Question re testing coils by unplugging each distributor
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Question re testing coils by unplugging each distributor
I'm trying to run down a low-power and rough running problem on my car ('96 C2) and want to cross coils off the list. For clarity, I'm going to refer to the main distributor as the one driven directly by the engine and the secondary distributor as the one driven by a belt.
So, with the engine idling warm, I unplugged the coil from the main distributor (engine running on secondary distrib) and there was no discernible change in how the engine idled -- none, nada. I have a slightly rough idle, so I wouldn't say it was idling perfectly. But taking the main distributor out of the equation didn't seem to change anything (I didn't drive it, just idle).
Different story with the secondary distributor. When I unplugged its coil (engine running on main distrib), the engine started making light popping backfires and the idle changed instantly.
Now the two distributors have different ignition timing specs so this may be perfectly normal, but I couldn't find anything in the archives on this, so I wanted to ask. Is it normal for the car to run well on the secondary distributor but not so well on the main one?
So, with the engine idling warm, I unplugged the coil from the main distributor (engine running on secondary distrib) and there was no discernible change in how the engine idled -- none, nada. I have a slightly rough idle, so I wouldn't say it was idling perfectly. But taking the main distributor out of the equation didn't seem to change anything (I didn't drive it, just idle).
Different story with the secondary distributor. When I unplugged its coil (engine running on main distrib), the engine started making light popping backfires and the idle changed instantly.
Now the two distributors have different ignition timing specs so this may be perfectly normal, but I couldn't find anything in the archives on this, so I wanted to ask. Is it normal for the car to run well on the secondary distributor but not so well on the main one?
Former Vendor
Joined: Jul 2018
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From: SoCal
I'm trying to run down a low-power and rough running problem on my car ('96 C2) and want to cross coils off the list. For clarity, I'm going to refer to the main distributor as the one driven directly by the engine and the secondary distributor as the one driven by a belt.
So, with the engine idling warm, I unplugged the coil from the main distributor (engine running on secondary distrib) and there was no discernible change in how the engine idled -- none, nada. I have a slightly rough idle, so I wouldn't say it was idling perfectly. But taking the main distributor out of the equation didn't seem to change anything (I didn't drive it, just idle).
Different story with the secondary distributor. When I unplugged its coil (engine running on main distrib), the engine started making light popping backfires and the idle changed instantly.
Now the two distributors have different ignition timing specs so this may be perfectly normal, but I couldn't find anything in the archives on this, so I wanted to ask. Is it normal for the car to run well on the secondary distributor but not so well on the main one?
So, with the engine idling warm, I unplugged the coil from the main distributor (engine running on secondary distrib) and there was no discernible change in how the engine idled -- none, nada. I have a slightly rough idle, so I wouldn't say it was idling perfectly. But taking the main distributor out of the equation didn't seem to change anything (I didn't drive it, just idle).
Different story with the secondary distributor. When I unplugged its coil (engine running on main distrib), the engine started making light popping backfires and the idle changed instantly.
Now the two distributors have different ignition timing specs so this may be perfectly normal, but I couldn't find anything in the archives on this, so I wanted to ask. Is it normal for the car to run well on the secondary distributor but not so well on the main one?
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Hi Loren -- it's lost in the foggy mists of my ever-fading memory, but I recall reading at some point that the plugs don't fire at identical times, one fires a few degrees different than the other. But of course that could be wrong. So if it idles fine on one distributor but not so good on the other, that means I have a problem?
PS I've been reading a lot of troubleshooting threads in the archives and your posts have been invaluable.
PS I've been reading a lot of troubleshooting threads in the archives and your posts have been invaluable.
Former Vendor
Joined: Jul 2018
Posts: 1,093
Likes: 180
From: SoCal
Hi Loren -- it's lost in the foggy mists of my ever-fading memory, but I recall reading at some point that the plugs don't fire at identical times, one fires a few degrees different than the other. But of course that could be wrong. So if it idles fine on one distributor but not so good on the other, that means I have a problem?
PS I've been reading a lot of troubleshooting threads in the archives and your posts have been invaluable.
PS I've been reading a lot of troubleshooting threads in the archives and your posts have been invaluable.
2. Even if the timing were just a few degrees, you'd never notice it by listening to how the engine idles.
3. The idle should drop a little when one of the two distributors is disabled, but not cause a mis-fire.
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From: Dallas, TX
Nope, they should be the same. You can pull the caps if you are concerned that the distributor belt is improperly mounted so the positions of the rotors are offset from one another. My understanding is that the only way the belt can slip is if it breaks and completely fails and so the belt drive rotor is stationary.
Also after sorting out the wires and plugs clean the idle valve.
Andy
Also after sorting out the wires and plugs clean the idle valve.
Andy
Thread Starter
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Ok so I just did the test again (this time cold start) and once again it idled very nicely off the secondary dist but poorly off the main. The popping backfire was from the left bank (left exhaust tip) about every two seconds and the idle was low. Interestingly, I looked at the power and ground wires going to the coil (the ones with ring terminals). On every wire there's a few mm of exposed wire between the insulation and terminal, and on one of the wires for this coil, around half the wires are broken where they enter the terminal. No idea why, but looks like they were flexed too many times.
Tonight I'm going to crimp on a new ring terminal and re-do the test. Fingers crossed this is the problem. The coils themselves only have a couple thousand miles on them -- replaced them last year. So in theory they should be good. I will also double-check wire routing from the caps to cylinders.
Tonight I'm going to crimp on a new ring terminal and re-do the test. Fingers crossed this is the problem. The coils themselves only have a couple thousand miles on them -- replaced them last year. So in theory they should be good. I will also double-check wire routing from the caps to cylinders.
Last edited by Churchill; Apr 4, 2022 at 03:43 PM.
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Former Vendor
Joined: Jul 2018
Posts: 1,093
Likes: 180
From: SoCal
Ok so I just did the test again (this time cold start) and once again it idled very nicely off the secondary dist but poorly off the main. The popping backfire was from the left bank (left exhaust tip) about every two seconds and the idle was low. Interestingly, I looked at the power and ground wires going to the coil (the ones with ring terminals). On every wire there's a few mm of exposed wire between the insulation and terminal, and on one of the wires for this coil, around half the wires are broken where they enter the terminal. No idea why, but looks like they were flexed too many times.
Tonight I'm going to crimp on a new ring terminal and re-do the test. Fingers crossed this is the problem. The coils themselves only have a couple thousand miles on them -- replaced them last year. So in theory they should be good. I will also double-check wire routing from the caps to cylinders.
Tonight I'm going to crimp on a new ring terminal and re-do the test. Fingers crossed this is the problem. The coils themselves only have a couple thousand miles on them -- replaced them last year. So in theory they should be good. I will also double-check wire routing from the caps to cylinders.
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From: Norcal
@Churchill a few comments
1. Did you open up the distributor caps and confirm the rotors are aligned correctly? Basically confirm what Andy suggested... are the rotors firing in the right sequence. Set the motor to TDC and check if the rotors are pointing toward the notches in the distributor housing correctly.
https://rennlist.com/forums/993-foru...his-at-z1.html
2. Double check your spark plug wires are going to the correct cylinder/distributor point combination for the misfiring distributor.
1. Did you open up the distributor caps and confirm the rotors are aligned correctly? Basically confirm what Andy suggested... are the rotors firing in the right sequence. Set the motor to TDC and check if the rotors are pointing toward the notches in the distributor housing correctly.
https://rennlist.com/forums/993-foru...his-at-z1.html
2. Double check your spark plug wires are going to the correct cylinder/distributor point combination for the misfiring distributor.
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Ok everyone, problem solved. And I am 100% to blame. I am embarrassed because this is some rookie ****.
Backstory is that my car started running poorly and throwing multiple CEL's including a #5 misfire and codes for lean running. It's got 150K miles and on the original fuel pump, which over the past year has grown progressively louder. I figured it was the pump and since I was due relatively soon for a tune-up, I did that too -- plugs, caps/rotors, filters, and cleaned the injectors. Got it all back together, fired it up, and....it still ran poorly! A slightly different kind of poor running, but largely the same. Down on power, stumbly acceleration, shaky idle. So I started going over various systems and crossing them off the list.
When doing the plugs, I had transposed the left bank lower #1 and #3 ignition wires. In fairness, somehow the grommet in the engine tin on the left bank got turned around 180 degrees and so this looked exactly right -- forward wire goes to #3, rear to #1, middle to #2. But since the grommet was reversed, that was wrong. And thrown in for good measure was this all happened after replacing the fuel pump, so when I fired it up, it ran a little better but still not great, leading me to assume I had not solved the original problem -- when in reality I had, but had then introduced a new problem whose symptoms were VERY SIMILAR to the old problem.
Many wrenching lessons contained in the above. And a good reminder to stay humble and question your own work even if you're experienced working on cars.
For future reference, I was getting codes P03035 (misfire #5), P1130, and P1124/26. All of these suggest a vacuum leak, MAF, or o2 sensors, and the plugs validated that it was running lean -- too white. I smoke tested the intake -- no leak. Tested o2 sensors, they were fine. Injectors perfect. MAF values on my Durametric were normal. To my mind the very loud fuel pump had to be the culprit, and I needed to change it anyway before it left me stranded, to I replaced it without running a pressure test. In the course of this I swapped ignition wires, but now that they're set right, the car runs perfectly. So it was indeed the pump.
Backstory is that my car started running poorly and throwing multiple CEL's including a #5 misfire and codes for lean running. It's got 150K miles and on the original fuel pump, which over the past year has grown progressively louder. I figured it was the pump and since I was due relatively soon for a tune-up, I did that too -- plugs, caps/rotors, filters, and cleaned the injectors. Got it all back together, fired it up, and....it still ran poorly! A slightly different kind of poor running, but largely the same. Down on power, stumbly acceleration, shaky idle. So I started going over various systems and crossing them off the list.
When doing the plugs, I had transposed the left bank lower #1 and #3 ignition wires. In fairness, somehow the grommet in the engine tin on the left bank got turned around 180 degrees and so this looked exactly right -- forward wire goes to #3, rear to #1, middle to #2. But since the grommet was reversed, that was wrong. And thrown in for good measure was this all happened after replacing the fuel pump, so when I fired it up, it ran a little better but still not great, leading me to assume I had not solved the original problem -- when in reality I had, but had then introduced a new problem whose symptoms were VERY SIMILAR to the old problem.
Many wrenching lessons contained in the above. And a good reminder to stay humble and question your own work even if you're experienced working on cars.
For future reference, I was getting codes P03035 (misfire #5), P1130, and P1124/26. All of these suggest a vacuum leak, MAF, or o2 sensors, and the plugs validated that it was running lean -- too white. I smoke tested the intake -- no leak. Tested o2 sensors, they were fine. Injectors perfect. MAF values on my Durametric were normal. To my mind the very loud fuel pump had to be the culprit, and I needed to change it anyway before it left me stranded, to I replaced it without running a pressure test. In the course of this I swapped ignition wires, but now that they're set right, the car runs perfectly. So it was indeed the pump.
Last edited by Churchill; Apr 4, 2022 at 06:59 PM.
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And just a note on fuel pumps: I was surprised the way mine went bad. The noisiness crept up so slowly and progressively over the last few years that I didn't really notice that it was too loud. It's the old frog in the pot of water problem. When I jumpered it with the hood open, it was LOUD. Did the same with the new one, and it had a nice, soft hum. The old one was loud and buzzy, easily audible in the driver's seat. Really can't hear the new one from the driver's seat. So if you have a higher-mileage 993 on an original pump, and it's loud, probably worth just changing it before you spend a weekend trying to diagnose a bunch of different CEL's like I just did.
