Posts Tagged ‘diagnosis’

I got in my Citroen ZX typed the code into the immobiliser keypad, cranked and cranked, but no start.  Panicked slightly, at this point it was 18.30 and I was 60 miles from home.  Gave it another go and voila fired up with the most almighty plume of blue smoke.  The car then drove flawlessly at 70 mph all the way home.

Next day I came to drive it and this time it just wouldn’t start.  Due to my flawless drive home only a day earlier, I knew it wasn’t injector issues.  If it was, the failure would have left me down on power and dropping clouds of black diesel smoke.  Due to running fine once started, it can’t be the last two reasons for not starting listed below:

  •  The glow plugs are not being powered – (won’t start, runs fine once going.)
  •  The stop solenoid is not lifting. (won’t start, runs fine once going.)
  • Air in the fuel (might start and cut out or idle/run erratically)
  • Low compression, ring or valve wear. (driveability and fuel consumption both suffer if you do get it started)

Don’t worry, their is no lift pump to fail, fuel is brought to the high pressure pump via the hand primer found on top of the engine mount on cam belt side, the only failure is for it to allow air into the sytem.

First things first, these engines will very rarely start from cold without red hot glow plugs – cranking simply does not generate enough in cylinder temperature.  They are really easy to check, all you need is:

  • 5 minutes
  • An 8mm spanner / socket
  • A multimeter

1) Check you have a decent battery.  I set my multimeter up to read in the 20v range and dropped my meter probes onto the battery terminal.  Expect to see well in excess of 12volts for a healthy battery.  With the engine off mine reads 13.14v.

healthy battery

2) Now you need to measure the resistance to ground for each plug.  To get an accurate reading I suggest removing the power supply screwed to the top of each plug.  Look closely in the image below, I have removed the power from the left most glow plug.

Disconnect the power rails to all glow plugs

3) Set your multimeter up to read low resistance, I set mine up for 200 ohm or less.  Now connect the meter’s ground lead to a suitable ground, negative battery terminal is fine.  The positive should go on the top where the power rail plugs, expect a resistance of less than 1 ohm.  My number 1 glow plug measured a corrected 0.7 ohm.  The correction is -0.1 ohm as this is the resistance measured when I touch the two probes together.  If  the glow plug has failed it will read open circuit, giving the resistance of thin air (very very high!!)

glow plug resistance

4) Assuming all plugs are in good shape, you MUST check that they are being powered when you turn on the ignition.  Do not skip this step because it is most likely that a sudden no start condition is caused by a bad earth or failed relay box, pictured below.

Relay and timers for powering the glow plugs

5)  So if I think back to school a couple of formulas can be used to theorise what voltage should be read from the power rail feeding all 4 glow plugs.  Assume all plugs measure a corrected 0.7 ohms.

~ Rtot =1/( (1/0.7) x 4)

~Rtot = 0.175 ohms

We can meaure the voltage across one plug, this is essentially acting as a potential divider :

– Vin is the supply voltage acting across the whole circuit, this is battery voltage = 13.14volts in this instance.

– Rtotal is the equivalent of Rtop = 0.175 ohms.

– Rbottom is the resistance measured earlier across one of our glow plugs.  Now we want to measure Vout across that glow plug.

Vout = (0.7 / 0.175 + 0.7) x 13.14

Vout = 10.51 volts

Furthermore we can apply kirchoffs 2nd law: “The sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop.”

Applied below, where x = potential drop.

13.14 – x = 4x

13.14 = 5x

x = 2.63

one potential drop is observed at 13.14 -2.63 = 10.51 volts.

In actual fact I measured a mere 9.43 volts across my glow plug when I turned on the igniton on during glow plug warming period:

The crucial step, power up the plugs and measure voltage on the power rail

The crucial step, power up the plugs and measure voltage on the power rail

So how much heat are these babies pumping out when they get switched on?

V = IR

I = 9.43 / 0.7

I = 13.5A per plug, giving a power of P = IV, P = 127 Watts.

Anyway I seem to have digressed.  Assuming you have power going to your rail and after approx 20 secs you see the voltage across a charged glow plug drop back to ground listen to the relay box as it clicks back.  If something is amiss, try bashing the relay box or simply unplug, clean up the contacts and plug back in.

If you absolutely must get home, I recommend spraying 1-2 seconds worth of easy start parrafin or perhaps even try some WD-40 into the intake!  This has a low flash point and will combust from cold without glow plugs, allowing diesel combustion to initiate.  Beware combustion occurs rather uncontrolled, it causes nasty knocking- potentially damaging big end bearings.  It can also cause the engine to over rev as it burns uncontrollably, that being said, use sparingly better too little than too much.

If those checks confirm glow plugs to be working, it is likely that the failure to start is caused by the stop solenoid. On my car it is used in conjunction with the immobiliser.  When the immobiliser is satisfied you have typed the correct code it sends a message to a unit hidden behind a tamper shield.  This unit, upon receiving the message, drops 12volts across the solenoid and lifts a plunger up.  This plunger is designed to stop fuel from being drawn from the fuel filter into the pump when down.  Check out the suction cup on the bottom, the action of the pump drawing fuel in, actually pulls the plunger on tighter, when in the ‘off’ position.  Here’s what it looks like when removed:

Stop plunger

The offending article

So with this stopping fuel flow, my engine was not ever gonna start!  To get to it is not elementary.  The stop solenoid is hidden under a hardened steel shroud.  There are 3 sheer bolts plus a nut and bolt at the bottom stopping any ‘would be’ thief from doing exactly what I needed to do – remove the plunger and therefore allow the engine to start.

I borrowed the services of a local handyman for the afternoon.  He got the shroud off in a couple of hours using a hammer chisel and centre punch to turn the sheer bolts.  And here it is off…

tamper shield

You can make out where the chisel went in to those bloody awkward bolts.  Don’t be afraid to disconnect the high pressure fuel lines and intake manifold to improve access for that chisel to work the bolts round ‘slowly but surely’.  I am told the most awkard of the three is the one at the bottom (blurred in this image, directly below the other two).  He used a mirror to get a visual on the bolt and hit it round with a centre punch.

With the tamper shield off, we pulled the stop solenoid out and watched to see if it popped in and out with the ignition and code.  It did but I reckon that was due to all the disturbance it had had.  I insisted we reassemble the pump with the plunger completely removed from the solenoid to see if the old girl would run.

We had to bleed the high pressure fuel lines as they had been taken out to improve access.  This was achieved, by loosening one at a time, the unions into the injector body and cranking the engine until fuel started coming out at the injector, repeat for all four.  Now I cranked for a further 3 – 4 secs whilst my assistant held the throttle open.  It fired and ran for the first time in a week!  I quickly removed the key from the ignition and of course it kept going.  Beware of doing this, I believe you remove the battery circuit from the alternator whilst it is still generating current, this is not advised and can damage stuff!

I cannot switch the car off any more with the key, I need to pull on the fuel cut off under the bonnet or simply stall the engine in gear.  I could wire a new solenoid to operate off ignition feed but where’s the fun in that?  Instead I simply tug on this rip cord when I’m finished with the car. (see pictures)

Manual engine shut off

FIXED!

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Before starting this tutorial on changing and installing a new differential for your MX5, have a look at  this link if you are interested / unsure what a differential is or how it works, this provides a good explanation.

Prepared for replacement

Diagnosing the problem

Diagnosing a problem is actually quite easy once you understand what’s going on.  Anyone who drives will have at some point heard a funny noise or felt something wasn’t quite right.  This is the first stage to fixing a problem- identify that one exists.  I will now work through the logic of diagnosis.

In my case the interior of the car was filled with an awful whining sound.  I know it can’t be a wheel bearing, experience tells me they make a lower “wub wub wub” noise.  Secondly this is usually only heard when the car is rolling, (regardless of acceleration, coasting etc).   Also, expect for wheel bearing noise to increase or decrease depending on cornering loads. In my case the sound went away when i rolled along in neutral.

We now know that by removing the power the noise goes, hence, it must be ‘powertrain’ .   Powertrain components essentially consist of engine and gearbox.  It can’t be the engine, the noise was not changing with engine speed.  Stop and think, if revving the engine does not immediately speed up the whine, it can’t be connected to the engine- and my sound wasn’t.  In fact, if the diff  is failing, the frequency and pitch of the noise must be directly linked to road speed.  Road speed is associated with gearbox and in particular the output of the gearbox, consisting of the layshaft, final drive and differential.

Identifying the component making a loud whining noise is not easy, the noise fills the cabin.  In the end I made an informed guess!  With some input from my Dad and a work colleague, it was decided that the most likely culprit on my car- a Jap import , was the early spec viscous limited slip diff (LSD).

Replacement

So I left work early on Friday and nursed it 250 miles in the slow lane, hanging out with the lorries.    Next morning, I headed to MX-5 Heaven, to collect my reconditioned diff.  It just had to be another LSD, I love the way the car handled with the viscous diff, a generous helping of opposite lock mid corner on the power was commonplace!

The upgrade employs a torque sensing design, ‘torsen’ diff.  This was originally fitted to a later model 1800cc NA MX-5.  It is actually a nice upgrade for those considering fitting a turboA far more robust and consequently larger form factor.

The recon

To upgrade an NA MX-5 (Miata) diff you must have the following:

  1. Replacement diff (obviously)
  2. Replacement drive shafts- shortened torsen spec  (due to larger size of the torsen replacement)
  3. Replacement prop shaft- shortened torsen specific
  4. 0.65 litres API GL-5 Differential/gearbox oil
  5. Goggles (I had to have rust removed from my eye, by the eye hospital during the job!)
  6. 23mm socket (Diff filler plug)
  7. 24mm socket (Diff drain plug)
  8. 14mm- removal of rear wheels & drive shaft to diff flange
  9. Hammer and chisel or equivalent to remove staking from hub nut
  10. 29mm socket (remove hub nut)
  11. Breaker bar and almost certainly some kind of extension
  12. Jack & axle stands
  13. Wire brush for cleaning duties
  14. Assortment of ‘standard’ spanners and sockets for other bits and piece
  15. Replacement exhaust gasket (as you will be splitting the exhaust)
  16. Long flat screw driver or equivalent (to remove rubber exhaust mounts)

Replacement parts

The night before starting, it might be helpful to spray the areas you need to loosen with some penetrating oil.  Once equipment is available you are ready to begin.

Start by simply slackening the hub nut (using your 29 mm socket), here is a picture of me in the process.  I applied so much torque, the wheel was rotating on the driveway despite the car being in gear with handbrake on!  Have someone stand on the foot brake to prevent this.

Remove hub nut

Once those nuts have been cracked, get the wheels off and car in the end.  Then remove the driveshafts from the diff using your 14mm socket.

Gain access to the diff for it’s removal.  This requires splitting the exhaust, up near the catalyst and dropping it off it’s rubber mounts (I used a long flat blade screw driver to lever it off).   Also, don’t forget to unplug the small plastic sensor found in this area.  My Jap import has a metal bar connecting the lower wishbones together for added rigidity.  Here I am working it loose with Dan’s help, the bolts were all corroded.

Remove lower arm brace

Disconnect the driveshaft from the diff flange, in preparation for their removal as shown (below).

Now to get the drive shafts out disconnect the top wishbone from the upright (below), then drift the splined shafts out allowing them to drop onto the waiting ground below.

 

The final problem is the Mazda power-plant frame (PPF)- this is designed to give good throttle response, by ensuring that the gearbox and the diff are rigidly connected.  I found that by loosening it, it is possible to slide it off the diff and with some string tie it out of the way.  Don’t be afraid to get rough with this sturdy old bit of girder!

Now you have access to the diff, it is simply a case of disconnecting the propshaft from the diff and removing the old unit.  Take note here, as far as I can remember the propshaft just slides into the gearbox.  Expect gearbox oil to seep out, when you pull it.  To avoid any further mess I would recommend supporting the car at an angle, (one hole higher on the axle stands at the rear) this helps keep the gearbox oil in the gearbox when you do remove the prop shaft.

The time had come and because Dan wanted to be a hero he bench pressed it out.  We loosened the single 17mm and  two 12mm bolts attaching the diff ‘arms’ to the vehicle.  Then, getting underneath it and undoing the final bit lowering it out by hand.  It’s fairly awkward but gravity is working with you.

To replace is the reverse of above, this time I would suggest utilising a jack so that you can line up the bolts without having to hold the heavier diff in the air.  Prepare for some swearing as you try and mate the PPF with the diff, whilst simultaneously offering up to the car for attacment.  Once up get it tightened up quickly, you don’t want to be looking for your 12mm bolts when its in position!

The offering

Now would be a good opportunity to replace the diff oil, for peace of mind.  I used 0.65 litres of API GL-5.  Unscrew both plugs and allow to drain.  Replace the lower plug and with the car level fill to the top of the filler (do not over fill).  Now replace the top plug, torque to approx 45Nm.

Fix the shorter driveshafts back on.  Smear the upright to top wishbone swivel nut in copper grease and reattach.  Reattach exhaust and all other things which are lying around on the floor!  Put the wheels back on and put the car back on planet earth.

I kept my old driveshaft nut, it is advised to replace however.  Now torque up the driveshafts to 260Nm and re-stake them.

All done

The scary thing now, was driving the car in hope that the noise did not persit- thankfully it didn’t, all was well in the MX-5 and the new diff gave the car a slightly different handling characteristic.  I have found that turning through a chicane has the effect of locking the outside wheel, this is great, except when you wish to finish the chicace, turning in the other direction.  It is impossible without lifting off.  Without lifting, the diff remains locked up and is effectively preventing the wheels from taking alternative radii, if this happens to you, you will feel the car’s front wanting to push on forward and the steering wheel becomes incredibly difficult to turn.

Hopefully this tutorial has been helpful.