So I'm working through my list of problems on my newly acquired 95 VDP and today I wanted to have a look at the misfire I can feel at idle. The misfire comes and goes fairly cyclically.

Using my Elm327 wifi tool and OBD Fusion on my iPad I got set up to read the engine data. However, some of the values seem quite strange and the car seems obstinately incapable of setting a CEL. I did have a stored P0300 code, random misfire, which I cleared. Now, even though I can hear and feel the misfire very clearly, the scan tool reports nothing amiss and the car does not illuminate the CEL.

I looked at the live data which also seemed strange.
The STFT on showed
B1S1 at -1% and 1.26v
B1S2 at -25% and 1.27v
B2S1 at -1% and 1.27v
B2S2 at -20.31% and 1.26v
The LTF%T on both banks showed 54.69%
RPM 739
Calc Load 18.16%
Engine Coolant Temp 192F
Ignition advance C1 8 degrees
Intake air temp 108F
Mass Air Flow 0.01 lb/min
Engine in closed Loop, using O2 sensor feedback to determine AFR

The O2 sensors voltages and % changed quite slowly, not sure if it's the sensors or the scan tool? I'll look at them with a DVM later. It seems like the ECM is at it's limit and can provide no more correction? but before I go and spend a bundle to have everything reoriented, I wanted to see if I could find obvious problems with the car or my scan tool.

Thinking it might be an issue with the O2 sensors, I disconnected the Coolant temp sender. The engine rpm rose ever so slightly but more importantly the misfire died away almost completely. However, when I checked the scan tool, no DTC was set and the CEL was off. The software was now displaying the engine temp as -40F!!!

Odd, so I then disconnected the AFM, same results in terms of misfire vanishing but also no CEL and no DTCs?

Is there some special set up procedure for OBDII on the MY95?

Also, I haven't checked out the ECM yet but I will, is it possible or of any benefit to upgrade to a later ECM that might be more compliant?

 

It took about 10 miles for my CEL to fire when my coolant temp sensor died. If that helps at all.

 

These cars are quite reluctant to set misfire codes. The S-Types will set misfire codes whilst running smooth as gas-turbines, yet these reportedly have to be chugging along on about 2 cyls before they'll set one! Only OBDII code my 95 has ever presented is the P0411 Secondary Air sys fault. Seems to be a more reliable procedure to change the plugs and buy an extra coil, start at one end or the other and move the new coil from station-to-station until the misfire goes away. Or run it in the dark with the coil-cover removed first to see if you have a lightning show - though many have reported a flickering instrument binnacle if they have that going on.

 

aholbro1 is right about the X300 misfire DTC. A slight misfire rarely shows as a cylinder number, sometimes just P0300.

The later the cars, the better the DTC reporting and the shear number of faults that are detected.

Read the DTC guides and notice the increased number of pages of DTCs with each successive model/generation of Jaguar.

The 1988/89 only had 5 faults that could be detected by the ECM.
Now there are HUNDREDS.

bob gauff

 

Thanks. I will try a longer run and multiple runs as well as I read that some faults require at least two trips to register as a fault. Very different from the AJ6 in my XJS. Disconnect the the CTS and you immediately get a CEL and an FF code.

I already tried the light show test, no joy. I will start an old school investigation later today or tomorrow but right now I'm leaning towards a wiring problem.

Thanks Bob, Yes the P0300 is the only code that I seem to get but I wouldn't say my misfire was slight or irregular. You can clearly feel and hear the misfire. Seems like at least one or two cylinders are dropping out on a regular basis for 5 seconds or so before coming back. The fault is cyclical and pretty regular. When the cylinders come back, idle is sweet and smooth.

That's what I was wondering. Is there an updated PROM available nor the ECM or is it possible to fit an ECU from a later MY car? I'm concerned that the data that I can read with the scan tool may not even be correct. For instance, it reports that my ignition advance is 8 degrees at idle (700 rpm) is this normal? Also MAF is showing at 0.01lb/min, normal? Both readings do change with increased rpm and loads.

I have the Fuel, Emission Control and Engine Management pdf open as I write this. Lots of good info but it's not exactly clear about when the DTC will be triggered. Most often the phrase like 'the DSM will decide wether to set the CEL and trigger a DTC' is used. But I will read it thoroughly and then get on with a step by step fault finding procedure

 

A few quick, before-my-morning-coffee remarks, all FWIW....

My '95 is also very reluctant to set codes. I've disconnected the CTS and the MAF.....no codes. Went for a year with the air injection unplugged...no codes. I have multiple coil failure...no codes.

OTOH, the OBD has tormented me with a P0430 at totally random intervals for years now

About a year ago I went thru absolute hell and high water with an idle misfire. Very long story cut short I replaced six appeared-to-be-perfect spark plugs (only a few months old) with six brand new spark plugs and the problem disappeared. I have no explanation.

As for coils, the "lightening show" test will prove a coil is bad. Lack of flashing from a coil doesn't prove that it is good, though.



Cheers
DD

 

Hi Doug,

Yes I just read some your threads on the misfire. I appear to have exactly the same issue. I have already replaced the plugs but to no avail The existing plugs were correct RC12's and I replaced them with the same. I set the gap at .035" but I'm going to try a smaller gap and see if that helps.

 

It is annoying to see that phrase again and again in the service manual, "the DSM will report to the ECU, which will then decide whether or not to store a fault and illuminate the CEL etc etc..." like it takes a community discussion and a vote as to whether a fault occurred!

There are some differences in the OBDII from 95 to 96, but in my opinion, changing the ECU to a later version would probably cause more problems than it solves.

Good luck, you will get it figured out eventually, and we will help as we can!

 

I agree!!!
I swapped a 1997 XJ6L ECM to a 1995 XJ6 to test for a starting fault another shop was having.

The fault was still there so we ruled out the ECM. The engine ran poorly with the swapped ECM though.

Upon reinstallation of the 1997 XJ6L ECM to my car, the idle was RACING so I had to perform a TPS and O2 sensor orientation with my WDS.

The reinstallation of the 95 ECM to the 95 car did not change anything.

If you do not have access to a WDS I would recommend against swapping modules between cars.

bob gauff

 

Thanks Bob, that's good to know.

So it's back to my trusty Avo, scope and amateur detective skills