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Cold Start Nose-over Problem

Cold Start Nose-over Problem

John Napoli, Brian Pel, Ian Grant, Earl Parkhurst, Brent Smith

Andy Morris wrote:

Here's one for you guys. My 87 XJ-6 runs beautifully ... as long as the temp is at, or above, 60C. When you first start the car, If you take off right away VERY gently it is OK, but if you try to put your foot in it AT ALL it "noses-over" and sputters. When it does this I have also seen slight puffs of black smoke from the exhaust. Once the temp hits 60C this stops and never happens again. It never did this until I put in new plugs,cap,rotor and air filter about a week ago.


John Napoli -- Who was correct

Did you accidentally disconnect a vacuum hose? Like the big one on the steel pipe under the air cleaner? Often, big vaccuum leaks can be compensated for by the ECU once you are in closed-loop, but fazool things when in open-loop, like when the car is cold.


Brian Pel -- Describing another possibility

Regarding Andy Morris's cold running problem, I attach a series of emails which discuss an absolutely identical problem I had with my 1981 4.2 litre. The capacitor fix works beautifully and I have had no additional problems to date. If it is of use, I can post the contents of a Jaguar Canada Technical Service Bulletin which details the problem and the fix. The bulletin provides somewhat different advice as to the connecting pins to use and the capacitor size. However, this may be due to the particular model year.


Ian Grant -- Rich cold idle

My problem seems to have been solved with the following fix received from Earl Parkhurst. This after many and horribly expensive efforts over many months and mechanics, and replacement of anything that might possibly have been the cause. Statues to Earl are being erected in the city as we speak, but thanks to all who offered other thoughts. I'm impressed with whoever diagnosed this, as most diagnostic equipment would have masked the problem when connected up (in a seperate communication Earl explains that the problem is a circuit resonance for the cold values of circuit components).

The only thing that I'd add is that the connector has 7 pins so there's no need to look for the pin numbering (which doesn't seem to be there anyway). Pins 3 and 5 are the same whichever end you start from. Also I assume that for longest life the capacitor polarity should be positive to the left side (working from the RHS of the car with the connector pointing towards you in the same alignment as when plugged to the airflow meter) but you can check polarity with a voltmeter first before you add the capacitor. Thanks to jag-lovers for connecting me to the solution.


Earl Parkhurst

I won't go into the electronic explaination, but if you remove the cap that covers the cable connection to the mass airflow meter, you will find a seven pin connector under the rubber boot. Solder a 25mfd electrolityic capacitor from pin 3 to pin 5 and it will clear up your problem. I had the same thing on my 84 and this cleared up my problem. Several others in our club also tried this and it cleared their problem. What is happening is you are "double fueling" (giving two pulses instead of one) the injectors when it's cold. I got my capaciitor from Radio Shack for a little over a buck. Go for no more than a 25V cap. as they start to get large after that. When it's installed slide the boot back down over the cap. and no one will tell the difference.


David Shield Offers a Clarification Regarding "No more than a 25V capacitor"

The voltage rating is the maximum rating. If you exceed the voltage rating on a cap, it'll fail. Exceed it by enough and it'll make a tiny explosion. Either one of the below caps should work. What's going on is by adding this capacitor, you're smoothing out a noisy voltage supply to the airflow meter, which in turn takes the noise out of the output. If the voltage supply to the meter was smooth (like when it was new) then the output would also be smooth. You're just restoring proper operation. You should also tighten up the major power connections, like all the ground connections etc. That's probably contributing to the noisy supply line.

The capacitance rating (22 or 32mfd) indicates how strongly the capacitor will resist a change in voltage. The voltage rating is the max voltage it will tolerate before failing. As either of these values increases, so doees the physical size of the capacitor. I strongly recommend using the Tantalum variety (dipped plastic, usually blue or some other bright color). They don't dry out. The electrolytic kind (tubular black) dry out, especially in such a hot place as under the bonnet. And then they're useless.


And Hunt Dabney Adds

The larger value will slow the response of the airflow meter a bit more than the smaller, so I'd try the 22 and see if it does the job. If you still have the problem, try changing to the 33.

BTW, I poped the cover off my airflow meter, loosened the screws to the resistive element and repositioned it slightly to get the wiper off of the worn area - gave me another 3 years before developing the noisiness.


And David Shield Contributes Again

Either the 22 or the 33mfd capacitor should be fine. More is better in this instance. What you're doing is filtering out the ripple in the line voltage to the airflow meter. More capacitance (up to a point) will provide more filtering.

Large capacitors like this, especially the commercial grade (that you'll find at Radio Shack) typically have a wide tolerance, like +/- 20% or more, so the exact value isn't critical. For example, if the 25mfd is on the high side of it's tolerance it could really be a 30mfd, and a 33mfd on the low end could actually be 26.4mfd, and both are within specs. For this application, such tolerances are OK.

Again, go through and tighten all the major connecting points like battery cables and grounding points. If these are loose or dirty it will be a major contributor to the noise source (that you're trying to filter out).


Brent Smith Helps to Clarify

Mark Moburg wrote:

I've seen references on the list to the 25 microfarad capacitor fix, and printed a posting that quoted the Jaguar of Canada service bulletin, which said to solder the capacitor across terminals 6 and 8 of the AFM multiplug. Trouble is, I can't figure out which one is 6 and which is 8.

I just installed a capacitor due to luke-warm starting problems. It seems to improve it, but not completely. Anyway, if you disconnect the meter cable and look at the connector, it will have seven pins. The positive end of the electrolytic capacitor gets connected to the third pin from your left as you look at the connector; this is pin 8 on the diagram and should have a pink/green wire connected to it. The negative end of the cap gets connected to the fifth pin from the left; this is pin 6 on the diagrams and should have a pink/yellow wire connected to it. By carefully sliding the rubber boot back you can solder (low wattage iron!) the cap in on the pins at the back side of the connectorand then cover it all up nicely with the boot. I used a Radio Shack 22mF cap with a 25V rating, although I think I might increase to 35mF and see if the problem goes away.


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