How to restore constant low power or rev limit on VW Jetta Golf TDI and no boost on turbo

Fixing constant low power or engine rev limited - VW and Audi TDI
back to 1000q: diagnose and fix limp mode

Introduction

This article shows possible causes of low power without limp mode on Volkswagen and Audi TDI. Limp mode is when the power level suddenly cuts out when you request power and is reset by restarting the engine. See 1000q: limp mode fix for the solution. Although you could be in permanent limp mode, it should be fairly obvious because engine error codes will be set and you may see the check engine light/malfunction indicator light. Read both articles since each situation has overlapping causes/solutions, this article is more for permanent low power without the sudden loss of power.

Since any cause of low power is possible in your specific case, this article is divided into more common, less common, and rare causes. The numbers are in no particular order, it's just for organization. Of course, if you have any error codes tripped or have a clue to the cause of low power you should address those causes first, so get the car scanned for error codes before throwing time and parts at it. This can be done for free at many auto parts stores. Further details for each each section are given in links to other articles. If you need more help, ask your question in the myturbodiesel forums.

If you have a problem that isn't caused by something obvious, you need a Ross tech VCDS cable. This is a laptop computer diagnostic cable to talk to the car's computer. Without it you cannot do the more advanced tests.

Note about generations - some generations have similar engines:
Mk3= 1996-1997 3rd generation Passat TDI or 1996-1999 3rd gen Jetta TDI
Mk4= 1998-2006 New Beetle, 1999-2005 Jetta, 1999-2006 Golf, 2004-2005 Passat TDI
Mk5= 2005.5-2010 Jetta TDI
Mk6= 2010+ Golf TDI

Procedure

Remember, an engine needs fuel, air, and compression to run. Low power is related to a lack of one of these or a sensor problem making the computer thinking there's a lack of these. Any sensor problem could also be caused by a bad ground or broken/chaffed wire so also check every section of the wiring of the suspect sensor for breaks.

More common causes of low power on TDI

1. Bad MAF sensor - very likely cause on the mk4 TDI. Not common on the mk3 TDI (1996-1999 Jetta/Passat). Early mk4 MAFs failed often, see 1000q: MAF FAQ and 1000q: mk4 MAF for more details and the part numbers. Error codes normally do not show up with a faulty MAF since the signal degrades instead of going out completely. Through VCDS, checking MAF actual vs. specified at idle, high rpm, and high load will quickly show a bad MAF or other problem causing a low MAF reading. Start the engine and click on "engine", then click on "measuring blocks" in the screen below.

You should see the screen below. Hit "up" until you see "group 3". Below is a bad MAF - the specified value and actual values are off. Bad MAFs don't go over 400-500 because the signal degrades. Also check for faulty wiring or a bad plug.

Below is a good MAF - the specified value and actual value are close and both can reach 800+.

2. Clogged intake manifold - carbon buildup chokes the intake manifold, starving the engine of air. Only ultra low sulfur diesel is sold in North America now so there should be much less buildup in the future. Always use good quality synthetic engine oil on your TDI. See 1000q:mk3 intake cleaning, 1000q: a4 intake clean, or 1000q: a5 intake removal for the detailed procedure.

3. Anti shudder valve shut or almost shut (does not apply to mk3 TDI, more for mk4 TDI) - there is a spring loaded valve right before the intake manifold. Newer TDI use an electronic valve and are not as susceptible to sticking. If there is excess carbon buildup, it could shut in a partially closed position. See 1000q: intake manifold cleaning for detailed pictures and operation.

4. Clogged snowscreen/air filter - a clogged air filter will starve the engine of air. A clogged snowscreen (large debris air pre-filter) shouldn't block off all air unless the aux-intake flap is also clogged. See 1000q: mk3 air filter, 1000q: mk4 air filter for details on changing them.

5. Clogged fuel filter - change interval is 20,000 miles but biodiesel use (cleans out old buildup) or bad fuel could clog it early, resulting in fuel starvation. Algae or bacterial growth in the fuel tank could also clog the lines. See 1000q: mk3 change fuel filter, 1000q:mk4 fuel filter, or 1000q: mk5-mk6 fuel filter for details on how to change it.

6. Boost leak - a cracked hose or loose connector lets measured air out. No air or major leaks = poor engine running or stuttering. A visual inspection may not reveal all the possible or hard to see spots where leaks can form. See 1000q: boost leak check for a foolproof method of finding all leaks. Common places for a boost leak to occur is at hose ends (the hoses pop off due to a loose connector or worn locking tab), the corners, or places where it rubs against something else. mk3 Jetta TDI tended to wear out the rubber hose before the intake manifold because it's close to the hot exhaust piping. mk4 Passat TDI tend to wear out one of the lower intake hoses because it rubs against a ground cable.

2004 and newer engines use a quick connect tab-spring connection. If it's only locked on 1 end, it can pop off halfway. For some reason, mk4 engines tended to wear the tabs at the intake manifold (probably due to a looser fit) which made the hose pop out. If it's loose you can use an aftermarket doggie collar to lock it in place or replace the hose. The BEW engine hose is VW# 1j0 145 838 t (from worldimpex). Other engines also have quick connect couplers at other locations.

7. Hose inside ECU (mk3 TDI only, does not apply to mk4 or newer TDI) - this hose leaks and normally sets a check engine light, see 1000q: ecu hose for the fix.

8a. Vacuum lines to/from turbo and n75 solenoid - these dry out over time and crack or can rub through. It's possible they are clogged. The n75 solenoid controls the turbo wastegate or VNT vanes with either vacuum or pressure. b4 Passat - on firewall above coolant reservoir, a3 Jetta - on pass side near air box, a4 Jetta/Golf - on firewall above brake fluid reservoir. Below are vacuum line diagrams for the mk4 ALH (below left) and BEW (labeled BEW) engines.

8b. Problem with the n75 solenoid, VNT actuator, VNT vanes, or vacuum lines. Only mk4 and newer have VNT. You should have already checked the vacuum lines, the below test will inspect the entire system. Start the engine and through VCDS, click on "engine"-->"measuring blocks"-->hit "up" until you reach "group 11". Compare Specified vs. Actual MAP. This compares what's actually happening and being observed from the boost sensor (barring a faulty sensor/plug/wire) to boost the computer is requesting (what should be happening). They should be relatively close. If they are far off this normally results in limp mode but it could also be contributing to the problem. If you have a mk3 you have a conventional turbo but you can still use this test to check the n75 solenoid, the wastegate, and vac lines. However, wastegates are much less susceptible to sticking vs. VNT vanes. The videos below show how it works. The lever on the outside is welded to a lever inside the turbo housing. This is how it moves the VNT vanes. See the below videos to see how smoothly and free the lever should move. It should not stick or bind at all. Vacuum is being applied to the can, not pressure.

The lever moves a ring and the ring moves the vanes. These vanes change the angle and speed of exhaust hitting the turbine wheel. If these vanes are sticking it will cause low power or limp mode, depending how how badly they're stuck and at what angle.

Below is a log of a turbo which was recently cleaned. Some initial spikes are normal but boost should level out. If you're quickly stomping on the acceleator/gas pedal, expect more spikes. If you're smoothly applying the accelerator pedal there should be less spikes. A faulty log could also indicated a faulty actuator, vacuum lines, or n75 solenoid.

8c. If the test shows poor response or no response at all, it could be sticky VNT vanes/actuator (mk4 and newer TDI only) - see 1000q: turbocharging for a description of what a VNT turbo is. The vanes or actuator can stick or fail to function, see 1000q: VNT actuator diagnosis and repair to check the actuator. The lever should move freely as shown in the above link and videos.

8d. If the actuator is fine, also check the n75 solenoid and vac lines. The n75 solenoid controls vacuum or boost to the vacuum line going to the turbo wastegate/VNT actuator. To test, apply voltage to the solenoid or swap with a known good unit. If you have a mk4 TDI, you can swap it with the EGR solenoid to test. Also check the plug for corrosion and the wiring harness for chaffing. If those are good, disconnect the VNT actuator rod and move the vanes by hand. If the vanes are stuck then remove the turbo and clean the inside of the exhaust housing to free then stuck vanes, see 1000q: turbo VNT vane cleaning and 1000q: turbo removal ALH BEW engine

If you need a new n75 solenoid, click the below links:
N75 1996-1999 mk3 body, VW #028-906-283-F (pierburg type n75)
N75 1998-1999 mk4 body, VW #1H0-906-627-A, for Up To Vin # 9 MX 999000
N75 1998-2005 mk4 body, VW #1J0-906-627-A, ALH and BHW engine, for From Vin # 9M Y 000 001, D shaped connector

9. Faulty injection pump's fuel injection quantity adjuster - these are occasionally set wrong from the factory or after seal replacement. It's also possible the fuel pump's internal quantity adjuster is faulty. Applies to 1996-2003 TDI only or TDI that use a Bosch VE injection pump (not pumpe duse or common rail). See 1000q: fuel injection quantity testing and adjustment and hammer mod for more details. Injection quantity should be 3-5 at idle and up to 36-38 at full throttle.

Less common causes of low power on TDI

1. Fuel pickup in fuel tank - it's possible algae or bacterial growth in the fuel tank has clogged the fuel pickup. It will show up as black sludge on the fuel pickup in the fuel tank. If there's clogging and growth, clean the fuel line all the way to the fuel filter and put some diesel fuel biocide in the fuel tank. After the next tank, change the fuel filter. Lift the rear seat and remove the access panel cover (3x phillips screws). Unplug the fuel feed, return, and fuel gauge sensor and unscrew the retaining ring. The fuel pickup will pull straight out. Remove it and clean it since there could also be a blockage in the pickup's tubes.

2. Electrical problem with injection pump/ecu internal problem - this is difficult to diagnose because an ECU internal problem can only be tested by replacing the ECU with a known good unit. A bad injection pump should throw codes or show erratic injection timing/no start/engine stumbles/null values through VCDS. More likely is bad wiring/grounds. Thoroughly check the ECU/ injection pump's plug and wiring harness for chaffing/corrosion.

3a. Stuck turbo wastegate/actuator: only mk3 TDI used a wastegate. As mentioned earlier, mk4 and newer cars use a VNT actuator. Stuck actuators were a common problem on newer cars because the vanes stuck. The simpler turbo on the mk3 means this isn't as common. Apply pressure to the vacuum hose at the N75 solenoid and see if the wastegate arm moves.

3b. If you have a VW Touareg TDI or Audi Q7 TDI, low power caused by the turbo could be a broken VNT actuator motor. Unlike the spring/vacuum actuator on other cars, your car uses an electric motor to move the VNT vanes. It's possible that the wiring, motor, or internal gears are damaged.

4a. 1996-2003 engines: If the timing belt was done within the last 5-10,000 miles, make sure the camshaft/crankshaft/injection pump are correctly indexed to each other. See 1000q: mk3 Jetta and Passat TDI timing belt removal, 1000q: ALH engine TDI timing belt -part 1 for more details. It's possible for just the injection pump to be off a tooth and the car may still run. This assumes that the camshaft and crankshaft timing is correct.

4b. 2004 and newer engines These engines have different fuel injection systems than the earlier cars. If the timing belt was just done, it's possible for it to be a tooth off if you did it incorrectly, even if you used the correct tools. See 1000q: BEW engine mk4 timing belt, 1000q: Passat TDI BHW timing belt, and 1000q: mk4 Jetta TDI BRM timing belt for more details.

5. Low compression - engine wear or other symptoms causing low engine compression can cause low power. If your engine has low compression you may feel more piston blowby. Open the oil fill cap and note how much pressure is coming out. Do not do the "add a few drops of oil to the cylinder" test that works on gas cars since a diesel can run off engine oil and because it doesn't take much fluid to hydrolock a diesel engine. See 1000q: compression test for more details.

6. Pumpe duse engines have a problem with camshaft lobe wear. If it gets really bad the engine valves won't work well and the engine can't breathe well. You may hear huffing at idle. See 1000q: PD camshaft inspection for how to check and a video showing idling. Non pumpe duse engines didn't have this common problem.

7. Parking brake seized/brakes seized - early mk5 Jetta had problems with misadjusted parking brakes. This can cause low fuel economy, hot rear wheels, excess rear brake wear, and low power. Remove the center console to adjust the parking brake lever.

8. Other sensor or electronic problem contributing to the problem - In measuring group 7, you can see fuel temp, intake air temp, and coolant temp. Below are normal readings on a warmed up ALH engine. Let the engine cool overnight and check these values again while the car is off and fully cooled - they should all read ambient temperature and be about the same.

9. If you have a 2009 or newer TDI (or 2006-2008 Touareg V10 TDI), your car is equipped with a diesel particulate filter. There are warning lights on the instrument cluster that show if it's abnormally clogged and can't regenerate "self clean". If it gets 75% clogged (abnormal) there will be noticeable power loss accompanied by messages on the instrument cluster. See 1000q: DPF particulate filter and Adblue FAQ for Volkswagen and Audi for more details on this system.

Rare causes of low power on TDI

Are you sure it's not the clutch slipping? If the engine RPM goes up but the car's speed stays the same or doesn't increase with RPM, then you have a bad/slipping clutch or transmission. If the engine RPM stays as sluggish as the car's speed the transmission/clutch is probably fine. See 1000q: clutch FAQ for more details on clutches used on the TDI and the dual mass clutch. If you have a DSG transmission and the PNDRL "shifter" gear selector lights are all on or blinking, you have a low battery (also check voltage regulator or alternator). Some DSG had defective transmission temp sensors which can cause false neutrals. See 1000q: DSG FAQ for the recall and DSG info.

If you have a newer VW Touareg V6 TDI, it has intake flaps that bypass the intercoolers for cold starts to help warm up. It's possible that they are closed.

Bad accelerator pedal - this should be obvious when reading the accelerator pedal value through VCDS. Turn the car on and plug in VCDS. It will tell you how far the accelerator pedal is being pressed. The pedal is drive by wire, meaning there's no cable attaching it to a throttle, it's just electrical wires and sensors. In the mk3 TDI the pedal sensor is a potentiometer. In the mk4 TDI there are 2 pedal sensors (g79 and g185 which are probably potentiometers), if one fails you should get an error code, if both fail the ECU will limit engine RPM to 1200. This should trip an error code.

Other fueling problem - first check and test the injection quantity. It's possible the fuel injectors or their nozzles are sticking or faulty. It's not likely that all 4 fuel injectors are equally bad because this should be accompanied by rough idling. You can check injector balance at idle through VCDS - if the values are far off then it's possible there's a problem. If your fuel injectors have been sitting dry outside of the car, diesel fuel can go bad and gum up the tiny components inside the fuel injector. If it's near freezing, is the fuel gelled? Since diesel fuel is an oil, it waxes over and gels if it's cold enough. Biodiesel gels at a higher temperature. Gel temperatures vary according to additives put in the fuel by the retailer and if the car was preheated.

Bad fuel shut off solenoid - the fuel shut off solenoid on the injection pump (8mm nut holding single wire) could be faulty and preventing fuel from reaching the fuel injectors. Because this is after the quantity adjuster where fuel is metered, it will not set any CEL or error codes. The normal resting position is closed, when 12V negative (the battery) is applied to the top of the solenoid, it should click and open, letting fuel go through. there's a spring and plunger inside, if they were stuck partially closed then it could cause fuel starvation. It's lubricated by diesel and there is some play in the shaft so this is rare. Clean the area well before removing the solenoid. Size is 15/16". Mk3-mk4 solenoids are slightly different but they can be swapped.

Other air supply problem - internal damage to turbo or paper towels stuffed into the intake path from previous mechanics. I try to use tape to cover holes instead of paper towels because they can get stuffed down and forgotten in the intake.

Clogged exhaust - it's possible the catalytic converter is clogged. WVO or bad biodiesel would increase the likelihood of unburned waste clogging the catalytic coverter.

Going back to the N75/vac line/VNT actuator problems which are more common, it's rare to have a vacuum system failure since it should produce other noticeable problems. If the vacuum pump at the end of the camshaft/tandem pump on pumpe duse engines fails, you won't have vacuum pressure and the actuator system won't work. If you don't have vacuum then the brake pedal will feel very hard since the brake booster also works on vacuum. It's possible the pump is bad or the vacuum line between the pump and other components are bad but you already checked the lines in more common causes.

Other problems - faulty ECU - if you have an mk3 TDI it's probably the hose. ECU failure is rare but it could cause mystery problems. The only way to check is to replace the ECU with a known good unit. Any car with the anti theft immobilizer may need synchronization to let the ECU work.

Coolant migration - there were a few faulty coolant reservoirs that leaked coolant into the wiring harness. Through capillary action, coolant migrated through the wiring harness, causing damage to all electrical components. This applies only to mk4 and newer VW.

EGR system leaking - aside from an external EGR leak, the EGR valve could be worn or faulty and let exhaust gas into the intake. Instead of the gas driving the turbo, it is leaking back into the intake. You should see the MAF sensor reporting low values since the exhaust gas is displacing intake air. Make a block off plate and isolate the EGR system to check. This will set a CEL/MIL light. See 1000q: EGR FAQ for pictures of block off plates and a description of the system.

Wobbly crankshaft sprocket - it's possible the crankshaft sprocket is faulty or out of round. This will mess up timing and should show bad idle, erratic timing.

Other misc engine fault - any suggestions?