MKII GTi 8v running rough, please help!

[Horney]

Hi Guys,

I posted this in MKII section, but on reflection it might be better in here.

I'm having a bit of a problem with my newly aquired GTi 8v . I shall endevour to explain but first another question. Which fuel should I use? It's an 87 digifant and the fuel cap says "Minimum 98 ron" or something like that anyway. Does this mean it should run on leaded? I had an 89 Digifant 8v a few years back and that was happy on any old fuel which brings me onto question number 2!

If I run it on standard uleaded it feels likes it missing and is down on power throughout the rev range. If I run it on Super unleaded (ESSO 97 RON) it's fecking quick till it gets hot in traffic and then it starts to miss again under acceleration, although no way near as bad as when on normal unleaded. I've given it a good dose of redex and that has made no difference. I've checked the plugs (Look perfect colour), leads (old but no too shabby), dizzy (again looks ok). Now I had the cambelt changed last week by the local garage, could it be the timing? I read on here (somewhere) that they can be a pain to time (something about timing marks moving).

Also the temp gauage is a bit dodgy and doesn't seem to work sometimes. When it does it never gets off cold until you hit traffic and then quickly runs up to just past where the red LED is. It's from here onwards it runs rough and even once cooled down it doesn't seem to fix te problem. I'm suspect the gauge maybe faulty and infact it's red hot  when it says it's half way. Also 've not heard the fan come on one in the last week which is odd. Not having owned a car with an electric fan in over three years I honestly can't remember if they should be coming on at this time of year.

Sorry for all the questions/ramblings but your help in diagnosing this fault would be greatly appreciated, before I shell out on new leads, plugs etc.

Horney
(Hoping not to get an scary "You need new injectors" type replies)

[davidhawkins_78]

Firstly I've got a Digi GTI too and run her on nothing but Optimax - luckily there's a few Shell stations local and this is the only fuel you can get that's true RON98.

As for the difference in performance this is only to be expected. The Digi GTI has a knock sensor and has the ability to alter the ignition timing to suit the fuel - the result is a tiny bit more power and crisper throttle responce.

As for this misfire, what have you checked so far?

Obviously the over heating is and issue - are all the fluids at there correct levels? No air locks?

With the over heating problems I'd say the coolant temperature sender might be getting some spurious readings - try unplugging the blue temperature sensor on the front of the head and see what happens.

Other than that it could be one of a few things ...

The Coil
The MAS (Mass Air Flow Sensor)
Air Leak

These are the most common things that cause a misfire - I had problems with it myself!!

[Horney]

Cheers dude,

Like I said I have checked the leads etc and all fluids are at maximum. I shall take your advice and have a nosey about tonight.

How can I test the electric fan switch to make sure the fan is coming on? Also you say "air leak" from whereabouts should I check for this?

I've read the thread about the MAF senser so I'll have a look at that too.

Many thanks,

[davidhawkins_78]

Check the fan for the number of wires that connects to it ... if there 3 wires then u have a 2 stage fan and the earlier is stage probably not working.

Best way to test the fan it unplug it and use some wires to use a straight connection to the battery - if that works then check the fuse and the thermostat.

As for the air leaks this is a complete bu**er as they could be anywhere!!!

Best start around the throttle body and the main air intake from tha MAS,

Hope this gets u started matey ... 

[Horney]

Nice one, Looks like a busy night for me tonight and there was me thinking I'd have the time to finish rebuilding the radius arms on the mini. Hey ho!

[Horney]

Well I've just come back from a test run having stripped and cleaned the MAS. It seems to be better but still not right. I folowed the instructions in the haynes for testing it and it all checked out ok other than the ohm readings while moving it were quite eratic, i was expecting a nice up and down movement of resistance but it seemed to jump about a fair bit. This maybe due to my cheapo ohmemeter but I still suspect that something with the ignition system is not right. I'm gonna take it for a long drive tomorrow and see how I get on.

[Horney]

I decided to stop being such a tight git and bought some new leads, dizzy cap and rotary arm. I went out and pooped in ?10 of normal unleaded, sure enough with in 5 mins she's running like a bag of poo. I stop at tescos, 5 mins and a lead swap and everything seems to be good! I'm gonna swap the cap and arm tonight to and then go for a burn, but I think I may have cracked it. Serves me right for not dipping my hand in my wallet!!!!!!

[Rain]

If I run it on standard uleaded it feels likes it missing and is down on power throughout the rev range. If I run it on Super unleaded (ESSO 97 RON) it's fecking quick till it gets hot in traffic and then it starts to miss again under acceleration, although no way near as bad as when on normal unleaded. I've given it a good dose of redex and that has made no difference. I've checked the plugs (Look perfect colour), leads (old but no too shabby), dizzy (again looks ok). Now I had the cambelt changed last week by the local garage, could it be the timing? I read on here (somewhere) that they can be a pain to time (something about timing marks moving).

Also the temp gauage is a bit dodgy and doesn't seem to work sometimes. When it does it never gets off cold until you hit traffic and then quickly runs up to just past where the red LED is. It's from here onwards it runs rough and even once cooled down it doesn't seem to fix te problem. I'm suspect the gauge maybe faulty and infact it's red hot  when it says it's half way. Also 've not heard the fan come on one in the last week which is odd. Not having owned a car with an electric fan in over three years I honestly can't remember if they should be coming on at this time of year.

Sorry for all the questions/ramblings but your help in diagnosing this fault would be greatly appreciated, before I shell out on new leads, plugs etc.

Horney
(Hoping not to get an scary "You need new injectors" type replies)

98ron is the best to use if you read the manual stats 98ron althought it will run on 95 ive found it lumpy and chuggy like misfires soon as i put optimax in ran perfect not even a couple of tanks was like day after i filled up and havent used 95 since.

as for temp use the digital clock on the mfa for engine temp i think this is oil not water but when it says 80-90 on the digital its just right of the led for water temp this seems normal running temp long as it dont go in the red or the light flash i wouldnt worry to much.

the rough running at high temps id have to look up cause i cant remember what causes it but i think its the mfa have the igntion and mfa setup right doesnt mean there faulty but just needs some adjustment.

[Horney]

Cheers DefectiveMK2,

All sorted now. Wel sort of. I took it for a good run last night and the leads have fixed the problem. It would seem the spark was weak and so running on standard unleaded it just wasn't strong enough, but on super it was just about ok. But as the engine bay temperature increased the leads got hot and became worse. New leads = no more problems.

 So after the test run last night I thought I'd swap the cap and arm as I'd bought new ones. Took the old cap off and it was manky as hell and popped the new one on. WHen I went to remove the rotary arm it was welded to the dizzy and need plenty of persausion with hammer/molegrips/pliers to get it off and it came off in several pieces. The only problem now is the new one I have doesn't fit as it's the wrong one. Going back to the local autofactors today to swap it for the right one!

[Horney]

New rotary arm fitted and car drives like new. YAY!