Mechanic says compression tests are outdated?

[silviaz]

I was calling a mechanic about doing a pre inspection for a car and he mentioned that mechanics don't do them anymore and it's not just as simple as pulling out spark plugs on modern cars but instead they do oscilloscope tests instead? I've never heard of this and I haven't found much information on it. Is this true?

Edited April 5, 2023 by silviaz

[Dose Pipe Sutututu]

Correct, on modern cars you can just use scanners such as Launch X421 or an Autel.

 

For our older shit boxes, compression tests are an ok way to determine even compression between cylinders, however motor with massive duration cams will read lower compression than say a motor with stock cams. Better to do a leak down test if you want to check engine health.

[silviaz]

33 minutes ago, Dose Pipe Sutututu said:

Correct, on modern cars you can just use scanners such as Launch X421 or an Autel.

 

For our older shit boxes, compression tests are an ok way to determine even compression between cylinders, however motor with massive duration cams will read lower compression than say a motor with stock cams. Better to do a leak down test if you want to check engine health.

Ah ok, but isn't even a 1997 car considered a modern car? That's why I was a bit confused. 

[GTSBoy]

1 minute ago, silviaz said:

Ah ok, but isn't even a 1997 car considered a modern car? That's why I was a bit confused. 

No. Ancient shitbox. The sort of diagnostics that your mechanic is talking about were probably only available on top end Mercs and other leading edge stuff.

A 1997 Nissan is barely more sophisticated than a 1987 Nissan.

[joshuaho96]

Even a Consult 1 Nissan has a relative compression test function, it's just super jank and you have to do all your own math. You can cut the fuel injection to each cylinder, average the resulting idle RPM in the logs. If one cylinder has relatively low drop when shut down you know something is wrong with it. If there's no misfire or funny business like that then the only conclusion I can think of is low compression.

Launch X431 is great stuff btw, probably the only scan tool I've seen other than maybe the real deal Consult that will let you reset throttle position idle switch on the RB26 instead of guessing blindly at how to do it like I did in the past. It is incredibly slow though compared to NDS1 which is disappointing and the logging functions are nowhere near as powerful I think.

[GTSBoy]

56 minutes ago, joshuaho96 said:

reset throttle position idle switch on the RB26 instead of guessing blindly at how to do it like I did in the past

It's never been difficult on that vintage of Nissan stuff. RB20 is same same. I just used to adjust it so that even the very slightest movement off idle would cause the injectors to do the old double pulse thing (which is the accel enrich thing that they do). You literally just listen for them to go click-click-click-clickclick.

[joshuaho96]

47 minutes ago, GTSBoy said:

It's never been difficult on that vintage of Nissan stuff. RB20 is same same. I just used to adjust it so that even the very slightest movement off idle would cause the injectors to do the old double pulse thing (which is the accel enrich thing that they do). You literally just listen for them to go click-click-click-clickclick.

The RB26 ECU in particular had some weird behavior, if the TPS switch was set to 0.2V when the ECU first gets power after discharging capacitors then it would decide the idle switch was off at 0.5V even though that was the factory spec. Seemingly the actual idle switch in the TPS is vestigial? Or it somehow can be reset, I never really got to the bottom of what was going on there. Regardless it was interesting to see how the scan tool also had a function to relearn the TPS idle position.

[GTSBoy]

8 minutes ago, joshuaho96 said:

Seemingly the actual idle switch in the TPS is vestigial?

It was my understanding that the RB26 was the same same as the RB20. The idle position was 100% determined by the switch on the RB20. The TPS potentiometer was not used for idle at all. And in fact, the potentiometer on the RB20 was only used for the auto transmission!!! The ECU paid it no heed.

Of course, I might be wrong about that on the 26. But seeing as the 26 ECU was just a slightly glorified version of the 20 ECU, I have been willing to believe they used the same strategy. And further of course, maybe because the 26 was ITB fed they found they needed to actually use the potentiometer for the ECU. I could look it up in the manual, but really cannot be arsed. I might have even been schooled on this point before.

It was only on the R33/RB25 era that Nissan abandoned a physical switch in favour of using the potentiometer's value as the determinant for idle position (Which was a pretty shit idea considering how much trouble it's caused people over the years. It was almost certainly done because it was cheaper than having a switch too.) It is this that has led to so much confusion between RB20 and RB25 owners.

[joshuaho96]

4 minutes ago, GTSBoy said:

It was my understanding that the RB26 was the same same as the RB20. The idle position was 100% determined by the switch on the RB20. The TPS potentiometer was not used for idle at all. And in fact, the potentiometer on the RB20 was only used for the auto transmission!!! The ECU paid it no heed.

Of course, I might be wrong about that on the 26. But seeing as the 26 ECU was just a slightly glorified version of the 20 ECU, I have been willing to believe they used the same strategy. And further of course, maybe because the 26 was ITB fed they found they needed to actually use the potentiometer for the ECU. I could look it up in the manual, but really cannot be arsed. I might have even been schooled on this point before.

It was only on the R33/RB25 era that Nissan abandoned a physical switch in favour of using the potentiometer's value as the determinant for idle position (Which was a pretty shit idea considering how much trouble it's caused people over the years. It was almost certainly done because it was cheaper than having a switch too.) It is this that has led to so much confusion between RB20 and RB25 owners.

I honestly don't understand what's going on in the RB26. It 100% has a physical TPS idle switch. Seemingly in the R33 at least it does not pay attention to it. I didn't experiment too much but it seemed like it was purely based on a learned voltage. Maybe I'll revisit it but as far as I could tell getting the idle TPS voltage to be about 0.45V was more important than whatever the idle switch trigger point ended up being.

[Dose Pipe Sutututu]

throw it all in the bin and convert to DBW.

[admS15]

On 06/04/2023 at 1:42 PM, Dose Pipe Sutututu said:

throw skyline in the bin and buy BMW.

FYP, lol😂

[Dose Pipe Sutututu]

1 minute ago, admS15 said:

FYP, lol😂

hopefully the bin shits out $60k in cash or directly deposits that money into my bank account - if so, done deal.

[Duncan]

2 minutes ago, Dose Pipe Sutututu said:

hopefully the bin shits out $60k in cash or directly deposits that money into my bank account - if so, done deal.

What's the $60k for? replacing broken computers to stop error lights on the dash?

[Dose Pipe Sutututu]

6 minutes ago, Duncan said:

What's the $60k for? replacing broken computers to stop error lights on the dash?

weekly trips to Eurowerke in Castle Hill LOL

[MBS206]

4 hours ago, Dose Pipe Sutututu said:

throw it all in the bin and convert to carbs

FYP :D