Hi Guys,

My check engine light came on this morning with a fuel fail 23 code on the VCM.

I know this is caused by the O2 sensor detecting a "too rich mixture" and have no doubt the car IS running too rich as my gas mileage seems to have gotten quite a bit worse lately.

I was kind of expecting the check engine light to come on as the car has been acting up for about a month now, first symptom was a hestitation when I gave it gas from a standing start, at first just when the engine was cold but now the hesitation sometimes happens even when warmed up.

The other thing I've noticed is that my temperature gauge has started behaving a bit erratically.

The temp gauge on the dash takes a long time to get up to temp and often sits about halfway between C and N, occasionally it will sit a N for a while but often drops back to between C and N even after a fast run. It never goes beyond N, although I get plenty of hot air from the heater so I don't think the car is actually running cold.

If the o2 sensor is working properly, I'm thinking the rich mixture is probably due to the car thinking it's cold even when it's warm and possibly the two issues, the gauge readings and the fuel fail 23 code might be related.

I didn't dive in too deep at first when I noticed the hesitation issue, just removed the intake elbow, cleaned out the gaiter (full of oily muck) and had a look at the air filter, which looked ok.

My feeling is the temperature sensor might be faulty and the TPS might not be in the best shape either!

Any suggestions for tests/checks before I start spending/replacing components ad infinitum?

Thanks

Lawrence,

Those temp sensors are terribly expensive, provided I'm looking at the correct one for your car...... (item 9 & 11)

Thermostat and Bypass Hose-4.0 Litre - Parts for XJ6 & XJ12 from (V)667829 to (V)708757 | Jaguar Classic Parts UK

Do a google search for DBC3728 and DAC2583, at least you can eliminate those at a reasonable rate. The TPS though is expensive.... might have to bite the bullet and try your hand at metering it and re-positioning if needed.

Good luck

Hey thanks Rob, I did as you suggested and found the OEM manufacturer for the 'stat (Behr) and an OEM temp sensor(Lucas), got both for $43, but they wanted $110 to ship to Canada!
Anyway, paid $6 to get them shipped to Seattle, easy for me to get them from there.

I'm going to install them first and see how it goes - the TPS? well, I can see I'm going to have to get a precision multimeter to set up all the stuff on the '40 - mine is ok for general stuff I guess, but from what I'm reading in the management .pdf, I'm going to need a pretty accurate unit ..I'm slowly finding out the car is really only "half mechanical" - electrics play a huge role in these old Jags, eh?

btw, Called the local Jag dealer first - they wanted $120 + 12% tax for just the 'stat - so that was out for sure!!

all the best

Who the hell was going to charge you $110!!! That's obscene. I want to know who it was so I don't shop there. Gonna stand with my Jag forum brothers on this one

Yes, very much electronic, thus my apprehension with the radio replacement. It's very hard to find the stock radio that works!!!

Living in Canada you get used to outrageous shipping, Rob!

Most folks in the US don't know that USPS First Class International mail is super cheap to Canada and quote "Priority" or even worse, some bogus Courier service. To ship the bits I ordered would be under 10 bucks with First Class, and could be a lot less as they can't weigh more than an ounce or two.

Funny thing is, First Class and "Priority" both take about the same amount of time (avg. 2 weeks) as they both have to go through the border and that's where the delay is. Actual time in transit is pretty quick.

The high quotes are more than likely a result of internet shopping carts which never offer First Class - probably a result of "upselling" by USPS.

NEVER ship anything over valued over $20 via UPS standard to Canada - that's when you get FULLY screwed - the delivery guy has to be paid a "brokerage fee" for the parcel before he'll hand it over and that fee starts at about $25 and goes up as the value of the parcel increases!!

Okay, end of rant.

cheers

Replaced thermostat, now car comes up to temperature properly, cleared FF23 code, test drive seemed OK - car ran smoothly. Going to pull clean and gap the plugs, no doubt they're going to be pretty mucky after a month of running so rich.

Let's see if that fixes it!

Hello:

What's the difference between these two parts?

Both are engine coolant temp sensors...so I wonder what is each of them for?

apart from prize (as DBC3728 is much more expensive than DAC2583), what's the difference?

DBC3728 is the sensor that relays the coolant temperature to the ECM which in turn uses this info (along with other sensor inputs) to determine fuel enrichment/injector pulse duration.

DAC2583 is the temperature sender that relays the coolant temperature to the gauge in the cabin; it has no bearing on engine performance.

Larry

Thank you Lawrence, perfect explanation

I replaced that sensor a few months back on my '88 VDP having the same issue you were having. I located one at my local parts store for about $25. I still got intermittent fault code readings and found that it was a faulty wire going into the socket connection. Replaced the connector and no more code popping up.

In my experience, that connector/connection is particularly troublesome on the 40.

I believe it may possibly be the type of wiring that was used on the XJ40's. The insulation surrounding the wiring seems to be very brittle compared to my X300,which is very similar underhood. I believe they also use the same sensor and connector but there aren't that many complaints about it under the X300. Maybe it's due to age?

On our '93 the only place I've had trouble with the wiring insulation becoming brittle is on the top of the engine, so while I'm sure age has something to do with it, heat must also.