Hello all, newbie here hoping for some advice. Have acquired a Vanden plas series 3 XJ6. Runs reasonably well but has a very annoying backfire through the intake but only when in drive. Will rev freely without this issue in park. I'm starting to pull my hair out ( what's left of it ) I have checked for vac leaks in the usual places, checked timing @ 4deg btdc, replaced plugs , checked cap and rotor. Anyone had a similar situation?

 

Your down here somewhere.

Please do an Intro in the New members Area when time allows.

Not 100% on S3 6cyl, but reading as I do.

The Airflow meter may be out of adjustment.
The hose/s between that Meter and the throttle body may be:
Ill seated/clamped
Cracked on the underside.

There would be a fuel enrichment switch/whatever somewhere to prevent that happening, and the S3 Guru's will chime in when the planets align a tad better.

 

thanks Grant, will look into that . And if anyone else has ideas ?

 

Agree with Grant on checking that duct between the air flow meter and the throttle body

Is this a USA spec car? What year? After '84 or so the timing spec was 17º BTDC

Cheers
DD

 

Australian delivered car , early 1983 sticker under the bonnet states 4deg btdc.

 

Doug,
On my 85,once warmed up and with timing set at 17 BTDC as soon as a gear is engaged she wants to die. If the AC is selected ON, then she dies for sure! Granted this is with the Pertronix distributor, which should make no difference at idle (ported Vac. source). So, following David's advice, I have it set to 8 BTDC and she's stable when idling in gear with AC on. But, she doesn't like to start once warmed up (but always fires up instantly from cold). I plan to move it to 4 BTDC to see if that improves the hot restart condition at all.

 

A backfire thru the intake typically suggests a too-lean running condition. If the previous ideas don't pan out I'd look at fuel pressure, fuel delivery, injectors.....

Cheers
DD

 

Over the years I've seen this crop up before and I did battle with such an XJ6 (an '86) myself, years ago.

These engines in full emissions trim (USA and Canada, for example) are tuned to run lean at idle to begin with. Any fault that reduces fueling further can mean that life can't be sustained at all. Marginal injectors, low fuel pressure, worn AFM. When cold, the ECU adds extra fueling so the problem is masked....until the engine warms up and the cold-fueling goes away.

Before going into the weeds on all that stuff, though, make sure your throttle body is clean and the throttle blade gap is at .002" and make sure you don't have any vacuum leaks.

On the '86 I worked on a long time ago I was able to tweak idle speed, idle mixture, and timing adjustments so that the car would stay running. The owner was happy but this didn't solve the actual fault; it masked it.

Cheers
DD

 

On an Aussie spec car I think the enrichment switch operates only at a wide throttle opening....like 80% or so? If the backfire is occurring only on heavy throttle then this switch would be on the list of suspects for sure. I think it's just a mechanically operated micro switch on the linkage

Cheers
DD