1990 Digifant idle at 3k, intermittent no-start *SOLVED*

[Robw_z]

My normally reliable 1990 2.1 has been acting up.

It consistently idles at 3k rpm. Sometimes it idles normally around 1100, sometimes it will go back and forth between 1k-2500 rpm at idle in a consistent loop, changing about once per second.

I cleaned the ISV and changed the spark plugs. Pinching the air line from the ISV makes no change. I cleaned the engine and looked for vacuum leaks, found none.

The brake booster does hiss, but only in winter. It's quiet now.

Temp II sensor and oxygen sensor are 5 years old with about 40k on them.

Sometimes it won't start, and the only way to get it to start is to push the gas all the way down, and turn the starter for 5-10 second intervals. It gets closer and closer to starting, wanting to, then finally does. If all else fails a push start does the trick. I checked for spark while it isn't starting and it has it, the starter sounds strong. The no-start issue always occurs when the engine is warm and has recently been driven, first thing in the morning it always starts.

Aside from the throttle body having a serious problem, I don't see why the idle can be so high, while relatively normal at other times.

Thanks for any suggestions.

-Rob

[DometicLover]

Is the Throttle Position Switch working?

[Robw_z]

It looks fine but I haven't tested it as the on/off nature of its function didn't fit with the vibe of the problem, in my limited view at least. It seems it may be the Temp II sensor, but I don't want it to be as I will further lose faith in "new" parts, although this one is 5 years old in my book that's still young.

-Rob

[bluebus86]

a maladjusted throotle switch can cause fast idle.

also consider an intake leak. easy to check with an aerosol can of flamanle carb cleaner with spray straw. idle van and squirt each intake joint, waiting a few seconds between each application to find if idle spead changes. also spray the rubber intake boots, intake vacuum hoses, etc..luct

also if you havent already, renew the vacuum hose on the fuel regulator with a high pressure hose, with clamps on both ends, this may prevent an engine fire if regulator diaphram fails. a wimpy vacuum hose with no clamps can be blown off when hit with near 100 psi of fuel, then you go BOOM! much better to contain the fuel, engine goes rich, stalls, and fuel pump switches off, with no fire risk.


good luck
_________________
Help Prevent VW Engine Fires, see this link.....Engine safety wire information

Stop introducing dirt into your oil when adjusting valves ... https://www.thesamba.com/vw/forum/viewtopic.php?t=683022

[Robw_z]

A trusted mechanic adjusted the switch some years ago and its been fine til now. The switch just detects zero throttle and full throttle, right? In my mind to idle at 3k you need more air, not simply more fuel?

-Rob

[jberger]

Unplug the Idle Control Box and see if the idle settles. They do go bad and will cause this exact symptom.

[Robw_z]

Turns out it was this little guy:

I replaced it 5 years ago proactively, the old one never gave me grief. At least I found some backed out CV bolts while under there.

Don't be under it when you pull it out. Just don't.

-Rob

[AtlasShrugged]

That sensor is one of the things I would get from the VW dealer, part number 025906041A..it was only $18 back in 2015. Now it is $24..still worth the money. There are a lot of junk sensors out there. Looks like you found one.



Get the OEM VW "O" ring too. Part number N90316802

[Robw_z]

I'm tempted to get a syncro thermostat housing and finish dealing with this style of sensor altogether. Not sure if the thread-in ones have a better reputation, but they do look more robust. Thank you for the recommendation, this one was a Meisterstatz, and shockingly only $4 due to a going out of business sale. I hope it lasts a year!

-Rob

[AtlasShrugged]

I did a comparison test of several temp sensors for the temp gauge..not the same as the temp sensor for the fuel injection..however..the same build construction and possible variances you get with several aftermarket sensors with the same part number; See the results here:

http://forums.pelicanparts.com/vw-audi-technical-f...nders.html

The Pelican sensor was boxed as Meisterstatz and was the best aside from the OEM VW sensor.