Poor idle and acceleration, 1987 XJ6

Got everything back together and engine is running rough. Poor idle, poor acceleration.

Throttle response is terrible. Just puttering off the line. This is the worst symptom.

Basically kind of puttering at idle. Especially when cold. Less so, but still noticeable when warm.

No smoke. I did get a faint sulfur smell on first start-up but that has since dissipated.

Past 2500 rpm its fairly smooth.

Fuel filter replaced in 2019 with less than 1000 miles. I added inline filters to the lines coming from the tanks. The fuel that was in the tanks looked perfectly clear. Injectors have been cleaned and tested by SD.

Here’s a couple videos where I’ve captured some of the symptoms in my garage.

Breather outlet looks unobstructed, but I will pull it off and clean the screen. I did get some oil seepage from the dipstick tube, so I thought this might be contributing.

I guess I need to investigate the ignition coil, distributor caps and rotor, spark plugs and wires, do a compression test and checking grounds.

Anything else?

I would say ‘all of the above’ but there might be some water vapor in the distributor cap from sitting. Just spray the inside with some WD40 it will disperse the water.*

Have you double checked the spark plug sequence? People have been known to have them going to the incorrect plugs.


Would explain running and sulfur smell (too rich, burning in the cat). Don’t torture it so much. They will be smooth at high rpm even with big issues.
Good first step is to immediately look at the spark plugs… are they wet, sooty, clean…?

I had that thought too. I’ll double check. I was in a bit of a hurry getting it all put back together, may have mixed one up in my rush.

If you mixed the spark plug wires up there will be two incorrect ones, not just one. :slightly_smiling_face:


At first I was going to say, “at least two” in response. Then I got to thinking, “how badly could you mix these wires up and the engine still run?” Mixing up three? What if they were all moved over just one position? Out by 180°?

What did you actually do, Jay - and how did the engine behave before you did it…?

A compression test is certainly one step - and will positively verify that the engine condition is not the issue.

And indeed, as Robin says - if you have disconnected spark plug leads; it’s incredibly easy to get the sequence wrong. The first step is to turn the engine to TDC and check dist rotor position - it should point to either the #6 (frontmost) or #1 plug lead.

If the first; trace the lead connections counterclockwise on the dist lid from #6 plug in ign sequence 6-2-4-1-5-3. If the latter; 1-5-3-6-2-4. And don’t skimp; trace each lead from the plug to the relevant position on the dist lid…twice. And do not(!) rely on previous set-up…:slight_smile:

It might be something else, but compression test and ign sequence are either correct or incorrect - there is no in between. Other tests are not as conclusive…

xj6 85 Sov Europe (UK/NZ)

The only things I did that I can imagine would have this effect would be something with the fuel injectors or something with the distributor cap, rotor, and wires. I also replaced the couple of wires that run across the front of the engine by the return fuel line.

I did spend the last 3 months cleaning the engine so accidently disconnecting a vacuum line, or bumping against a connector is entirely possible. Nothing is standing out as out of place though.

Before that it ran very well. I’m not a mechanic, but to me it seems unlikely that all of the sudden there would be a significant enough problem that a compression test is necessary, but reading other posts it seems it could, at least, be informative. I’ve only done one compression test before so I’m still doing some reading on the procedure.

Tomorrow I’m picking up a rotor and cap from the parts store. The current cap and rotor appear to be fairly old. I’ll double check the wire routing. I’m 98% sure of their locations on the cap prior to moving them. If that doesn’t help then I’ll start reading up on finding TDC.

Just remove the plugs and insert a piece of dowling or a long screwdriver into the front cylinder (no6 on the XK) and then slowly rotate the crankshaft until the rod/driver stop rising, this is going to be TDC however it may be on the exhaust stroke, the cam lobes need to be pointing up and away to be on TDC firing. If they aren’t you need to do another rotation of the crankshaft. From here you can follow Franks instructions for the plug leads.

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When rebuilding my 75 BMW 2002 motor a few tears ago my brother attached plug wires wrong. This is a 4 cylinder and it started idled and we drove it… all crappily till we discovered plug sequence wrong

After fixing she purred… as well as a 4 banger can purr!!


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I agree with the suggestions given.

Just for the heck of it check the air ducting between the Air Flow Meter and the throttle body. Is is tight and leak free? Not ripped? An air leak here can cause all manner of weird running problems.


Circa 58 I got my scratch built from junk T speedster’s engine to run. Not bad, but not right. Only 3 firing1. recognized from way back my first t had that proclivity.

On a WAG, i swapped HT 2 & 3. Yahoo all four running sweetly.

By some odd means, i swapped in a Jeep four distributor in lieu of the four wood coil boxes…

Much better…

You were right Robin. I had #2 & #6 swapped. Running perfectly now. Cap and rotor seemed to help too, running better than ever.

I really appreciate all the help and responsiveness from everyone.

Next up, replacing the transmission oil sump gasket.

If someone has overtightened the bolts (trying to stop a leak) the transmission sump has been deformed, so hammer the pan flat before installing a new gasket should it be bent around the bolts!

Even better bend the holes slightly the other way around, so when tightened will go flat.

How old is the Thermo Time switch on the manifold?
This controls the cold start valve.
It could be something like that is over looked.


What color is the paint on your '87 XJ6? The photo looks like cirrus grey, which is a 1986 color. Perhaps your car was repainted? The underside of your hood looks white. Although your car is '87, I see your coil has a ballast resistor, which my '87 does not have, but which my '86 does. Perhaps the folks in Coventry utilized whatever they had on the shelf at the time. I had the same experioence on one of my cars replacing spark plug wires in the wrong places. I’m relieved to know I’m not the only one who erred,

It’s Talisman Silver. Might just be the weird lighting in my garage that’s making the underside look white.

I’ll have to look into the ballast resistor. Wouldn’t even occur to me that it is out of place.

As problems go, luckily this was one of the easiest to remedy.