CIS Low RPM Misfire on Fresh Build
06-30-2017, 10:19 AM
#1
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CIS Low RPM Misfire on Fresh Build
Hello All!
My apologies for what may seem to be a duplicate post. I am starting a new thread in the hopes of not only soliciting your excellent advice and input but hopefully to help myself and those after to document step by step the process of tuning and troubleshooting a low RPM misfire. I am basically going to start from scratch troubleshooting this issue.
Let me give you some background.
1979 928 purchased in 2013 as a basket with 74k true miles. SEE THIS THREAD https://rennlist.com/forums/928-foru...-spd-save.html
Rebuilt entire CIS system from both fuel pumps, accumulator, fuel distributor, injectors and seals. Every vacuum line, every line, every hose. Rebuilt WUR. Was installed on original engine with intake refresh. Unfortunately, that engine had low compression in one cylinder and was pulled in July of 2016.
1981 M15 engine chosen as donor, good running engine with excellent compression across the board. It was completely resealed with new head gaskets and rebuilt heads. CIS system moved over to this engine.
CIS system pressures cold control 17.5 System pressure is 74, rest pressure after 30 minutes is 42 and warm pressure is 45. Seem to all be well in spec. A/F adjusted with ColorTune and tweaked multiple times. Plugs show nothing out of the ordinary. A/F meter not utilized.
Three different wire sets tried, two sets of plugs. two caps, two different coils and three different distributors. Engine timed multiple times. SEE THIS THREAD http://https://rennlist.com/forums/9...fferences.html
Runs like it has a vacuum leak, spent hours chasing down small leaks, can't find any more. Have not used smoke machine, may try that in this thread. SEE THIS THREAD https://rennlist.com/forums/928-foru...cuum-leak.html
Valve timing checked, all good.
SO....here I go, going to start from scratch tonight with NEW fresh compression tests. Then I will move on to CIS pressure tests again. Input always welcomed and appreciated, but at the very least. This thread will be my therapy and journal, cause I may be going crazy
My apologies for what may seem to be a duplicate post. I am starting a new thread in the hopes of not only soliciting your excellent advice and input but hopefully to help myself and those after to document step by step the process of tuning and troubleshooting a low RPM misfire. I am basically going to start from scratch troubleshooting this issue.
Let me give you some background.
1979 928 purchased in 2013 as a basket with 74k true miles. SEE THIS THREAD https://rennlist.com/forums/928-foru...-spd-save.html
Rebuilt entire CIS system from both fuel pumps, accumulator, fuel distributor, injectors and seals. Every vacuum line, every line, every hose. Rebuilt WUR. Was installed on original engine with intake refresh. Unfortunately, that engine had low compression in one cylinder and was pulled in July of 2016.
1981 M15 engine chosen as donor, good running engine with excellent compression across the board. It was completely resealed with new head gaskets and rebuilt heads. CIS system moved over to this engine.
CIS system pressures cold control 17.5 System pressure is 74, rest pressure after 30 minutes is 42 and warm pressure is 45. Seem to all be well in spec. A/F adjusted with ColorTune and tweaked multiple times. Plugs show nothing out of the ordinary. A/F meter not utilized.
Three different wire sets tried, two sets of plugs. two caps, two different coils and three different distributors. Engine timed multiple times. SEE THIS THREAD http://https://rennlist.com/forums/9...fferences.html
Runs like it has a vacuum leak, spent hours chasing down small leaks, can't find any more. Have not used smoke machine, may try that in this thread. SEE THIS THREAD https://rennlist.com/forums/928-foru...cuum-leak.html
Valve timing checked, all good.
SO....here I go, going to start from scratch tonight with NEW fresh compression tests. Then I will move on to CIS pressure tests again. Input always welcomed and appreciated, but at the very least. This thread will be my therapy and journal, cause I may be going crazy
06-30-2017, 07:29 PM
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Well, that wasn't too hard to figure out. Cylinder 1 is 130. The reminder at 170. I can't even believe this. I guess I will move on to leak down. This car is starting to beat me.
07-01-2017, 08:35 AM
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07-01-2017, 01:45 PM
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Thanks Guys!
Leak down showed dead center of the "moderate" level. You can easily hear the blow by out the breather tube. I tested a few other cylinders and experienced no blow by.
So how did an engine with great compression prior to reseal, get bad Rings? I am thinking a piece of debris may be stuck in the ring.
I tried a little water injection to cause some detonation in the hopes of freeing something up. No dice so far. Put a little MMO in the cylinder and did a wet compression test and got 155.
filled the cylinder at TDC with MMO and will let it soak overnight.
This is nuts!
Leak down showed dead center of the "moderate" level. You can easily hear the blow by out the breather tube. I tested a few other cylinders and experienced no blow by.
So how did an engine with great compression prior to reseal, get bad Rings? I am thinking a piece of debris may be stuck in the ring.
I tried a little water injection to cause some detonation in the hopes of freeing something up. No dice so far. Put a little MMO in the cylinder and did a wet compression test and got 155.
filled the cylinder at TDC with MMO and will let it soak overnight.
This is nuts!
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07-01-2017, 10:11 PM
#8
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Something fell down the intake port and damaged the bore/piston?
07-02-2017, 08:34 AM
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I don't think so, Greg. I scoped it and it looks as good as it did when I assembled it.
my only theory is that a piece of old gasket or carbon got lodged in the ring land prior to my final assembly
a teaspoon of oil brings the compression right up to 160. I have it soaking in MMO but I am not optimistic.
any ideas? Thanks !
my only theory is that a piece of old gasket or carbon got lodged in the ring land prior to my final assembly
a teaspoon of oil brings the compression right up to 160. I have it soaking in MMO but I am not optimistic.
any ideas? Thanks !
07-02-2017, 04:47 PM
#10
Former Vendor
I don't think so, Greg. I scoped it and it looks as good as it did when I assembled it.
my only theory is that a piece of old gasket or carbon got lodged in the ring land prior to my final assembly
a teaspoon of oil brings the compression right up to 160. I have it soaking in MMO but I am not optimistic.
any ideas? Thanks !
my only theory is that a piece of old gasket or carbon got lodged in the ring land prior to my final assembly
a teaspoon of oil brings the compression right up to 160. I have it soaking in MMO but I am not optimistic.
any ideas? Thanks !
You have a couple of choices:
1. Go run it hard and see what happens to the compression. If your theory is correct, the problem will go away.
2. I've found that compression is very sensitive to how individual cylinders are working. For example if a cylinder is either rich or lean, the ring seal will be compromised and compression will suffer. I'm very conservative....plus I have always had enough "real" work on things that are "really broken" that I've never been forced to "hard sell" any questionable repair, which apparently is common at other shops. I very, very rarely would even consider tearing into an engine from one compression test....unless I could verify that there was a terminal issue.
The CIS injection system, because of the mechanical nature of the system, is prone to varying injection volumes, more than the later electrically controlled injection.
Whenever I have a cylinder running poorly, I remove all eight injectors and put them into some sort of a fluid collection device. I then jumper the fuel pump and operate the injectors by moving the sensor plate. This enables me to visually check how each injector is functioning....through the entire operating range and compare each one with the other seven. This works very, very well and is quite the time saver. (Note: No sparks allowed while raw fuel is being sprayed and collected in the fluid collection device. Regardless, I always keep a Halon fire extinguisher very handy when I'm doing this test....VERY HANDY.)
07-03-2017, 09:31 AM
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If your theory is correct, nothing soft like a chunk of gasket or carbon is going to survive for very long. his makes me a bit dubious that this is your problem.
You have a couple of choices:
1. Go run it hard and see what happens to the compression. If your theory is correct, the problem will go away.
2. I've found that compression is very sensitive to how individual cylinders are working. For example if a cylinder is either rich or lean, the ring seal will be compromised and compression will suffer. I'm very conservative....plus I have always had enough "real" work on things that are "really broken" that I've never been forced to "hard sell" any questionable repair, which apparently is common at other shops. I very, very rarely would even consider tearing into an engine from one compression test....unless I could verify that there was a terminal issue.
The CIS injection system, because of the mechanical nature of the system, is prone to varying injection volumes, more than the later electrically controlled injection.
Whenever I have a cylinder running poorly, I remove all eight injectors and put them into some sort of a fluid collection device. I then jumper the fuel pump and operate the injectors by moving the sensor plate. This enables me to visually check how each injector is functioning....through the entire operating range and compare each one with the other seven. This works very, very well and is quite the time saver. (Note: No sparks allowed while raw fuel is being sprayed and collected in the fluid collection device. Regardless, I always keep a Halon fire extinguisher very handy when I'm doing this test....VERY HANDY.)
You have a couple of choices:
1. Go run it hard and see what happens to the compression. If your theory is correct, the problem will go away.
2. I've found that compression is very sensitive to how individual cylinders are working. For example if a cylinder is either rich or lean, the ring seal will be compromised and compression will suffer. I'm very conservative....plus I have always had enough "real" work on things that are "really broken" that I've never been forced to "hard sell" any questionable repair, which apparently is common at other shops. I very, very rarely would even consider tearing into an engine from one compression test....unless I could verify that there was a terminal issue.
The CIS injection system, because of the mechanical nature of the system, is prone to varying injection volumes, more than the later electrically controlled injection.
Whenever I have a cylinder running poorly, I remove all eight injectors and put them into some sort of a fluid collection device. I then jumper the fuel pump and operate the injectors by moving the sensor plate. This enables me to visually check how each injector is functioning....through the entire operating range and compare each one with the other seven. This works very, very well and is quite the time saver. (Note: No sparks allowed while raw fuel is being sprayed and collected in the fluid collection device. Regardless, I always keep a Halon fire extinguisher very handy when I'm doing this test....VERY HANDY.)
After soaking overnight in MMO, I blew out the excess and started on a test drive. Once the MMO burned off, I took it on a spirited test drive. Getting up to 5000rpm for the first time. Car drove amazing but it always did above 2200 or so. After about 10 minutes or so, the car smoothed out at low rpm. I was so excited. It was 100 percent perfect. So I pulled back into the driveway and retested the compression. Still 120, which shocked me somewhat. So I took it out again and pushed it hard, still drove perfectly. Then I drove normal for another 30 minutes. Smooth and consistent.
So apparently while the compression may be a contributing factor to my low rpm miss, it's not the origin. Not sure why the car came alive all of a sudden. I am going to continue to drive this week to see how it reacts. I am also going to test the A/f just to see how close to spec I am.
fingers crossed!
07-03-2017, 01:13 PM
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07-03-2017, 02:13 PM
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Guy, don't get too excited yet....I believe this is a terminal issue, so, just park it, and write it off as a total loss, and as a favor to you, I will come and haul it away for you so you will no longer have to look at it! Sorry buddy....
Ok, ok, can't blame me for trying.....
Hoi is right....drive it...I bet it will build back up and be just fine!
Ok, ok, can't blame me for trying.....
Hoi is right....drive it...I bet it will build back up and be just fine!