Strange Relative Compression VW Transporter

Ask for and share advice on using the PicoScope kit to fix vehicles here.

Strange Relative Compression VW Transporter

Postby RYM6746 » Tue Jul 11, 2017 3:02 pm

Customer complaint is misfire at low revs (mainly under load). So I verified the complaint on road test, I can replicate it easily. When idling it is a little lumpy. Smooth running values are positive 3mg on cylinder. 1 and negative 2.5mg on cylinders 3 and 4. This is a 5 cylinder engine code AXE fitted with the PD injectors all under the cam cover (none of it easy to get to) so the easiest place to start was with a relative compression test. I admit the capture would be better with a sync and I will try to get another one with a sync when time allows. I was not expecting to see the "steps" I am GUESSING that it is something to do with the PD injector springs, extra cam lobes or compressing of the fuel inside of injector? Not really sure as I have never seen it before? Could someone help? Am I looking at a mechanical fault or two or no fault at all?
Attachments
Volkswagen_Transporter_2004_Diesel_Relative compression with PD injectors.psdata
(972.17 KiB) Downloaded 190 times
User avatar
RYM6746
TwoWaves
TwoWaves
 
Posts: 99
Joined: Tue Nov 01, 2016 10:58 am

Re: Strange Relative Compression VW Transporter

Postby STC » Tue Jul 11, 2017 10:28 pm

That is indeed a strange starter current waveform. It is difficult to pick out the 5 separate cylinders and any repeating pattern.
The good news is that your capture does not reflect an engine with a slightly lumpy idle. The capture / set up has to be more at fault than the Engine if that makes sense or I might be eating those words some time soon.

Although we cannot see the peak, I am not so sure that the Starter Inrush Current is anywhere near where it should be.

I would
1. Look in the library for a waveform from any VAG PD if an AXE is not there.
2. Do a relative compression test on a known good diesel, PD if possible to confirm your scope, amp clamp and set up.
3. You already know this - add CKP or CMP for Engine Position
4. Scope on CKP and Injector 1 for Cylinder Identification. Add a RPM Math Channel. At idle and while it falters.

Hope that helps.
User avatar
STC
TwoWaves
TwoWaves
 
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Re: Strange Relative Compression VW Transporter

Postby RYM6746 » Wed Jul 12, 2017 12:07 am

Will do thanks for the help I'll keep you posted
User avatar
RYM6746
TwoWaves
TwoWaves
 
Posts: 99
Joined: Tue Nov 01, 2016 10:58 am

Re: Strange Relative Compression VW Transporter

Postby ben.martins » Wed Jul 12, 2017 10:37 am

Hi RYM6746,

As STC's states it is an odd waveform for a 5 cylinder vehicle. I have had a look on the waveform library and there are lots for a 4 cylinder diesel but not the 5. One thing we can see from the initial capture is the starter speed, which in my opinion seems a little low. I don't suppose this has any bearing on the fault and maybe nothing but just looking at things that stand out! What does it sound like when it turns over? Any metallic or unexpected noises?

The sync would definitely help to see where these peaks actually sit in relation to engine position but maybe tricking with the PD engine as you say. From what I remember the main wiring for injectors and glow plugs sits on the back of the head. Being transporter I would have thought it was FWD so it might be easier to get wiring diagram, find cylinder 1 on this loom and use a 20/60A clamp to capture sync. Not a lot of help at the moment I know but I will also post something on the Facebook group to see if anyone has one in and could gets a known good capture to assist.

Just to confirm this is a 2.0l or 2.5l? Your waveform notes state 2.0l yet everything I've found on AXE engines is a 2.5?

Keep us updated!

Kind Regards

Ben
ben.martins
Pico Staff Member
Pico Staff Member
 
Posts: 25
Joined: Tue May 16, 2017 3:02 pm

Re: Strange Relative Compression VW Transporter

Postby RYM6746 » Wed Jul 12, 2017 5:11 pm

Hi my mistake it is a 2.5L. Thanks for your help. I will try and get a sync with waveform but it will be tricky as according to wiring diagram the live and control wires for all injectors comes from ECU. and to stop it starting I am pulling the ECU fuse. I may be able to pull in tank
fuel pump fuse (if fitted). I am also pressed for time at moment but I will try my best. Thanks for your input
User avatar
RYM6746
TwoWaves
TwoWaves
 
Posts: 99
Joined: Tue Nov 01, 2016 10:58 am

Re: Strange Relative Compression VW Transporter

Postby STC » Wed Jul 12, 2017 11:33 pm

as according to wiring diagram the live and control wires for all injectors comes from ECU


Indeed they do, the PD injectors don't share a power supply or ground. As Ben has pointed out, at the back of the cylinder head (gearbox end)will be a big round plug, the injector and heater wires are in there. They will have to exit the head somewhere.
User avatar
STC
TwoWaves
TwoWaves
 
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Re: Strange Relative Compression VW Transporter

Postby RYM6746 » Thu Jul 13, 2017 12:20 am

I can see the loom and I will be able to get on an injector wire my main problem is stopping the engine from starting but still having an injector pulse to sync from. I need to disable the fuel supply somehow (easily being my priority)
User avatar
RYM6746
TwoWaves
TwoWaves
 
Posts: 99
Joined: Tue Nov 01, 2016 10:58 am

Re: Strange Relative Compression VW Transporter

Postby STC » Thu Jul 13, 2017 12:40 am

Clamp the fuel supply line after the filter. There are no pressure or flow sensors that can call the ECU Police ?

Or connect the Feed to the Return at the filter
User avatar
STC
TwoWaves
TwoWaves
 
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Re: Strange Relative Compression VW Transporter

Postby RYM6746 » Thu Jul 13, 2017 9:08 pm

So I did another relative compression test this morning and would you believe it. It looks perfect GRRRRR :evil: . it must of been equipment error (e.g amps clamp not shut properly or something). Anyway compression looking relatively good, I went after fuel. Turns out after much digging there is a port in the tandem pump that I could attach the WPS500 too. I think you will agree that the misfire can be seen in the pressure dips seen on channel 1 :D . I also did another capture using the zoom feature on the WPS500. Which is making me feel much more comfortable about the 4.2 hours involved for injector replacement. Any extra feedback is welcome as always
Attachments
Volkswagen_Transporter_2004_Diesel_Tandem pump pressure with misfire seen.psdata
(1.85 MiB) Downloaded 35 times
Volkswagen_Transporter_2004_Diesel_Relative compression with PD injectors GOOD.psdata
(900.02 KiB) Downloaded 18 times
Volkswagen_Transporter_2004_Diesel_Tandem pump pressure ZOOMED level 2.psdata
(2.47 MiB) Downloaded 24 times
Volkswagen_Transporter_2004_Diesel_Injector current GOOD.psdata
(1.86 MiB) Downloaded 21 times
User avatar
RYM6746
TwoWaves
TwoWaves
 
Posts: 99
Joined: Tue Nov 01, 2016 10:58 am

Re: Strange Relative Compression VW Transporter

Postby STC » Thu Jul 13, 2017 10:57 pm

Hi Rym

You have a 4 Channel Scope, using only 2 Channels at a time is not going to make it last any longer :D :D :D :D

Your Capture "Tandem Pump with Misfire Seen" I see no evidence of any misfire there ???
Am I missing something ?
User avatar
STC
TwoWaves
TwoWaves
 
Posts: 790
Joined: Sun Sep 13, 2015 4:10 am

Next

Return to Diagnostic discussion

Who is online

Users browsing this forum: No registered users and 0 guests