Weird off-throttle behavior
06-25-2017 | 05:48 PM
#16
Rennlist Member
Joined: Feb 2012
Posts: 1,397
Likes: 190
From: Under Cruella's Thumb
First, some background. I just replaced my ISV (ICV, whatever) and chased down some incorrect vacuum routing, and some self-created vacuum leaks. Thread https://rennlist.com/forums/928-forum/992389-isv-icv-replacement-86-5-a.html
I was doing that to fix a very high idle and a tendency for the RPMs to dive very low when clutch in, very occasionally resulting in a stall.
I now have mostly-correct vacuum routing (plugged off some emissions-related stuff due to not having an air pump and having a broken nipple on the thermoswitch) and my smoke test doesn't show any leaks, although I measure about 16 in-Hg at the EZF (20-21 at the brake booster) so I may have a small leak somewhere still.
Now, my idle is still a little high (1000-1100) but (ducks head in shame) it's always been that way since I owned it. Haven't messed with the idle adjustment screw yet, so I will see what that does. My idle switch itself passes the click test and I can see it operating via DMM at the LH connector). The thing is, if I have revs up 2500-3000, and push clutch (e.g. as I'm coming to a stop) the revs do the dive, but only down to about idle, then jump up to maybe 1600-1800, then slooowly settle back down to idle. This seems weird. Additionally, hard to explain but coming off throttle and coasting seems to have a delayed jerk, that wasn't there before. Tach doesn't really show this, but it's very perceptible. Almost like there's some lag before it REALLY comes off throttle.
Here is a vid of the tach, just sitting in my driveway, when I give it some gas and then release, to simulate the behavior:
https://youtu.be/yaT241V1oVw
I was doing that to fix a very high idle and a tendency for the RPMs to dive very low when clutch in, very occasionally resulting in a stall.
I now have mostly-correct vacuum routing (plugged off some emissions-related stuff due to not having an air pump and having a broken nipple on the thermoswitch) and my smoke test doesn't show any leaks, although I measure about 16 in-Hg at the EZF (20-21 at the brake booster) so I may have a small leak somewhere still.
Now, my idle is still a little high (1000-1100) but (ducks head in shame) it's always been that way since I owned it. Haven't messed with the idle adjustment screw yet, so I will see what that does. My idle switch itself passes the click test and I can see it operating via DMM at the LH connector). The thing is, if I have revs up 2500-3000, and push clutch (e.g. as I'm coming to a stop) the revs do the dive, but only down to about idle, then jump up to maybe 1600-1800, then slooowly settle back down to idle. This seems weird. Additionally, hard to explain but coming off throttle and coasting seems to have a delayed jerk, that wasn't there before. Tach doesn't really show this, but it's very perceptible. Almost like there's some lag before it REALLY comes off throttle.
Here is a vid of the tach, just sitting in my driveway, when I give it some gas and then release, to simulate the behavior:
https://youtu.be/yaT241V1oVw
I have an early '86 and had that behavior with mine as well. I was able to address via adjustment to the idle bypass screw. I would begin screwing it closed (clockwise) about 1/4 turn and see if that makes a difference. In certain situations I still see a little bounce but I think your situation is that you need to work on adjusting the idle bypass screw and that will help.
Last edited by 928NOOBIE; 06-25-2017 at 05:49 PM. Reason: Please Delete.
06-25-2017 | 05:54 PM
#17
Rennlist Member
Joined: Feb 2012
Posts: 1,397
Likes: 190
From: Under Cruella's Thumb
OK, so back to this issue...if I warm the car up, then jumper B and C at the diagnostic port, I hear a change, but there is not a drastic change in idle speed. I actually think with no jumper, the idle speed varies slightly up and down every few seconds, but not enough to register on the tach, if that makes sense. I think that stopped with the jumper in place, but what I really noticed was a loud whistling noise, that I hadn't really processed before, went away (or at least is drastically reduced in pitch and volume).
Theory: Vacuum leak in the ISV or (more probably) the hose connections to the ISV. Does anyone know if, engine off, the ISV is in any particular configuration, which would cause a smoke test to pass, but open up a leak when the engine is on and ISV active? It makes sense in theory, but I don't know the state of this valve in various conditions.
Also of note: Idle speed control did nothing even with the jumper in place. Anyone know why that would be?
Can I change those ISV hoses without removing the whole intake? I really do not want to open up that can of worms...
Theory: Vacuum leak in the ISV or (more probably) the hose connections to the ISV. Does anyone know if, engine off, the ISV is in any particular configuration, which would cause a smoke test to pass, but open up a leak when the engine is on and ISV active? It makes sense in theory, but I don't know the state of this valve in various conditions.
Also of note: Idle speed control did nothing even with the jumper in place. Anyone know why that would be?
Can I change those ISV hoses without removing the whole intake? I really do not want to open up that can of worms...
I have several areas on mine and use a variety of things to address. Loctite 518 works ..it helps get in those very small places but requires care when tightening down or the plenum will just scoot off. White plastic plumbers tape is working well for me also, without the fiddling you have to do if you use loctite 518 or similar.
If you are using the standard rubber plenum/intake connectors the rubber is soft and there is a limit to how much you can tighten the clamps...tightening the clamps pushes the rubber outward on both sides from under the clamp....result is rubber pushes against the raised metal and pushes the clamp off the clamping surface..also it pushes the rubber in towards the center and can wind up splitting the boot...catch 22...
I want to get everything polished..no more enamel/paint for me...saving pennies and hoping to do it over the winter. I've seen '86 intakes with everything just polished and I like the look...all metal.
06-25-2017 | 06:18 PM
#18
Thread Starter
Rennlist Member
Joined: May 2014
Posts: 3,488
Likes: 57
From: South Florida
Based on my smoke test results, I did originally have a massive leak at the boot on the center T, but taking it apart and putting it back together again seems to have solved that. I didn't see any smoke after fixing that, although I want to try it again, preferably with the air box in place (need to keep smoke from coming out of the air box somehow, though) which makes me think the ISV in it's off state is not letting smoke reach the culprit...maybe? It's a theory so far, not sure exactly how to test an intake leak that isn't present when the car is running...
06-25-2017 | 08:15 PM
#19
Rennlist Member
Joined: Jan 2004
Posts: 1,014
Likes: 186
From: Peoria, IL
The ISV on these is about the stupidest logic you can get. Apply 12V to the close pin, and it snaps shut. 12V to the open pin, full open. The LH modulates the duty cycle of the signal so it's constantly vibrating back and forth as the voltage jumps between the open and close pins, even when it's supposed to be open to a constant position, it's still vibrating back and forth (which is why they wear out so easily).
All of that is a long way of saying that who knows where the valve is going to end up when you shut off the engine.
Connecting the test pins, though, does close it completely.
All of that is a long way of saying that who knows where the valve is going to end up when you shut off the engine.
Connecting the test pins, though, does close it completely.
06-26-2017 | 11:19 AM
#20
Thread Starter
Rennlist Member
Joined: May 2014
Posts: 3,488
Likes: 57
From: South Florida
Thanks for that info. Sounds like I can be sure of testing it closed, and anything else is a crap shoot. I suppose I could run the car, shut it off and smoke test a couple of times...but given the pretty obvious leak-when-running that changes when the ISV is disabled, I think what I need to do is revisit the ISV installation, and probably replace those hoses.
Does anyone know if that can be done without removing the entire intake? I found that replacing the ISV itself was not so difficult, with the air box, MAF and side intake plenum boxes all removed, and the center T pulled out of it's boot and moved up and out of the way (wouldn't easily come out all the way on my car without pulling the rest of the intake) but I'm not sure about those hoses.
Does anyone know if that can be done without removing the entire intake? I found that replacing the ISV itself was not so difficult, with the air box, MAF and side intake plenum boxes all removed, and the center T pulled out of it's boot and moved up and out of the way (wouldn't easily come out all the way on my car without pulling the rest of the intake) but I'm not sure about those hoses.
06-26-2017 | 02:40 PM
#21
Rennlist Member
Joined: Jul 2006
Posts: 5,198
Likes: 395
From: Bloomfield, MI
If you know that all the fittings are snug (don't over tighten them) and not leaking and you have installed a new ISV, then get a known good LH and see what happens.
As Stan has already pointed out a badly worn ISV will take out the driver circuit in the LH, and the engine Idle will sit at about 1200 to 1500 RPM.
Also don't try to use 30 year old rubber, buy new bits, they seal much better
Dave K
As Stan has already pointed out a badly worn ISV will take out the driver circuit in the LH, and the engine Idle will sit at about 1200 to 1500 RPM.
Also don't try to use 30 year old rubber, buy new bits, they seal much better
Dave K
Last edited by davek9; 06-26-2017 at 04:04 PM.
06-26-2017 | 03:06 PM
#22
Thread Starter
Rennlist Member
Joined: May 2014
Posts: 3,488
Likes: 57
From: South Florida
I've been assuming, since a change is seen when jumpering the diag port (even though it's not a huge change) that the circuit must be working. It seems like a failure of that driver circuit would be an all-or-nothing thing?
It's starting to become more obvious that a proper top-end refresh may be in order. I've avoided it because obvious problem areas were done before I bought the car, and some of those items (e.g. injectors) soon after, but I think there is a considerable amount of old rubber in there still that could be causing problems. Ugh...
It's starting to become more obvious that a proper top-end refresh may be in order. I've avoided it because obvious problem areas were done before I bought the car, and some of those items (e.g. injectors) soon after, but I think there is a considerable amount of old rubber in there still that could be causing problems. Ugh...
06-26-2017 | 04:29 PM
#23
Rennlist Member
Joined: Feb 2012
Posts: 1,397
Likes: 190
From: Under Cruella's Thumb
Have you replaced injectors? Just a thought, but I did..and I did not do a good job of using any kind of agent to assist the sealing of the O-rings. Result was I had massive vacuum leaks and got to install them again (and on the passenger side....again after that for some wierd reason)....
I used Loctite 518 on them...then I let them set for about a couple hours then good.
On the plus side...I can go from popping the hood to having the passenger bank of fuel injectors in my hand in 20 minutes now
I used Loctite 518 on them...then I let them set for about a couple hours then good.
On the plus side...I can go from popping the hood to having the passenger bank of fuel injectors in my hand in 20 minutes now
06-26-2017 | 09:27 PM
#25
Thread Starter
Rennlist Member
Joined: May 2014
Posts: 3,488
Likes: 57
From: South Florida
I did the injectors about 2 years ago, with DC111 on the o-rings. This was the single best thing I've done in terms of making the car run better, and solved many problems I was having at the time.
Have you replaced injectors? Just a thought, but I did..and I did not do a good job of using any kind of agent to assist the sealing of the O-rings. Result was I had massive vacuum leaks and got to install them again (and on the passenger side....again after that for some wierd reason)....
I used Loctite 518 on them...then I let them set for about a couple hours then good.
On the plus side...I can go from popping the hood to having the passenger bank of fuel injectors in my hand in 20 minutes now
I used Loctite 518 on them...then I let them set for about a couple hours then good.
On the plus side...I can go from popping the hood to having the passenger bank of fuel injectors in my hand in 20 minutes now
06-26-2017 | 09:29 PM
#26
Thread Starter
Rennlist Member
Joined: May 2014
Posts: 3,488
Likes: 57
From: South Florida
