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Fri Apr 20, 2007 10:09 am

on a cold start my bus really struggles with a low idle (almost dying) until it warms up. I thought it may be the Aux Air Regulator, but I replaced it with a new one and I am still having problems.

My idle use to shoot up to 2000rpm when warm, but I recently set the idle at 850(+-75)rpm when the engine was fully warm, so now my idle at "cold start-up" struggles considerably, but when fully warm the idle is ok, I think.

My thoughts are to check the power coming to the AAR valve. Does that come through the double relay? should I check something on the double relay as well??

Also in my "idle" search here in the forum, someone mentioned checking the gap of the spark plugs. I have never done that to my plugs, but I have changed them a few times in the life of my bus. I always thought they were pre-gapped. do i need to gap them?

Would a vacuum leak create this low idle at a cold start that eventually creeps up?I don't know if this is related, but when I take the vacuum hose of the Distributor (SVDA) there is no change in the engine and i feel no vacuum in the hose. I tried revving the engine with the hose off and my finger over the hole in the hose, but felt very little, if any, vacuum. That doesn't seem right to me. On my '71 bus (with a 34PICT3 carb), if I do that at the distributor, the engine will die unless I cover the vacuum hose with my finger. and i can definitely feel the vacuum.

HELP! I would love to have a smooth idle from cold to warm.

Fri Apr 20, 2007 10:30 am

To get a warm idle, did you have to screw the idle screw all the way in? Maybe it's too far in and it's cutting off the air supply at cold throttle? This is a condition I'm noticing on my engine as well. To help, I too replaced my Aux. Air valve, but also put hose clamps on the cracked hose and filled the cracks with some sealant...

How are the 4 braided hoses on the air plenum? Maybe it's drawing in false air from some where?

Fri Apr 20, 2007 10:32 am

I think the hoses are ok. I replaced them. But would a vacuum leak cause the idle to creep up as the engine gets warmer?
what about the no-vacuum issue at the dizzy?

Fri Apr 20, 2007 11:11 am

BusBerd wrote: I think the hoses are ok. I replaced them. But would a vacuum leak cause the idle to creep up as the engine gets warmer?
what about the no-vacuum issue at the dizzy?
I thought the vacuum would increase as the throttle increases...perhaps a clogged vac. line to the throttle? Pinched hose? There is no vacuum at idle, I don't believe.

Fri Apr 20, 2007 11:30 am

I think the bentley has a method for determining if the thermotime switch is actually working in order to tell the CSV to give you a squirt when the engine is cold.

Fri Apr 20, 2007 11:36 am

also take a look at your cylinder head temp sensor. they tend to go south after a while.

when I DD'd my bus, I replaced it once a year.

Fri Apr 20, 2007 12:02 pm

The cylinder head temp sensor is critical to good engine performance, particularly during starting and warm-up.

The cold start valve can be checked by hot wiring the valve and watching the change in fuel pressure on the gage.

for the thermo-time sw. - operate starter, test light connected on the cold start valve harness should go on for several seconds and then go out if the thermo-time sw. and harness are OK. Hope this helps!

Fri Apr 20, 2007 3:07 pm

Thanks for the help guys!

VWBusrepairman wrote: The cylinder head temp sensor is critical to good engine performance, particularly during starting and warm-up.
this sounds promising. I have not checked or replaced this yet. Just to be clear: this is the sensor that is screwed in (13mm nut, I think) by the #3 cylinder air intake.

VWBusrepairman wrote: The cold start valve can be checked by hot wiring the valve and watching the change in fuel pressure on the gage.
I thought the cold start valve shuts off right after the bus has started. Is it invovled in the idling of the bus too?

VWBusrepairman wrote: for the thermo-time sw. - operate starter, test light connected on the cold start valve harness should go on for several seconds and then go out if the thermo-time sw. and harness are OK. Hope this helps!
I need to get a test light with small enough clips. :?

Fri Apr 20, 2007 4:31 pm

BusBerd wrote: Thanks for the help guys!
this sounds promising. I have not checked or replaced this yet. Just to be clear: this is the sensor that is screwed in (13mm nut, I think) by the #3 cylinder air intake.

yes.
I think this will help you a bunch. let us know how you make out.

Fri Apr 20, 2007 4:49 pm

Quote: I thought the cold start valve shuts off right after the bus has started. Is it invovled in the idling of the bus too?

Just like riding a bull. 8 seconds is all it takes.

Sat Apr 21, 2007 7:50 am

Randy in Maine wrote: Quote: I thought the cold start valve shuts off right after the bus has started. Is it invovled in the idling of the bus too?

Just like riding a bull. 8 seconds is all it takes.
those 8 or so seconds make a huge difference as gasoline engines are less efficient when cold, so an extra "sptriz" of fuel is required and this would be similar to the choke function on a carburated engine. Likewise, the aux. air valve supplies extra air to the mixture, which increases the idle on an initial cold start. God luck.

Sat Apr 21, 2007 2:40 pm

VWBusrepairman wrote: Randy in Maine wrote: Quote: I thought the cold start valve shuts off right after the bus has started. Is it invovled in the idling of the bus too?

Just like riding a bull. 8 seconds is all it takes.
those 8 or so seconds make a huge difference as gasoline engines are less efficient when cold, so an extra "sptriz" of fuel is required and this would be similar to the choke function on a carburated engine. Likewise, the aux. air valve supplies extra air to the mixture, which increases the idle on an initial cold start. God luck.

the idle is rough for more than just 8 seconds when starting from a cold engine.
I have ordered a temp sensor from Bus Depot, and I am going to try to find a test light with prongs tiny enough to fit on the FI harness connections so I can check the power coming to the Aux Air Regulator and the Cold Start Valve.

Sat Apr 21, 2007 3:10 pm

Just plunk your volt ohm meter on the wiring harness. that will tell you the voltage right now.

Fri May 11, 2007 7:28 am

Just to wrap this thread up, I'll post the my fix of the idle.

Well, I just got my temp sensor II in the mail a few days ago.

I took out my old one and put the new one in and it idles great now. I really didn't think it was going to make a difference, but I was wrong. thankfully.

It was an easy fix.

Thanks to all for your help and insight!

Fri May 11, 2007 8:41 am

Suggestion: Always keep a new spare Temp Sensor II in your tool box. they are cheap to buy and have as a spare.

The old ones will also bench test "OK" when they are really "bunk".