Golf Mk3 (1992) Misfiring???

I wouldn't even entertain that thought until ignition is checked out. On a car with so few miles that shouldn't be the case.

that could be it but so could a lot of other things. What sort of injection system is it? We didn't get a Mk3 1.8 in the US so I don't know for sure what you have. I believe Mk3 1.8s sold in canada and south america and mexico were Digifant but I'm not sure if that's the case with yours.

Thanks for the reply Matt! Don't think it's Digifant, as it's an early Mk3 built late '92. Found this page

which refersto Bosch Mono-Motronic ECU for Golf GL 1.8 litre 66kW (ABS). Have seenit referred to as Single-point fuel injection system but this doesn'tmean a lot to me, currently learning about engines the hard way! Ran compression test last night and all cylinders seem ok. Had friend turn the engine over 6 times each cylinder with fuel pump disabled and ignition coil grounded to engine block. All 4 cylinders reported close to 240 PSI so I don't think there's a valve/piston ring/cylinder head gasket problem.

Also checked RPM when removing spark plugs from each cylinder in turn. Noticed for cylinders 1-3, RPM dropped off slightly when spark plug removed, picking up again when plug replaced. When removing plug from cylinder no. 4, no change. Looks as if cylinder 4 not firing at all when idling, although car feels like it drives normally at speed. Could this be a fuel injection problem??? Spark plug definitely seems to be sparking when removed from cylinder 4, judging by current passing up my assistants arm! Does this indicate ignition system is OK and that something else is causing the problem???

Good. On a car this young (didn't you say it had 70K miles or something like that) I wouldn't expect such a problem.

:) As long as you're getting spark to it, then yes...the other half of the equation is fuel. Spark + fuel = combustion so if you have spark and it still isn't firing, it's fuel.

Clogged fuel injector maybe?

You might need to locate a shop manual and see if there are any electrical tests you can do on the injector to see if the wiring to the injector is faulty or not. If the wiring to the injector is fine, then the injector itself is clogged or defective.

Not so sure it's a fuel issue - have been finding out about single point injection systems and now I realise this means the fuel is injected by a single injector into the intake manifold so I don't get how an injection issue can cause regular misfire on one cyclinder in this configuration! Maybe it's a dodgy intake valve but I would have thought that was unlikely again given low mileage - yes it is 70k by the way! I guess most modern engines would have separate injectors for each cylinder in which case I would focus on an injection issue now!

Going back to the vacuum leak possibility, there is a large rubber hose coming out of the right hand side of the engine block, slightly higher than the level at which the spark plugs are fixed. I'm guessing this is the brake power booster vacuum hose but this disappears into the bulkhead - I would have thought it would head over somewhere near to the brake fluid bottle if it was the brake booster vacuum. If this was leaking, would that be likely to cause a misfire on the nearest cylinder but not affect the other three??? Really confused now :-!

Nice testing.

Does the #4 plug look/smell different from the others?

A plug could be firing in air, but not under pressure. You could try swapping plugs and maybe even plug wires (for testing) if the length is close enough.

I don't know if that car has a distributor or a 4-lead spark pack. If a spark pack, I *think* you could swap wire 1 with wire 4 at the pack, or cobble an extension, and cross connect wires 1 and 4 at the plugs. I think plugs 1 and 4 fire simultaneously if there is no distributor, but only the one with fuel fires. However I would wait until those with more knowledge have had a chance to post why my idea is faulty. Of course if you have a distributor rather than a coil pack, this proposal does not even apply.

Thanks T, Just to clarify, this one does have a Distributor! Also sparks were replaced during recent service (2 weeks ago) but not sure when HT leads/distributor cap/rotor were last changed (if ever!). See what your saying about plug possibly not firing under pressure, but would not have thought this could be a problem with new plugs!

I am not saying this is probable, but consider a gap at the distributor cap or plug wiring system of 1.2 times the spark plug gap. I am thinking that in atmosphere, the plug gap would break down first.

As the compression came, suppose the plug firing voltage rose to 1.5 normal. (I have not tried to figure out what would be realistic numbers). Then the distributor cap gap could break down first, reducing the voltage below the firing voltage of the plug.

Just called local VW dealer to get price for new set of HT Leads. Apparently they are =A320 each (x4) plus the same again for a new king lead and another =A315 for a new distributor cap on top! Think I'll be checking out the prices down at Halfords instead!

On a more useful note, he said that the ABS engine in the 1.8 GL is known for eating distributors and this is a likely cause for misifiring in cylinders 3 or 4. He suggested removing it from the engine bay and checking it's vacuum advance function whatever that means. Guess I'll be checking the Haynes manual again tonight! Any tips???

Just been doing some more investigation! Jammed the throttle open slightly whilst engine running. Unplugged No. 4 HT lead from distributor and still no change in RPM which I guess suggests this cylinder isn't even firing at speed. Sparks are flying nicely between HT plug and distributor though. Strange thing is it doesn't seem to feel any different from normal when driving at speed!

Another thing I've just realised and haven't mentioned before is that there appears to have been some oil leaking from the rocker head gasket area above the no. 4 cylinder. Noticed this quite some time ago and the black area seems to have spread. Suspect this may be a clue to the problem. What does this mean given that compression test seems ok??? Wrecked valve maybe??? Help!!!

All advice gratefully accepted - this is now starting to worry me a bit!

You engine is indeed monomotronic, so a fuel problem is difficult.

I would start by changing number 4 spark plug again. Sometimes they are just bad out of the box. If that does not fix it, change the HT wire for that plug, then the distributor cap and rotor. That SHOULD fix it.

I neither of those fix it, you are looking at either a large leak in the intake manifold near that cilinder, or a leaky or lazy valve on tht cilinder.

And try non/VW cables. I just bought a set for US$15, brazilian made.

Dist. cap might be cracked

-- Kaveman

Crack in the distributer cap

-- Kaveman

Thanks for all the help guys! Got there in the end, unbelievably the problem was caused by a split in a rubber connector, about 4-5 inches long which was clipped onto the intake manifold right by no. 4 cylinder. The other end attaches to the vacuum tube for the brake power booster and the split was allowing too much air to be pulled into the cylinder hence the misfire! So after new plugs, HT leads, distributor cap, rotor arm, the missing link was a =A33 piece of rubber and two hose clips!!! Engine running as new now though so what can I say, live and learn!!!

Hi Stevo,

Been reading the thread with interest, seems you have solved your problem at last!

I have an almost identical problem with a 1992 mk III driver ABS engine, however it's done high mileage at 130k.

It runs very well at speed but has intermitent idling, searching between 850 -1000 rpm, but usually (but not always) fine after being driven for about 20 mins. When starting from cold it often stalls and often feels jerky on accelleration/deceleration, and running slow in traffic is a pain in the a*se.

I replaced the rubber tube next to the rear of the rocker cover which was also perished, so i'm pretty sure no unmetered air is entering, but still it runs poorly.

The stepper motor - throttle valve potentiometer - needed replacing last year, as this is working overtime searching for the correct idling speed.

I have gone round the houses replacing fuel filters, HT leads, throttle body parts are all spotless, etc, each time i think i've found something I think i've cracked it.

Compression test is fine, however i'm thinking of trying to make it someone elses problem for a few hundred quid....

Wondered if you or any others had any bright ideas