V12 airflow imbalance

V12 airflow imbalance

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anonymous-user

Original Poster:

53 months

Wednesday 15th February 2017
quotequote all
Question concerns a 1992 Jaguar XJR-S 6.0 V12.

I have just changed a coolant hose and was standing around waiting for the car to warm up at idle. I noticed that at idle the airflow to the LH (passenger side) bank through the intake hose was much higher than the RH bank. If you block the LH air inlet you can stall the engine, if you do the same thing with the RH intake it makes absolutely no difference.

If you rev the engine the RH does have airflow but it remains substantially lower than the LH bank.

I have checked that there is spark on all cylinders on the RH bank and there is. Removing the plugs causes it to stumble so all seem to be fuelling and firing.

Air filter is relatively new and clean.

The exhaust from both side puff like a chimney seemingly equally, and if you stand at the back with a hand over each pipe they do feel to be flowing evenly which seems very odd. Very hard to gauge this though.

If the car is down on power it's not by a huge amount, it'll rip up to the rev limiter in 2nd and pulls hard at 120+ in 3rd. It revs freely and there is no stink from the exhaust beyond the usual. Fuel economy is terrible but not unexpectedly so (15mpg on last tank calculated by me).

My thoughts currently are:
1 - Blockage on the RH exhaust system;
2 - Lambda fault on the RH bank so it's running lean;
3 - Inlet butterflies imbalanced, but I would have thought that airflow would be pretty equal once you start opening the throttle;

I'll try and get it on a gas analyzer before the end of the week, but does anybody have any other ideas?

melhookv12

958 posts

173 months

Thursday 16th February 2017
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Don't worry. On warm up the auxiliary air valve draws air through the ns air box and it creates this effect.

Recheck when it's hot they should be more or less equal. But as you will see there is a huge balance pipe between the inlet manifolds.

melhookv12

958 posts

173 months

Thursday 16th February 2017
quotequote all
Check mixtures as suggested. Always go off the lowest figure assuming the worst.

A v12 exhaust should be smooth, but XJRS always had a different sound and feel at idle due to sequential injection.

Just check over you work ensuring all connectors are secure.1 connector on each air box measures each side of air intake temp. Easiest way to spot a real Zytec car

anonymous-user

Original Poster:

53 months

Thursday 16th February 2017
quotequote all
Thank you, I'll make sure it's properly warmed up and repeat the exercise.

Interesting you should mention idle quality, the idle on my car varies between "quite good" and "quite poor" but is never anything like as smooth as my XJ12 was. I gather from other owners and from contemporary reviews that the idle quality of the XJR-S was never brilliant.

melhookv12

958 posts

173 months

Friday 17th February 2017
quotequote all
Could you take a picture of the under bonnet. So I can refresh the ignition system in my head.
The idle is hard to describe but it's shouldn't be rough. Zytec cats were just different to normal 12's. Sorry for being an anorak.

You should obviously see virtually no co, but high hydro carbons will indicate a misfire which will very quickly burn out a cat.

Please get it checked by someone who knows what they are looking at.

anonymous-user

Original Poster:

53 months

Friday 17th February 2017
quotequote all
I'll get some pics tomorrow, is there any area in particular?

I put it on a gas tester earlier and it was fine (by the expected standards, by modern standards it was a death factory).

I read the service book on the auxiliary air valve. I knew it let extra air in from the LH airbox when the coolant was cold, but I hadn't realised that this was the source of all of the idle air supply. This of course explains the extra air flow to the LH side when cold, the extra air flow when idling, and the ability to stall the thing only my blocking the LH inlet. I think too many years of electronic throttles where everything is under ECU control has made me dim witted.


melhookv12

958 posts

173 months

Saturday 18th February 2017
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Just in V, dizzy and leads.

anonymous-user

Original Poster:

53 months

Saturday 18th February 2017
quotequote all
2017-02-18 15.46.54 by davidedney123, on Flickr

It uses the old Lucas distributor cap, I understand that the distributor itself is a gutted version of the Lucas one with hall sensors added for the engine management system and all the vacuum advance kit removed. It uses a single coil although there was a TSB to "upgrade" to dual coils like the older cars.

melhookv12

958 posts

173 months

Sunday 19th February 2017
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Twin coils is a good idea. Or at least move yours out of the heat of the V.
Those coils are notorious for failing. I'd carry a spare if I were you.
Check all injector connections, then can be dislodged quite easily.

Check for air leaks, there were a few take off's under the throttle bodies where the blanking plugs would just fall off.

Hope this helps.