Questions tagged [diagnostics]

Use this tag for generic questions related to tools, techniques and methodologies for diagnosing problems. Do not use this tag for case-specific problems, the "troubleshooting" tag is better suited for that.

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20
votes
2answers
2k views

What possible causes for the “check engine” light can I check for myself?

The "Service Engine Soon" light came on yesterday and isn't going away on its own. The car drives fine, no weird noises, smells, or other symptoms. The manual is not very helpful, just directing me to ...
18
votes
4answers
14k views

How to test a clutch

I have just bought a second hand car, and I have the feeling that the clutch is worn out. How can I do a quick clutch sanity check?
17
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4answers
73k views

How to test for defective glow plugs?

On a diesel engine, how can I know which glow plugs are defective, and can I change these only?
17
votes
2answers
3k views

Is there a rule of thumb for estimating volumetric efficiency?

Here's a situation I encounter quite often During diagnosis, I want to determine if the mass air flow sensor(s) [MAF(s)] is/are healthy. I hook up an OBDII reader to a vehicle that I'm trying to ...
16
votes
3answers
13k views

How can I tell if I have bad cats?

No, not that cat! Catalytic converters - or "cats" - have a nasty habit of failing with age. What are the symptoms associated with a cat that's gone bad? Are there any tests that could definitively ...
15
votes
3answers
5k views

Afraid to crank engine - how can I tell if my cylinders are flooded with water?

Backdrop We had really heavy rains here today. I was in my '99 M5, idling in traffic when I noticed the brake pedal feel became really notchy. When I let go of the brakes the pedal would not travel ...
13
votes
10answers
106k views

How can I detect vacuum leaks without access to a smoke machine?

In the course of helping my friend sort out a lean condition with his Mercedes GLK280 we have to pinpoint the source of a vacuum leak in the intake plumbing. The interwebz tend to recommend having ...
13
votes
1answer
3k views

What exactly are fuel trims?

What is meant by the term "fuel trim"? Why are they so important in diagnosing engine operation issues? How do short-term fuel trims differ from long-term fuel trims?
12
votes
5answers
44k views

How to diagnose high speed vibration?

I get a vibration at high speeds (50+), can feel it particularly in the steering wheel, front seats, dashboard. I have been trying to figure out what's going on, keeping in mind that: All 4 shocks ...
11
votes
3answers
3k views

Diagnosing a worn wheel bearing

I've noticed an increase of road noise while driving my car and I'm wondering if it might be the wheel bearing. I think it's coming from one of the rear wheels. Is there a way to identify and confirm ...
11
votes
3answers
2k views

In Cylinder Pressure Testing With a Pressure Transducer and a Oscilliscope

Recently I've been looking into buying a higher PSI pressure transducer to do in cylinder pressure testing. I've encountered vehicles which would of benefited from this approach and wish I had thought ...
11
votes
1answer
462 views

Are there any plans to update OBD-II?

OBD-II is ancient It predates the Porsche 911 GT1, Windows 95 and implementation of the Schengen Agreement. I would have thought that SAE would be working to add more diagnostic coverage for ...
10
votes
3answers
55k views

How to diagnose the cause of no power reaching the spark plugs when power reaches everything else, after checking fuses?

I've been trying to get my long-suffering Toyota RAV4 first generation (1998) to start after numerous different problems, the latest of which I've narrowed to the spark plugs not firing seemingly due ...
10
votes
1answer
235 views

How to Tell if an IAC needs to be Cleaned?

Is it possible to diagnose an IAC as needing cleaning without actually taking it apart? For example, I recently was having some idle issues with my 99 Nissan Almera 1.6L which were solved by ...
10
votes
1answer
990 views

Are resistance/capacitance based brake fluid testers reliable?

I would like to test my brake fluid to see whether it needs replacing rather than spend the time and money on it unnecessarily. I've seen inexpensive brake fluid testers on the market which work on ...
10
votes
1answer
370 views

More In Cylinder Pressure Testing Questions

So I was looking at a 06 Pontiac G6 2.4l with a complaint of low power on hills. Had been at another shop to have the transmission checked and they referred the customer to us. Road test confirms car ...
10
votes
1answer
154 views

Is firing order sufficient to tell if a crankshaft is cross-plane or flat-plane?

If I know the order in which the cylinders on a V-engine fire, can I know for sure whether the engine has a cross-plane or flat-plane crankshaft?
9
votes
6answers
4k views

Can I confirm clutch slippage by shifting without disengaging the clutch (while stopped)?

This question got me thinking about a possible test that anyone driving a manual transmission could perform to diagnose severe clutch plate slippage: Engage handbrake, make sure there is nothing in ...
9
votes
2answers
3k views

What is the “propane test” and how is it carried out?

This test is often mentioned in the context of figuring out what's wrong with a car engine's operation (misfires, hesitation, poor fuel economy, etc.) What is this test and what does one need to ...
9
votes
2answers
934 views

Why are the spark plugs from the same 2 cylinders corroding faster than the others?

On my engine ('99, 360 v8 Magnum), I replaced the spark plugs less than 15k miles ago, because of misfires in cylinders 2 and 8. Those two plugs were badly corroded. Today I had to replace them again, ...
9
votes
2answers
14k views

What can I do with an Inline Spark Tester?

Due to my recent problem with weakly firing plugs, I'm looking at inline spark testers. I've seen three basic types: The simple flashing one which goes in between the wire and plug with the plug in ...
9
votes
2answers
256 views

Firing a Coil on Plug

I was wondering if there is a simple way of firing a coil on plug outside of the car. The coil in question is just a two wire coil on plug and I don't know if its possible to just hook up a 12V ...
8
votes
1answer
6k views

How to check if spark plugs are firing? Coil-on plugs

While trying to diagnose a tricky car not starting problem I was asked to check if the spark plugs were firing at all. I've found plenty of resources on inspecting the condition of spark plugs after ...
8
votes
2answers
964 views

Issues cold starting diesel van

Our van has been progressively getting worse at starting up in the morning. It only has issues when the engine is cold. Initially I thought it might be the glow plugs. I tested their resistance and ...
8
votes
1answer
409 views

Exactly how much refrigerant is in there?

A concern that comes up regularly when troubleshooting A/C problems is whether an A/C system has too little refrigerant (hello leaks) or too much of it (hello overcharging). The traditional way to do ...
7
votes
1answer
302 views

Can I use a digital multimeter in resistance mode on a “no-load” circuit?

While this question is probably more at home on EE.SE, the implications can be quite profound when troubleshooting automotive problems. If I want to test for continuity on a 12 V circuit with next to ...
7
votes
3answers
9k views

Best practice to tell if an interference engine has bent valves?

I have a 2003 Mitsubishi Galant 2.4L SOHC engine with 251k miles on it. While I was driving it all of sudden the engine just died. I pulled off the road and then had it towed to a local mechanic. ...
7
votes
4answers
176 views

Basic Automotive Maintenance Repair - Compiling a Toolset

I am looking into getting more serious about doing my own automotive repair and maintenance, and am working toward gathering the appropriate tools. However, I am admittedly can be a bit of an ...
7
votes
1answer
149 views

Is an injector an injector?

In general terms at least - lately the engines I've been working on are diesels and I've never had to troubleshoot fuel injection problems on gasoline/petrol engines. So, I'm wondering, how many ...
6
votes
4answers
36k views

Technique to test an alternator - fact or fiction?

A friend explained that the following is a simple way to test an alternator to see if it is working: Turn on the vehicle. While it is running, carefully remove one of the 12V battery leads. See if ...
6
votes
2answers
30k views

How can I test my thermostat?

How can I tell whether a mechanical thermostat works as it should? Is there a test that can definitively tell me if a thermostat is good or bad?
6
votes
1answer
5k views

What is a smoke test and what kind of tools do I need to do it?

I've seen 'the smoke test' referenced several times in answers to OP's. I would like to ask; What is a smoke test? What kind of tool(s) do I need to do a smoke test? What is the proper procedure to ...
6
votes
3answers
6k views

Does overheating that goes away under high RPMs indicate a sticky thermostat?

Sitting in traffic, my car's thermostat goes up to the red. If I put it in neutral and rev it, it goes up a tiny bit, then suddenly drops back to normal. Is that the thermostat sticking, or something ...
6
votes
2answers
10k views

What is the “dollar bill” test?

One of the questions recently made reference to an exhaust failing the "dollar bill" test. How does one perform it and what does it tell you?
6
votes
2answers
4k views

Is this the correct procedure for a stator test?

Stator Test I have a 2007 DL 1000 V-Strom: I followed the following process while checking the stator: Put multimeter to VAC Tested the three phases of the stator connector plugged into a new ...
5
votes
4answers
7k views

How to Test an Ignition Coil with a Multimeter

I have a 98 Mazda 626 GF and I'd like to test the ignition coils with a multimeter to see if any of them are starting to fail. The car has ECU controlled timing / ignition and there are two coils, ...
5
votes
3answers
14k views

Used Car Condition - Is a Compression Test a good proxy of engine health? (and what's a leak down test?)

I'm looking to buy a used car and aside from the advice here (which is mostly functional) I want to know if the oil has been changed "enough" to prevent gunk build up that could starve parts from ...
5
votes
2answers
35k views

How to diagnose excessive coolant consumption?

Whats the best way to determine the source of coolant consumption? How should one differentiate between say a leaking radiator, leaking waterpump, leaking hoses, a blown head gasket, or crack in the ...
5
votes
2answers
4k views

P0300, P0302, P0303 on 2003 Honda Civic 1.7

I'm in the process of helping someone with their 2003 Honda Civic, 1.7L engine, 161k miles on the clock. Pulling the codes from the vehicle I got three specific codes: P0300, P0302, P0303. The codes ...
5
votes
3answers
5k views

Why is a volume test necessary when diagnosing fuel delivery problems?

I suspect my fuel pump causing intermittent driveability issues. I checked the fuel pressure at the fuel rail and it seems like the pressure is fine: ~2.4 bar at idle, 3.1 bar at engine off (3.09 bar ...
5
votes
1answer
5k views

Subaru clunking and whining - differential?

I have a 2002 Subaru Outback with a 5 speed manual and 160k miles on it. The previous owner claims to have replaced the front differential, because it was locked up. Now, I have been hearing whining ...
5
votes
1answer
679 views

How to Improve My Diagnostic Process

I had some problems which I suspected were related ( diagnostic index at pg. 150 in 98 US WSM ): idle speed too slow and excessive engine shake pg. 167 Hesitation / Stumble: Momentary pause at ...
5
votes
3answers
2k views

Testing radiator fan

I took the fan out from my engine compartment and am now ready to test it. It consists of two fans, one larger than the other connected I believe in series with two large wires, one red, one brown ...
5
votes
1answer
3k views

Does Fiat need dedicated FiCom OBD hardware and software for diagnostics?

I got an ELM 327 with Bluetooth from my brother in law. We have tested it in his Chevrolet Aveo with several apps and it works like a charm. When I tried it on my Fiat Panda 2012, the apps couldn't ...
5
votes
2answers
5k views

Why does Honda Civic 05 overheat when water reservoir gets full?

My father in law's Honda Civic overheats when the water reservoir is full or to the top. He is wondering what could be the problem if the radiator needs water when the reservoir is full. The ...
5
votes
3answers
9k views

Possible EGR problem on Fiat Idea Multijet 1.3 Diesel

I've been having issues with my 56 plate Fiat Idea for a while now. It seems to drive normally up to about 2500-3000 rpm where the car lurches a little, as though i shifted down a gear, the MIL light ...
4
votes
1answer
332 views

Fuel rail pressure - how long should it hold?

My factory service manual states fuel pressure should be 50 psi with the engine running at idle. This is a six cylinder 2008 Subaru 3.0R engine with a return-less fuel line to the injectors. ...
4
votes
1answer
74 views

Coil On Plug, Testing The Primary Coil

Are there any alternatives for scoping the primary coil on a COP system other than using a low amp probe? I'm guessing backprobing doesn't work since the ignition control module is built into the coil ...
4
votes
1answer
582 views

Why would a lambda sensor fail?

I had an early failure of one of the two lambda sensors in my 2011 Toyota Yaris that was driven for approximately 40 000 - 50 000 km at the time of failure. Fortunately, this was replaced for warranty....
4
votes
2answers
2k views

What controls O2 sensor/lambda cycling?

I just replaced the lambda on my 98 Mazda 626 GF 2L because it was showing a signal that was constantly pegged lean and I suspected it was defective. After replacing the sensor I took the car for a ...