[T3] '72 FI won't idle down

[David]

Well, Jim – you owe me a beer 😉 In my original post I said “ I have the ’72 decel valve installed & working, but plugging it for a test showed no issues.” Yeah, I do know about this valve & its function, which is why it took it out of the equation from my initial investigations. In fact I have the big hose under the IAD removed & capped off, so I eliminated both the decel & the AAR from this investigation. I even capped off the now open end of the AAR.

I know you are suggesting crankcase leaks, but the part that does not make sense to me is the simple volumes of air flow. Let’s say this engine is brand-new and has zero leaks anywhere. The PCV hose to the IAD is a .5” inside diameter hose to take air from the crankcase, but there are TWO .5 inside diameter hoses supplying fresh air to the crankcase, so there should always be an overabundance of fresh air going to the crankcase. What difference would it be if there was a little more air allowed into the crankcase if there’s already more air going IN that can be drawn OUT? Even Bentley says: “ The crankcase vapors are drawn into the IAD, then into the cylinders where they are burned. A PCV valve in the hose regulates the air flow.”

Jim, you asked about the air entering the 2 fresh air breather hoses. At the 1330 rpm idle: I can put my thumbs over these hoses and I can feel suction on both hoses but it made zero difference on idle speed. (This makes me question just how much air can get around the rear bearing that has no seal). I adapted a vacuum gauge to one hose while I plugged the other hose – and I got a vacuum reading of 1.5” mercury. Then I used my “credit card in front of the intake” trick to get the idle down to 850 rpm, and still got the same at 1.5” mercury. This engines intake pulls 18” of mercury at idle, which according to Gemini – it says that signifies a very healthy 1600 CC VW engine.

So, here’s where I’m at today. Put the original idle adjustment screw back in and turned it down all the way to bottomed out. Warm engine idling at 1330 rpm. Next, I made a plug to fit tightly inside the .5” hose from the PCV to the IAD, and it has a 3mm hole through this plug. Adding this plug changed my idle speed of 1330 rpm’s to 1590 rpm’s!!! Then I took the plastic cap off the breather box to expose the PCV valve and plugged the PCV with my thumb. This dropped the idle down to 1000 rpm! So, I put the plastic breather box cap back on, and I took the 3mm plug back out of the PCV hose and the idle went back down to 1330 rpm (as it was before this test). Now removing the plastic breather cap dropped the idle down to 1300 rpm, and now plugging the PCV with my thumb dropped the idle back down to 1000 rpm again. Wow….

Here’s some other info. to add to this idle issue: with a hot engine (oil at 180) – if I shut it off with the key, wait like 5 or 10 seconds – then only turn the key to start: it idles at 900 rpm and gradually works its way to 1330 rpm within 30 seconds. If I again shut the car off with the key and come back 30 minutes later and just turn the key to start – it starts at 850 rpm, and then it takes 2 full minutes to reach 1330 rpm. One more thing: I do have a spare temp. sensor for the IAD, but exchanging it into the IAD made zero difference. I do have one extra head senser, but I have no idea what it came off of, and the threads on it are suspect, so I don’t want to use it and bugger up the threads on my head. 

Otherwise this engine runs so good & strong. I know this all sounds like a leak on the intake side, but what if it is too much fuel being sent instead of too much air?

Dave Pallo

’72 Square ~ Elwood

Fairport, New York

 

 

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