Idling mystery?

Hi all, I have been struggling with serious idling problems on my 1984 Mk1 GTi for quite some time now. The block has been converted to 2L (dx block, 82,5mm pistins, VW 2L crank), and the cylinder head is flowed with what I was told is a 288 cam. I just built the sub assembly and the head was redone by a reputable engineering shop in my area. I was hoping that rebuilding the engine and replacing all relevant gaskets would take care of what seemed like a vacuum leak causing the idling problems, but sadly the idling is still the same.  I swapped out throttle body, all vacuum hoses, all injectors, even the dizzy with my other GTi to no avail. Finaly in desperation I pulled off the ht leads to see if I could spot any problems there, and to my utter surprise the number one, and number four cylinders are dead on idle. 8O  Pulling off the ht leads from there makes no difference to the idling whatsoever! What makes this even more strange is that the car has huge power when you drive it, thus obviously running on all four cylinders. The offending cylinders do have both fuel and spark, yet no combustion?? :dontknow:  There is a flat spot on take away, and then things seem to sort itsself out from about 2000 rpm and the car pulls strongly. A compression test shows that number one and number four are about one bar below number two and three cylinder. If anyone has any idea what is going on here I would realy appreciate your input.

Anybody with any sugestions please??

camshaft timing out

agree problem with cam shaft, although if the timing was out surely all cylinders would be equally as bad unless it is some sort of funky scatter cam :-) have you checked valve clearances or is it a hydraulic head? maybe 1 and 4 are too tight?

The cam timing is spot on, the co's are correct at 1.5% although its difficult to set with erratic idling. The distributor timing is also correct. The head is a shim head, so I will have to check on the valve clearances as you sugest thanks. The cam is a 288 as far as I know, and I have heard that they can cause idling problems, but I am sure that this is something else.

check that your getting fuel from the injectors whilst the engine is ticking over,its a similar problem to what happened to mine ,turned out to be a blocked gauze filter in the metering head.

injectors could be dodgy but it doesn't explain the low compression test readings which correspond with the dodgy cylinders :-)

Injectors were checked and all four are woking fine, just for good measure I also swapped over the injectors from my other GTi which runs fine with no problems and problem persists, I also checked them while the engine was running (trying to idle) and both dead cylinders have fuel and their spark plugs are firing.

that only leaves the basics

1 piston rings
2 valve seats
3 valve clearances, this being most likely as you say it makes good power although if these were at fault it would make better power once shimmed correctly

or badly ground cam . . . . .

Hi, thanks for the feedback slowstarter.... you dont seem all that slow to me. I will be checking the valve clearances somewhere over the weekend as i havent had the time just yet. I dont realy think there will be any problems with the sub assembly. I just fitted brand new KS pistons and rings together with a rebore and hone. I agree with you that the problem lies in the head somewhere.... hopefully not the cam, but you never know. Anyway I only have the word of the guy I bought the car from that it is a 288, there are no markings on the cam to identify it.

Hope for valve clearances then, thats the easiest to remedy :-)

The larger overlap cams do indeed start to cause problems with k-jet, 288 should be fine although I think 295 onwards would probably be a bit undriveable as it basically forces a bit of gas back up the induction ports at lower gas speeds which can cause the metering head to flutter at lower revs, obviously the best way of smoothing out the airflow on the bigger cams would have to be a turbo or super charger

if you know anyone locally that can do a leak down test then that would more than likely remove the rings head gasket and valve seats from the equation, but at least checking the valve clearances is a ten minute job :-)

I checked the valve clearances this weekend and it seems the "reputable" engineering shop messed up in a big way. :?  Five of the eight valves had the wrong clearances, including the inlet valves on number 1 and four that seemed to have no clearance! 8O Luckily I managed to find the correct size shims and sorted it all out. The good news is that the car is now running on all four cylinders at idle, the bad news is that the idling is still extremely bad. She just barely manages to idle, and the idle screw doesnt make any difference.

I am quite sure that there is no vacuum leak as I have listened to every join in the vacuum loop including the inlet manifold with a thin rubber pipe and everything seems quiet. The distributor timing is correct as well as the cam timing, and the co's are correct. The fuel pump measures the correct pressure, and the injectors have a clean spray. Is there perhaps something else that I am missing? :dontknow:

have you done another compression test since re shimming the valves?

Thats cool, easy fix to poor compression! I am sure the engineering shop couldn't have got it that wrong, maybe there is some other factor at play here!

I second recommending  you recheck compression :-)

When you say that the idle adjustment screw makes no difference, does it actually increase the revs? if not then does opening the throttle slightly with the vacuum pipe off and the hole on the throttle body blocked make it idle nicely? if that works then you need to clean the idle bypass on your throttle body, probably gunked up.

Does the fast idle work properly from cold?

The compression is back to normal now thanks, and I have just cleared up the mystery of the non existant idling :mrgreen:  :clap: . I swopped over the fuel distributor from my other gti on to this one, and the car now idles fine. Does anyone have any sugestions as to what may be wrong with the fuel distributor? Or should I just cut my losses and try and find another one??

Thanks for the advice so far guys