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Thread: Misfire under maximum throttle -

  1. #1
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    Misfire under maximum throttle -

    apologies for posting when I need something...Ill make amends in future. Been a Pistonheads user.
    My 98 XJR is causing me a problem. I did post this in the engines section but advicesends me here....

    Under max acceleration the car is blowing / misfiring. It has got gradually worse. I cannot get a code from it because its saying P1601 incorrect module fitted, which is a bit weird because nothing has been touched.

    Its a well maintained 70k miles car.

    I have had a lower pulley upgrade done very recently but prior to this the misfire effect was present but barely there.

    Im not the worlds best DIYer but decided to change plugs and coils which has made it more severe.

    I can put the car in third and it will sit there at 5000 rpm and not miss a beat, I can accelerate to a high speed with my foot halfway down on the accelerator and its fine.

    When I give the car full throttle in any gear Im being shook all over the place, feels like a Lada Riva.

    I have cleaned the MAF with the right spray, however I noticed that where the MAF connects to the snorkel, post filter, someone had sealed it with a silicone, so I put it back after cleaning and just inserted into the tube. Why would they do that, should there be a gasket ? but I wonder whether this could be the cause, I doubt it.

    I am really struggling with this now and any direction would be really helpful

  2. #2
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    P1601 ECM/TCM Serial Communication Error - which I believe means that the engine and transmission control modules aren't talking to each other, so there might be the place to start but further than that is beyond me, sorry.
    2 bikes, 2 cars, too little time to do them all justice. Yet.

    2000 XJ8 3.2 exec in Cardinal Red and Oatmeal. 127,000 miles

  3. #3
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    I appreciate the response, does this point at a wiring fault do you reckon ?

    Should I look at swapping the ECU first ?

    Scratching my head here, if the ECU is duff, that would possibly explain the severe blowing through the exhaust /misfiring / jerking about too, wouldit not ?

  4. #4
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    Quote Originally Posted by zippyworld View Post
    I’m not the worlds best DIYer but decided to change plugs and coils which has made it more severe.
    The above may be a clue.

    The wiring to the coils gets very brittle over the years. You have to be VERY careful when disconnecting the wiring from each coil, it is very easy to crack the wiring.
    2000 Jaguar X308 4.0
    1973 Jensen Interceptor Mk III 7.2
    1969 Jensen Interceptor Mk I 6.3

  5. #5
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    I'll throw a couple of suggestions into the pot. "Misfire under maximum load". First, could that be related to a faulty second fuel pump which only the XJR (and not the XJ has ? As I recall the second pump is only used on full throttle. Secondly, have you tried cleaning the throttle body?

    Are you saying that the problem causing your code P1601 is preventing the code reader from reading other codes ? You could try checking that by disconnecting the MAF sensor or one ignition coil and then see if the relevant code for those faults comes up on start up.


    In relation to code P1601, I'd try removing the modules from under the toolkit by the bulkhead and giving the connectors a clean with contact cleaner.
    Present car: 2000 XJR Platinum Silver 200,000+ miles

    Previous Jaguars: 1977 XJ6L 4.2
    1985 Daimler Double Six V12
    1990 Jaguarsport XJR 4.0

  6. #6
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    OK, I will explore the wiring, hiw can I find out what type / value of cable I would need to replace ?
    The second thing, I may have been looking at this from the wrong direction. This feels like fuel starvation to me, I am also getting a bit of coke in my tailpipes.

  7. #7
    Senior Member Andy K's Avatar
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    Only use the same brand plugs as jaguar use. NGK. Did you use NGK.

    Try lowering the gap. Try 1.0mm and go down to see if you get an improvement.

    Good point about the 2nd fuel pump. Should be able to test it removing the relay No.2 and bypassing contacts. Do you get a little spark? This is good.

    Remove fuse for pump1 (bridge relay for pump 2) and see if the engine will run for 1 minute
    XJR 2001 Pacific Blue
    Range Rover Sport 2005 Red 2.7 TD

  8. #8
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    Quote Originally Posted by Andy K View Post
    Only use the same brand plugs as jaguar use. NGK. Did you use NGK.

    Try lowering the gap. Try 1.0mm and go down to see if you get an improvement.

    Good point about the 2nd fuel pump. Should be able to test it removing the relay No.2 and bypassing contacts. Do you get a little spark? This is good.

    Remove fuse for pump1 (bridge relay for pump 2) and see if the engine will run for 1 minute
    Yes NGK were used.
    Looking at fuel pumps and filter tomorrow.

    I will report back with the findings.

    Thankyou for the ideas

  9. #9
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    Finally got to the bottom of this.

    Two things, the first was a knackered fuel pump.

    The secondary issue was a real head scratcher, it turns out that the Lucas coils ( which I thought were OEM ) well one of them was faulty, despite being brand new. Techician said they are still cheap chinese imports.
    It didn’t show up on the dash because it was only losing a spark at full throttle.

    The final repair was replacing with used genuine coils from a breaker, too pricey from Jag.

    The car had a used pulley replaced on both the waterpump and AC unit. It also got a new alternator and it still ****** squeals when started from cold.....!

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