Rough idle with vacuum advance - JeepForum.com
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post #1 of 14 Old 08-23-2019, 06:03 PM Thread Starter
Wolfpack83
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Rough idle with manifold vacuum advance connected

Hi all,

I have a 258 with a Sniper EFI conversion that I finished late last year. Everything else is pretty much stock and I've had the Jeep about a year. During that time, I've driven it about 300 miles, and it runs and idles pretty good since the EFI conversion.

Last week I began replacing some old vacuum lines with new ones. In doing so I discovered that there was a BB in the distributor vac advance line the whole time I've had it, so I've been getting no vacuum advance, only mechanical. I recently put in the new line and have it hooked directly to manifold vacuum, as it was before (but was blocked off by the BB). When I started it up after putting the new line on I noticed it began to idle rough as it started to warm up. I drove around less than 5 miles and it almost died on me a couple of times, which it didn't do before. In the days after, I checked the vac advance on the distributor with a vacuum pump to confirm no leaks and it does advance with the handheld vacuum pump.

With everything hooked back up I ran it again today, and the same thing happened - rough idle as it began to warm up, and continued to do so long after reaching normal operating temp. Thinking about how it idled fine with no vacuum advance before, I plugged the manifold and distributor like you would when setting the initial timing (which is 8* BTDC). I cranked the Jeep back up and it idled fine once again and drove the way it did before putting the new vac line on. Any ideas what could be going on here? Runs fine on no vac advance, runs rough on full manifold vac.


1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #2 of 14 Old 11-23-2019, 09:54 PM
johnnytaco
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Did you ever figure this one out, Wolfpack83?
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post #3 of 14 Old 11-24-2019, 04:46 PM Thread Starter
Wolfpack83
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I have not, I've just been running no vacuum advance, only mechanical. I only drive me CJ about 30 miles / month, so not a big deal right now. I need to devote some time to tune it to run advance though. I've also been thinking about just swapping the distributor out for the one Holly recommends go with the Sniper EFI:
http://https://www.holley.com/produc.../parts/565-307

1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #4 of 14 Old 11-24-2019, 10:38 PM
BagusJeep
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It should not be dying, that is not a sign of too much advance. Have you converted from a CEC computer version? If so, was the mechanical advance head changed?

Have you watched the linkage between vaccuum capsule and distributor and plugged and unplugged the vac at idle? It should go back and forth about 1/2".

When you look at the ignition timing at idle you should get advance about 20 degrees. If you disconnect the vac advance, As you rev up to about 1750 rpm the mechanical advance should be kicking in.

If all this is working, it is not a question of timing. More likely, as you suspect, there is a vaccuum leak

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post #5 of 14 Old 11-25-2019, 12:08 AM
Nucking1Futs
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Try using ported vacuum and see what happens.

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post #6 of 14 Old 11-25-2019, 08:56 AM Thread Starter
Wolfpack83
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Quote:
Originally Posted by BagusJeep View Post
It should not be dying, that is not a sign of too much advance. Have you converted from a CEC computer version? If so, was the mechanical advance head changed?

Have you watched the linkage between vaccuum capsule and distributor and plugged and unplugged the vac at idle? It should go back and forth about 1/2".

When you look at the ignition timing at idle you should get advance about 20 degrees. If you disconnect the vac advance, As you rev up to about 1750 rpm the mechanical advance should be kicking in.

If all this is working, it is not a question of timing. More likely, as you suspect, there is a vaccuum leak
It has been "nuttered" but I did not do anything yet with the mechanical advance head.

It's been several weeks since I messed around with it, but when i did, I had the engine off and hooked up a handheld vacuum pump to see how much vacuum it would take to fully advance the vacuum capsule. I took some notes. I am at work and away from my notes at the moment, but I think the advance was all in at 14" (with engine off and using vacuum pump it would move all of the way advanced by 14"). With the engine on and warm and using the handheld pump, the engine didn't seem to like anything over 10 or 11" if I remember correctly. I'm pulling 20" at idle per my vacuum gauge in the cab.

I was able to confirm mechanical advance as you said with vac advance plugged.

As for how much timing advance at idle with vac advance hooked up to manifold and engine warm, I think was over 20* and closer to 26* if I remember. I'll have to look at my notes again and double check.

I am hoping to get some time over the Thanksgiving weekend to mess with it some more.

EDITED to try and clarify more

1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #7 of 14 Old 11-25-2019, 09:00 AM Thread Starter
Wolfpack83
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Quote:
Originally Posted by Nucking1Futs View Post
Try using ported vacuum and see what happens.
I have not tried that yet, but was considering it. That would likely solve the problem at idle, since there would be no advance. It seems most folks are in favor of manifold, so I would lean towards that If I can make it work. Are you running ported?

1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #8 of 14 Old 11-25-2019, 09:11 AM
Dryseals
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I don't have the Sniper but understanding some of the way these systems work, I would guess your Learn Comp Limits has adjusted for the no vacuum advance and can not compensate enough to over come the normal driving mode. You are probably in the NoLearn mode still and need to go back to the learn mode and let the system retune itself. It needs to rebuild the base table.

From Holley

Learn comp limits: This value is set to 100% by default, and should remain there until ample driving time and tuning
has occurred. The LEARN COMPENSATION LIMIT is a parameter that ECU is allowed to work within when making
changes to the fuel map based upon CLOSED LOOP operation. Unlike the CLOSED LOOP LIMIT which is a set
parameter for commanded changes to actual fuel flow based upon the O2 sensor reading, LEARN COMPENSATION
LIMITS are the percentage of change that is allowed to actually be saved as a modifier to the fuel map

Transfer Table: When run is pressed, it will transfer the Learn Modifier table to the Base Fuel Table and Zero out the
Learn Values. When “Run” is pressed the Sniper EFI will prompt if you would like to smooth the fuel table with the
learned values. It is recommended you allow the Sniper EFI to perform the smoothing
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post #9 of 14 Old 11-25-2019, 09:53 AM Thread Starter
Wolfpack83
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Quote:
Originally Posted by Dryseals View Post
I don't have the Sniper but understanding some of the way these systems work, I would guess your Learn Comp Limits has adjusted for the no vacuum advance and can not compensate enough to over come the normal driving mode. You are probably in the NoLearn mode still and need to go back to the learn mode and let the system retune itself. It needs to rebuild the base table.

From Holley

Learn comp limits: This value is set to 100% by default, and should remain there until ample driving time and tuning
has occurred. The LEARN COMPENSATION LIMIT is a parameter that ECU is allowed to work within when making
changes to the fuel map based upon CLOSED LOOP operation. Unlike the CLOSED LOOP LIMIT which is a set
parameter for commanded changes to actual fuel flow based upon the O2 sensor reading, LEARN COMPENSATION
LIMITS are the percentage of change that is allowed to actually be saved as a modifier to the fuel map

Transfer Table: When run is pressed, it will transfer the Learn Modifier table to the Base Fuel Table and Zero out the
Learn Values. When “Run” is pressed the Sniper EFI will prompt if you would like to smooth the fuel table with the
learned values. It is recommended you allow the Sniper EFI to perform the smoothing
This is a very good point! I had thought about this too and how the ECU had been learning with no vac advance prior, and then I introduced vacuum advance. This very well could be playing a big part in the problem.

From what I understand though, it still sounds like I may need to either adjust or change my distributor to a pre-computer controlled one to get the correct amount of mechanical and vacuum advance. This is the part I'm needing to try and wrap my head around and understand.

1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #10 of 14 Old 11-25-2019, 07:16 PM
Nucking1Futs
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Quote:
Originally Posted by Wolfpack83 View Post
I have not tried that yet, but was considering it. That would likely solve the problem at idle, since there would be no advance. It seems most folks are in favor of manifold, so I would lean towards that If I can make it work. Are you running ported?
Yes I am using ported vacuum. I am running the Howell system, and abiding by CA smog regulations.

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post #11 of 14 Old 11-26-2019, 06:08 AM
Dryseals
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Quote:
Originally Posted by Wolfpack83 View Post
This is a very good point! I had thought about this too and how the ECU had been learning with no vac advance prior, and then I introduced vacuum advance. This very well could be playing a big part in the problem.

From what I understand though, it still sounds like I may need to either adjust or change my distributor to a pre-computer controlled one to get the correct amount of mechanical and vacuum advance. This is the part I'm needing to try and wrap my head around and understand.
The Computer doesn't really care if your distributor is mechanical/vacuum or electronic. The main variables are going to be Throttle position TPI , Manifold pressure MAP, Engine RPM and coolant temperature the O2 sensor acts as the feed back to create the fuel maps. Fuel maps in the learn mode are initially built from what you tell the ECM your engine parameters are. Then as you drive it makes corrections based on RPM, MAP and TPI. In the GM world these corrections were logged in a table in the ECM as the Block Learn Mode (BLM) and would bias the fuel based on where it thought it needed to be. Back then they used 8 bit CPU's so they had a correction rate of 256 bits with 128 being perfect. The correction rate varied by vehicle and was related to another table for percent correction, so it had its limitations on how far it could go. I strongly suspect Holley is using the same type CPU as they have to be durable enough to take the heat cycles and I doubt they went out and recreated the wheel. So once you have told the ECM you are through tuning, its going to save the existing fuel tables and the corrections are going to be much smaller in percent. With Vacuum advance working, you are going to get a much better burn and right now the ECU is trying to correct things based on the old fuel maps.

You can view the correction rates before you retune, I'd be curious to know which way they are biasing the fuel.

For what its worth, an electronic controlled spark would probably be the better choice for an everyday driver trying to squeeze the best fuel economy out of the motor. But for the amount of driving you do, I think I would just keep the distributor you have and roll with it as your vacuum advance (VA) is looking at the same pressures your MAP sensor is and the VA curve was engineered to run with your motor. Same goes for the mechanical advance. So those variables are going to more fixed, curve wise, something the ECM will like.
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post #12 of 14 Old 11-26-2019, 07:30 AM Thread Starter
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Quote:
Originally Posted by Dryseals View Post
The Computer doesn't really care if your distributor is mechanical/vacuum or electronic. The main variables are going to be Throttle position TPI , Manifold pressure MAP, Engine RPM and coolant temperature the O2 sensor acts as the feed back to create the fuel maps. Fuel maps in the learn mode are initially built from what you tell the ECM your engine parameters are. Then as you drive it makes corrections based on RPM, MAP and TPI. In the GM world these corrections were logged in a table in the ECM as the Block Learn Mode (BLM) and would bias the fuel based on where it thought it needed to be. Back then they used 8 bit CPU's so they had a correction rate of 256 bits with 128 being perfect. The correction rate varied by vehicle and was related to another table for percent correction, so it had its limitations on how far it could go. I strongly suspect Holley is using the same type CPU as they have to be durable enough to take the heat cycles and I doubt they went out and recreated the wheel. So once you have told the ECM you are through tuning, its going to save the existing fuel tables and the corrections are going to be much smaller in percent. With Vacuum advance working, you are going to get a much better burn and right now the ECU is trying to correct things based on the old fuel maps.

You can view the correction rates before you retune, I'd be curious to know which way they are biasing the fuel.

For what its worth, an electronic controlled spark would probably be the better choice for an everyday driver trying to squeeze the best fuel economy out of the motor. But for the amount of driving you do, I think I would just keep the distributor you have and roll with it as your vacuum advance (VA) is looking at the same pressures your MAP sensor is and the VA curve was engineered to run with your motor. Same goes for the mechanical advance. So those variables are going to more fixed, curve wise, something the ECM will like.
Lots of good insight here, Dryseals. Thanks for taking the time to explain. I'll see if I can find out how much is being corrected before trying to reset the base tune.

Good point about keeping my current distributor vs going electronic. I'm pretty much thinking the same, if I can get what I have working correctly. I'll try to mess around with things this weekend and update here.

1983 CJ-7 / 258 / T4 / 32" BFG mud terrains
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post #13 of 14 Old 11-26-2019, 08:03 AM
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When you get a chance to work on it again, take a picture of your handheld display showing the air/fuel display after you get the engine warmed up at idle.

Unless you've actually changed the learn compensation limit % and closed loop compensation limit % values in the handheld it should be able to tune itself just as if it were straight out of the box. Just remember it will not learn(modify the fuel map) until the engine is up to operating temp.


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post #14 of 14 Old 12-21-2019, 07:53 PM
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Over advance will cause rough idle and potential stalling (when hooked to manifold a vacuum port). Normally, a vacuum can should add 10 to 12 degrees of timing at the crankshaft. In the last 20 years or so, I've found that many replacement vacuum cans add up to twice that amount and I've had to add aftermarket limiters to prevent an over advance from the vacuum can.
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