GTD Cold Running Issue

[BugginPaul]

I've got an interesting one..

My '91 Golf/Rabbit GTD has an interesting problem.... I've not set the pump up yet, but i know that it needs doing as cold start always needs the lever out... it's on my to-do list

The issue is that once it's started (fires up first turn of the key) that sometimes it drops down to three cylinders, and you have to wait for the block to warm through and the 4th to kick in before you can drive it... it also doesn't like going above 2krpm (driving) when then block is cold or it'll do the same and drop down to three cylinders again

I'm a little stumped as to what the issue is 

[8v-of-fury]

Glow plugs and timing I'd say, or low compression on one or two cylinders.

A bad glow plug or two will not get that cylinder up to sufficient temperature during cold mornings and make the engine run badly. Low compression will also do this, as it too will take longer to warm that cylinder up to sufficient temps.

Retarded timing will cause rough cold starts as well, even with good compression and good glow plugs. Pulling the cold start lever advances static timing of the injection pump. If it runs better with it pulled out, your pump could do with a few mm of advance. I run mine at 1.03mm.

good luck

[BugginPaul]

Glow plugs are new - put brand spankers in as i had to rebuild the head when i got the car

will check the compression when i time the pump.. but it looked like a good lump when the head was off...

we will see

Thanks

[BugginPaul]

I've checked the timing, and it's set bang on 1mm (1.00mm)

Now, if i stick a multimeter on one of the glow plugs when i switch on, should i see 12v, or will i see less due to the current draw?

If i see less and it should be 12v, then what possible causes would cause the drop?

Ta

[Vincent Waldon]

You will generally see less than 12V at the glow plugs since they are drawing 50+ amps thru all the little resistances along the way... 10V is more common.

In terms of "runs on 3 cylinders until it warms up a bit" it's been my experience that it's one of three things... in approximate order of occurrence:

1) a completely dead glow plug.  Can't be detected using by measuring the voltage... best to remove the plugs and test them on the shelf.  In your case you have brand new ones, so this is unlikely.

2) a worn injector that is mis-spraying or not spraying at all until it is warmed up.  Have your injectors pop-tested when cold... stick 'em in the fridge even to make it really obvious.

3) an engine where one or more cylinders have weak cold compression that gets better as the engine warms up.  The standard compression test misses this, since it's generally done on a warm engine.  Instead, compression test your engine when stone cold.  Then re-install the injectors, warm the engine up, and repeat the test,  looking for one or more cylinders with a big gain in compression.

80/20 it's an injector...   ;-)

[Rabbit on Roids]

a couple of my engines did this same thing. and it turned out to be bad compression. ran pretty rough when cold, then smoothed out with a little throttle, or smoothed out when it was a little bit warmed up.

not trying to contradict you, but just stating my personal experiences. i know mine wasnt caused by injectors. my audi and 2 VW diesels have new glow plugs and fairly recent injector rebuilds..

[BugginPaul]

You will generally see less than 12V at the glow plugs since they are drawing 50+ amps thru all the little resistances along the way... 10V is more common.

In terms of "runs on 3 cylinders until it warms up a bit" it's been my experience that it's one of three things... in approximate order of occurrence:

1) a completely dead glow plug.  Can't be detected using by measuring the voltage... best to remove the plugs and test them on the shelf.  In your case you have brand new ones, so this is unlikely.

2) a worn injector that is mis-spraying or not spraying at all until it is warmed up.  Have your injectors pop-tested when cold... stick 'em in the fridge even to make it really obvious.

3) an engine where one or more cylinders have weak cold compression that gets better as the engine warms up.  The standard compression test misses this, since it's generally done on a warm engine.  Instead, compression test your engine when stone cold.  Then re-install the injectors, warm the engine up, and repeat the test,  looking for one or more cylinders with a big gain in compression.

80/20 it's an injector...   ;-)

Cool, would make sense that it could be an injector... it smokes like a *** if you try and drive it... but if you do drive it, it does clear, but you feel it stuttering..

i'll get a set of recon injectors into it and see how it is (won't hurt as the speedo says it's done 153,000miles)

[rabbitman]

I've had one GP go bad and it'll fire right up but miss on that cylinder for less than a minute before smoothing out.

Shortly after that another one went bad and I had to crank for 10-12 seconds and when it started it would really shake for about a minute.

All this was with 400-420 psi compression which is barely above minimum.

Just saying it's probably not a GP causing your problems, even lower compression would exaggerate it.