Member Since: 24 Oct 2023
Location: Cornwall
Posts: 14
Underboost fault after MOT
Hi guys, I recently got my 2004 auto D3 back from the MOT, it passed (after replacing the front left inner driveshaft CV joint and an EPB module service), but on driving it away from the garage it had absolutely no power whatsoever, it felt like when the turbo actuator arm seizes and there's no boost.
I checked my iid tool and got the underboost fault code P0299-00.
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This weekend I have checked and relubed the actuator arm (it felt like it was moving fine anyway), checked and cleaned the MAF and MAP sensors, visually inspected the main intercooler hose (looked fine, it's a terrafirma silicone one so presumably won't split anyway?), but it's still driving the same, I also noticed the smoke out the back looked blue / white.
One thing I did notice was the bottom of the intercooler hose looked a little oily but I'm not knowledgeable enough to know whether this is a "normal" amount or whether it indicates a problem.
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So I brought up the following live values in the iid tool, this is sat with the engine running at idle:
Click image to enlarge
And then these values were taken at stationary in neutral at full throttle:
Click image to enlarge
The first thing I noticed while revving is that even at full throttle it won't rev past around 2600rpm, which presumably is a symptom of the underboost fault, secondly I noticed that the actuator was reading "N/A" the entire time, and thirdly I noticed that the manifold air pressure drops significantly with the higher revs.
So my questions to the forum are: am I right in thinking the manifold pressure should significantly increase with higher revs, not decrease as is shown here, or is it because I'm simply revving in neutral that this is happening, do I need to run the live values again but while moving? Should I be concerned that the actuator reading is constantly "N/A" even though it's moving freely, or is this normal for older models? Is the amount of oil at the bottom of the intercooler hose normal or does this suggest a split or leak in the intercooler? Is there anything else I can check before assuming it's the turbo itself that has died? Could the MOT repairs to the front left CV joint have loosened or broken something down near the turbo area?
I have a portable smoke machine coming later this week so I can check the system for splits and leaks.
When you cycle the ignition on and off can you hear the turbo actuator moving, get somebody to turn the key while you watch the actuator arm. Its stuck or there is a leak somewhere, check that the connector has not been disturbed during inner cv replacement.
Flack
22nd Jan 2024 8:45 am
Kernowek
Member Since: 24 Oct 2023
Location: Cornwall
Posts: 14
Thanks Flack, I'll visually check that the arm moves during ignition on / off. I'd read that the actuator moving at ignition is only on newer models and the old ones like mine didn't have the firmware to do it, is that not the case?
The actuator arm was moving very freely back and forth with me physically moving it, so I was confident it wasn't seized, though a visual check would certainly confirm this.
I have a small smoke machine coming this week so I can check more thoroughly for leaks.
Regarding your last comment, which connector could have been disturbed? Do you mean there is an electrical connector to the turbo which could have been disturbed, or do you mean one of the physical joints / connectors between the boost system pipes?
22nd Jan 2024 11:23 am
Disco_Mikey
Member Since: 29 May 2007
Location: Dundee, Scotland
Posts: 20844
You MAP readings are too low. You have either a faulty sensor, or something restricting the air intakeMy D3 Build Thread
Member Since: 24 Oct 2023
Location: Cornwall
Posts: 14
Like something physically blocking the airflow? I've got a smoke tester coming this week so I can check for any obvious air leaks along the airflow path. After that I'll try a new MAP sensor if you think that could be an issue.
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