Ultra-rich condition in 87 XJ6

Got the '87 XJ6 back from the shop today because the shop can’t figure it out and has reached the end of their rope. We’re a little hamstrung because we lack the tools to completely diagnose the problem and are in the throwing-parts-at-it stage.

This is what has been thrown at it so far:

  • One complete, new set of injectors
  • Two sets of spark plugs
  • Cap & rotor + 2 sets of plug wires
  • 2 coolant temp sensors
  • 1 airflow meter from Everyday XJ (used)
  • Cleaned/reoiled the K&N air filter

EDIT: I also forgot the following has been changed/replaced:

  • Cold start solenoid (twice)
  • Fuel pressure regulator

The car basically is a brick. It is dumping black smoke out both tailpipes. It builds revs very slowly and is hesitant to rev beyond 3,000 rpm, but will do it. It will not pull a hill of any kind. The in-dash computer, which I’ve found to be quite accurate on these cars, is reading MPG of between 5-12.

We also believe now that the catalytic converter has been ruined from all the raw gas being dumped into it. At the end of a 10-minute drive this afternoon, it began dying at stop signs, and it had trouble going up a 2-foot incline (no exaggeration) into my garage. When it finally got in the garage, I was greeted with a strong smell of unburned fuel and catalytic converter.

I have no shop around here that has Jaguar diagnostic tools from a S3 car, so we’re stuck in that regard. The shop wants me to get another airflow meter, or maybe a new computer. Something is causing the car to dump raw gas, wide open at all times. The car actually runs BETTER if you disconnect three of the fuel injectors.

As simple as these cars are, there doesn’t seem to be a lot of components involved in this problem. It would seem to me it’s either a computer issue, airflow meter issue or there’s compromised wiring somewhere. I’m open to suggestions here but as you can see from the list above, we’ve already thrown quite a bit at it.


Did they not look at the coolant sensor?
Edit, overlooked.
Injectors won’t do it. The trip computer knows about it so it isn’t the separate cold start injector but the engine does think it’s cold or consumes tons of air.
Try disconnecting the coolant temp sensor (not to gauge but the one with the bosch plug), does it die?
What is the fuel pressure? Take the brown switch on the fuel line out of the equation by directly connecting the pressure regulator to manifold vacuum. Make sure the fuel return isn’t blocked.
One set of 3 injectors grounded, unlikely. Does it idle ok?
Welcome to the forum and good luck. The cat converter may be shot by now.


Thanks for the response. You reminded me that we’d also replaced the cold start solenoid/injector and the fuel pressure regulator.

I will try undoing the CTS later tonight after it cools down a bit. The cat, I’m sure, is clogged badly. I’ve got to wait until it, too, cools off a bit for the passageways to open up enough.

It idled OK until everything heated up and then it slowly got worse. This car has never been a great idler since I’ve had it, though. It sort of lopes like it has a big cam in it.

But 3 months ago, I was getting 24 mpg on the interstate. The reason I knew I had a problem is I started having to fill up more often and checked my MPG on the computer and it was running around 16-17. Now it can’t get over 12, so this is a deteriorating condition for sure.


I read that you changed spark plugs - question - what gap are you using on the spark plugs - almost sounds like you may have too large of a gap on the spark plugs since seems to not be burning all the fuel - evident in the black exhaust and the failure to increase revs under load going uphill and the excessive smell of fuel - just a THOUGHT.

I’m not sure where the shop gapped them, but remember that the problem started suddenly with the old set of plugs/wires in it. So for it to have started so abruptly, I don’t think the gapping on the old plugs would suddenly have been the problem (same for plug wires, cap and rotor).

This feels more like sensor failure, or something internal to the airflow meter, computer or wiring in between the two. Also, I don’t think we have replaced the O2 sensor yet.

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Had a similar issue years ago with an XJ6 and it turned out to be a bad battery! I wouldn’t have believed it either, but, it worked! We’d made some phone calls and were told this by a jaguar dealer!

I had similar situation with my V12…turned out to be bad ECU.

I can believe a bad battery, but the ECU rarely fails and has no internal pressure sensors etc. that can break, unlike the V12.

If it idles ok and gets way rich when in gear (closed loop) it can be a misfire causing the mixture to be perceived lean, then it’s fattening the mixture to compensate. Which could be ignition related in the end.
That is a common issue your shop should be knowing about. When they throw parts at your cars always keep what they tore out despite working fine. Put a multimeter to the coolant sensor (values here on the forum) and learn what disconnecting the o2 does because I don’t know what an XJ6 does. Make a „stall test“ when warmed up and immediately pull the plugs and take pictures of them.

I presume it has electrolytic caps, though. Those things are notorious for causing issues with age. They’re so cheap that some advocate just opening the case, warming up the soldering iron and replacing all of them just to see if it helps.

Have you actually measured the resistance between the pins on the CTS, Jess…?

It’s purpose is to vary mixture with engine temps; a cold engine needs richer mixture, leaning out as engine warms up. You are certainly running too rich, but maybe not so right about ‘few components’ being involved. What you need is some basic diagnostic tools, in this case; a multimeter and a vacuum gauge. Most components and their actions can be easily checked out. Swapping components is fair enough, but only if they are verified as working - and ‘new’ does not mean that they are…

After checking the CTS resistance; check resistance between the two wires on its connector. It should read some 2 Kohm - but importantly; there should be no variations when the connector/wires are bent and twisted. This test verifies connection to the ECU…

Clamp the CSI hose; it will positively eliminate any fuel delivery from this source. If no change; the CSI is innocent - but fault finding is as much about verifying which components work as which do not. Working components cause no faults…:slight_smile:

As per David; open the filler lids and verify, with the pump running, that you can hear fuel returning. If fuel return is blocked; the fuel pressure will rise - which certainly causes overfueling…

Disconnect the vacuum hose at the fuel pressure regulator and run the pump; there should be no fuel leaking out - if it does; the replacement regulator is faulty. With the engine idling; connect the vacuum gauge to the hose - ‘normal’ reading is some 15 - 18" Hg. You probably have very low idle vacuum, then increase rpms to some 1500 rpms - then to 2000 etc. At steady rpms the vacuum readings, high or low, should be steady; if vacuum gradually drops - it indicates clogged exhaust/cat.

Have you actually checked spark quality? Use a spare spark plug, triple gapped, connected to any plug lead and crank. The spark should be strong and blue - and while the shop certainly should have checked ignition (and indeed ign timing) testing it yourself is simple. Likewise; it would be very remiss of the shop not to verify fuel pressure - but they would not change, or recommend changing, the fuel pressure regulator if pressure was to spec…

With the engine idling; disconnect the multiplug on the AFM, observing changes. Which are not conclusive, but there seems to be some sort of ECU fall-back fueling when the AFM is faulty…

Fuel consumption while driving is somewhat inconclusive - with a fault we tend to push pedal for more power, and fuel consumption will increase.

I very much suspect a clogged exhaust, although the shop should certainly have checked this. Plain overfueling should not in itself limit rpms; the engine burns the amount of fuel the air allow and eject the rest of the fuel unburnt. However, a clogged exhaust will gradually build up back pressure, limiting rpms and power - the engine may idle ‘fairly well’. That the problem has gradually increased also points toward this diagnosis - clogging begets more clogging…

The crude way of verifying this is to disconnect the downpipe from the manifold. It is extremely noisy, but conclusive…

Ultimately, the ECU may be the culprit - though it is very reliable it cannot be conclusively eliminated except by swapping it with a working one…

xj6 85 Sov Europe (UK/NZ)

I have to correct myself, if it’s idling well, fuel return can be ruled out. Pressure regulator to manifold is unlikely but worth a try.

If the problem can be switched on and off by putting it into gear it should be a lambda issue, cause to be found. Clogged exhaust is possible.