Idle drops when in drive

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Idle drops when in drive

Post by SonOfTheGrim on Sun Jul 16, 2017 1:47 am

So I've been dealing with this problem the entire time I've had this engine, back when it was in the Camino and the whole time its been in the Chevelle.

When sitting in park I have my engine idling at around 1100rpm. I know you are thinking thats really way too high to idle. I know that. The problem is that when I shift into reverse or drive the rpms drop by about 300-400 so the only way I can end up around 750rpm in drive is to ramp up the idle.

I have asked local mechanics and searched many many hours on google and have found others with this problem but never a solution. A mechanic here told me I might look at the torque converter but thats the only advice I've gotten. I've checked vacuum going to the trans, adjusted the carb, and wished upon stars.

Anyone ever delt with this problem or solved it? If I remove the bell housing lower cover how can I identify the torque converter? Maybe its an aftermarket stall converter?
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Re: Idle drops when in drive

Post by Joe73 on Sun Jul 16, 2017 6:50 am

What size cam are you running? And are you running a PCV valve ?
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Re: Idle drops when in drive

Post by Damon23 on Sun Jul 16, 2017 8:54 am

I had this very issue with a quadrajet and it was a problem with the carb.

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Re: Idle drops when in drive

Post by SonOfTheGrim on Sun Jul 16, 2017 9:15 am

Far as I know its a stock cam and I am running a PCV.

I also thought the carb might be the issue so I JUST replaced my old Edelbrock 1406 with a Holley 670 street avenger. The Edelbrock had play in the throttle plate shafts and was pretty worn but the newcarb didn't change the issue at all.
What was the actual issue or did you just change the carb and it fixed it?
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Re: Idle drops when in drive

Post by Damon23 on Sun Jul 16, 2017 9:23 am

I had the carb rebuilt and replaced all the old vacuum hoses while I was at it. Would a bad vacuum advance cause this issue? I remember when the one in my 75 was going out it was causing the car to idle weird and stall out.

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Re: Idle drops when in drive

Post by SonOfTheGrim on Sun Jul 16, 2017 9:27 am

I rebuilt the Holley before putting it on. And my vac advance has been replaced about a year ago I think. I've also checked for vacuum leaks already.
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Re: Idle drops when in drive

Post by 73ss on Sun Jul 16, 2017 10:18 am

Sounds like weak springs, heavy weights in the dist. Un-hook the vac advance & plug the port. See what the timing is at idle. Have an assistant drop it in gear while watching the timing. If it drops dramatically you need stiffer springs on the weights. I would suspect your advance is starting to come in at 1100 rpm.
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Re: Idle drops when in drive

Post by Joe73 on Sun Jul 16, 2017 10:36 am

I'd be checking for vacuum leaks first. But first, I'd remove the PCV hose from the carb and cap the port on the carb. PCV is supposed to be a controlled vacuum leak. When you change a cam, the odds are not good that your stock PCV will be correct for your setup. the spring inside the pcv comes in all different weights. Thats why I'll be running a fixed orifice PCV with my new motor. No ball or spring inside. Just a small hole on the bottom. ACdelco CV4000C Fram FV410 Autozone 1009.
PCV can cause all kinds of carb tuning issues.
Just something I like to take out of the equation first. Then play with carb and timing.
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Re: Idle drops when in drive

Post by Hs1973 on Sun Jul 16, 2017 12:42 pm

Sorry to highjack. But i have a quick question on the pcv valve to Joe. Are you talking about removing the insides of the valve, ball, springs etc so it is open all the time? And if you try and run an original pcv on a slightly hotter cam it could cause problems with the idle etc. I have a 355 with a little bigger cam and have always had the pcv valve on it, it has a lumpy idle and stumpel sometimes when i punch it. Sorry again for highjacking your tread.
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Re: Idle drops when in drive

Post by clanceman427 on Sun Jul 16, 2017 1:46 pm

I'd say that a stumble when you punch it would not be caused by any vacuum related things like PCV and vacuum advance because as soon as you stomp it you go to WOT zero vacuum situation. The stumble would be momentary fuel starvation or too much fuel at that instant.
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Re: Idle drops when in drive

Post by bigredlaguna on Sun Jul 16, 2017 2:00 pm

I agree about the timing being the issue. The new engine in my daughter's car does this to a smaller degree. I do have weak springs in it on purpose. Check the condition of the springs or just replace them with new ones right away, and verify the initial timing.
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Re: Idle drops when in drive

Post by Joe73 on Sun Jul 16, 2017 2:07 pm

HS1973. No need to remove or modify any pcv. The part numbers I posted are fixed orifice (tiny hole with no ball or spring). Only because there are no charts in existence that list all the vacuum parameters of pcv valves.

I still recommend just taking the pcv out of the equation because it is a controlled vacuum leak. Your issue is going to be carb, vacuum or timing. If you decide to change springs in the distributor, I'd so some research as to what you already have and what you should be going too. There are no "one spring fits all" spring. Everything has to work together as a unit. Initial timing, advance and total timing. And total timing should "come in" when your cam is "coming in."
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Re: Idle drops when in drive

Post by SonOfTheGrim on Sun Jul 16, 2017 2:38 pm

HS, no problem hijacking the thread. I am curious about the PCV too. I'll isolate it to see what happens.

I completely rebuilt the distributor just a few months ago so I know the springs are the stock ones. Everything down to the magnetic pickup (except the housing and shaft) is new. New bushings, weights and stock springs.
Maybe I need to use heavier springs?
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Re: Idle drops when in drive

Post by bigredlaguna on Sun Jul 16, 2017 3:36 pm

What is the initial advance set at? If it at the factory spec, you can advance it a few degrees. I like to set mine at 10 to 12 degrees, depending on what the engine likes best. If the timing is good and the springs are strong enough to prevent fluctuating at idle speeds, then it would be time to look at carburetor and vacuum issues.
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Re: Idle drops when in drive

Post by pila on Mon Jul 17, 2017 9:25 am

Another question : Is the problem occurring after a cold start ? Or all the time ??....

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Re: Idle drops when in drive

Post by SonOfTheGrim on Mon Jul 17, 2017 11:58 am

This happens no matter what.
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Re: Idle drops when in drive

Post by SonOfTheGrim on Mon Jul 17, 2017 1:59 pm

Thinking about it I thought of some other possible symptoms of this issue. Maybe these are related, maybe not.
The car shifts out of 1st gear very quickly and also shifts pretty hard. I know the fluid is where it needs to be and relatively new and so is the modulator.

I am putting a starter on the car today but I'll check the timing after that and report back.
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Re: Idle drops when in drive

Post by SonOfTheGrim on Tue Jul 18, 2017 1:46 am

So I've checked the timing and it is 11 and doesn't change with vacuum advance at idle. I have checked everything for vacuum leaks. I plugged the PCV and the nipple it was run to. No change at all to the idle dropping when in gear.

I have decided that what's happening is the transmission is trying to pull the car even at such low rpm as idle. When I put the car in gear it lurches and the rear wheels shift no matter how hard I hold the brake.
I am thinking the torque converter is the problem after all though I've never had a bad one to diagnose. Any input?
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Re: Idle drops when in drive

Post by bigredlaguna on Tue Jul 18, 2017 2:47 am

SonOfTheGrim wrote:So I've checked the timing and it is 11 and doesn't change with vacuum advance at idle.

This is a bit confusing to me- are you saying it is at 11 degrees with the vacuum advance unhooked, and it stays there after hooking it back to its source?

If that is the case, you either have a bad vacuum advance can, or you have the vacuum advance hooked up to ported vacuum. I suspect it is on ported vacuum, because as soon as you open the throttle it will gain as much as 20 degrees advance. That is enough to make it feel like it wants to giddy-up when you crack open the throttle. This situation could also make you open the throttle blades more than necessary for the idle speed you want, which uncovers the ported vacuum slot and advancing the timing which makes the idle that much faster. THEN when it is put into gear, the open more than necessary throttle blades create a low vacuum situation which could retard the timing which would make the idle slow down more than it needs to.

So long story short, make sure you have the vacuum advance can hooked up to manifold vacuum and adjust the idle as necessary.
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Re: Idle drops when in drive

Post by Hs1973 on Tue Jul 18, 2017 6:26 am

Thank you for info. I have been in the game 30 plus years, but i still need to learn about engine related things. Im a certified body man and have concentrated mostly on that, so im always on the lookout for things to learn and little good to know advise. A friend and i are going to play with my carb and dissy later to see if we cant make it perform even better. Hope you get your issue fixed also. I would like to say though that i bought a new hei an put it in. It made a total difference in how the car acted. It ran ok before but felt slugish now its Way more pep. Even if my mechanic Said my old dissy was fine, i guess it still had a issue.
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Re: Idle drops when in drive

Post by Joe73 on Tue Jul 18, 2017 6:34 am

I'm with Bigredlaguna. No change when you reconnect the vac. advance?
Thats an issue. Vacuum needs to be connected to full vacuum which would be base plate of the carb or below.
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Re: Idle drops when in drive

Post by SonOfTheGrim on Tue Jul 18, 2017 12:02 pm

I know that's been debated many times and I have moved the line back and forth several times (with no change to my problem) but if you ask Holley they designate a ported vacuum nipple just for distributor advance.
If you Google Holley vacuum diagram you'll see that every one has marked the outlet for timing advance and they are all above the throttle plates.

Point being I've tried that and it doesn't help my problem.
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Re: Idle drops when in drive

Post by 73ss on Tue Jul 18, 2017 9:09 pm

What rpm did you check timing? It needs to be as low as you can get it to check initial timing, 600-650 if it will idle down that low. the springs should be fully re-laxed at a low rpm.
Vac advance: As stated above, If it's not adding 20 or so degrees when connected to full vacuum, It's not working. You mentioned you re-built the dist. Did you attach the end of the plunger to the mag pickup plate?

How does it idle? Does it have a lope? Maybe you do have a cam that requires a loose/stall converter.
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Re: Idle drops when in drive

Post by Joe73 on Tue Jul 18, 2017 9:18 pm

When you have a bunch of time, this is an excellent read describing how timing works and how to set it up. Its a very long read but excellent.

http://www.chevelles.com/forums/13-performance/189195-ignition-101-a.html
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Re: Idle drops when in drive

Post by SonOfTheGrim on Wed Jul 19, 2017 3:16 am

I lowered the rpms to a normal idle speed to check timing by adjusting the curb idle screw. I do also have the vacuum can connected to the mag.

Joe, I've actually read that entire thread before and skimmed over it tonight too. I have to ask though, if that is the case then why do both Holley and Edelbrock install a nipple on their carburetors for vac advance that is ported instead of direct manifold? Not arguing, just asking. What is that timed vacuum port for if not vac advance?


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