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Strange blower speed / PCV issues


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After been away from the forum for a while for several reasons, one of them being not using the cars for quite some time, I thought it would be time to start using them again. Having quite some nice weather right now and seeing other people taking their classics for a spin, made me decide that it would be time for me to enjoy them also.

The last posts were about some idle issues. Being kind of rough. It still isn’t as smooth as the 350 in my Lesabre, but also not so bad I’m afraid taking it out for a drive. But still rough enough to keep me fiddling around trying to figure it out.

By now I noticed a couple of very strange things, maybe related, but I can’t figure it out.

My car  is equipped with air conditioning, and when it put the blower switch(on the AC control) on “high” , and the lever on  “vent”, the blower motor starts blowing at maximum speed. So far, so good.

But when I put the car in Drive, the blower speed will decrease?? Looks like it goes back to medium.

This only happens when I put it in Drive. When I put it in Reverse, no problem, full blower speed?? Don’t get it.

The other thing, and maybe it’s related so that’s why I put in in the same topic, is my PCV valve.

I’m 100% sure it not clogged or something but when I take it out of the valve cover, and block it with my finger, I feel, and hear, the valve closing. But no RPM drop whatsoever??

I thought, (and still think) that my rough idle problem had something to do with my vacuum. Looking for a leak or something I noticed those two strange things as mentioned above?

Anyone any idea what to look for, or what could cause this?   

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                    The lower blower speed in Drive is very strange, but one thought....are you sure that you have a good tight ground strap

between the firewall and the engine block? Bad grounds can cause very weird electrical problems, and the ground strap not being a good ground

can be affected by what gear the car is in. the purpose of the ground strap is to ground the body of the car and if the body doesn't have a good ground, the blower motor operation will be affected as it get's it's ground from the body. In my shop we get old cars coming in all the time with the strap either broken, disconnected or missing all together. Just had a 76 Trans Am come in yesterday with a broken ground strap and electrical gremlins. The idle speed normally doesn't drop noticeably from putting your finger over the end of the PCV valve in my experience. The best way to check for a lean condition is put your hand over the throat of the carb when the engine is fully warm and idle mixture screws adjusted to their best setting. If the engine picks up and smooths  out when you do this, you have a vacuum leak somewhere or the idle circuit is plugged up in your carburetor.

Edited by Seafoam65 (see edit history)
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I am using a hand vacuum pump to create vacuum and to monitor if it holds it. I am connecting it to the different components or systems (like vent flaps actuators) one by one,  to shoot the problem.


Edited by George, buick-riviera.pl
typo (see edit history)
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When I bought the car I had my share of bad grond problems. I now that very strange things can happen if you have a faulty / bad ground connetion.

So I replaced all the ground straps and added some extra. That took care of most of the problems. The other one's were also gound related but it was a bad connection at the light housing and so on.


Yesterday I decided to take the blower / AC controls apart. I've kind of looked at them before, but decided this time I would dig in (a lot) deeper. Pfff, what a lot of vacuum hoses. Looks like they have been replaced since they are all plain black, no colours, and very fexible. I Noticed that there were 2 loose connections, but I don't know if I did that taking the controls out, or if it was like that already. Never heard a "hissing" sound from a vacuum leak.


I took the blower switches apart and wondered that it worked at all. very filthy greasy contacts. Cleaned them, connected them and tested it. Worked fine.

Haven't put it all back together, hope to do that later this week and I'll keep you posted about what I found out.


About not having an RPM drop when closing the PCV valve, in almost every case they tell you that is the way to test the valve. Closing it would result in an RPM drop. So that's why I tried it like that.


First getting the controles back together, hopefully it makes a change. (for the best.)


See if I can find a hand vacuum pump around here. Lookes like an easy way to look for vacuum leaks.


Thanks for your thoughts, keep you posted.

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Just a thought, you may want to check the blower motor resistor that sits in the blower duct under the bonnet (hood) and the connections that plug to it. There is also a ground strap on the blower motor too.


The motors do come apart easily and all that tired grease can be renewed and the brushes checked and cleaned, good for peace of mind. That way you eliminate one more suspect!?

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Well, after sorting out the vacuum hose spaghetti, I put everything back together.

At the first startup I noticed that the speed drop when shifting into Drive was gone. Probably a faulty contact I guess. Couldn't reproduce it, so problem solved.


But then I also noticed a vacuum leak at that black rubber vacuum hose connector thing that is pushed on the .... Probably has a name but can't find it, but anyone who has ever worked on it / seen it knows what I mean.

Coudn't get it fixed so for the moment I just blocked that connection. No more noticeable vacuum leak (didn't hear a hissing sound) and I got the slight Idea that the engine ran kind of smoother.


It has to do until I get a vacuum pump.


Went for a nice ride last Monday and enjoyed the weather (and an ice-cream ;)).

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