Polishing pressure analysis skills question

Brian Diagnostician Willoughby, Ohio Posted   Latest  
Discussion
Driveability
2011 Chevrolet Cruze 1.8L (LUW) 5-spd
Vehicle Overheating

This vehicle came in with two concerns. One that the check engine light was on and the second concern was that the vehicle would overheat at idle. 

The vehicle had a bunch of lean fuel system codes and low voltage oxygen sensor which I quickly Identified as a ruptured crankcase ventilation diaphragm in the valve cover. That isnt what this post is about however.

The second concern of it overheating was looked at next. I performed the typical tests you'd perform on a cooling system including a pressure test ( no leaks found). I then started the vehicle and wanted to watch the behavior of the thermostat, cooling fans, engine coolant temp sensors etc. 

Once operating temperature was reached, I noticed coolant temp continuing to rise. It got to approximately 235 and I noticed the electric cooling fan would not come on. I performed a "percussion test" by tapping on the electric cooling fan motor. Wahhh lahhh the fan is working I'm a magician! I checked connections just to make sure I didn't disturb a poor connection. All is well, Needs a fan,

This vehicle has been overheated many many times according to the customer in our oh-so important interrogation interview when the vehicle was dropped off. So lets make sure the engine was not damaged. The vehicle smells "charred" to me so I had a sneaking suspicion that the vehicle may have a damaged head/head gasket.

A simple block test was performed with the magical color-changing blue fluid and within seconds, it changed from blue to yellow. I know this test isnt the most reliable when it DOESNT change colors, BUT when it does, You know for sure you have combustion gasses entering the cooling system. 

OK so, this vehicle needs a cooling fan and head removed for inspection/gasket replacement. 

Now to the purpose of this post. Pressure analysis is a fairly new diagnostic technique that we ALL can benefit from and also ALL need more practice on. So, being the nerd I am. I naturally will whip out my scope any chance I get to perform some tests even if I already have a vehicle diagnosed. 

So I get out my pico and WPS and make my connections by pulling the hose off of my pressure tester pump and place the hose onto my WPS pressure transducer. I like to suspend the transducer with a bungee to avoid it rattling against engine components and also decrease any noise on the signal. 

I have a sync on #1 ignition coil for a 720 degree of engine rotation reference as well on channel B (Red)

As the vehicle is warmed up, I am noticing the flat line of zero pressure I had when it was cold, begin to rise and the cooling system warms up. As its rising slowly, I also notice pulses slowly beginning to present themselves in the capture. The warmer it gets, the stronger the pulses get. Once operating temp is reached I get a capture that looks like this

I add Driveability Guys TDC Software ( found at driveabilityguys​.​com ) overlayed onto my capture to help reference the firing order and make more sense of my capture

To me, It appears that I get a pressure pulse in the cooling system shortly after # 3 Ignition event. 

I spoke with a friend of mine Mario Rojas about this after pulling the vehicle out (going to scrap vehicle) and he suggested unplugging the injector on the suspect cylinder (#3) to prove it is the cause of this pressure pulse. Great idea! Wish I still had the vehicle here to do so :(

My question to y'all would be, am I on the right track as far as identifying a combustion leak into the cooling system via this method? Have you done this yourself? Any advice?

+7
Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Seems like a pretty sound analysis Brian. I think the fact that it was near flat line when cold and then started to pulse when it got hot confirms it. It would have been great to see what happened when disabling the cylinder. Next time right? ;-)

0 Default Ð Bounty Awarded
Ben Technical Support Specialist
Eaton Socon, United Kingdom
Ben Default
   

Hi Brian, As you've mentioned, pressure analysis is a hot topic and one that keeps opening our eyes to new things and different ways of testing and I've tried doing something similar in the past when I was working at Toyota. We had a car come in which we had already diagnosed as a blown head gasket using the leak detector fluid. However, like so many of us sometimes I just wanted to know more

+2 Default Ð Bounty Awarded
Brian Diagnostician
Willoughby, Ohio
Brian Default
 

Awesome! Great minds think alike. However, you said the pressure pulse exists during #4 compression. How can we confirm its that cylinders compression stroke vs. Potentially the POWER stroke of the cylinder before that (#3)? Combustion pressure is obviously higher than compression so I'd imagine a failure of a head gasket would display a failure during a power stroke vs. Any other preasure

+1 Default Ð Bounty Awarded
Ben Technical Support Specialist
Eaton Socon, United Kingdom
Ben Default
 

I love how this posts start the brain ticking! Yes I think by disconnecting injectors would change the way the pulses look if we have a head gasket failure. You maybe right in saying that combustion pressure is higher than compression. However, (I'm just throwing this out there as to be honest I'm really not sure!) would this actually be correct? I'm just thinking about the piston position

0 Default Ð Bounty Awarded
Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Hi Ben, If you think about it, combustion pressure has to exceed compression pressure or you wouldn't get any power output from the engine right? Here is a great explanation of the pressures. BTW, I think using the injector for reference could cause you to misidentify the cylinders sometimes since injectors don't always fire at a consistent time. This is cool stuff.

0 Default Ð Bounty Awarded
Walter Technician
Sarasota, Florida
Walter Default
 

I've been playing around with this using a homemade pulse sensor connected to the radiator using and old stant pressure tester hose and cap. I installed a small tee because I was only interested in seeing a pulse and not pressure. Like above the test vehicle was already proven to have a bad head gasket but I wanted to try my set up and see if I could pin point the cylinder. Like above my scope

0 Default Ð Bounty Awarded
Bill Technician
Rosetown, Saskatchewan
Bill Default
 

Hello Brian What were the operating conditions during this capture. RPM, Load etc. Were the "puffs" noticable from the surge tank? Just casually observing the pressure waves. It seems fairly similar to pressure waves that are created by combustion swelling the cylinder. I am maybe unfamiliar with this sync, but why does it not pull to ground?

0 Default Ð Bounty Awarded
Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Hi Brian, I stumbled across this post again and I was curious if this cylinder head was removed for inspection? The reason I ask is that I think your capture shows a blown head gasket between cylinders 3 and 4. The pressure spikes on cyl3 combustion event then levels off for a moment and then spikes again on cyl4 before dropping back to baseline. I would put my money on the gasket blown

0 Default Ð Bounty Awarded