Basics of DIY Diagnostics – aka the joys of owning an old haggard Jeep

If you’ve been a fan of ECOA and the NHT series for a while you’ll know my seemingly ceaseless plight of vehicle problems.  I love my 2004 Jeep Wrangler Unlimited, I really do.  I’ll never sell her.  She’s the last newest Jeep I’ll ever own.  That said, she can be a real pain in the ass sometimes.

2015 No Highways Tour – Western North Carolina
Jeep is down on power and not running “great”
Opted to swap in a new TPS
Good thing I pack my own tools.

One thing I get asked a lot is, “Why?”  Why do I tolerate the problems?  Why don’t I buy a Toyota? (Kevin!)  Why don’t I just take it to a professional?  Well, in short I’m a masochist.  I jest, but I honestly enjoy the problem solving aspect of being my own mechanic.  That’s not the only reason though.  Read on for more as well as some tips and tricks for your own DIY diagnostics…

Let me start off with a much needed disclaimer: I am by no means a professional mechanic.  Just like you shouldn’t take medical or legal advice from the internet, it’s probably not a great idea to take mechanical advice from the internet, or at least not all of it.  However, you’re already here and chances are you know that already, but I feel like I had to say it anyway.

The Check Engine Light

This li’l light of mine, I’m going to let it shine.
The question is… why is it on?

If you don’t currently own a code scanner, go buy one.  The basic ones aren’t super expensive.  There are even some that work with your phone/tablet via bluetooth (my next investment).  A code scanner is the first step in figuring out what is wrong with your engine.

So… many… codes…
Also a reason not to use cheap parts off the internet.

Usually an onboard computer can tell you something is wrong with a reliable degree of accuracy.  For instance, if your oxygen sensor goes bad the computer will usually throw a code stating as much.  I say usually a lot because it’s not always the case.  That’s why a little problem solving and a basic understanding of how an engine works helps.

Engine basics

To start diagnosing problems you must first have a basic understanding of how an engine works.  To really simplify things, an engine is just a giant air pump.  It sucks air in and pushes air out. Along the way fuel is added along with an ignition source to make a small explosion which is used as power.  I know that’s super basic, but when you break it down there are only three things an engine needs: air, fuel, and spark.  From those three basic elements you can connect the dots to the other vehicle systems. Those are also the three basic elements from which most vehicle problems occur.

The basic four-stroke engine cycle.
Air + fuel + spark = BOOM!
Those booms create rotational energy measured as Torque.
Horse power is a function of torque and RPMS.

Now I cannot, nor will not, cover every vehicle system.  That’s not why I’m here.  Remember, the goal here is basic DIY diagnostics.  What I can do is focus on my own experiences and common problems I’ve experienced on the trail.

Air Problems

An engine is just like a human body, it needs air to survive and work properly.  If an engine isn’t getting clean dry air it won’t run right.  Modern fuel injected engines are super sensitive to air quality due to the all the sensors they have.  Also, if an engine isn’t getting air out, in the form of exhaust, it can also cause some problems.

Getting the exhaust out of and engine efficiently is just as important as getting good clean air into an engine.
In this case I was limping home on an engine that couldn’t breath.
Turned out both upstream catalytic converts has blown out and were clogging the downstream converter.

Yeah, that’s not supposed to be empty like that.
With three new catalytic converters and four new oxygen sensors the Jeep was up and running.

In terms of quality air in, that’s where the bevy of sensors on the intake side of things can complicate things.  The 4.0L in the LJ has a throttle position sensor (measures the volume of air coming in) an air temperature sensor (self explanatory) a MAP sensor (measures the ambient air pressure) and a Idle Air Control Valve (let’s air bypass the throttle when it’s closed to keep the motor running at idle).  If any one of these sensors is faulty, which is common on a Jeep that sees dirty dusty air while off road, it can cause the motor to run bad.  I’ve had all of them go bad at one time or another.  A faulty TPS can mean power loss throughout the RPM band or even a hesitation/dead-spot at a particular RPM.  A bad IAC can cause a motor to stall out when you approach things like a stop sign.  As you take your foot off the gas to apply brake the IAC should open but if it’s sticky it can not open fast enough and cause the motor to stall.    A bad MAP sensor can mean power loss at elevation (like when I was in CO last year) or when the weather changes (like a pressure drop during a storm).  I chased a bad MAP sensor for a while.  Jeep would run find on a warm sunny day… but once it would rain the motor would run rough.  Swap out the MAP sensor and it ran fine all the time.

Similarly things like oxygen sensors on modern fuel injected motors play a huge role in keeping an engine running healthy and efficiently.  A lot of people think they can get away with bypassing or “tricking” an engine’s computer by either removing an oxygen sensor, spacing it out, or replacing it with a “dummy” sensor.  Truth be told, those tricks worked on older EFI engines.  However on modern engines with multiple sensors it’s getting harder and harder to “fool” them.  As such, a faulty or bypassed oxygen sensor can cause a motor to run lean or rich which brings me to the next part of the equation, fuel.

Fuel Problems

Fuel problems come in two forms, quantity or quality.  Too much fuel is just as bad as too little fuel and bad fuel can cause a host of issues.  In various Jeeps I’ve owned I’ve dealt with clogged fuel filters, clogged/faulty fuel injectors, clogged/faulty fuel pressure regulators, and bad fuel pumps all of which cause a motor to run like shit in a blender.

Was running into some fuel issues.
Thought it was the pressure regulator. Sadly the pump/pressure-regulator/filter unit is all one assembly.
The one I pulled out was a non-OEM one.
Probably faulty since there was a recall, but I opted for a solid unit and did the work myself with some help from a friend.
(Also, don’t wear cloth gloves when working on a fuel system.  Bad idea.)

Another issue is fuel quality.  While on the 2015 NHT I got some gas from a very sketchy gas station in the middle of nowhere in Maine.  The fuel was bad.  Good enough to keep me moving but bad enough the Jeep didn’t like it.  Eventually some fresh gas and some fuel system cleaner was enough to smooth things out, but it had to be some of the sketchiest nastiest fuel I’ve ever used in the states.

Fuel quality is important because it can impact how well a motor runs both in terms of power and in terms of fuel mileage.  Poor fuel quality means pour fuel economy.  It can also cause premature part failure, like with spark plugs.  That brings me to the next part of the equation.

Problems with Spark

I was once told that if you have a problem at high RPM chances are it’s fuel related (pressure and/or flow) and if you have a problem at low RPM chances are it’s spark related (timing, plug quality, etc).  I’ve never been able to fully confirm that old wive’s tale, but it makes sense.  Spark is controlled by a few things.  First you have the plugs themselves.  A quality plug will give a nice spark at the optimal temperature for the type of gas the engine runs on.  Too hot or too cold a spark can cause misfires.  Too weak a spark means you may not achieve full combustion.  So having good plugs of the right kind – as recommended per the OEM – is a good idea.

I’m not too great at “reading” spark plugs.  That said, some of these look better than others.
I made the mistake of replacing them with fancy plugs and the LJ didn’t like them.
Older 4.0’s loved nicer plugs.  However the PCM and the coil-pack in the new 4.0’s is super sensitive and very picky.
Lesson learned.

I’ve learned the hard way that the LJ is very picky about it’s spark plugs and sensors.  In the past my XJ and ZJ’s would use just about any part you through at it.  However I’m finding out the LJ is a lot pickier.  The wrong kind of plug caused the Jeep to run rough, misfire, and idle like crap.  I also lost about 2 miles per gallon.  Once I put the right kind of plugs in things returned to normal.

As far as when the spark is ignited, that is controlled by the distributor on older type EFI engines (like my ZJ) or the coil pack on newer engines (like the LJ).  Through a complex relationship with the Crank Position Sensor, Cam Position Sensor, and the Ignition Coil the right time for the spark is determined.  Too early and you have premature detention.  It’s very bad to have the spark plug go off while the engine is still in the compression cycle.  Those how motors become paper weights.  Too late and the combustion is ineffective.  Even a single occasional misfire in cylinder can cause an engine to run rough.  This about it.  At 1800 RPMs (average cruising speed). That means the engine is firing off every spark plug 1,800 times a minute.  That works out to be 30 times a second.  A FREAKING SECOND.  So if you have a six cylinder engine that means there are 180 tiny explosions going on in your engine every second.  You can see why having the proper timing is important.  That why something like a crank position sensor can sideline a Jeep.  Or the “Oil Pump Drive Assembly” in the newer 4.0s.  If the engine computer gets the wrong information as to the timing cycle of the engine then the spark goes off at the wrong time and bad things happen.

Problems with Other Systems

Explosions are hot.  That’s why an engine has a cooling system.  Some older smaller engines (like a Volkswagen or a lawn mower) use an air cooled system to keep the engine running at optimum temperature.  Larger engines use a liquid cooling system.  Major elements in this system are the radiator (exchanges heat from the hot fluid with cold air), water pump (circulates the fluid from the engine to the radiator), the thermostat (controls the flow between the two), the cooling fan (moves air through the radiator when the vehicle isn’t moving fast or at all), and of course the fluid itself.

Yup.  That’s a crack… and a leak.
Why Chrysler thought it was a good idea to make a PLASTIC radiator end tank I’ll never know.

Jeeps run hot.  That’s no secret.  Some other EFI engines run a little cooler and some run even hotter.  As such, Jeeps are often plagued by heat issues.  I’ve dealt with cracked radiators (usually messy), bad fan clutches (leads to overheating while idling or trail riding), bad water pumps and faulty thermostats (also causes overheating).  Finding out why your vehicle is overheating is always a chore.  If there’s a leak (like seen above from the radiator; from a hose; or the weephole of the water pump) that’s obviously the first place to start.  The next is the cooling fan.  If it’s electronic, check to see if it’s running and that it comes on at the right time.  Mechanical (ie crank driven) fans have a clutch assembly (either a spring or viscous) that locks up at a certain temperature.  If I had a dollar for every faulty fan clutch I’ve replaced on a Jeep I would be a lot less broke than I am now.  Problems with the internals (pump and/or t-stat) are a little harder to figure out, but if it’s a high mileage motor it’s not a bad idea to replace them.  I once pulled a water pump out of a high mileage motor that had ZERO fins left on the pumps impeller.  No wonder it was overheating.

That’s not a good sign when the key is turned.
You can read the full writeup on this here.

Electrical problems are also a common thorn in my side.  Everything to bad ground wires to dead starters has sidelined me at one time or another – and the LJ’s done both to me.

Bad solenoid on the starter motor meant the LJ got another ride on a rollback.

A lot of people are intimidated by electrical problems, and rightfully so.  That said, it’s not hard to do a visual inspection and spot a problem.  If you read the battery wire install you can see photos of how nasty the stock wires looked.  Even if you don’t understand that corrosion means power loss and that bad-grounds can drain batteries you at least tell when something doesn’t look right.  Same with the starter.  It was dirty, greasy, and a lot more disgusting before I took that photo.  Probably a safe bet that something that runs on electricity and is vital to getting a motor running shouldn’t be so disgusting.

Here are a few pics with some other things I’ve deal with:

Faulty belt tensioner and idler pulley.
These help transfer the rotational power of the cranky to other things like the water pump, power steering pump, and alternator.
If the belt is bad and/or pulley and/or tensioner is bad it could mean one of those things not spinning fast enough…
…. or worse yet, not at all.

I could probably write an article just on transmission problems…
… maybe I will.
Either way, a hot transmission is an unhappy transmission.
Even though the LJ has a factory cooler in the bottom of the radiator, an aftermarket auxiliary cooler was deemed necessary.

Remember that time I blew my shocks out from overloading the LJ?
Fun times.
“Death Wobble” is a common problem on solid axle vehicles.
Jeeps are notorious for bad track-bar’s or bushings causing things like bump steer and death wobble.
However, did you know control arms and bad control arm business can cause death wobble too?
I do… ask me how I know (cheat, you can look at the above photo).


If I have any measurable advice at all for you it’s two fold: First, don’t neglect your vehicle.  It’s not kind but as an overland adventurer your vehicle is your lifeline.  You can’t always call AAA or find the parts you need out in the wilderness.  My only saving grace right now is that the vast majority of my problems happen when I’m at home.  More often than not I’m trying to fix something preemptively.  Admittedly that can sometimes backfire on me, but for the most part I can catch problems when they are small before they become huge.  Second, take the time to do something right the first time.  I’ve learned the hardware cutting corners and trying to save a buck doesn’t always work.  My older Jeeps were a lot more forgiving that the LJ has been in terms of what parts I use.  I’m finding the off-brand/discount parts I could throw at the ZJ just don’t work on the LJ.  The PCM and the tune on the engine has such tight tolerances that it seems OEM sensors and stuff are my only option.

May seem silly, but routine maintenance like oil changes are key.
Not going to take sides on the “every 3,000 mile” debate.
That said, doing your own oil changes on a regular basis is a good way to check your vehicle every few months.

Again, this is just a primer on some basics of DIY diagnostics.  I fully admit it’s mostly anecdotal stuff from my own experience, however if you’ve learned nothing and take nothing away other than reading about some of my misadventures then I hope at the very least you were entertained.  Driving an old Jeep isn’t for everyone.  I get harassed and hounded by nearly everyone I know every time one of my Jeeps act up.

A good set of tools goes a long way in doing it yourself.
I’m a bit of a tool junky and, arguably, carry way too many tools.
I however deny such a problem.

I also get harassed and hounded by nearly everyone I know for “not taking it to a real mechanic.”  As I said, I’m a glutton for punishment for trying to work on my own stuff first.  Additionally I enjoy the experiential learning process of problem solving and trying to figure out things on my own.  Sure I’ve made a lot of mistakes over the last decade-plus of owning Jeeps.  Yet the world still turns. I think I’m better off for it too.  Thomas Edison once said, “I have not failed. I’ve just found 10,000 ways that won’t work.”  I’d like to think all of my mistakes and misadventures put me in good company.  As such, let that be an inspiration for you to start doing your own vehicle work.  Even if you’ve never turned a wrench before you can, over time, learn a lot.  I knew nothing automotive related when I was younger.  I didn’t start tinkering with vehicles till I was in college.  Over the last 15+ years I’ve learned a lot from the people around me.  That’s the best place to start.  Make friends.  Bribe them with food (or beer if they’re into that kind of thing) and keep an open mind.  Don’t be afraid to make mistakes.  If you don’t want to learn on your daily-driver or your overland adventure vehicle, search Craigslist for some beater and work on it.  Take it apart. Tinker. Put it back together.  Probably the best couple hundred bucks you could spend in the long run.

All that said, owning an old haggard Jeep as taught me a lot of lessons. Patience being one of them.  There’s nothing like pondering life’s mysteries sitting alongside the road while waiting on a AAA tow-truck to put things in perspective.  And with that, I leave you with the following photo montage:

Injector fault and misfire…

… caused by a faulty PCM…

… Took a few trips to figure that out.
“Yeah, just throw it back there, he has his own parking spot.”
Blown Transmission Line

Blown out upstream catalytic converters clogging the downstream one.
Insert Jeep A into Garage Bay 2.
Repeat if necessary.

(This was for the bad starter.)

I think that was all of them, but I might have missed a tow-truck trip or two.  Thank god for AAA.

0 thoughts on “Basics of DIY Diagnostics – aka the joys of owning an old haggard Jeep

  1. Luckily for me, I don't know the tow truck driver on a first name basis. But after I got my license and then my first car as a teenager, I really didn't have the money to pay someone to work on my car. So instead of paying someone else. I bought my first set of Craftsman tools. Since then I have upgraded to Matco and Snap-On tools and boxes. Funny how something being a necessity turned into a career, at least for a few years, to be able to pay the bills.

  2. Luckily for me, I don't know the tow truck driver on a first name basis. But after I got my license and then my first car as a teenager, I really didn't have the money to pay someone to work on my car. So instead of paying someone else. I bought my first set of Craftsman tools. Since then I have upgraded to Matco and Snap-On tools and boxes. Funny how something being a necessity turned into a career, at least for a few years, to be able to pay the bills.