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I have 22 years experience with vehicles and have been full time European auto tech. This is the first time I've been stumped 100%
I have a 2004 audi a4 quattro 1.8t, I have replaced the head and put a new turbo on due to a cracked water passage on the head. Since I've put it back together I have not had it keep boost. I have smoke tested, compression tested the engine and all vaccum and boost lines and checked for codes. I have no fault code or engine light. I can hear the turbo spool and the diverter valve open. I have checked the wastegate pressure, it falls on its face at 3k rpms and I am stumped.
Have you checked exhaust back pressure or checked the turbo oil feed line for coking of oil? I doubt the latter is an issue but you never know. Does maf, map, tps, iat or ect drop out or glitch through the rpm range? Did you double check your sensor grounds are tight?
Was this while driving or in park revving engine? if in park might be a rev limiter
It does have a catless downpipe and was told stage 1 tune., fuel trims are within spec, I have no sensors that read out of range in live data so far, driven about 1500 miles since the repairs and repeated all tests, turbo had oil feed. Lines were clear. The issue with no power is in driving the car on road feels good in first gear to about 3k then just runs out of breath, the turbo gets warm but not glowing. It will rev through all rpms but produces no power past 1-2 gear or past 3k rpms. I have to replace the boost gauge but before it broke was showing 17psi then falls flat to 4psi when driving. I have sent video of data to an audi tuner and have not found anything to date
This might be a trans issue i would check trans line pressure to see if its in spec (if its an automatic) and look at shift adapts in data. If its excessive in 2, 3 and 4 thats what it could be.
Edit:
Only other thing i can think of is fuel pressure dropping at higher rpm from a weak fuel pump.
Found 2 issues in live data but they did not set a fault , stuck -.004 resistance on rear 02 sensor. It in fact shows resistance range when using ohmmeter.
And channel 132 in cooling map shows a stuck -54.5 farenheit reading but both inlet and outlet temp sensors are functional and showing live temps.
Last edit: 4 years 9 months ago by Jettadaddy. Reason: Spelling
What does engine timing do when you hit 3 k rpm cam timing chain tensioner comes to mind as well as belt tensioner. If you have a incylinder pressure transducer would look at waveform to see if your getting valve float or opening early or late at those rpms also oil pressure i am curious about. Just a thought
Cam phase shows 0 deg. Timing set is new and all Mark's are lined up, has vvt and is in spec at all rpms and tensioner works with output tests. I did not check actual oil pressure readings at filter housing and oil and coolant temps stay normal. I do not have the cyl pressure transducer.
Fuel pressure was a though of mine too. No codes or data to support that so far. Just had a new fuel pressure tester delivered. And they tell me they cant get me a fuel filter till Monday on this car
Same way just rev engine to 2500 -3000 or tape gauge to windshield and drive it. Does cam phaser change timing at 3000 rpm? Or stay at 0 i think it should change.
Does the turbo waste gate activate via a solinoid controlling vac applied and are the vac hoses hooked in the right spots. Not sure if that model uses timed vac ports or not.
The wastegate, diverted valve , and seems like a secondary air valve (but can't remember for sure) run off the vsv's under the intake I would bet something is hooked up wrong there such as instead of opening something for secondary air its opening wastegate you can probably find something online that shows which valve is what I vaguely remember going through something like that years ago on a 1.8t
I believe there are PIDs for the timing advance of each cylinder where you could see if any are being pulled at that time for some reason. I don't think it would cause it to dump boost like that though. Maybe you could fully open the wastegate to prevent building boost and see if it still goes into limp mode under the same conditions? No overboost codes?
Do you have access to an oscilloscope? Check wastegate vsv control and if you have a low pressure transducer you could check how much pressure is going to wastegate (maybe a bad vsv thats opening under pressure or maybe a check valve) , is it throwing any codes for limp mode or overboost? When you let off and re any throttle does it have boost again or does it need to be shut off or cleared to regain boost?
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