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Omar M. Shams
Grand Master
Username: omar

Post Number: 1460
Registered: 4-2009
Posted on Tuesday, 10 April, 2018 - 03:37 am:   Edit PostDelete PostView Post/Check IP

My 96 Azure is amongst the best cars I have ever owned in my life. I love this car. It drives like a dream and looks a million dollars. It sometimes idles rough however. Other times it is ok. When she idles rough the engine speed fluctuates from 500 to 1000 rev/min and stays hunting in that range. If it ever dips below 500 rev/min the check engine light comes on. I then need to clear the fault and the fault is always "P300 Random misfire detected".
The local Bentley specialist tells me that this is a common fault on Zytec systems and that there are diodes and transistors in the ECU that fail over time. He suggested swapping ECUs with one of my other cars (which I will try to do one day). He also says that there is a company in the UK that repairs these ECUs (Bowling Co) and that they are inundated with customers all asking for the same repair.

What are your thoughts chaps?
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Jeff Young
Grand Master
Username: jeyjey

Post Number: 369
Registered: 10-2010
Posted on Tuesday, 10 April, 2018 - 03:51 am:   Edit PostDelete PostView Post/Check IP

24 years would be a little on the short side for diodes and transistors, but not unheard of if they're in a hot environment (the ECU, not your climate -- although that might contribute as well).

My 07 Arnage had a similar issue but without the hunting. Couldn't pin down when it was rough and when it wasn't. Dealer suggested new plugs. I was skeptical but dang if they didn't do the trick. (It shifts much smoother now too -- the only thing I can think of is the old plugs were keeping the ECU from retarding the ignition during a shift.)

Anyway, if you haven't tried new plugs yet it might also be worth an experiment.
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Omar M. Shams
Grand Master
Username: omar

Post Number: 1461
Registered: 4-2009
Posted on Tuesday, 10 April, 2018 - 04:18 am:   Edit PostDelete PostView Post/Check IP

Many thanks Jeff.
I put new iridium plugs in the car when I first commissioned it a year ago. The plugs were new and appropriate. However it is not uncommon for new parts to fail I suppose. The car has behaved this way the whole time I have had it.
The random misfire and the fact that it is not confined to one cylinder makes me think it may not be the plugs. Having said that I am happy to try any of these cheap fixes if there is merit.
What plugs would be the best of the best?
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1839
Registered: 9-2004
Posted on Tuesday, 10 April, 2018 - 06:17 am:   Edit PostDelete PostView Post/Check IP

Omar, I would first whack some fuel cleaner additive in the tank, then if no good then give the MAF sensor a good clean, if it dose not do the trick then I would check for any vacuum leaks and also the fuel pressure.
The above faults could cause running weak with hunting.

If all the above dose not sort it out scope the electrics.
All IMO.
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Jeff Young
Grand Master
Username: jeyjey

Post Number: 370
Registered: 10-2010
Posted on Tuesday, 10 April, 2018 - 07:13 am:   Edit PostDelete PostView Post/Check IP

Yeah, it doesn't sound like plugs if it's acted the same since you changed them.

Swapping the ECUs sounds like a sound plan....
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Larry Kavanagh
Prolific User
Username: shadow_11

Post Number: 164
Registered: 5-2016
Posted on Tuesday, 10 April, 2018 - 08:14 am:   Edit PostDelete PostView Post/Check IP

Bad plugs would be one cause of intermittent random misfire but this seems to have been ruled out. Provided you're happy that the plug leads are good the next test would be for vacuum leaks at inlet manifold gasket and all vacuum pipes by squirting carb cleaner on them to see if there's any change in revs which would indicate a leak. If that doesn't show up anything then the next test would be for fuel pressure, a fuel pressure gauge would be needed but it may simply be a case of needing to renew the fuel filter rather than the pump. If that doesn't cure it then the battery may be defective, it may have enough power to start the car but not enough to run all the electrical components especially if the air con is being used. Fuel economy would normally be adversely affected by any of the above issues.
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Colin Salmon
Yet to post message
Username: spurfatherfigure

Post Number: 1
Registered: 3-2018
Posted on Tuesday, 10 April, 2018 - 05:39 pm:   Edit PostDelete PostView Post/Check IP

O2 sensor is a place I would check.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1840
Registered: 9-2004
Posted on Tuesday, 10 April, 2018 - 06:25 pm:   Edit PostDelete PostView Post/Check IP

I had thought about the 02 sensors however Omar stated the code P300!

If a 02 sensor DTC is shown in most cases the code showing is reference to bank 1 sensor 1,2,3.
bank 2 sensor 1,2,3, or whatever.

Battery failing etc will also throw up fault codes.
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Robert J. Sprauer
Experienced User
Username: wraithman

Post Number: 26
Registered: 11-2017
Posted on Tuesday, 10 April, 2018 - 09:27 pm:   Edit PostDelete PostView Post/Check IP

I would seriously check the plug wires . After 22 yrs of service they would be suspect. In some cases you can see the arcing in a very dark location. You have to look carefully. Many wire sets are not replaced because in many cases its not an easy procedure. In some cases there is corrosion at the terminals.
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Patrick Lockyer.
Grand Master
Username: pat_lockyer

Post Number: 1841
Registered: 9-2004
Posted on Tuesday, 10 April, 2018 - 10:35 pm:   Edit PostDelete PostView Post/Check IP

Yes, that would be a case of the scope electrics
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Omar M. Shams
Grand Master
Username: omar

Post Number: 1463
Registered: 4-2009
Posted on Wednesday, 11 April, 2018 - 02:41 am:   Edit PostDelete PostView Post/Check IP

I am most grateful for the support gentlemen. Many thanks.
I have replaced the plug leads as these are easy to change on this car. They go straight form the individual coils to the plugs.
The O2 sensor would throw a different code as already stated - thanks for that suggestion too.

I like the possibility of a vacuum leak as these cars have difficult induction systems to disassemble/build. Before I take any of that off I will get a kit from Paul Yorke to allow me to put new seals/o rings during rebuild.

I also like the fuel filter option - Paul I will need one of those too.
Once I have the induction system off I will look at everything including a possible leak in the intercooler matrix.
I have many batteries I can try at a later date too, but I have already been there and this has not shown any difference.
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michael vass
Grand Master
Username: mikebentleyturbo2

Post Number: 452
Registered: 7-2015
Posted on Thursday, 12 April, 2018 - 12:43 am:   Edit PostDelete PostView Post/Check IP

Hi Omar
I think the error code is a red herring, it's coming on because the engine is nearly stalling so not turning over fast enough to fire.
I wonder if any of these points could help?
https://www.autoserviceprofessional.com/article/92420/The-dreaded-engine-surge?Page=3
I think if you can stabilise the idle speed it will stop the error codes.
Just my opinion, hope it helps
Mike

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