no sign of blockage of any of the honeycomb inside the CAT. Seems not to be blocked but unsure of any catalyst material or function. 

5. Ran the engine without CAT It was very very noisy sounded great but I am not sure the neighbours where too impressed!

Captured a diagnostic log of the usual functions (rpm, coolant, injector, engine load).

   
From the graph you can see that sensor warms up quickly dropping down to a lean level of 0.1 / 0.2 engine does richer upon acceleration. I would expect the engine to run lean as the lambda sensor is fitted just before the CAT so would be quite lean exposed so close to air.

If I compare this to a typical run with full exhaust fitted it normally goes straight to rich 0.8 / 0.9. Did not dare take it for a drive but would have been interested in knowing if any changes to injector timings would happen.

Removing CAT and having an unrestricted exhaust does improve mixture (all be it a little too lean)

6. While I was under the van working on the exhaust I thought I would treat my van to some new exhaust hangers.

7. Carried on not sure if to put on new CAT but decided if I did not change anything then I would just get the same result again. New CAT was similar but looks like to be a different shape. Maybe a replacement from the original Brazilian one.

8. Once new CAT was fitted took the van for a drive along my "typical" A6 drive which gives two dual carriageway constant speed sections.

Logged the data as before.

Did not manage to capture the initial warm up but still the two humps at the start are the same sections as other logs.

As before there seems to be running yet again as constantly rich (0.8/0.9)

Changing the CAT has seemed to have no effect on Lambda levels. So I suspect its running the same as before

so what I have learnt? I think the following:

1. CAT does not look restricted but running rich for so long will reduce effectiveness.
2. Running free non restricted exhaust does not show a rich mixture.
3. If exhaust is restricted then it must be CAT or two boxes further.
4. To confirm I will remove the CAT to BOX 1 connection and re-test and remove connection from BOX 1 to BOX 2 and re-test.
5. Try the back pressure measurement. No idea of how much back pressure is acceptable.

I think there is a possibility of a restricted exhaust. so will need a new exhaust but the choices are:

1. Stock steel - VW heritage sell Box 1 / Box 2 for approx £250
2. Longlife Stainless - £359 for stock but stainless.
3. Remove all including the CAT and opt for a S/S vintage with Lambda port £410

I am tempted by the vintage speed BUT concerned without a CAT it could fail. I think Stainless Steel Longlife is good BUT expensive £££ but at least will last and an investment.


UPDATE - Did the back pressure measurement - Connecting into Lambda port.

NO BACK PRESSURE AT ALL - ZERO PSI at idle and only 1-2 PSI on acceleration etc. SO no back pressure.