Basic Engine Diagonostics Thread

[slideways...]

i keep coming upon many many instances of just poorly running cars and usually come up with the same answers, so i was thinking i could make a sticky (hopefully) that would answer the majority of these problems. ill include some basic engine diagnostic techniques, and if anyone has anything else to add please feel free. dont add anything to this thread unless you know it works.

1. fuel system
there are only 3 parts to the fuel system - fuel pump, pressure regulator, injector assembly. these are each easy to diagnose.

fuel pump - people will tell you that you can hear it turn on when you cycle the key into the on position. this can help, but to be sure you should check to see if things are actually working like they should. first, make sure your getting fuel pressure to the rail. hook up a pressure guage to the rail, with the ignition on you should be getting somewhere in the range of 40-50 psi. if its less, check the pump first. with the ignition on, you should be getting battery voltage at the positive wire going into the pump. (the pump is in the gas tank. 240s have an access port in the trunk, some other cars you need to get underneath and find the harness on top of the gas tank). if theres battery voltage at the pump and no fuel pressure at the rail, your pump is bad.
fuel pressure regulator - this is basically a vacuum operated pressure relief valve. same concept as a wastegate. there are really only 2 ways to diagnose a bad regulator. remove the vacuum hose and it should be dry. any fuel smell is a bad seal in the regulator. also hooking a vacuum pump up to the regulator will tell you if its bad. there should be no leak, i.e. pump it up to 15-20 psi and it should stay steady.

fuel injectors - basically electric solenoids connected to a simple valve. the solenoid has a coil with a set resistance so not just any voltage can activate it. 240s have high impedence injectors, which means that if you put an ohmmeter to it (while unplugged of course) you should get around 12 ohms. usually 10-14 ohms is acceptable, but check the specs on each car. if this is good, you can check the physical condition of the injector. to listen if the injector is cycling, run the car and listen for the ticking with an automotive stethoscope. to check the mechanical condition of the injector, pull the harness, pull the spark plug for that cylinder, and look into the cylinder while your buddy cycles the key on and off without starting the car. there should be no fuel going into the cylinder. if there is, its a bad injector. CAUTION: dont start the car with your eye up to the plug hole. youll get a nasty blast of shit in your eye.

2. ignition system:
first of all, to all you bandwagon riders, just because everyone and their grandma put an MSD ignition on their civic, doesnt mean there was a good reason to do so. there is actually more reason not to on almost every modern car out there, and even more so with imports. almost every modern ignition system can handle more power than you have. dont waste your money.

ignition systems are all pretty much the same. ecu sends a signal from your cam/crank sensor, through a control module to a coil telling it to send spark. if you have a distributor, this sends the spark to each cylinder, physically timing it with engine operation. if you have coil packs, the timing is done electronically. ka24s have an external coil. this uses electromagnetic induction to create a spark, and then uses a secondary coil to amplify the spark so that its powerful enough to ignite a spark plug. this is all highly controlled as far as how much resistance is in the ignition system, so putting bigger plug wires on for no reason will almost never help, and sometimes hurt driveability. coil over plug systems like on jdm SR20s are the same basically, but have less reliance on the coil packs than you might think. all the computing and signal transfering is done in the ecu with data from your different engine sensors. checking the condition of a coil is as simple as checking the resistance of the 2 coils inside the coil and matching them to spec. checking a distributor is more dependent on what car you have more than anything, but basically its job is just to send spark to each cylinder. you can check the pulse signal and reference signal from the control module/ecu to the distributor(its actually the other way around but for simplicity's sake lets pretend its this way.) to see if the distributor is being timed correctly. otherwise diagnosing an ignition problem is basically all about checking voltage and resistance at each system and comparing to spec.

3. this also goes for the various sensors in the car. each has a reference voltage with the ignition on, and each behaves in a preset way as far as voltage it puts out. diagnosing sensors is hard without a FSM or alldata or something.

4. the air part of the equation is a bit more obscure, and a bit simpler. you always have air, but having properly compressed air of a proper volume is what an engine needs to be concerned with. there are 2 basic ways to make sure this is happening correctly. one is with a speed/density system, the other is with a mass airflow system. both have pros and cons. the speed/density system uses a map sensor to tell the ecu how much air is in the engine. this is done with preset calculations in the ECU concerning the density of air, usually with a base equation based on the density of air at sea level. this is further modified by a signal from the 02 sensor to adjust for any condition.

the mass airflow system is what most non-honda imports use. based on the theory that a given mass of air will cool a piece of metal with known heat retention characteristics a specific amount, this system is less dependant on peripheral sensors to determine air/fuel mixture. the downside of that is if the MAF sensor is malfunctioning, the car has to reset into a limp home mode, which is much more limited than other systems. this is because the car then has no way of knowing how much air is actually entering the engine until the 02 sensor reads AF mixture after combustion. this can be dangerous if the car isnt tuned very very well, so for engine safety, the ECU wont let you do anything that might hurt the car.



it all boils down to the basic 5 things that any otto cycle internal combustion engine needs to operate: air, fuel, spark, compression, and timing. things get more complicated when emissions and driveability are concerned, but it all boils down to the same thing.


if i have more ill add to this post later.

[S13Driftfiend]

good stuff/idea. i vote sticky

[slideways...]

if your serious about working on cars, there are a few basic tools you really should own. these are: compression tester, a good test light, a fuel pressure guage, a vacuum guage, a timing light, and a DVOM (digital volt/ohmmeter). having these can simplify any problems you are having.

compression test is pretty easy, but for anyone who doesnt know how, ill explain it.
first remove all plug wires, keeping track of which goes where. then unplug the distributor or ignition module to prevent an errant spark from lighting your engine on fire. remove the first spark plug, screw the compression tester into the plug hole, and crank the engine until the pressure builds as high as it will go. it should take a few seconds to do. the pressure shouldnt drop basically at all until you relieve it with the button on the side of the tester.
after this, replace the plug and move on to the next cylinder. each cylinder should be within around 10% of each other. in other words, if the highest cylinder is 180 psi, the lowest cylinder should be no less than 162 psi. this is a rough measurement used for normal cars. higher hp or highly tuned cars should be closer.
one thing that most people get confused about is when you check compression, the guage reads usually a hundred or two psi. but when you hear about a compression ratio, its totally different. the reason is that the compression pressure and compression ratios measure two completely different things that are only somewhat related. compression ratio is basically the ratio of the volume of the combustion chamber at BDC compared to the volume of the chamber at TDC. compression pressure that you measure is just the pressure of the compressed air in the cylinder.


checking timing is done with an inductive timing light. it simply makes sure that your getting spark at the right point in the engine's mechanical movement cycle. to check timing, most cars will have timing marks on the crank pulley and a tab or some sort of reference mark on the block. connect the two power cables on the timing light to the battery, and hook the inductive lead around the #1`plug wire, making sure that its facing the right way. spark physically travels through a plug wire so if you get the lead backwards, the timing light will think that somethings wrong and your reading will be off. after you have it hooked up, simply point and shoot at the tang on the block, and the gun will flash once with each spark through the wire, creating a strobe effect. the crank will appear to be still and you can check exactly how far off the pulley's timing marks are off in relation to the tang.
if you have a distributor ignition, loosening and turning the distributor can adjust the timing. if you have a distributorless ignition, something is wrong with your ignition or timing belt/chain if the timing is off.

a vacuum guage actually can be used to tune your car. i dont know all the specifics about this but basically at idle you should have around 15-17 inches of vacuum on a normal running car. turbo cars are different, and vacuum guages arent used much on them, so this is mainly a tool for a n/a car.

a fuel pressure guage is pretty self explanitory. before the regulator you should have around 40-45 psi of pressure. if you dont have a schrader valve on your rail, you have to tee into the high pressure fuel line(the one going to the rail, not the one coming from the regulator). teeing in at the fuel filter usually works well if your filter is easily accessable.

the DVOM is a must have for any diagnosis that even has very simple electronics. modern engines are so full of modules, sensors, reference voltages, timing voltages that you cant really figure out if a part is really wrong without making sure everything around it is good first.

[slideways...]

testing engine peripherals

alternator -
people might say that just unplug the battery with the car running. if the car dies then the alternator is bad. this test isnt very accurate or safe though. theres a thing called spike voltage that can occur and wreck wiring, blow fuses and possibly blow your alternator. it has to do with the law of inertia. it also applies to electric current. when you interrupt an electric circuit, the electricity very much doesnt want this to happen, so it will spike at the interruption point trying to keep the circuit complete. if theres no place for it to go, it could travel back up the wiring and start frying things.
testing an alternator isnt hard though. break out your DVOM, install a known good battery, and you should have between 13.5 and 15.5 volts at the battery with the car running. if you dont, check and make sure you have power at the positive terminal bolt on the alternator. if theres battery voltage there, you can pretty much safely say that you have a bad alternator. theres a chance that the harness or wiring could be bad also, this you can also check with a DVOM by back probing the alternator harness and comparing your findings to the specs for your car.

starter-
banging on the starter to see if it helps is sort of a misleading diagnosis procedure, but it works sometimes(key word: sometimes). the easiest way is turn your key to the on position, and put a cable from the positive battery terminal to the positive terminal on the starter solenoid(usually the big one on top). your car should start. if it does, its either the ignition switch or relay. if it doesnt, and your battery is good, your starter is bad.

checking relays is as easy as replacing it with a known good relay from another system, and seeing if the faulty system works then. a relay is no more than a switch that uses voltage to turn on and off instead of your finger.

[slideways...]

ECU trouble code retrieval for pre-OBDII DOHC 240sx.

1991 Nissan-Datsun 240SX L4-2.4L (KA24DE)

Displaying & Reading Trouble Codes Fig. 69 Trouble Code Example

READING DIAGNOSTIC CODES
The Electronic Control Unit (ECU) monitors several engine sensors and actuators. If a problem occurs the information is stored in the ECU's memory for retrieval.
The codes are displayed by the red diode on the ECU. The codes are interpreted by the number of times and duration the diode flashes.
The red diode will flash for 0.6 of a second with 0.6 of a second delay between flashes "X" number of times, this will represent units of ten.
Then there will be a delay 0.9 of a second and the red diode will flash for 0.3 of a second with 0.3 of a second delay between flashes "X" number of times, this will represent units of one.
There will be a delay of 2.1 seconds between codes.
For example, if the red diode flashes once and then flashes twice rapidly this should be interpreted as code 12. The codes are displayed in numerical order starting with the lowest and ending with the highest.

Fig. 35 Selecting Modes

EXTRACTING DIAGNOSTIC CODES

1. Access the ECU from below the panel under the carpet on the passenger side floor board and position it for observing the diode.
2. Turn the ignition switch on (do not start the engine).
3. Confirm that the "CHECK ENGINE" lamp in the instrument panel illuminates. If not, check the bulb and replace if faulty.
4. Turn the diagnostic mode selector on the ECU fully clockwise for at least two seconds then turn the selector fully counterclockwise. The ECCS system is now in mode 2.

Fig. 36 Inspecting E.C.U. Lamp

1. Observe the diode and note the code(s) displayed. Note: Code 55 indicates all systems are OK.

^Refer to the chart above for code identification and proceed to DIAGNOSTIC CHARTS for trouble code diagnosis.

ERASING DIAGNOSTIC CODES

1. Turn the ignition key on.
2. Turn the diagnostic mode selector on the ECU fully clockwise and wait more than two seconds then turn the selector fully counterclockwise. The ECCS system is now in mode 2.
3. Turn the diagnostic mode selector on the ECU fully clockwise and wait more than two seconds then turn the selector fully counterclockwise. The ECCS system is now in mode 1. ECU memory is now erased.

[nissan_240sx]

sticky x2


More useful info for the new guys..

http://www.240sx.org/faq/index.html

[AWDSR20]

NICE JOB BRO! sticky ^^^^^^

[93-240SX-COUPE]

I have an s13 service manual in PDF format. U guys know how I could post the entire PDF files?

[D-Bo]

^ that would be a waste. post it as a file, not individual pages. its easier to download the whole thing and look through it that way. it sure as hell was easier for me.

[jon@af]

NOTE TO ALL POSTERS IN THIS THREAD: IF the content is not congruent with the purpose of this thread - i.e. providing basic information for the new members - your posts will be deleted. We don't want to clutter this thing up with useless posts.

[stairweitoheaven]

the pictures are gone. can you relink them? thanks

[slideways...]

sorry ill work on it. the stuff is off alldata so ill have to host the pics elsewhere.

[Clutch_kick_94]

Does anyone know if a RB25 engine swap for the s14 is difficult??