Misfire in Damp/Wet Weather

In winter last year and as it’s getting damp again this year (in Australia) I’ve been having trouble with misfiring under load (accelerating or uphill) after the engine (4L in 1990-model XJ40) has warmed up. The problem seems to be slowly getting worse.

I first thought it might be damaged insulation on the spark plug leads, but on a dry day I tried spraying water all over the engine bay and it didn’t miss a beat when I revved it. Then I found that the wires to the oxygen sensor had been rubbing against the exhaust and burnt the insulation off one side, exposing the signal wire and one heater wire. I taped them up thoroughly and secured them better, but the problem remains.

I tested the oxygen sensor after replacing the blown exhaust manifold gasket (in case that was leaking enough to throw off the readings), and it swings the full range from 0.1v to 0.9v as expected after it’s warmed up, but (on a cold day) that took over five minues whereas Haynes suggests that it should start swinging after about two minutes. Anyway at around five minutes it’s also about the same time since starting from cold after which the misfiring starts happening under load. The voltage to the oxy sensor’s heater, and the heater resistance, both checked fine.

I am pretty certain it’s related to damp/wet weather. I drove ~100Km in heavy mist one morning and it was getting upset at the slightest hill. Then by the afternoon the weather had become sunny and dry for the trip back, and it went fine even at full throttle up a steep freeway entry ramp. Similarly the more dampness the worse the misfiring is (and the less load is needed to cause it). Also a few seconds after going through a big puddle the engine stalled altogether once.

In spite of the fairly successful test, I still suspect the oxy sensor, which has done at least the 60,000Km life I see quoted online. I’ve also seen reference to splash shields for these but I don’t see anything on mine.

Or does this remind anyone of another fault? I replaced the coolant temperature sensor (and its connector, which crumbed in my hands) with a new one recently to cure troubles with starting (which seems to have worked), but anything else is suspect. I have checked for water in the spark plug tubes.

I have had an intermittent error code 22 that I’ve ignored all the time I’ve had the car, only happening occasionally, but it doesn’t correspond at all to when the misfiring happens.

It does sound HT/damp related, but could be low coil output voltage.
If your engine still uses a distributor cap, check the inside isn’t suffering condensation build up.

Is the coolant thermostat operating properly, warming the engine quickly?

The XJ40 does utilise a distributor cap, I would also check the spark plugs for corrosion where the leads interact with the plugs.

I seem to recall a J-Ler a while back experiencing a misfire due to arcing at the high tension tower of the coil, best observed in a dark garage or at night. Which could occur only with, or be worsened by, damp conditions, methinks.

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Have you checked the MAF for being clean - since you stated that the problem surfaced going through some fog, I am wondering if the MAF is getting wet and affecting the intake air signal - Tex.

We had a very rainy week in early June. It was raining for almost a whole week. My car sat for three days because I didn’t go anywhere. Three days later it was cranking but didn’t start even after like 10 tries. I removed the battery, charged it and next day started right up. But lately I feel pushes coming from the engine like the combustions are not complete. It goes away after the engine/engine bay warms up. No CEL.

Condensation accumulation, from different air temps of night vs day, building up on a distributor cap, will cause arcing, and therefore a misfire, and will cause carbon tracks for future misfires - also may cause cracks on cap, inside or out, - learned about this living in Texas and experiencing cars with moisture affects - before your next morning start, try removing the cap and use a hair dryer inside and out and then reinstall it - watch the order of the wires before removing so all gets reinstalled correctly - even before drying off, look closely at the cap, inside and out, for any signs of cracks or carbon tracks, and also check the rotor for carbon tracks - Tex.

Thanks for the good advice. Will do that. I keep a spare cap and rotor in the trunk though.

We suffer a lot with wet weather here in the UK even in most Summers we can have weeks of heavy rainfall. I’ve always used WD40 to repel moisture. I spray it all over the distributor cap and use a rag soaked in it to wipe over the HT leads. I also use it on all the visible relays and electrical connectors under the bonnet and to wipe down the aerial mast. In 20 plus years of driving 2 XJ40’s I’ve never had a misfire caused by water/damp.

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The spark plugs and distributor cap looked OK. My problems happen after the engine has reached “N” on the temperature guage, so I doubt much moisture would be left around the ignition parts then, plus spraying in the engine bay with water from a spray gun didn’t reveal anything.

The MAF is something I haven’t considered. I live on a gravel road so it sucks in lots of dust, quickly clogging up the air filter (which I blow out with compressed air between replacements, no change in this behaviour afterwards). I’ve never had the MAF sensor out before so maybe it’s a mess inside, and that would explain why spraying inside the engine bay didn’t reveal anything. I’ll give that a look this weekend, or maybe sooner.

Thanks everyone!

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I cleaned the MAF sensor with MAF cleaner spray, cleaned its electrical connector, and swapped the distributor cap and rotor over from my parts car. But no change to the misfiring issue driving yesterday in the drizzle. I think it’s worst for a few minutes after the engine has reached normal temperature, then not as bad later when the whole engine bay might have warmed up more too.

The MAF sensor had some dust inside which I cleaned off, but I couldn’t actually clean off the dark bits of the sensor wires. I tried rubbing them lightly with a cotten bud but nothing seemed to come off, so is that a problem? I could try swapping the MAF sensor over from my 80s-model parts car.



I didn’t get around to testing the signal from the MAF sensor as described in the Haynes manual, and I think I might test the throttle potentiometer and inlet air temperature sensors too when I get the chance. When my coolant expansion tank cap failed last year and coolant overflowed, I think liquid in the throttle pot or its connector might have been why the car went into limp-home 3rd-gear-only mode (that was one of my warning light bingo trips), so maybe that’s a hint about a sensor with moisture issues.

I assume by your mention of the Haynes manual that you do not have Jaguar’s AJ6 4.0L Engine Management Systems and Diagnostic Guide. It has a lot of good information including troubleshooting and testing flow charts. The file is too large to attach to a post, but if you would like to have it here is a link to the document file in my Dropbox account:


Possibly not a cause of your misfire but I once had a non-start during a significant rainy spell. A significant clue was no needle movement on the tachometer whilst cranking. I separated the connectors for the crankshaft position sensor lead, spritzed them with WD40, and reassembled them, which solved the problem. Usually a failure of the CPS (or wiring/connectors thereto) is a total no-go, but I once had a failing CPS cause occasional misfires/stutters before it died, so it might be worth cleaning/drying that connector.

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Thanks, it seemes I have downloaded that guide before, and forgotten all about it! Time to have a proper read.

Sounds like swapping over the CPS from my parts car might be worth a try as well.