1G Cylinder Pressure Waveform interpretation

[barza21]

Hello All,

I need some help interpreting some of the waveforms that I was able to capture using my labscope. The vehicle is a 1991 TSi AWD. The waveforms pertain to in cylinder pressure test while running (at idle only. I have snap throttle events if requested). Based on what I see on the captures, it seems like the exhaust cam is advanced by certain amount of degrees. I believe this since there is a rise in pressure towards the end of the exhaust stroke and the pressure in the cyclinder stops dropping off too early in the expansion stroke). I thought that I had timed this engine correctly, but it seems like I didn't according to this. I am wondering if my interpretation of this is correct. If anybody could take a look at the images and share what they think, it would be really helpful seeing as I would like to know if I have to tear this head apart or if it is just a simple re-timing.

A little background as to why I am here. I have been battling a misfire that does not pop a CEL; however, I can hear it at idle consistently. I am running ECMlink on speed density. I have tested and changed injectors (cleaned and tested PTE 1000cc), coils, PTUs, and wires. I have verified fuel pressures, boost leaks, compression checks, and leak down tests. All of those tests come back good. Cylinder leak check came back at 98% and compression checks came back at around 160-165 all around. I have checked the wiring throughout and it checked good. Grounds for ECU and sensors have been verified clean. The signal coming out of the CAS has been verified and smooth. I have look at ignition patterns on primaries, secondaries, etc., and they look similar across all cylinders. However, they didn't look as smooth as I would like, hence, what led me to dig deeper.

I am learning to interpret these waveforms, so if I am misinterpreting them, please feel free to correct or share knowledge. The images were picked up with a pico 2400a with the maximum amound of captures possible and a SSI pressure transducer was used to collect the pressure data. If this helps me solve this issue, I will probably share the set up here for future reference and troubleshooting.

Thank you in advance,

Oscar

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[steve]

Interesting data but how about some clear pictures of your timing marks when the engine is at TDC cyl #1?

 

[barza21]

Hey Steve,

Thank you for the reply. I will try to get some images later and post them.

Oscar

 

[barza21]

Steve,

Here are the pictures of the timing. I messed around with it for a while and no matter what I did I could only get it off by about 1/2 a tooth on between both can sprokets when the crank was at the TDC marking. I also used a long extension in the cylinder to verify I was reaching the peak travel of the piston and it lined up with the crank sprocket marking. I am scratching my head with this, . This contradicts all the data captured with the scope. I apologize for the crank sprocket picture not being as clear, but I didn't have time to ake off the belt cover. I could make out through the hole that it is lining up with the marking, though.

Oscar

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[steve]

That looks pretty much spot on to me except for where the belt is sitting on the crank sprocket. There is usually a little play in the disk that holds the crank timing mark that fits behind the sprocket. It shouldn't be riding forward off it but maybe that's an artifact of pulling the harmonic dampener to take the picture.

 

[barza21]

Steve,

Yeah. I am at a loss. My only thought is that maybe I have an exhaust cam that is not supposed to be in there? Is there a massive difference in can profiles between the 1g year cams that would cause this type of problem? What if I have a 1ga intake cam along with a 1gb exhaust cam? Would that be too much of a difference in the pairing? I am reaching to a potentially reason for this data.

Oscar

 

[steve]

I don't have any personal experience with the various cams. Here's a few threads on both Factory and Aftermarket versions.

Cam differences cam identification cam ID cams camshafts camshaft degreeing timing

Whats the differences in the cams between the 91-6 bolt and the 93-7 bolt blocks? I did a head for a friend and we used the 91 head on the 93 bglock. I would of changed the cams also when I did the head, but the cams were "beat" out of the 93!Steve. :confused:

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[danl]

Is your misfire only at idle or does it also manifest off idle?

 

[barza21]

danl,

The misfire is other than idle as well. If I hold the rpm at 2k or 2.5k, I can consistently hear the popping. Here is a video I took a month ago.

Thank you

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[danl]

Is the car drive able? If so while driving does it have trouble driving off boost?

 

[barza21]

Danl,
The car is perfectly driveable. There is no issues off or on boost boost. On the highway, it is smooth and doesn't act up. I can't hear the popping when driving due to all the other noise. I only hear it at idle and when I rev and hold at a stop.

Oscar

 

[ist dwa]

Is the head over milled?

 

[DSMPT]

We can't know if the valve timing is off by a tooth or two only by seeing those pics. Because even the valve timing is set as it should, the waving pattern should still look like that and if early or late would even differ by the cam profile and engine speed, too.
If incorrect valve timing is the cause of misfire, you should already have seen a huge difference in drivability. To verify the actual valve timing accurately, you have to use a degree wheel and indicator. I would check the base ignition timing first.

Maybe I didn't understand well what you meant but as I understood, what you described about the things that made you believe that the exhaust is advanced at least sounds normal to me.

I believe this since there is a rise in pressure towards the end of the exhaust stroke

Because the exhaust valves are already being in closing event after the middle of exhaust stroke towards the end of exhaust stroke alongside piston position towards TDC. And then intake valves are already being in opening event before TDC.

the pressure in the cyclinder stops dropping off too early in the expansion stroke

Because the exhaust valves are already being in opening event after the middle of power stroke towards the end of power stroke alongside piston position towards BDC.

 

[barza21]

Is the head over milled?

I am not sure if it is. The head came with the car when I bought it some time ago. The previous owner didn't mention any milling of the head. The head is, however, a 1gb head. I know that much from the ID marks on it.

DSMPT,
Thanks for the reply. I see what you're saying. The reason I believe there is an is something going on in the head is because I compared my pressure waveform to other waveforms over the Internet. The ones that looked similar to mine all had issues with some type of obstruction or timing of the cams.

 

[danl]

Danl,
The car is perfectly driveable. There is no issues off or on boost boost. On the highway, it is smooth and doesn't act up. I can't hear the popping when driving due to all the other noise. I only hear it at idle and when I rev and hold at a stop.

Oscar

If the timing were off it would not drive well. Are you sure it’s not just a case of a lumpy idle? These cars never were known for great idle quality and then we add cams and double the injector size.

 

[barza21]

Danl,
I don't think that it is a lumpy idle since when I hold the RPM at 2000rpm, the popping increases with the rpm. Link doesn't display any really hiccup that I can see coincides with this problem. I see a hiccup in the AFRest sometimes, but it is not consistent enough for me to say that is it.

Oscar

 

[steve]

I see a hiccup in the AFRest sometimes, but it is not consistent enough for me to say that is it.

Could we see that log anyway?

 

[barza21]

Steve,

Yes. Attached are the logs. One is an idle log and the other one is a log from a drive to work. You can see random AFRatio spikes in both of them and quick changes in the direction of the O2 voltage. Those quick changes are usually correlated with pops that I hear. I made a post some time ago about random AFRatio spikes that I was having, but never found a conclusive reason for them. I thought I had figured it out when I started increasing airflow smoothing, but it ended up not being a solution.

Oscar

 

[steve]

I've only looked at the idle log so far and I see the O2 spikes, need to look more and think about what going on in the place where you blip the throttle a little because it seems like it takes a long time to settle. I also don't like how slowly the O2 is switching compared to what I'm used to seeing.

 

[barza21]

Steve,
Thank you for looking at it. One of the logs might've been with the old injectors. The idle log is with the used/cleaned and tested injectors. There was no difference between the two sets of injectors. The result was the same. All the logs have been with NB simulation. I just had a bung welded on a couple of days ago and reinstalled the stock O2. I have to redo the wiring at the ECU and route the wideband. I will log and re-upload logs when I complete that.

Oscar

 

[danl]

If you force the car into open loop and maintain 13-14.5 aft at idle does the misfire remain?

 

[barza21]

danl,

I am going to say that it does. However, I can't accurately remember. I will finish putting the car together today and try it out. I will update once that happens.

Oscar

 

[barza21]

Hey Everyone,

Here is an update to the logs after I added the factory O2 back in and relocated the wideband to a welded-on bung further down on the downpipe. There is no change to how the car idles and reacts. I can still hear the popping at idle and when holding 2k RPM. danl, I set the car to open loop and held the RPM at approximately 2k and it still heard the misfire/popping. Here is an attached log of a cruise on the highway and a slow cruise with idle. Also, attached is a video of what it sounds like when I hold 2k RPM.

Oscar

Here is the video for the throttle held at 2,000 RPM.

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[danl]

In your first log on monday at 296 seconds did you do anything during the idle like turn on the headlights? Through all your logs I see a lot of weird behavior in your battery voltage. It was similar to what mine looked like until I replaced a bad "new" reman alternator with a good one. I also had some strange issues but they were slight different from yours and honestly, I just forget what they were.

In the screen shot below (from monday at 296 seconds) voltage goes from 14.4 volts to 13.0 volts in a couple tenths of a second. Try disconnecting the alternator and idling the car on just a healthy full size battery, perhaps from another car. See if the popping and crackling remains. I'm trying to isolate systems to see if we can begin to find where the problem is.

My theory is that the alternator output is jittery (you can't pick it up via the log) and this jitter is messing with a sensor or the ECU's operation, causing your symptoms. Another option is to put a scope on the alternator output.

It does not seem mechanical to me. If it were mechanical it would be bad all the time (like during cruise). It seems intermittent electrical but the struggle is finding out which electrical system it is since it is intermittent.

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[barza21]

Danl,
Thank you for the idea. I'll put the scope on the battery while it's running and check it. I'll check the grounds with the scope and see if anything is there as well. I'll update as I get info.