Low power / Rough Warm Idle

I picked up a 1987 V12 XJ-S last week, and now that I’ve been driving it for a bit I’ve noticed there’s nowhere near as much power as I’d expect - 0-60 is taking 13+ seconds! It’s like you get increases in power upto about 50% throttle, then further increases make a lot more intake noise (its had the air filter inlets modified to be noisier) but doesn’t appear to produce any more power.

It also seems to miss slightly at idle when it’s warm. Idle seems to be either 1000rpm or more like 300rpm depending on what mood the car is in - I suspect the AAV is defective!

The previous owner barely drove it and didn’t service it in years, which may be a factor.

I decided to service it, and when I drained the coolant I noticed some gunk under the filler caps and the coolant was rather cloudly - oil was pretty horrible but no sign of coolant contamination. Bought a test kit for combustion gases in the coolant and it reads consistently negative. I did a compression test and most cylinders showed around 200psi except one which was more like 175psi, but I don’t think one cylinder at a lower compression would cause such a significant power issue.

So far I’ve replaced the fuel filter, checked the TPS (it was slightly out, 0.25v at idle, so adjusted), checked the throttles are set correctly (they seem to be), checked the centrifugal advance wasn’t stuck (it wasn’t) andchecked the coolant temperature sensor.

From reading the forums I’m wondering if this a fuel pressure issue, or an ignition issue (it’s still on the 2-coil system and the spark plugs look quite old). I have noticed a fairly loud hiss from the LH bank at idle but not been able to locate an obvious vacuum leak.

Aside from the slightly rough idle when warm and lack of power, the engine seems to behave itself, starts up fine etc. Idle when cold is fine, and it passed the MOT emissions test last week with flying colours.

Any thoughts would be greatly appreciated!

Restricted exhaust? Cat melt downs on non-Marelli cars are rare but I supposed it could happen. Or maybe a pipe is collapsed somewhere?

Forgot to mention its a UK car, so no cats (and also no closed-loop fuel injection…)

Exhausts don’t appear restricted - plenty of air coming out of both at the back.

Take a look at the screen filter, which is inside the sump tank below the battery. If that is semi-plugged it will severely restrict your fuel supply. (and power) Welcome to our world!

Interesting! Does that involve draining the whole fuel system?..

That would be the safest way. If your fuel hoses in the boot are nice and flexible (unlikely) the appropriate hoses could be clamped, but draining is better. A failing secondary ignition coil will limit your RPM’s, too! You pick!

Doesn’t seem to be an RPM issue - it will still rev up to a high RPM and a reasonable road speed, it just takes forever to get there!

If the car has been badly neglected, you really do need to ensure that your fuel supply is not leaning out. Engine damage could be around the corner.Are you familiar with Kirby Palms Book on all things XJS? It is available for download on this forum, and is free. Plenty of advice there.

I am, I have read it in quite some depth already! Fantastic resource!

I know the previous owner did have the main tank replaced, and that and the sump tank both look in good condition externally, but I guess checking the screen filter wouldn’t be a bad idea!

Do you smell brake pads? They could be dragging.

Also, try staying in 2nd gear when you time accelerating. I’ve noticed mine shifts into 3rd way too early, you have no power in that gear. I was able to go 0-50 in about 7 secs, but by staying in 2nd. Exhaust sounds terrible, like a Cadillac whishing along. I can’t wait to remove middle resonators and cats!

Also, new spark plugs should help a lot, especially with idle

No brake smell and the front wheels both spin freely (much to my annoyance when I tried to remove them with the car jacked up without having loosened the nuts first!).

Replaced the coil, HT leads and plugs to no avail, so gave up and took it to a specialist.

They immediately diagnosed it as incorrectly set timing (too far retarded) and having corrected this I’m told the power is back to normal.

Thanks all for the tips!

That would do it!

Yeah, timing these beasts is not fun. You need at least two people, three for perfection. One in the car to keep it at 3000 rpm, one to adjust distributor, and one poor soul to lay under a hot engine/fan roaring at 3000 rpm pointing the timing gun.

Got the car back with the apparently fixed timing - it does feel a bit more responsive but still ~10 seconds to 60 which is way down on what it should be…

I’m really running out of things to check now!

You said that you have already done a compression check. What spark plugs are you using, what gap have they been set to, and how did the plugs look when you removed them?


Im using NGK BR7EF plugs, gapped to what the book said to gap the to (which they pretty much were when they arrived, I only had to adjust a couple).

The old ones didnt look particularly out of the ordinary, no obvious signs of lean or rich running, and all looked the same.

Having said that, being a UK car it has no cat and so mixture adjustment is manual and is one of the few things ive not yet looked at…

Is the kickdown working?

Yes, and ive also tried holding it manually in the lower gears.

Have you tried an Italian Tune-Up?

I haven’t - do they actually make any difference?..