Erratic idle

[Turftech]

Well, maybe "erratic" is a bit strong, but my 90 Jetta TD (MF) has always had a weird idle issue.  I bought the car 6 years ago and never really paid it much attention as it ran perfectly well enough except for a high (over 1000) idle when cold.  It idles down to ~800 when at operating temp.  Ran back and forth 150-200 clicks a day with little in the way of trouble.  4.7-5.0 liters per 100 when I wasn't standing on the throttle through Abbotsford/Langley traffic.
 
I retired the car to pasture last fall, and in December I pulled the head and replaced a lightly blown head gasket.  Was leaking a bit of pressure into the cooling system, but nothing else.  Piston #1 was a bit pitted but within reason.  I polished and cleaned and repaired or replaced anything I found.  Not much really.  Head was dead flat, and I re-shimmed the valves.  New T belt, tensioner, head and exhaust gskts.  Didn't do intake gskt as turbo bolts were seized and I didn't want to wreck them. (Yet)  Left turbo on during re+re.  Re-torqued after a heat cycle etc.  Timing at .102 as per max Bently spec.  I had noticed a bit of jerkiness when accelerating, so I reset the pre-charge, min idle and idle boost screws.  Again as per Bently.  These adjustments were way out, and needed a lot of finessing.  The enrichment screw was also out by a country mile.  It always did smoke a fair bit before, and even during boost, so thanks to the info I finally found here, I believe I finally got it set up good the other day.  Just a low to moderate amount of smoke now and much better power out of the hole.  Everything is stock parts and specs, and she is running like a Rolex except for this idle business.  The pump has a very slight leak at the distributor block to pump body seal.  (I'm going to have another pump rebuilt for replacement)  Injectors all pop tested in Dec. '09.  Some slight dribbling, but fairly decent.  Pop pressure and spray pattern good.  Cleaned nozzles and replaced heat shields at the time.  Mobil1 5W40 synthetic for the past 6 years. 

Hundreds of other tiny fixes and fiddles but still the idle changes from cold to hot!  Is this a sign of a worn pump?  Is it the fuel temperature changing?  When will the Canucks ever win Le Coup de Stanley???

You guys have any ideas?  This is driving me sane.

Cheers!

[blackbird82]

do you notice a sudden drop when it hits 70 degrees (engine coolant temp)
look on top of the pump, if you see a small diagram with a vacuum line it has an automatic cold high idle circuit. at a certain temp it shuts off and whammo, lower idle.
both are adjustable.

[Turftech]

do you notice a sudden drop when it hits 70 degrees (engine coolant temp)
look on top of the pump, if you see a small diagram with a vacuum line it has an automatic cold high idle circuit. at a certain temp it shuts off and whammo, lower idle.
both are adjustable.

There is no diaphragm other than the LDA.  No vacuum line.  Just the pressure line from the intake manifold.  Does the LDA provide a cold high idle automatically?  I'll be dipped in doggie doo if it does...     I believe mine only has the manual cold start.  I spent a lot of time getting those set screws corrected the other day.  (Backed off minimum idle stop screw two turns.  Adjust idle linkage to ~800.  Turn min idle screw until just touching idle stop.  Tighten locknut etc)  I wish I could post a pic!  I am guessing I need to set up some kind of web space to host my pics and insert the link between the [img] code tags right?  Anyway, the idle kind of slowly goes lower as it warms up.  It will rise again if driven at speed with minimal load, running on the cool side of normal.  Goes very low when shut down when at operating temp and restarted within 5-10 mins, then will rise when "cooler" after driving easy for a bit.

Cheers!

[Turftech]

Anyway, I have to leave for a couple days on a job.  Maybe it'll settle down after driving it 2000 kms. with proper settings?  It was way out of whack.

Cheers!  I appreciate all the info available here.

[blackbird82]

if yours is the non electric idle, then it might be doin the same thing mine does. When u start it warm it idles low eh? well the vane pump in your injection pump that sucks the fuel from the tank is like any pump, not 1005 efficient. when the fuel is warm and thin, it slips past the vanes easier. My theory is the high idle when cold vs. warm is fuel temp in pump causing lower internal pup pressures which make all the difference in everything. After a bit of driving to "cool" it you are basically getting fresh cooler fuel from the tank. After a shut off, the pump heat soaks from the engine, and your fuel in there gets pretty hot.
Just my theory.
Pete
John deere