ADS Diagnostic Services

04/22/2022

What if you could "See" electricity?

That's where the scope comes in handy. I use it as much as I can to learn how to use it, connect it, read the results... I have a single channel stand-alone device that's really good for "quick" checks, a professional grade 2 channel "Pico" that uses software on the laptop to display captures, a 4 channel (lower quality) that is also software based, and a 4 channel portable unit that is also my "scanner" for tapping into the auto computers for diagnostics.

With the appropriate probes, a scope can read voltage, amps, pressure, vacuum, temperature, and a host of other types of signals. I use it most often for voltage and amp captures (snapshots).

CASE STUDY - 1998 Lincoln Town Car 4.6L V8 engine with a single cylinder misfire.

I'm going to share some waveform captures with you, but I feel the need to tell a story first. The Lincoln "hole in block" engine was replaced with a used unit. This unit started and ran pretty well except for a surging idle and a "fish-bite" misfire. I figured I'd attack the misfire first, then address the idle.

Since the ignition system is very difficult to access when everything is assembled, it was rather difficult to determine "which" cylinder was causing the issue. My scanner does have "misfire counters" for this vehicle, but unfortunately wasn't picking up the misfire events. Typically the misfire counters will indicate which cylinder is misfiring and how often. I made the decision to disassemble enough components to access the ignition coils and fuel injectors for testing. However, the engine couldn't be run in this state. Resistance tests on the coils and injectors proved nothing exceptional. (I don't know what the specs are but I typically look for the "odd man out"). Since I have a single cylinder misfire, I would expect one particular component to be different from the rest. Not the case with the resistance readings. (Crap!) Pulled the spark plugs looking for anomalies. Again, nothing out of the ordinary. But "hey", since I'm already here, how about an engine compression test?

I performed an engine compression test which was pretty dismal. 3 out of the 8 cylinders had compression lower than 60 PSI. One was at 30 PSI. (Used engines are kind of a crap shoot). I then made the decision to pull the motor back out, and also sent an email to the seller to see if they had, maybe, a 90 day warranty or something. I have yet to hear back from them. Regardless, the engine needs to come back out. Engine on cart - while waiting for a reply from the seller, I performed an engine "leak-down" test on all the cylinders. The 3 low compression cylinders also had severe pressure loss during the test. Exhaust ports were very vocal also. Days later and no response from the seller, I decided to pull the heads and inspect the valves. A head gasket kit was ordered for reassembly. All the while thinking about how the low compression could be affecting my symptoms.

Heads pulled and disassembled - all exhaust valves looked like they've been sitting in a junkyard for a few years (go figure). Intake valves weren't terrible, but they weren't "like new" either. I lapped all the valves, cleaned everything up and reassembled the heads. After installing heads on the engine, and prior to installing timing components, I reran the cylinder leak-down test, with much more suitable results. I felt good about the whole repair. I reassembled the engine and reinstalled it in the vehicle. It fired right up, and ran smoothly on all "7" cylinders. ("Son of a *****"!). Did my misfire symptom change? Well... yes. It now misfired "ALL THE TIME". But "hey", at least I know all the timing components are installed correctly. And, by the way, the idle "surge" was gone.

Ok, time for a different approach. I got out the scope and was determined to find the exact cylinder that was giving me grief. My initial (pre-disassembly) scope testing was using an amp clamp across fused jumpers on the fuses for the ignition coils and fuel injectors. But I was not "seeing" the issue in these waveforms. This time I rigged up some leads (long needles) to access the coil and injector connectors (with the engine all assembled and running) and used a single channel on the Pico scope to test each coil and each injector individually. I was actually looking for a fuel injector that was possibly missing its "hump" in the waveform, indicating a "stuck" injector. After some tangled leads and a few burnt fingers, all injectors were firing properly. Onward to the ignition coils.

Based on the "feel" of the symptom, it really wouldn't surprise me if it was a secondary ignition issue. I just need to find it. After managing to "tap into" all the ignition coil connectors, I had the "Ah Ha" moment. All ignition traces looked good (Ford triple strike) except for one, which I couldn't determine (due to lack of experience reading secondary waveforms) what was causing the unusual capture. But at least it was different from the rest of them. Ford made the spark plugs and coils difficult to remove from this engine without some "top end" disassembly. However, there is one coil ( #1, I believe, which happened to be the one I was questioning) that looked like I could almost squeeze it out with little difficulty (what are the odds). I successfully removed the coil & spark plug and inspected both. I could not see any particular issues with either, and swapping these to another cylinder (which is typically my next step, to see if the problem moves) was not really an option on this engine since everything is buried. I do, however, still have all the removed components from the used engine, which included all ignition coils and spark plugs.

I snagged a coil and good looking spark plug from the "scrap" pile and installed these into the engine. Scope trace on ignition coil, we started it up and the misfire was COMPLETELY GONE, scope capture looked like every other cylinder.. Enough verification for me. I buttoned everything back up and called it "done". The engine starts and runs great. All data PID's on the scan tool are right on par. What a "pi**er"... I almost feel like it was a complete waste of time pulling the engine out again for the head work. However, I know I won't be chasing down compression problems or valve issues later on, so I guess it wasn't a waste of time after all. Had I tested using the second approach the first time around, I may not have found the compression issues and I'm sure they would have come back to bite me. So it all worked out as it should have. Can I put the hood back on now?

Scope captures included here -
Ford triple strike secondary waveform (normal)
Ford triple strike secondary waveform (abnormal)
Relative compression test (after completing the head work)

Being able to (what I call) "See" the flow of electricity is just SO COOL!!!

02/27/2022

Got a Code? Change a part. Not So Fast…

Within the past (2) years I’ve run across (2) separate situations in which the Check Engine Light trouble code was not what it seemed. Both happened to be a P0420 code (catalytic converter issue).

2008 Jeep Commander, 80K+ miles - “the other guy said it needs a catalytic converter”… A road test showed a very erratic O2 sensor signal on one bank only, When diagnosing a failing catalytic converter, I would expect the downstream O2 sensor to mimic the upstream sensor. In this case the downstream sensor had a mind of its own. The upstream sensor was working normally.

2019 Buick Enclave, 60K_ miles - Ok, so this is way too early in this vehicles life to be having a failing Cat, but, Ok, stranger things have happened. Again, a road test showed a downstream O2 sensor that just didn’t act like it should. Not the “lazy” signal I would expect, nor did it mimic the upstream O2 sensor. Being within its warranty miles and year, I recommended that this vehicle be sent back to the dealer for diagnosis / repair.

The Buick went to the dealer, got a new catalytic converter, and had another check engine light within a week of being returned to the customer. (go figure…)

Thankfully the diagnosis of the Jeep led me to the diagnosis of the Buick much faster.

Why the crazy O2 sensor signals? Well, how does an O2 sensor function? I don’t have the scientific details, and I really don’t need to know. But what I do know is that, when at normal operating temperature, the O2 sensor analyzes the oxygen content in the exhaust gases that are blowing past it. It generates an electrical signal based on the amount of exhaust gas oxygen. Wait a minute - let me rephrase that - “It generates an electrical signal based on a comparison of oxygen content between the exhaust gas and non-exhaust gas (outside air)”.

In other words, it needs a “base line” reading of non-exhaust air oxygen content, and then compares the exhaust gas to this base line to know how different the exhaust is, and generates a signal according to the amount of difference. With that in mind, that would mean that an oxygen sensor samples “outside air” to get its base reading. So maybe the O2 sensor no longer has the ability to sample the outside air. A quick test would be to swap the downstream sensors side to side, which was done.

Road test readings were unchanged, The side with the erratic readings remained erratic. Ok, not the sensor. I’m not convinced it has a failing cat, and now now I’m pretty convinced it’s not a failing O2 sensor. If the O2 sensor needs an “outside” sample of oxygen, what if that source is contaminated? Let’s check for exhaust leaks near the sensor. A smoke machine up the tailpipe revealed an exhaust leak (on both vehicles) near the downstream sensor in question, right at a “Y” pipe weld. The theory - with the engine running, the “sample” air is being contaminated with the very air it’s supposed to be comparing to. But since it is open to the atmosphere, and the vehicle is rolling down the road, the outside air is being mixed with exhaust, creating the erratic signals I saw on the scanner.

Obviously, fix the exhaust leaks. A welder should do the trick. Road test - “Viola!”. Smooth, steady, “lazy” downstream O2 sensor readings. Mucho better… Clear codes and return vehicle to customer, never to return again with said code.

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Trouble codes generated in automotive computer systems DO NOT indicate that I part needs to be replaced. Don’t get me wrong, often times a new part will resolve the issue (parts do fail). Trouble codes are generated because a computer programmer wrote in some “normal range values” for a sensor, and that sensor has fallen out of the “normal” range. Trouble codes are a guide to determine which sensor the computer didn’t like, why it didn’t like it, and to diagnose the reason behind “out of range” sensor values. When a sensor fails, it most certainly has fallen out of the “normal” range. but most often there are other reasons that sensor values get “skewed”.

A lot of money gets spent at the parts stores for vehicle repairs, based on trouble codes. Hell, they even read your check engine light trouble codes for free. “What part should I replace?” It seems that the dealers may also tend to lean that way, as well.

Really?

Really?

—ADS—

06/20/2020

I conquered the Cat. The 955k is back to leaping tall trees with a single bound.

04/29/2020

Caterpillar 955k track loader mopping the floor with me. Next round I come prepared.

06/08/2019

Been sitting awhile, you think?

02/18/2019

Seems I've acquired a few Cadillac's for my ToDo list. SRX, CTS, STS. I guess I have a soft spot for Cadillac's.

01/19/2019

Who woulda thunk it'd be so difficult to buy a car with a clean title.

07/26/2018

GMC Acadia Air Conditioning Leak Issue - I just finished repairing a 2012 GMC Acadia with a customer concern of the air conditioning system not blowing cold air. We typically expect to find a low system charge of refrigerant and a leak in the systems somewhere, and this vehicle was no different. I have had a previous experience with a GMC Acadia with the same concern and the fault was difficult to pinpoint. Having had that experience, the troubleshooting of this vehicle went much quicker, and the fault was worth mentioning here in this post..

Being that this was the second Acadia with the exact same failure, I thought it might be worth spreading the wealth of knowledge here, thinking that there may be many of these vehicles with the same problem. The GM parts houses seem to be quite aware of the failure as they all seem to have the parts in stock for the fix.

Let's start by hanging some AC pressure gauges on the system to verify the low / no system charge – verified. The system showed “0” psi of pressure. A quick visual under hood inspection shows green oil residue around the lower right front of the engine compartment, near the bottom right of the AC condenser. Knowing GM uses yellow / green refrigerant dye in the factory charge is certainly a plus, as we are quickly dialed in to the approximate location of the source of the leak.

Using an AC refrigerant leak detector in this area provided no definitive pin-point source for the leak. It did appear to be a rather large area of oil, which I found odd. Almost as if, instead of “dripping” or “seeping” from a leak, it was “spraying” from the source. This should “light up” the detector, but since the system was completely empty of refrigerant, I assumed that any residual had already dissipated, so adding some to the system was the next logical step.

For those not too familiar with vehicle AC systems, the AC refrigerant charge contains a specified amount of a specific type of oil to the system to lubricate the compressor during operation. This oil circulates throughout the system during normal use. When a refrigerant leaks develops, some of this oil escapes via the leak and is visible, aiding in the diagnosis. Since oil attracts dirt, this typically shows up as a damp area of “slime” near the location of the leak.

I thoroughly cleaned off the oily residue from the area of the suspected leak, added enough refrigerant to the system to get the compressor to turn on and, using the AC leak detector, went about attempting to isolate the exact source of the problem. Considering how large the oily area was (and several “drips” of oil were noted), I figured it shouldn't be too difficult to narrow it down. After several minutes of “Sniffing around”, I was surprised that the leak checker was not “excited” about anything in the suspect area. This is were having the experience of diagnosing this original Acadia came in handy.

The first Acadia also had a failed front engine mount, that would allow the engine to “roll” when accelerating from a stop. Thinking maybe something was “flexing” in the engine compartment during this engine “roll”, I had a helper “brake torque” the engine while inspecting for an AC leak. Wha-Lah! No leak checker required! (use safety glasses when working on any AC repair job).

During the “brake torque” test, the front of the engine would “roll” approximately 3-4 inches upwards, and refrigerant could be seen “spraying” out of the AC line, soaking the immediate area. I could see how one could empty the system in a hurry just by driving around town for a day. I could also see how having this failed engine mount may have been a contributing factor in the failure of this AC line. The line in question is the AC compressor output line that goes to the condenser. Not difficult to remove / replace, and contains the high pressure sensor fitting (a new sensor comes with the new hose).

Next steps: recover refrigerant, replace hose assembly, vacuum and recharge system, verify operation, recheck leaks at suspected area, all is good! I should have done a video on the diagnosis and repair since I really couldn't find much information on this particular leak using YouTube (maybe I just wasn't looking hard enough). I suspect it's a rather common issue on the Acadia, which might give me the opportunity to record the next one that rolls in. Both Acadia's had approximately 70k-90k miles on the vehicle, and both had failed front engine mounts, which also needed to be replaced. Unfortunately, replacing this mount was not near as much fun to replace as the leaking AC line.

09/16/2017

07/27/2017

An introduction to automotive diagnostics using a scope.

07/13/2017

Commentary weak, material pretty good. Repairing a common coolant leak failure on GM 3.8L engines

07/08/2017

Failed battery diagnosis using PICO scope starter current and battery voltage scope trace. 2005 Chevy Silverado 4.8L VIN V. This is a before repair capture.