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.
Questions tagged [diagnostics]
159 questions
21
votes
2 answers
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 take it to a professional for service. Before I…
Robert
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19
votes
5 answers
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?
Alekk
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17
votes
4 answers
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?
jfoucher
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17
votes
2 answers
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 diagnose and get access to real-time mass air flow…
Zaid
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16
votes
3 answers
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 confirm that a cat has gone bad?
Zaid
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15
votes
1 answer
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?
Zaid
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15
votes
3 answers
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 back up easily and the engine started sputtering…
Zaid
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14
votes
10 answers
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 the car taken to a mechanic so that they can perform…
Zaid
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13
votes
5 answers
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 replaced in April
Had a side swipe accident, they…
Anton Pugachevsky
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11
votes
3 answers
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 if a wheel bearing is worn, and therefore in need…
BG100
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11
votes
3 answers
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 to try this in the past.
I've been watching…
Ben
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11
votes
1 answer
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 features that are virtually essential to most passenger…
Zaid
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10
votes
1 answer
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 the basis of detecting water percentage based on…
GdD
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10
votes
1 answer
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 doesn't go anywhere fast.
I didn't really think…
Ben
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10
votes
3 answers
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 to no power reaching the spark plug wires.
So, my…
user56reinstatemonica8
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