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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[jw4free]

How tight is your exhaust after the down pipe? At low flow maybe you are getting an air infiltration that affect the exhaust note and sounds like a misfire.

 

[barza21]

Jw4fre,

I will check the exhaust when I jack it up. I recr tlu took it apart and installed new gaskets. Didn't see anything out of the ordinary.

 

[barza21]

Hello All,

So, here is the update. @steve , as you suggested, I checked the voltage at the battery with the car running. The voltage is pretty steady at 14.2V and after a while it works itself down to 13.9V-14.0V. I also did an AC ripple test on it and my delta between the high and low AC peaks is between 70-80mV. According to some data online, that seems to be the norm. Subsequently, I disconnected the alternator's field connector and let the car run on battery power. It did not solve the popping at idle or at 2k RPM. After checking all of that I am pretty comfortable saying that the alternator is providing clean enough power.
The next step that I took was to disconnect each individual fuse from any accessory system. I am just trying to eliminate any outside system that might be causing some type of noise for the ECU. I disconnected any fuse available to access in the engine bay and the cabin. No change on any of those. Additionally, I also disconnected my wideband and all the gauges. No change there either.
Another thing that I did was increase the idle air smoothing. I took it from 25% up to 50% and it did no change.
I messed around with the global, deadtimes, and SD table to no avail. No matter what combination I selected that would put the AFRatioEst at 14.0-14.7AFR, it wouldn't change the idle. Furthermore, decreasing that AFRatioEst to high 12s to low 13AFRs also did nothing. Interestingly enough, though, the wideband reading would hover around low 15 to mid 15 AFRs; it didn't matter how rich I made the AFRatioEst. Any insight on this? That was the one piece of data that was very interesting and might point in some direction.
I have ordered a valve cover gasket kit and will be taking the valve cover off to see what's going on in there. I suspect that I verified those cams when I put that head together years ago, but I can't remember after all the stuff I have checked.

@jw4free , I verified the exhaust is clamped down correctly and made sure it was tight. No changes there either.

Again, thanks for all the help and any insight that will be shared.

Oscar

 

[barza21]

@steve ,

Here is a log after all those checks were made. This was on the way to work today. Those AFRatioest spikes are a lot more aggressive now. Also, I can now feel them while I am cruising. That did not used to be the case before. I will try to take a look around the engine bay tonight when I get home. Nothing really has changed from last time. Not sure what happened.

Oscar

 

[danl]

You can change the O2 switchover point in dsmlink to get a richer idle if you are simulating narrowband and using the o2 input for the 0-5v wideband. That will force the car to run closer to 14.7 at idle, though in my experience, 15.5 wouldn’t cause an issue.

Your testing looks good, I’m stumped why your battery voltage had sudden drops in your one log.

 

[barza21]

@danl ,

I am not sure. There is a chance I was messing with something. I will look into changing the switchover point just as a troubleshooting step.
I Have a question regarding those afratioest spikes? I have some pretty noticeable ones going on. What does the ECU use to calculate that? Airflow, pressure, estimated fuel flow, estimated fuel injector duty cycle? There was a pretty big spike in that last log. It was big enough that I felt it in the cab.
I will pull the fuel filter and change it just in case there is something in there. I doubt it, though, seeing as I just replaced it a couple of years ago.

Oscar

 

[We're on Boost]

There was a pretty big spike in that last log. It was big enough that I felt it in the cab.

Can you pinpoint where exactly this particular spike is in the log (as in XXX.XX seconds)?

I Have a question regarding those afratioest spikes? I have some pretty noticeable ones going on. What does the ECU use to calculate that? Airflow, pressure, estimated fuel flow, estimated fuel injector duty cycle?

Actually I have this question too because when I look at injector duty% and Ontime, and airflow, at some of these spikes, I don't really see why there should be such a big spike there. Although I haven't tried to calculate any of those points for a "check".
So I'd like to look at this one that you felt.

 

[steve]

Can you pinpoint where exactly this particular spike is in the log (as in XXX.XX seconds)?

I'm guessing @ 12.29 sec then a bunch after 65 sec. I don't see spikes in the MAP just in the AFRatioEst and WBFactor and a small rise in LC1WB AFR shortly after the spikes.

 

[barza21]

@We're on Boost and @steve ,

The event that I am talking about is at 701.898. There are several events before and after that. I have been seeing those spikes for some time now. I am not sure what is causing them. I will scope out of the MAP signal again, along with the intake temperature signal. I am guessing that injector duty cycle is determined by current RPM and the MAP signal. There has to be some hiccup in there that is causing this. When I have looked at this before the signal was clean. Might as well double check.

Based on the data provided by ECMtuning, AFRatioEst is determined by airflow and calculated massfuel flow. So, I am guessing one of those two elements in my tuning is hiccuping somehow. These spikes happened when I was running a MAF and have followed when converted to SD (along with the popping sounds shared in the videos).

Oscar

 

[We're on Boost]

The event that I am talking about is at 701.898.

Oh yeah that's a good example. Because you are steady on the throttle cruising there, and the engine has been warmed up for quite a while. Everything should be wonderful. But at that one instant the logged FuelFlow is essentially 0, so the AFRatioEst is essentially infinity (29,893).

With FuelFlow only 0.07 cc/min it must be that the InjOn time of 0.89 ms there is basically entirely "dead time". And that is pretty much what you get when you add your global deadtime to your InjBatteryAdj ( 0.240 ms + 0.648 ms).

The MAP jumps from a steady 13 inches of vacuum to only 5 inches of vacuum for just that one instant. What does that mean? A backfire into the intake manifold?

The spike I'm looking at is showing 702.238 seconds on the timer though.
This is in your 2025.05.28-01 log.

 

[barza21]

@We're on Boost ,

Yeah, you're correct. The spike at 702.238 is the one I was referring to. I listed the other time just to indicate the beginning of the event. A backfire into the intake is possible. How could I test for that? If I took the intake pipe at the throttle body, should I be able to feel those pulses? If that is the case, then what is a cause of this? Mechanical timing being out of wack somehow even though everything is lined up (I will be taking the valve cover off this weekend to see what is going on in there). Incorrectly timed injector and spark timing? I will try to scope the TDC signal tonight and plot it against the secondary ignition pickup on #1 cylinder. Any additional ideas would be appreciated.

Oscar

 

[We're on Boost]

Incorrectly timed injector and spark timing? I will try to scope the TDC signal tonight and plot it against the secondary ignition pickup on #1 cylinder. Any additional ideas would be appreciated.

This seems like a good thing to do.

Mechanical timing being out of wack somehow even though everything is lined up (I will be taking the valve cover off this weekend to see what is going on in there).

And it might come down to this. So you can verify when the exhaust valves start to move and when they stop moving.

 

[barza21]

@We're on Boost , @danl , and @steve ,

I was able to get a few things checked out over the weekend. First I will start with the cams. I got the valve cover off and nothing seem awry from looking and touching anything. I didn't get a chance to measure the lobes seeing as it was late, but the markings stamped on the CAS side of the cams are as follows:

Intake: RB and 12 OR 0 and 12
Exhaust: R2 and 08 OR 0 and 28

I went through some of the threads on cam identification markings and was not able to find something specific to these markings. Other similar ones listed were like R7 or R9 and consensus seem to be that they were stock. Can somebody confirm?

Next, there was the testing to the TDC and CAS signal. I was going to try to couple the TDC signal to the #1 cylinder ignition secondary, but my scope (pico 2000 series) shares grounds between the signals and the ignition secondary pickup was introducing a lot of noise to the TDC signal. So, I had to isolate the CAS signals coming out. The results are seen here (also, I picked up the signal at the back of the ECU in case there was something wrong with the cabling):

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The other CAS signal is pretty similar. The spikes, low and high, do seem to correlate with the popping I heard. I would think, though, that ~5.4V high signal should be acceptable by the ECU. Now, I am not sure about the low portion of the signal. I have no reference to go on here. What do you guys think?

Next, I kept checking different sensors to see if I saw spikes and instability in signals and came across the tach signal coming from the PTU (pin 4 of PTU). Somebody stated that it is just used by the ECU to know that the PTU is working. With that being said, what is that signal voltage and signal supposed to look like? The pops I heard correlated with jumbling of the signal coming out of that pin. The voltage would peak out at 11.42V. Once again, I am not sure what this is supposed to look like.

I recalled that I had a check engine light (that I had turned off on link... that is my bad for not mentioning that) for the fuel pump relay circuit malfunction. I checked the voltage at pin 13 of the ECU and got the image below. The voltage is hovering somewhere around 16.5V on the high side to 13.3V on the low on average. The spikes, which correlate with the popping, would go up to 16.7V and down to 12.97V. Isn't this supposed to be a clean 12V or what ever the alternator voltage is? When I turn on the key, without the engine running, I can see battery voltage without any of the signals on it:

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Zoomed in:

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Digital signal processing added (lose peaks but gain a better image of the waveform):

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Is that supposed to look like that? When I look at anything on that fuel pump circuit, like pin 3 or 2 on the MFI relay, the signal looks like that. Consequently, that signal looks like that going into the PTU pin 6. I thought that voltage coming from the ignition switch to pin 110 of the ECU should be battery/alternator voltage? Why a signal that looks like an ignition primary on a supply voltage line to the ECU just to tell it its on? I am not sure if this is the problem, but it doesn't seem to be a clean 12v signal to the ECU to pin 110. The pops that I would hear from the exhaust would line up with those high spikes seen in the pin 13 un-zoomed image; it would follow when I would rev up the engine.

Have I found something significant or am I still this gremlin? This is starting to become really frustrating. I don't want to pull the head seeing as there is no clear indication that there is anything going on in there. The cylinder leakdown checks come back good and the compression checks look good. The only thing that is pushing me that way is the running compression test I did.

Any additional information and input would be great,

Oscar

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[barza21]

Here is another piece of information. I am curious/concerned about this reading. I did a ground check between sensor ground and chassis and I got a voltage ranging from -280mV to +360mV. This would seem kinda high to me for a ground voltage. Especially seeing as the baro sensor hovers around 330mV.

Oscar

 

[We're on Boost]

Here is another piece of information. I am curious/concerned about this reading. I did a ground check between sensor ground and chassis and I got a voltage ranging from -280mV to +360mV. This would seem kinda high to me for a ground voltage. Especially seeing as the baro sensor hovers around 330mV.

A range of 640 mv? That does seem very high.
Where exactly did you have the meter probes to check this?
Does your MAP sensor ground wire go to the sensor ground or to a chassis ground?

 

[barza21]

@We're on Boost ,

The test probes are going from the sensor ground pins in the back of the ECU to the chassis frame that the MPI relay is attached to. My aftermarket gauges, wideband, and radio are all disconnected. There are no accessories on either.

Oscar

 

[danl]

Is your intake manifold grounded to the firewall? Same with the battery? I also run an extra ground from the alternator to the firewall even though the intake manifold should cover it.

If you disconnect the alternator power wire do you still have voltage spikes and thus popping?
With the key on and engine off, turn on the fuel pump with link and see if the DC voltage looks clean or changes.
Between battery neg and firewall, battery neg and engine block, battery neg and alternator what are the voltage drops look like with the engine running?

 

[barza21]

@danl ,
The intake manifold is grounded to the battery. Another portion of the intake manifold is grounded to the chassis by the brake master cylinder. I have disconnected the grounding cables individually to see if it makes a difference. The only grounding cable left was the OE one. The popping continued.

Regarding the popping with the alternator disconnected, it continued when I disconnected the alternator field wire and when I disconnected the alternator completely from the system by removing the main fusible link.

As far as voltage drops are concerned, they look pretty much solid around 14v. I checked it when I did the AC ripple test on the alternator last week.

I checked pin 13 at the ECU with the engine off and ignition on, and it read ~12.4V. I then turned the pump on through link and it dropped a little bit, but the signal didn't get crazy how you guys are seeing. Once again. I am not sure what that signal on pin 13 is supposed to look like. I would think it should just be alternator/battery volts. However, it is showing that it is going up to 16.7 and dropping into the 13volts. That's a really big swing. I am not sure if that erratic voltage is getting induced into the ECU and it is not being filtered and bleeding over to the noise floor and bringing it up. I think I need to figure out where the noise is coming from exactly... Maybe? Lol. I took the valve cover off and have to put it back together. I'll try that today or tomorrow. Summers up here take up a lot of our time because it is a race to do as many outside activities as we can in 3 months, .

Oscar

 

[DSMPT]

Just another possibility, if you wouldn't be able to figure out the cause, maybe should try to replace the HLAs and/or valve spring with higher seat pressure ones (or maybe you can shim the springs to see if it would make difference). When the HLAs stop adjusting the lash properly or can't follow cam shaft rotating speed for some reason or seat pressure is not sufficient, it would cause misfire even at idle.

 

[steve]

I checked pin 13 at the ECU with the engine off and ignition on, and it read ~12.4V. I then turned the pump on through link and it dropped a little bit, but the signal didn't get crazy how you guys are seeing. Once again. I am not sure what that signal on pin 13 is supposed to look like. I would think it should just be alternator/battery volts. However, it is showing that it is going up to 16.7 and dropping into the 13volts. That's a really big swing. I am not sure if that erratic voltage is getting induced into the ECU and it is not being filtered and bleeding over to the noise floor and bringing it up. I think I need to figure out where the noise is coming from exactly... Maybe? Lol.

Pin 13 on a 1G is a ECU input (it's not drawn that way in the diagram) used to check that the fuel pump side of the MPI relay is providing power to the fuel pump when the ECU commands it (via pin 56). I need to refresh my memory but with the ignition on and the engine off there shouldn't be any voltage on pin 13 until ECU pin 56 goes low at which point pin 13 should go high (battery voltage). Any voltage on pin 13 should also be present at the fuel pump (or the relay input if it's been rewired) and if high enough would try to run the pump. Are you sure it's pin 13 (Lower left most on the large connector Black/White wire)?

If I get into the basement today and clear the workbench I'll double check.

 

[barza21]

Just another possibility, if you wouldn't be able to figure out the cause, maybe should try to replace the HLAs and/or valve spring with higher seat pressure ones (or maybe you can shim the springs to see if it would make difference). When the HLAs stop adjusting the lash properly or can't follow cam shaft rotating speed for some reason or seat pressure is not sufficient, it would cause misfire even at idle.

@DSMPT ,
Yes. If I get to the point when the head has to come off, I will check that out. The head that that the cams came out of had 80k miles on it and the HLA I bought brand new when I rebuilt a few years back. Since then, I've put 30k miles on them. I will replace them again, along with the rockers. We shall see what happens. Thank you fro the advice.

Pin 13 on a 1G is a ECU input (it's not drawn that way in the diagram) used to check that the fuel pump side of the MPI relay is providing power to the fuel pump when the ECU commands it (via pin 56). I need to refresh my memory but with the ignition on and the engine off there shouldn't be any voltage on pin 13 until ECU pin 56 goes low at which point pin 13 should go high (battery voltage). Any voltage on pin 13 should also be present at the fuel pump (or the relay input if it's been rewired) and if high enough would try to run the pump. Are you sure it's pin 13 (Lower left most on the large connector Black/White wire)?

If I get into the basement today and clear the workbench I'll double check.

@steve ,

Yes, you are correct. Pin 13 is an input to the ECU (I apologize with how I explained it) and it should go high when pin 56 is grounded internally by the ECU. All of that only happens when the ignition switch is at position IG1. When I have the key to IG1 position, I see 12.4V there. When I activate the pump through link to always on, the voltage stays about 12.3-12.4V.
Regarding being sure if it is pin 13 the one that I checked, I am pretty sure it was. I followed the wire in the diagram, checked the output from pin 2 of the MFI relay and it looked all erratic as well. I checked the input to the MFI relay, pin 3 (which should be coming from IG1), and the signal looked exactly the same (erratic). My next step, after I get the valve cover on, is to check the main fusible link (30A fuse) for the ignition switch and see what the signal looks like there. If the voltage is clean, then I will move down to the ignition switch. I am dreading that area seeing as it is small and confined. I am not as nimble as I used to be 10 years ago, .
Steve, should that signal coming to pin 13 be a clean battery/alternator voltage signal (12.4V - ~14.8V (on the high side))? The reason I ask this is because that IG1 cable shares power to the coil and the PTU. Would that affect the shape of the signal I am seeing. From testing that I have done, I believe that I shouldn't see anything on the positive side of the coil other than straight source voltage. All the signal switching should be seen on the negative side of the coil. Is my understanding in correct?

Oscar

 

[steve]

Yes, you are correct. Pin 13 is an input to the ECU (I apologize with how I explained it) and it should go high when pin 56 is grounded internally by the ECU. All of that only happens when the ignition switch is at position IG1. When I have the key to IG1 position, I see 12.4V there. When I activate the pump through link to always on, the voltage stays about 12.3-12.4V.

I don't think it should unless you have configured ECMLink to prime the fuel system. Normally you would see anything there until the key was in START or after the CAS started pulsing.

Steve, should that signal coming to pin 13 be a clean battery/alternator voltage signal (12.4V - ~14.8V (on the high side))? The reason I ask this is because that IG1 cable shares power to the coil and the PTU. Would that affect the shape of the signal I am seeing. From testing that I have done, I believe that I shouldn't see anything on the positive side of the coil other than straight source voltage. All the signal switching should be seen on the negative side of the coil. Is my understanding in correct?

It should be clean and under no case be 16v or more. How is your fuel pump electrically connected? Do you see any of his noise at the battery terminals?

 

[barza21]

I don't think it should unless you have configured ECMLink to prime the fuel system. Normally you would see anything there until the key was in START or after the CAS started pulsing.


It should be clean and under no case be 16v or more. How is your fuel pump electrically connected? Do you see any of his noise at the battery terminals?

Steve,
Regarding the activation of the pump with the ignition on, I will check today or tomorrow. I am pretty sure I saw voltage there, though. I will take an image and verify the pin I was monitoring.

Regarding the 16v at the relay points and pin 13 of the ECU, I can attest to that. If looking directly at the battery terminals, I see 14.2V. I did an alternator ripple check and it checked out good as well. I can see the voltage drop at the battery when the fan comes on, but then it recovers shortly after that. I don't know where that noise could be coming from, then. I will start working myself up the line until I don't see that erratic voltage anymore.
I'm going to look at the diagrams and see what else is possibly on the 30A fuse that provides power to the to the pump.

 

[steve]

Just making sure there isn't something like a boost-a-pump raising the voltage to the pump and possible backfeeding the ignition circuit that the pump is on.

 

[barza21]

Just making sure there isn't something like a boost-a-pump raising the voltage to the pump and possible backfeeding the ignition circuit that the pump is on.

I see what you're saying. I am wondering the same thing. I think I didn't answer your question last time. The pump has been rewired with a thicker gauge wire and a relay.
I can also try disconnecting the pin 13 from the ECU connector and see what that does. This is a direction that I can look at and it is progress for me. Hopefully it leads to a resolution.

Oscar