Mis-Fire

[JamesBall]

My 944T has developed a mis-fire under anything more than about 1/2 throttle over 2,500 rpm. I have already a split vacuum hose replaced which initially seemed to solve the problem but it returned. It has now been diagnosed by my local specialist as requiring Ignition lead set and I am being recommended to replace the Distributor cap and Rotor arm at the same time, which comes to £360+VAT including labour and diagnosis.

Any thoughts on whether the extra items are necessary/sensible to do at the same time? I have a friendly, much cheaper mechanic who could probably do these at a later date unless it really does make sense to do them all at the same time.

 

[barks944]

Replacing the damaged item will fix the problem. The benefit to replacing the lot is you know where you are with it. I replaced an ignition component only to have another fail a few months later. Additionally you may be loosing spark quailty without knowing it so it might be a good idea even if the really obvious fault is fixed by replacing just the leads. Also the dizzy and rotor arm are relatively cheap, its the leads that cost the ££. Theres a decent ammount of labour and diagnosis charge in there too. So really the dizzy and rotor arm aren't adding too much to the cost.

 

[Lowtimer]

When were they last replaced? If the car has been running around on them for a decade or more they need to be replaced as a matter of course. These things need to be regarded as 'lifed' items if you want to have any sort of reliability.

 

[JamesBall]

I need to check my paperwork at home to see when they were last replaced. It would be been by a previous owner.

Thanks for the thoughts.

 

[JamesBall]

I've just had a quick look at prices online for ignition leads and they seem to vary from £170+vat which my indie is quoting down to £40 at GSF. Do they vary a lot in quality? Any reason not to just go for the cheapest?

 

[zcacogp]

Yes, lots of reason not to go for the cheapest; ignition components are things that it is not worth skimping on.

Having said that, the leads for the Turbo are not that expensive as they are the same as for the lux, and don't have the expensive ends on (that the S/S2 have). About £40 from ECP sounds about right. Look for ones made by Beru (which is what ECP sell, I think) and you'll have OE quality.

To be honest, if you have misfire problems and the parts are cheap (which they are for a turbo), I'd replace the leads, plugs, cap and rotor arm as a matter of course (should all fit within £100 without too much bother) and start diagnosis after these have been changed. And changing them is no more than a 20 minute job, and easy for even the most beginner mechanic - Porsche even give you the tools to do the work in the car toolkit.

Advice? Go to ECP, buy the bits, change them yourself this weekend, then come back on here if there is still a problem. (Of if the problem is solved, for that matter ... )


Oli.

 

[ukmastiff]

Even a mechanical nobody like me should change those bits himself !

I finally bought myself quite a nice trolley jack and axle stands the other week as I am sick of being ripped off by garages for simple jobs. First job this weekend is front pads on theT5 with just the internet as my guide so wish me luck.

Mas

 

[JamesBall]

Thanks all. The price I was quoted for the parts was wrong so it is a good bit cheaper than first thought. Given the diagnosis and other bits and pieces the garage has done I will get them to fit these parts but will bear in mind how easy it is for next time!

I am also looking at having to recondition the steering rack and replace the track rods but have the option of some 2nd hand parts. I have almost paid the purchase price in servicing and fixes in my first 6 months of ownership!

(particualrly if I secumb to the temptation to fit KWv3s!)

 

[zcacogp]

Second hand steering rack and track rods? Daft idea.

KWv3's may be great, but using worn parts elsewhere in the geometry will negate a reasonable chance of their brilliance. Save the suspension cash and get the basics right first.


Oli.

 

[JamesBall]

Surely steering racks don't wear that much - I mean how often do people replace steering racks as part of a suspension/handling overhaul? And track rods are just a straight rod of threaded metal (or in my case bent but of course I wouldn't fit another set of bent rods).

Or are steering racks regularly replaced?

 

[zcacogp]

Every surface-surface interface between you and the road has a bearing on how much control and feel you have from the steering wheel. If those interfaces have any effect on the geometry then they will have an effect on the handling as well.

The steering rack, when worn, will introduce play into the steering system. The track rods (which have ball joints on the end of them), when worn, will allow play into the steering system, and allow the wheels to vary their track unintentionally. This will have a negative effect on the steering feel as well as the grip, the tyre wear and the handling. And may, in some circumstances, cause other components to wear prematurely as well.

That's before asking why you are changing these components in the first place, and how are you going to make sure that the replacement (second-hand) parts aren't suffering from the same maladies as the parts you are replacing.

As with HT leads, don't skimp on these parts.


Oli.

 

[Lowtimer]

I've replaced racks several times: twice on Mark II Escorts, once on a Manta and once on my old Scimitar. They definitely wear. Track rod ends even more so.

 

[sawood12]

ORIGINAL: ukmastiff

Even a mechanical nobody like me should change those bits himself ! 

I finally bought myself quite a nice trolley jack and axle stands the other week as I am sick of being ripped off by garages for simple jobs. First job this weekend is front pads on theT5 with just the internet as my guide so wish me luck.

Mas 

You're right of course, but I find that now i've not got access to a double garage I tend to be avoiding even the simplest jobs on any car. I've been talked into replacing the disks and pads on my mates series 1 Leon Cupra R to save him some dosh (and you think Porsche brakes are expensive!!) and I must admit, though it is a very easy job i'm not relishing the prospect of doing it.

By the way, that lower attachment on the dizzy cap is a real PITA to change on a turbo. Best make sure you have a very short and stubby screwdriver with quite a wide blade.

 

[ukmastiff]

Well I'm clearing out my old front wooden garage and removing the duck house in there to make way for car repairs so anyone nearby will always be welcome to a days car working space , no ramp im afraid it's not high enough but a good trolley jack and axle stands lol

Regards Mas

 

[JamesBall]

That is a nice offer for anyone in the area.

I haven't even got the excuse of no garage - just shortage of time and fear of making problems worse (plus limited tools).

 

[ukmastiff]

You know you can always use anything I have here James just dont expect any mechanical know how lol

 

[pauljmcnulty]

Well I'm clearing out my old front wooden garage and removing the duck house in there to make way for car repairs

Not able to claim expenses any more? [][]

 

[peanut]

ORIGINAL: James Ball

My 944T has developed a mis-fire under anything more than about 1/2 throttle over 2,500 rpm. I have already a split vacuum hose replaced which initially seemed to solve the problem but it returned. It has now been diagnosed by my local specialist as requiring Ignition lead set and I am being recommended to replace the Distributor cap and Rotor arm at the same time, which comes to £360+VAT including labour and diagnosis.

Any thoughts on whether the extra items are necessary/sensible to do at the same time? I have a friendly, much cheaper mechanic who could probably do these at a later date unless it really does make sense to do them all at the same time.

The last thing you want to do is bolt on lots of new parts in the hope that eventually one of them will make the problem go away.. It can be extremely expensive and frustrating. What you should do is follow a troubleshooting proceedure and eliminate and pinpoint as you go. It could be the coil , distributor cap, plugs, ignition amplifier, rotor, high tension leads, shorting low tension cables , poor electrical connections ,fuel delivery system etc etc

First thing I would check is the vacuum lines then I'd check the main earthing points (see electrical schematic below). Some of the vacuum lines and connections go below the inlet and are difficult to access and test. You can buy a simple vacuum test guage at halfords or ebay for £15.00 or so which would give you an instant readout of your vacuum.

A simple earth test would be to connect a jump lead direct from the battery - (negative black ) terminal and connect the other end to a good clean earth point on your engine . See if that makes any difference. Any change in performance would indicate an earthing problem.
The key earthing points are GP1, GPll, GPlll and the strap between engine and chassis (RHD strap)

One other remote possibility is a broken valve spring which a compression test would show up.

 

[Black Beauty]

If the above issues, don't resolve your mis-fire, you may want to do the following.

If you have a multimeter, check the resistance of your crank position sensor and your speed sensor, both cold and after running, and if they are very different, this may well be your problem. The resistances that they should come back to are shown in the clarks guide.

My position sensor was fine when cold, but when hot the resitance dropped off a cliff and when I replaced the offending item, no more mis-fire - she run's like a dream now. I went through the whole process of checking vacuum hoses, leads, d cap, plugs, computer blah blah blah and it was the simplest thing.

If you need a link to the correct article in Clarks guide and any more advice, let me know.

Spence

 

[peanut]

A timely reminder about the crucial crank position sensor Spence.[] its even more difficult to diagnose an intermittant or temperature related fault like yours.

It just goes to show that rough running and poor starting on a 20+ year old car can be down to almost anything. Theres no substitute for preventative maintenance , thorough servicing and a systematic troubleshooting sequence.