Do you see the problem?

Bob from East Longmeadow Diagnostician Posted   Latest  
Discussion
Driveability
2007 Cadillac Escalade ESV 6.2L (8 L92) 6-spd (6L80)—1GYFK66887R234259
No Power
Runs Rough

There is a problem with this vehicle and this cranking waveform shows it. Do you see it?

process​.​filestackapi​.​com/resize=h:1000…

I had posted this on another site so if you saw it already please hold your comments for a bit so others can take a stab at it. I have more data but this capture is a hugh clue.

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Joe from Jersey City

 

Diagnostician
 

Hey Bob thanks for the challenge. I see this is a cranking waveform and this appears normal. I am still learning the ropes with in-cylinder waveform interpretation. I see your ignition trigger is either exactly at Tdc or slightly after. So this would cause the fuel to burn later. I know during cranking the ignition timing is retarded. I am going to say the timing of the ignition is incorrect and this is the problem you would like us to pinpoint.

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Bob from East Longmeadow

 

Diagnostician
 

Joe, look at the details in the capture carefully and then remind yourself that this engine is cranking. I would recommend reading through Bernies series on transducer testing to find some clues. diag​.​net/msg/m27jar2apt…

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Joe from Jersey City

 

Diagnostician
 

Ok will do.

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Joe from Jersey City

   

Diagnostician
   

I see on your snap the exhaust is restricted. Now as I look at this picture for a 30th time, I see that the exhaust valve opening has no definite spot. The exhaust valve opening appears wicked advanced. The rounded pocket clues me in on the valve not opening period. Due to this flaw the in cylinder pressure ramps up very early and mimics the look of an advanced exhaust cam profile. The high pressure on the snap is 305 psig. Sounds like the in cylinder volume has no release. Vacuum gauge would have a rhythm pulse going on I bet.

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Bob from East Longmeadow

 

Diagnostician
 

The cam is advanced but not due to a problem with the exhaust valve. The pattern looks very different when a valve doesn't open. Here's an example. process​.​filestackapi​.​com/resize=h:1000…

As far as the advanced valve timing, see my latest response to Ray. I believe this is a normal condition on this engine. 

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Danny from Cleveland

 

Technician
 

looks like the timing is advance 

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Bob from East Longmeadow

 

Diagnostician
 

The timing is just about tdc which is a normal condition during cranking.

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Stacey from Brantford

 

Technician
 

Restricted exhaust on that bank should never see increase in pressure on the exhaust stroke during cranking. Hard to judge how much without scaling. Do you have a running waveform? Other clue is the pressure continues to rise at the end of the exhaust stroke pressure should be decreasing 

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Bob from East Longmeadow

 

Diagnostician
 

Here's a good cylinder for comparison. 

process​.​filestackapi​.​com/resize=h:1000…

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Ray from North York

 

Diagnostician
 

I'm with Stacey, exhaust looks partially plugged.

Ray

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Bob from East Longmeadow

   

Diagnostician
   

Ray and Stacey are on the right track. Here is an idle capture with overlay and a snap throttle. Also, let me know what you think of the cam timing and why it may be so.

process​.​filestackapi​.​com/resize=h:1000…

process​.​filestackapi​.​com/resize=h:1000…

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Ray from North York

 

Diagnostician
 

In the idle capture, the cam timing looks advanced, the EVO too early and the EVC, IVO too early.

The WOT snap should not be used to determine if the exhaust is plugged because on some vehicle makes, the higher exhaust pressures at a WOT snap is normal.

A better method to test for plugged exhaust with the engine running, is to do a brake torque at 1500 rpm and the exhaust pressure should not exceed 5 psi.

Ray

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Bob from East Longmeadow

 

Diagnostician
 

Ray what kind of pressure under snap do think could be normal? In my capture it's rising to about 46psi. Plus, at idle it should be close to zero and I had approx 4psi.

Now regarding the valve timing, I don't have a know good to check but I'm suspecting that is normal. After reading service information it states that the variable cam is normally parked in the advanced position when not activated. I believe it said it was 17 degrees advanced when parked or not activated.

I watched the cam position pid on the scanner and it is zero degrees at idle so I think the advance seen in my idle capture is normal. Don't know this for a fact so I would welcome anyone else chiming in that may be more familiar with this system. I would love to see a known good pattern for comparison.

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Ray from North York

 

Diagnostician
 

I've seen many types of engines that show high exhaust back pressure on snap throttles, when the exhaust is not plugged.

During cranking and at idle,the exhaust pressure should be at zero psi.

To measure the valve timing accurately, the ckp sensor is more accurate than the overlay, to mark the 180, 360, 540, 720 crankshaft degrees, and to measure exactly where the EVO in crankshaft degrees before the 180 TDC and where the EVC, IVO at the 360 crankshaft degree position.

Sometimes you can find specifications of exactly where the valves open and close in crankshaft degrees.

Ray 

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Bob from East Longmeadow

 

Diagnostician
 

I believe the snap throttle works for back pressure testing if you know how to analyze it. I understand that you could see some back pressure on some engines under a hard snap but the place to look is at the exhaust valve opening point just as the throttle transitions from closed to open. When there is a back pressure problem you will see an immediate rise in pressure as the throttle is cracked open. Here is an example of bad. process​.​filestackapi​.​com/resize=h:1000…

Next, here is an example of good from the same vehicle. When you look at the initial throttle opening it becomes very obvious. The exhaust valve opens and the pressure basically stays at zero. process​.​filestackapi​.​com/resize=h:1000…

Now the other issue with my running waveform was the indication that the camshaft is advanced. From the capture it is obviously advanced. My question is, what's normal for this engine? I haven't seen a know good for comparison but after reading service information, I'm thinking this is normal. Here is a snippet from description and operation of the camshaft.

"During engine OFF, engine idling conditions, and engine shutdown, the camshaft actuator is held in the park position. Internal to the CMP actuator assembly is a return spring and a locking pin. During non-phasing modes of the camshaft, the return spring rotates the camshaft back to the park position, and the locking pin retains the CMP actuator sprocket to the camshaft. For the Gen IV small block engines, the park position for the CMP actuator and camshaft is, 8.5 degrees before top dead center (BTDC), which is equal to 17 crankshaft degrees BTDC, to the next cylinder in firing order. The engine control module (ECM) can only command the CMP actuator to retard the valve timing from the park position, or advance the valve timing back to the park position. The total range of valve timing authority is 31 degrees of camshaft rotation, which is equal to 62 degrees of crankshaft rotation. The control range is from the park position of 8.5 degrees camshaft, or 17 degrees of crankshaft BTDC, to 22.5 degrees camshaft, or 45 degrees crankshaft, after top dead center (ATDC)." (courtesy identifix)

As you can see from this information, the camshaft is normally advanced 17 crank degrees when in it's non-phased position. So it seems to me this is saying that at idle speeds, the camshaft will be 17 degrees advanced. That seems to agree with my capture. Thoughts on this?

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Ray from North York

   

Diagnostician
   

This waveform is from a 2015 Ford Focus 2.0.

The red CH B is the ckp sensor at 944 rpm.

The green CH C is a WPS500 in cyl 1

The first step is to disable the cylinder's injector, before cranking or running the engine, so as not to wash the cylinder wall and not to add fuel into the oil pan, then use the scope to count the number of ckp teeth in 360 crankshaft degrees, or between the missing crank teeth, which, for this engine, is 58 crankshaft teeth. Some engines don't use 58 crank teeth between the crank shaft's missing teeth.

If we add the 2 missing crank teeth to the 58 crank teeth, we get 60 crank teeth in 360 crankshaft degrees.

So 60 crank teeth in 360 crank degrees is 6 crank degrees per crank tooth. Each ckp toggle is 6 crankshaft degrees.

The second step is to count the number of crank teeth between the missing crank tooth to the pressure transducer's compression stroke at Top Dead Center.

On this engine, the Top Dead Center is at 20 crankshaft teeth. 20 crank teeth is 120 crankshaft degrees.

If there is a leak in the cylinder, either from a leaky valve or from leaking piston rings or a fuel fouled cylinder wall, on the compression stroke, the compression tower could be leaning, in which case we would no longer count 20 crank teeth.

If an engine will not start, the cylinder walls and piston rings could be fuel fouled and your compression tower could be leaning.

On a no start, check the oil level and ck if the oil level is too high. Do an oil change.

If you need to measure where a good cylinder's TDC should be, put your pressure transducer in the engine's "companion cylinder" to measure the number of crank teeth to the pressure transducer's compression TDC.

In this engine, cylinders 1 and 4 are companion cylinders and cylinders 2 and 3 are companion cylinders, so if you have a misfire in one of the cylinders, you can measure and compare the position of where the compression stroke at TDC using the crank sensor. 

With a firing order of …

1 8 4 3

6 5 7 2

Cylinders 1 and 6 are companion cylinders, 8 and 5, 4 and 7, 3 and 2 are the companion cylinders

Using the cursors, we can measure where the 180 crankshaft degrees is, which is the piston at Bottom Dead Center and then measure where the Exhaust Valve Opens Before Bottom Dead Cylinder, in crank degrees. 

Using the crank sensor, we can then measure where the 360 degree Top Dead Center is and where the Intake Valve Opens, and where the Exhaust Valve Closes. Then we can measure where the 540 crank degree is, along with where the Intake Valve Closes.

Ray

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Bob from East Longmeadow

 

Diagnostician
 

Ray that's a great explanation of how to measure pressure waveforms but you didn't address anything in my previous post. 

Do you have any comment on the L92 engine and the cam being parked in the advanced position? Do you have any pressure waveforms from one of these engines?

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Ray from North York

 

Diagnostician
 

Hi Bob, I don't have any wave forms for the 6.2 engine.

You can look at your scanner VCT cam desired and actual pids at idle and graph the scan data off idle on a test drive or you can use the scanner Function Mode to advance and or retard the cam timing off idle to keep the oil pressure high enough to turn the phasers.

I would like to see a waveform of the running compression of when the Exhaust Valve Opens in crank degrees using the crank sensor.

Ray

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Bob from East Longmeadow

 

Diagnostician
 

I guess I'm not being clear with my question, let me try again. I'm talking "theory" here. I'm not trying to analyze my specific waveform at the moment but rather, I'm interested in exploring what we are likely to see, or should see based on the particular engine/camshaft setups.

Service information states that on this engine, when the cam IS NOT being phased, it will sit in the fully advanced position. It also states that this position will be 17 degrees advanced. Since this is the case, wouldn't you agree that the idle pressure waveform will normally show advanced camshaft timing on this particular engine?

The cam PID does show zero degrees at idle which essentially means that the cam is in the fully advanced position. I'll paste in the service info again for reference.

"During engine OFF, engine idling conditions, and engine shutdown, the camshaft actuator is held in the park position. Internal to the CMP actuator assembly is a return spring and a locking pin. During non-phasing modes of the camshaft, the return spring rotates the camshaft back to the park position, and the locking pin retains the CMP actuator sprocket to the camshaft. For the Gen IV small block engines, the park position for the CMP actuator and camshaft is, 8.5 degrees before top dead center (BTDC), which is equal to 17 crankshaft degrees BTDC, to the next cylinder in firing order. The engine control module (ECM) can only command the CMP actuator to retard the valve timing from the park position, or advance the valve timing back to the park position. The total range of valve timing authority is 31 degrees of camshaft rotation, which is equal to 62 degrees of crankshaft rotation. The control range is from the park position of 8.5 degrees camshaft, or 17 degrees of crankshaft BTDC, to 22.5 degrees camshaft, or 45 degrees crankshaft, after top dead center (ATDC)." (courtesy identifix)

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Ray from North York

   

Diagnostician
   

Your right, on this 6.2L engine, the cam sits at 17 degrees advanced at idle.

This car is a 2016 Kia Optima 2.4 and the first wave form, which is at 1,543 rpm, is at start up.

The blue CH A intake cmp rises at 40 ckp degrees after the first ckp toggle

The red CH B is the exhaust cmp and it's 2 ckp teeth after the first ckp toggle 

The green CH C is the ckp and I've marked a 360 crank rotation. Each ckp tooth is 6 crank degrees.

The tan CH D is the COP 1.

The second GIF is at a WOT snap, near 3,500 rpm

The blue CH A is the intake cmp has retarded and it is now at 65 ckp degrees after the 1st ckp toggle.

The red CH B is the exhaust cmp and it's still at 2 ckp teeth after the 1st ckp toggle.

The green CH C is the ckp with the 360 ckp degree marked.

The tan CH D is the COP 1 and it has retarded.

The ignition timing and the cams will advance and retard while driving depending on the loads, acceleration, deceleration, cruising, etc

Ray 

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Joe from Jersey City

 

Diagnostician
 

So the initial problem has been discovered by Ray. If so thanks Ray.

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Ray from North York

 

Diagnostician
 

Stacey Fleming found that the exhaust was partially plugged before I did.

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Joe from Jersey City

 

Diagnostician
 

I tip my hat to all. Very large pool of knowledge that weighed in on this question. Thanks to Bob and Ray for the hints and screenshots.

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Bob from East Longmeadow

 

Diagnostician
 

I repaired this truck this week and wanted to put up a little more data. Unfortunately I was up against the wall late on a friday so I didn't have time to put the pressure transducer in it but I'm sure it would have looked similar to the good bank waveform I already posted. 

When initially diagnosing this I suspected a restriction on one bank. I had mode 6 misfire counts on all bank 2 cylinders. I wanted to watch trim and look for a bank to bank skew to confirm the problem. This thing would go open loop pretty quickly so I didn't have long to look at trims. I did manage to catch a trend shortly after one restart. It did seem to confirm a single bank restriction

This is fuel trim on a road test after a new Cat converter and spark plugs.

I didn't get an initial capture of VE partly because the vehicle was almost undrivable. I wanted to be able to investigate the misfire issues before putting a new cat on it so I loosened up the bolts on the bank 2 exhaust flange and drove it. I was now able to accelerate the engine and get a VE reading.

Here is the VE after a new cat and spark plugs were installed.

And fuel trims and Cat efficiency chart on road test with new parts.

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Ray from North York

 

Diagnostician
 

Thank you for the update and the graphs!

Ray

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