Vehicle details: Vauxhall Astra
Year: 2001
Symptom: Engine misfire,
Author: Nick Hibberd | Hibtech Auto-Electrical Diagnostics

Vauxhall Astra | Misfire under load

This next problem comes from a 2001 Vauxhall Astra 1.6 with a misfire problem under load. The repair shop had diagnosed the coil pack as faulty and fitted a new unit with plugs, but this made no difference to the problem. Before they went any further I was called in for a second opinion. Not least, the tech involved wanted to know how his checks that he’d relied on for countless previous ignition problems had somehow let him down with this one.

Let’s start from the beginning. When I arrived at the car, the new coil pack was still fitted and the engine MIL was flagged. Checking for DTCs revealed the dreaded P0300 GENERAL MISFIRE logged – absolutely no use whatsoever. Retrieving the freeze-frame data only pointed to the same DTC with the engine at moderate load and throttle part open, again not a lot to build on, but as this misfire was present and consistent there was no need to start chasing a particular operating or faulting condition.

I start with ignition as it’s often the most accessible. If direct HT access is awkward then monitoring mutual induction through the coil primary can yield big results. This vehicle’s ignition system has four individual COPs lined up in one sealed unit and placed over the plugs, and the coil drivers are external to the coil pack and built into the ECM, making the coil primary available for analysis.

A quick test was to current-clamp the shared ignition live feed to all four COPs. The purpose behind this is to note and compare the amps consumed by each coil primary: if a good current trace is displayed then we have established the circuit supply voltage, ground path and circuit resistance. Needless to say, each captured trace should be very similar, having a good self-inductive ramp, a good peak current, and a clean release.

Included in any coil resistance is not only physical ohms of the wire wound inside the coil, but also a brief period of self-inductance which opposes current flow and acts as a resistance. Hence we see the current gradually building up rather than flowing at full strength instantaneously.


At a glance they all appeared normal, yet when examined closer I found every 4th current ramp displaying different characteristics. Fig.1a shows a good trace overlaid on our suspect trace. The sudden hike at the start of the inductive ramp suggests a winding short inside the coil, so further monitoring of the HT burn-time is needed. This investigation was taking an unexpected turn towards a coil failure, but this was the new coil pack I was testing. Were there two failed coil packs on the same vehicle?

  • ChA (blue trace) is measuring ignition power supply shared amongst coils
  • ChB (red trace) is hooked up to CYL3 coil primary command
  • ChC (green trace) is hooked up to CYL1 coil primary command
  • ChD (orange trace) is hooked up to CYL2 coil primary command
  • (CYL4 has been sacrificed for the benefit of including the current trace)

Fig.2 was captured during a typical snap throttle test. Engine speed is calculated to be about 2400 rpm and was ascending during the WOT state. The problem cylinder is identified as No.3, with no burn-time, indicating that no spark exists at all. Perhaps not surprisingly, neither are there any coil oscillations. Although the primary inductive kick was found at about 250 V and at a level comparable with the remaining firing cylinders, it is a poor indication of the secondary voltage available. Monitoring secondary information through the primary is only useful for burn-time analysis. This problem is simply a weak coil and confirms what the current probe picked up earlier; in fact, this coil pack is displaying various ignition issues. Looking at the firing cylinders, I picked out the best burn time at 0.57 ms for CYL1, but expect double that for a healthy coil: at least 1 ms. And even on these firing cylinders there are no coil oscillations present when the burn-time finishes, so secondary energy is used up very quickly.

Premature coil failure (and you cannot get more premature than a brand-new coil failing) is likely due to external influences. For example, having the coil firing open-ended puts it under tremendous stress and pushes insulation beyond normal limits. However, I don’t think such an extreme is the case here, but I think this unit was ready to cause problems from the moment the box was opened. I recommended another new coil pack and was asked to stick around by a very cautious and slightly sceptical garage owner.

With the new coil pack in place, Fig.3 was a typical capture taken at snap throttle and just off idle speed. What a difference! The key points of interest include the extinguished hike at the beginning of the current ramp; the peak current that has increased to 14 amps, a full 2 amps up on the previous coil; most importantly a burn time that’s present and holding for a good 1.3 ms; and finally the presence of coil oscillations before and after the burn time. The other three coils all showed similar improvements.

If you buy a new part or component, it’s not unreasonable to automatically expect it to work. If electrical troubleshooting isn’t challenging enough then a faulty new component could send you on a costly ghost hunt. I can only guess that the dud coil pack came from a Friday afternoon batch.

In response to the tech’s question about why his own checks had let him down, I replied “They haven’t”.


10 comments | Add comment

August 23 2016

You could try posting this query on our forum, you are likely to be seen by more end users there. We have a section named “Help me fix my car” -

If you have any PicoScope traces, you can upload them to the forum for an even better response.

Good luck!

Tony Pennell
August 19 2016


Astra G 2003 with 1.6 8V engine.  110 000 miles.  Recurrent misfire on No 2 plug (only) that oils up about every two weeks.  Clean the plug and it’s OK for another two weeks.  I’ve tried everything i can think of that might only affect one cylinder:  Change plugs - no cure.  Change coil pack - no cure.  Change plug leads - no cure.  Check compression - a bit low on 2 and 4, so head off, decoke, grind in valves, fit new valve stem seals, etc. - no effect.  While head off, also fitted new ring set and glaze-busted bores. - no cure, but compression pressures now fine, though a bit smokey for a few days until rings bedded in. Also cleaned EGR valve and throttle unit as someone said it might be them (though i can’t see why that would only affect one cylinder).  People have also suggested crankshaft sensors, etc, but why only one cylinder affected?

I don’t know how I could check the fuel injectors.  Possibly swap No 2 into another cylinder and see if the misfire moves with it?

The persistent misfire has probably wrecked the cat, but I’m not going to fit a new one until I’ve cured the misfire!

Codes not helpful:  ‘misfire on No 2 cylinder’ (and EGR valve, of course - it’s now blanked off!).

Any ideas, anyone?  I’m getting a bit desperate.

March 15 2015

My astra is a duel fuel and when on gas it runs on all four. Once it goes on petrol it only fires on three .......any ideas is it likely to be the coilpack?

January 09 2014

Hi Mark in response to your Astra misfire. You really need to establish the type of misfire you are experiencing. (Fuelling-Ignition-Compression) The information in the case study is invaluable and an excellent path to follow. Assuming you have access to a Scan tool and of course a PicoScope, Evaluate your fault codes, you originally discovered 6 codes, did they return are they relevant or a consequence of the misfire? Before going in to deep are you happy with the mechanical condition of your Astra, the fuel quality, hoses, connections etc.? Once these are in place you have to pursue as the case study indicates. Don’t throw the towel in yet Mark, Good luck

January 06 2014

I have an Astra 53 plate, bought from an auction about 2 weeks ago, it was misfiring on first drive, I had it hooked up to a diagnostic, about 6 faults, my mate said just replace coil pack and leads, which I did, no difference, just done a service, new plugs, filters etc, no difference, Is this going to be costly?? I see its a common fault, I feel like putting it back through the auction in all honesty…

Linda Fuller
February 16 2011

Our 55 plate Astra had a new coil pack 2 years ago. Last Friday the Exhaust light came on, my husband took it to the garage and they fitted another new coild pack.  Not sure why they did that as the exhaust light was on but assumed they had fixed the problem.  Two days later the light is back on and has now gone back in.  Any ideas, why they would have fitted a new coil pack whent he exhaust light was the problem?


Tony House
October 11 2010

I have had sooooo much trouble with my 51 plate Astra, I simply don’t know WHY I have still got it!
New Cambelt, New Plugs (oh, so many), New coil pack and..NEW ECU. Where does it end? Just when I think, nothing else could go wrong.. yep, you’ve guessed it- all over again.!
Misfiring AGAIN, MORE plugs, HT leads now in the offing, then (perhaps) another coil pack to, eventually, a NEW ECU..AGAIN !!

John Wood
May 07 2010

I have the same problem with my 53 plate Astra 1.6, I’ve had 2 new coil packs fitted over the past 2 years I’ve owned it and the car still feels as though it’s misfiring. I’ll print out this page and take it with me to the garage to see if they come up with similar results.
I’ll keep you posted…
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Larry Johnson
March 08 2010

I understand the pause and confusion when it comes to scopes however I think you will find once like all obstacles when you overcome them you find satisfaction and welth of knowlege thats what I found as Ive only been into scopes @ 3yrs

November 22 2009

Unluckily so far never had a scope to test a replacement part after which same problem persists as you said mate the wild goose chase would get to the state of yes as everyone who has experience brand new items do fail.
Mehanical/hardware!!? just one of many oddies A brand new water pump in a truck only to find out that a threaded hole was left plain only to remove so many bits and pieces again to put things right—a simple tap obviousely did the job and text books problems are the most often overlooked in the old days it would be possibile to have a wing attached to the main body by bolts and nuts where earth contact would not be sufficient to light a 21 w An alarm system was having a voltage drop so much to the ign SW in the 6v region that solenoid chatter with the occasional reluctant hold in coil accepting the ever elusive ammount of volts   and mind you only at particular mild lower temperatures mornings and late evenings only the new owner of the car was told that the alarm had been dissconnected Only discovered after fitting an aux relay to energise the starter sol and still no diff whatsoever only sol chatter! and 6v plus was reg at the aux relay just fitted the alarm module/unit was found under the dash and had an external switch in the off poss Well only when physically dissconected some wires the alarm was actually OFF and the engine fired like never before instantanuosely well i am not so sure if i will be able to handle a scope which i seem to be hooked upon i am old school and out of dedication to the car repair trade and challenges i am seriousely cosidering some significant investment cheers to all

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Case study: Misfire under load