Wondering why AAZ would spin up to 2200 rpm idle on 90 degree (F) days.. - Troubleshooting - VWDiesel.net The IDI, TDI, and mTDI source.

• Wondering why AAZ would spin up to 2200 rpm idle on 90 degree (F) days.. by Vandoka on 14 Jul, 2021 17:43
• Hi All,
  
  I have a 1987 Doka with a recent 1.9 AAZ rebuild. The truck had a great winter piling up 5000 miles in the mountains with no major issues...a few leaks around the oil filter manifold and oil feed line.
  
  Recently on a few hot days I have noticed the truck starts fine and then after City driving for 20 minutes it will get what feels like a run-a-way idle. I take it out of gear and the rpm's linger at 2000-2200. At times they hang there for 10-20 seconds and descend, other times they stay high and I have to put the truck in gear to use the brake to slow the motor down.
  
  On a cooler day, like this morning, the temps were in the 60's and the issue did not occur.
  
  I have tested the throttle cable and it is not stuck. Wondering if others have seen this symptom before?
  
  Thanks.
• #1 by libbydiesel on 14 Jul, 2021 20:08
• What rpm is your idle set to?  Lower it a little and it should fix it.
• #2 by Vandoka on 14 Jul, 2021 22:27
• The idle is set at 900 +/- rpm. It runs smoothly at this setting most days, I can adjust it to see if your solution changes anything.
  
  Thanks!
• #3 by libbydiesel on 15 Jul, 2021 19:14
• Is your AAZ pump stock?  If so then it should have the slow/fast idle when the cold start lever is pulled.  If that is the case, then lower the 'residual fuel screw' which is the screw that the accelerator rests on when the pedal is fully released.  Just a small amount (e.g. 1/4 turn) should fix it.  If it does not have the fast/slow idle, then the same screw is the idle screw and the same fix is required.  What I am saying is not to mess with the slow/fast idle screws that are the stops for the little lever on the back of the top of the pump.
  
  Post pics if in doubt.
• #4 by Vandoka on 23 Jul, 2021 10:36
• Hello Mr. Libby,
  
  Thanks for the post..you may recall you helped me get my pump sorted out on thesamba after it imploded....Anyhow, I cannot figure out how to post a pic. The pump came off the original 1.6 td of the Doka. It has the idle screw on the drivers side of the pump, it intercepts the throttle lever/lobe piece.
  
  There is a fuel adjustment screw with a lock collar on it located in the middle of the pump. Then there is another screw that seems to limit the extent of the throttle lever/lobe piece.
  
  If there is an easy way to add pictures it would make my description more clear.
  
  
  
• #5 by libbydiesel on 23 Jul, 2021 19:18
• If it has the JX pump, then it does not have the fast idle cold start setup.  The accelerator lever stop screw that runs horizontally at the back of the pump is the idle screw.  Try turning it down a little so the accelerator lever can travel a little farther. 
• #6 by Ohiobenz on 29 Jul, 2021 22:38
• Posting pics on here requires that they are posted elsewhere so you can provide a link.
  It's obnoxious and ancient IMO.
• #7 by Vandoka on 18 Apr, 2022 21:32
• Hi All,
  Just wanted to update this topic because the issue has not been resolved by changing the idle screw. Just to refresh this is an original 1.6 td injection pump fitted to an AAZ. The motor runs well during winter months but as the air gets warm, even just over 60 degree F. The motor may intermittently rev high when the clutch is depressed. This may be after driving along at 1500 rpm, 30-35 mph range. If the motor is showing signs of this increased rpm I can depress the clutch and the motor quickly ascends to 2000 rpm. It may stay that way for a moment, 5 seconds, then it descends. 
  
  Any other options for troubleshooting? The fuel lines and fuel filter are new, motor and pump are newly rebuilt. It fires up and idles smooth so I think it is something to do with the increase in temps.
  
  
• #8 by fatmobile on 13 May, 2022 23:42
• Got some white smoke?
   Got clear fuel lines?
  
   Sometimes it's fuel left over after the burn, caused by pissing injector.
   Sometimes the fuel is turned up too high and the RPMs will hang.
• #9 by Vandoka on 05 Jun, 2022 22:43
• Hey Fatmobile thanks for the note. I do not have any white smoke and the fuel lines are clear with no air. I will look into the fuel screw being turned up too high and see if that resolves the issue. With summer almost here the motor is doing the rev. thing again after driving a few thousand miles during the winter without any of the heat symptoms I am seeing.
• #10 by Vandoka on 10 Jun, 2022 09:20
• I have put a little more time into investigating the symptoms of this issue...I removed the crank vent and put it into a can yesterday. Around 2:00 p.m. it was 85 degrees F. I took the truck out for a drive. It fired up normally and idled normally at 900 rpm. After only 10-15 min. of driving when I would push in the clutch the rpm's would linger (if I had been driving along at 2000 rpm and push in the clutch then the rpms would stay high for 5-8 seconds then slowly descend) and slowly drop down to idle. I got to my destination and the truck sat for about an hour. Then on my return home it was scary how bad things were. Driving along the motor just wanted to run at about 2200-2400 rpm. I would have to put it in gear and use the brake a bit. When I would drive along, say in 3rd gear at about 1800 rpm if I pushed in the clutch the engine would rev up to 2400 rpm. This time it would not decrease though. It also had a high idle around 1100 rpm. This was not the case when I initially started it.
  
  The throttle cable moves just fine. I also took it for a drive this morning, temps are around 68 degrees F. It ran fine, idled around 900 rpm the whole time, rpm's did not linger and it had no run away symptoms like yesterday when it was hot outside. This is how the truck normally runs in cooler temps. I have accumulated 8000 miles like this. So I am not sure what is going on during the hot days that is making this motor behave badly.
  
  I have also consulted Giles in the event it were the pump. We feel like it is not based on the normal running.
  
  Other ideas would be that oil is causing some of this run away. This too is a puzzle. The block is rebuilt from Quality German (I know they have some questionable stories out there but the compression has shown to be solid and no smoke)
  
  Another thought is that the old K03 may have some leaks and is delivering oil into the motor when it is hot. Not like I am driving it hard or even using boost when it revs away like this.
  
  I will continue to investigate...but am also open to suggestions for possible causes. Thanks!
• #11 by oblique on 26 Jan, 2023 20:18
• Curious if you managed to resolve this.