Why you need your deceleration valve
05-14-2007, 12:52 AM
#1
Rennlist Member
Thread Starter
Why you need your deceleration valve
So, I got my new motor in the car, and got it running
https://rennlist.com/forums/showthre...ht=compression
It was running very nicely, clean exhaust, clean oil, clean coolant, but when compressing braking down hill I had huge clouds of white smoke out the exhaust. Since several people were suggesing I should have changed the head gaskets, I was a more than a little worried.
After sleeping on it a night, I remembered that someone had removed the deceleration valve from this motor. I had not really paid attention to this since I was not sure why I needed it and I have had trouble with them before. Sure enough, during compression braking it was pulling so much manifold vacume that it was sucking oil out of the crankcase and into the intake. Fortunatly I had one sitting around, was easy enough to put on. Now: no smoke, but lots of BWAAAA!
https://rennlist.com/forums/showthre...ht=compression
It was running very nicely, clean exhaust, clean oil, clean coolant, but when compressing braking down hill I had huge clouds of white smoke out the exhaust. Since several people were suggesing I should have changed the head gaskets, I was a more than a little worried.
After sleeping on it a night, I remembered that someone had removed the deceleration valve from this motor. I had not really paid attention to this since I was not sure why I needed it and I have had trouble with them before. Sure enough, during compression braking it was pulling so much manifold vacume that it was sucking oil out of the crankcase and into the intake. Fortunatly I had one sitting around, was easy enough to put on. Now: no smoke, but lots of BWAAAA!
05-14-2007, 01:29 AM
#2
Rennlist Member
i made a point to modify my part euro to match the US Ljet system , including the decel valve. (i had to drill two holes on either side of the throttle plate to connect the decel valve to the throttle body)
after learning what it really did, and that the euro didnt have one, we didnt do it on Scots engine. doesnt seem to be an issue, but if if it fixed the smoking issue, thats good for you!
MK
after learning what it really did, and that the euro didnt have one, we didnt do it on Scots engine. doesnt seem to be an issue, but if if it fixed the smoking issue, thats good for you!
MK
05-14-2007, 01:40 AM
#3
Inventor
Rennlist Member
Rennlist Member
Vacuum limiter: CIS, yes. L-Jet auto, no. (L-Jet manual, no, if idle switch disconnected.)
I'd guess that the white smoke may actually be gas sucked through the CIS injectors. L-Jet injectors aren't affected by vacuum. L-Jet manual cars may stall without the vacuum limiter, if you don't disable the idle switch, in order to stop the decelleration fuel injector cutoff.
I'd guess that the white smoke may actually be gas sucked through the CIS injectors. L-Jet injectors aren't affected by vacuum. L-Jet manual cars may stall without the vacuum limiter, if you don't disable the idle switch, in order to stop the decelleration fuel injector cutoff.
05-14-2007, 01:37 PM
#5
Race Car
Might depend on M/Y. I'm not sure earlier cars had it?
My 83 Euro did have one. And now it has a brand new one. Fitting/securing those new hoses was a bear though!
My 83 Euro did have one. And now it has a brand new one. Fitting/securing those new hoses was a bear though!
05-14-2007, 08:53 PM
#7
Inventor
Rennlist Member
Rennlist Member
PET shows the 84-86 LH equipped euro 16V had a vacuum limiter (#2) in some markets, too. It must be a real rats nest under the plenum!
Australia, Sweden, Switzerland LH2.2 vacuum limiter
Australia, Sweden, Switzerland LH2.2 vacuum limiter
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05-14-2007, 09:59 PM
#8
Mine has been removed from the 79' for about 2 1/2 years and have noticed no ill affect, i've noticed no smoke on engine breaking, but i've never seen behind my car while it's driving. The rpms do how ever crash down on throttle closing, but stop where ever i put the idle at using the air by-screw.
05-15-2007, 12:17 AM
#9
Instructor
'81 928 Australian delivery.
I found an air leak at the diverter valve servo line, so I blocked that and this made the vacuum limiter start to operate, but it was bad, took far too long to return to normal idle speed, hard on brakes as well, so I blocked the vacuum signal to it. Any problems with doing this?? Engine runs just fine without it in my opinion.
Just need to remove it from under the plenum assy, if I can get to it that is without pulling everything else off .
Glenn
'81 9-2-8
I found an air leak at the diverter valve servo line, so I blocked that and this made the vacuum limiter start to operate, but it was bad, took far too long to return to normal idle speed, hard on brakes as well, so I blocked the vacuum signal to it. Any problems with doing this?? Engine runs just fine without it in my opinion.
Just need to remove it from under the plenum assy, if I can get to it that is without pulling everything else off .
Glenn
'81 9-2-8
05-15-2007, 03:17 AM
#12
Rennlist Member
just remove the wires to the idle switch. that fixes the off idle rpm drop. i did this on my Ljet and my LHjet. actually, its almost essential for racing to have it disconnected.
MK
MK
05-19-2007, 03:43 AM
#13
Well I did some testing after reading Karl's post, due to my deceleration valve is out.
And I can't belive I never noticed it before. I could see smoke out the back window when I floored it to red line and quickly let off on the peddle. Thanks for posting your finding Karl! Now SMTCapeCod where did you get your new decel valve from?
And I can't belive I never noticed it before. I could see smoke out the back window when I floored it to red line and quickly let off on the peddle. Thanks for posting your finding Karl! Now SMTCapeCod where did you get your new decel valve from?
05-19-2007, 10:12 AM
#14
Administrator - "Tyson"
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Oh my, how did I miss this thread? 
Take a look at this:
http://video.google.com/videoplay?do...323&q=928+dyno
L-Jet - no vacuum limiter, check out the white smoke at the end when the throttle is closed. Yes the idle switch is connected - I had to. We chatted about this in a different thread.
Previous dyno runs with the car I saw the same white smoke with the idle switch disconnected.
Friend of mine who has a pretty tuned up C4 LT4 Corvette puffs out white smoke like that on they dyno as well. He said "So, that is because there are not any CATS". My car doesn't have CATS either. Just tossing that in there.
There is an old expression in the hot rod world. Modying a car will take a small issue and make it a big one. A small vacuum leak will turn into a massive one. A slightly bad injector will become a huge problem etc.....
In the case of my car, 9psi on a stock engine bad things happened without the vacuum limiter & the idle switch. I was seeing very lean spots (17+) when doing a low throttle, low load roll on. Around town cruising you could feel the car was not happy, made it a real bitch to drive to the grocery store. WOT it was perfect, but that is from the aftermarket fuel add-ons for the blower.
As mentioned before, I connected the idle switch back up as suggested by Z. Most of my lean spots dissappeared, still not perfect but much better (then off to the above dyno). A few days later I put a vacuum limiter back on - went on a 450 mile road trip. What can I say, the car is now 100% easier to drive. I agree on a race car you would not want these items on a car. For the street, they are there for a reason. The L-Jet system was not designed to see more than 19 inches of vacuum.
I wonder if my issue was the stock regulator being pulled beyond where it was supposed to go due to all the accessive vacuum, causing the lean spots? I dunno, total guess.
Originally Posted by PorKen
Vacuum limiter: CIS, yes. L-Jet auto, no. (L-Jet manual, no, if idle switch disconnected.)
I'd guess that the white smoke may actually be gas sucked through the CIS injectors. L-Jet injectors aren't affected by vacuum. L-Jet manual cars may stall without the vacuum limiter, if you don't disable the idle switch, in order to stop the decelleration fuel injector cutoff.
I'd guess that the white smoke may actually be gas sucked through the CIS injectors. L-Jet injectors aren't affected by vacuum. L-Jet manual cars may stall without the vacuum limiter, if you don't disable the idle switch, in order to stop the decelleration fuel injector cutoff.
http://video.google.com/videoplay?do...323&q=928+dyno
L-Jet - no vacuum limiter, check out the white smoke at the end when the throttle is closed. Yes the idle switch is connected - I had to. We chatted about this in a different thread.
Previous dyno runs with the car I saw the same white smoke with the idle switch disconnected.
Friend of mine who has a pretty tuned up C4 LT4 Corvette puffs out white smoke like that on they dyno as well. He said "So, that is because there are not any CATS". My car doesn't have CATS either. Just tossing that in there.
There is an old expression in the hot rod world. Modying a car will take a small issue and make it a big one. A small vacuum leak will turn into a massive one. A slightly bad injector will become a huge problem etc.....
In the case of my car, 9psi on a stock engine bad things happened without the vacuum limiter & the idle switch. I was seeing very lean spots (17+) when doing a low throttle, low load roll on. Around town cruising you could feel the car was not happy, made it a real bitch to drive to the grocery store. WOT it was perfect, but that is from the aftermarket fuel add-ons for the blower.
As mentioned before, I connected the idle switch back up as suggested by Z. Most of my lean spots dissappeared, still not perfect but much better (then off to the above dyno). A few days later I put a vacuum limiter back on - went on a 450 mile road trip. What can I say, the car is now 100% easier to drive. I agree on a race car you would not want these items on a car. For the street, they are there for a reason. The L-Jet system was not designed to see more than 19 inches of vacuum.
I wonder if my issue was the stock regulator being pulled beyond where it was supposed to go due to all the accessive vacuum, causing the lean spots? I dunno, total guess.
