Loss of power with no CEL light?

[Rapidrob]

'96 Passat TDII was cruising along at 65 MPH for 40 miles and came to a nine mile long 15% grade hill where the speed limit went up to 75 MPH. I set the cruise to 75 MPH and was rolling along. About 1/2 way up the hill my engine slowly lost about 20% of its power. It was N O T in limp mode. I have had LIMP mode happen over the decades and there was always a CEL light. When it had gone into limp mode it was very abrupt with very little throttle response.
I pulled off of the interstate and the engine idled normally. I accelerated away and once again felt a loss of power as the engine was increasing in RPM to about 3,000 RPM RPM but the pull was at least 20% weaker than normal. It was cool but not cold as in jelled fuel.
I got to my destination OK and parked the car for several hours and then drove the 50 miles back home where the speed limit is 65 MPH up some very steep mountain roads. The engine never missed a beat. I did pass several slower semi-trucks at 75 MPH with no issues.
I did check all of my vacuum lines with no issues found. I have no idea why the engine just started to slow down very gradually under a steep incline/load?
A clogging fuel filter?
A dying turbo?
Any helpful ideas what to look for?
Thanks.

 

[Mongler98]

So, unless you scan the car with vcds with a proper vagcom cable and get some log graphs in 3rd gear preferably though the entire rpm range with this happening, it's just guess work for no reason.
Do you have a vagcom cable?
Sounds like a fuel delivery issue, like a low tank cracked fuel line to me.

 

[Rapidrob]

I have a HF code reader that's all.

 

[turbodieseldyke]

Sounds like a boost problem. If not the turbo, maybe a cracked hose that realigned itself for the return trip.

 

[Mongler98]

Get the vagcom one. Its a tool you need more than a few times to keep your car in good shape. The old style ones can be had for about $100 and a cheap laptop is like $20.
Do it

 

[Rapidrob]

It did it again today. Climbing a steep hill at 65 MPH I had to pass a semi and floored it. The engine went from 2,500 RPM to 3,000 RPM and was pulling hard for about five seconds and then started to loos power again. It stayed that way for two miles until I could safely pull over. I shut the engine off for 30 seconds,restarted the engine and all was back to normal for boost.
What is causing this. I drove home and there are no codes being thrown.
This is not the typical vacuum leaking hose going to the computer because it is not in limp mode, it just looses some of its power but recovers on a re-start.

 

[DivineChaos]

Sounds like an overboost issue. You can always get obdeleven. But I reccomend vagcom

 

[Mongler98]

sounds like a fuel issue to me. was the tank full or close to E? Do you see lots or little or no air bubbles in the fuel line clear session? when was the fuel filter changed last.
Any overboost will store a code and trip the CEL. so either the bulb for the CEL is out or its something else.

 

[Vince Waldon]

It's pretty common for there to be no codes thrown... the ECU is mostly worried about monitoring emission-related systems.

A loss of power that comes back once the engine is restarted is actually the very definition of limp-mode... good step-by-step in my sig for tracking it down one step at a time.

 

[KLXD]

I agree with Vince. Gone into limp many times with never a CEL. Quick cycle of the key while rolling in gear fixes it.

 

[Rapidrob]

The wagon has done this every time I drive at 75 of faster ( passing a truck). In the past, when the engine went into limp mode, it was a cracked or broken vacuum line going to the computer. The lines are new silicon now.
Before,the engine would loose almost all of its power, now it is loosing about 25% of its power.
Were should I start to look for the problem?

 

[Vince Waldon]

good step-by-step in my sig for tracking it down one step at a time.

 

[Rapidrob]

I spent the last six hours repairing an old laptop I bought in 1998 that has VAGCOM on it and plugged it into my Passat.
Only one code showed up.
00575: Intake Manifold Pressure
17-10 Control Deference Intermittent
no idea what that means or how to correct it. MAP sensor?
The other more advanced functions require a pin code I used to have but long lost years ago. I'm not paying VAGCOM again for a pin code.

 

[KLXD]

Did you even look at the link in Vince's sig?

Ever clean the intake manifold or inspect it?

Ross Tech's site has good info on Trouble Codes.

 

[Rapidrob]

00575 - Intake Manifold Pressure: Control Difference
Possible Symptoms

• Reduced Power

Possible Causes

• Solenoid Valve for Boost Pressure Control (N75) defective
• Hoses incorrect connected, disconnected, blocked or leaking
• In 1Z/AHU engines, stuck/rusted wastegate or actuator.

Possible Solutions

• Check Solenoid Valve for Boost Pressure Control (N75)
• Check Hoses
• In 1Z/AHU engines, clean rust scale off wastegate/actuator to allow lever to move.

I'm not sure how to test the wastegate t osee if it is not openinging up/closing properly?

 

[Rapidrob]

Yes, I read all the info links posted. Yesterday was the first time I was able to get the old laptop to work for VAGCOM.

 

[KLXD]

AHU uses manifold pressure through the N75 to control boost so a bike pump on the actuator should do it.

 

[Rapidrob]

I'll give that a try.

 

[MukGyver]

Here, watch this. sounds like could be N75, actuator, or sticking waste gate vanes

 

[Rapidrob]

Very well done vidio on the turbo. Thank you for the link.

 

[KLXD]

I only skimmed through the video without audio but it seems to cover the vacuum operated VNT used on the the ALH motors. AHU uses a pressure actuated wastegate so it doesn't apply. Other than maybe theory on what a malfuncfuncfunctioning TC can cause if he discussed that.

 

[MukGyver]

I only skimmed through the video without audio but it seems to cover the vacuum operated VNT used on the the ALH motors. AHU uses a pressure actuated wastegate so it doesn't apply. Other than maybe theory on what a malfuncfuncfunctioning TC can cause if he discussed that.

Good catch. So sounds like the system is similar but with pressure. maybe a blockage of the pressure circuit/passage or sticking vanes? ?

 

[ninedee_golf_tdi]

The passat has a waste gate turbo.

 

[MukGyver]

The passat has a waste gate turbo.

Please explain passat's. "waste gate" is just an old term general reference to the boost control.

 

[KLXD]

Always a guy ready to jump in and repeat what's already been said with nothing constructive added.

Anyway, it's a valve that lets exhaust bypass the turbine to limit boost. Older systems just had a line from the intake to the actuator on the wastegate. The TDI is more sophisticated with the ECU controlling the pressure to the actuator through the N75.

Not really similar to the VNT or variable nozzle at all which changes the angle of the vanes or nozzle that impinge on the turbine wheel to control boost.

 

[Rapidrob]

The problem is still an issue. The engine behaved until last week.
I noticed my "outside / inside" air actuator slowly stopped working. The "door" is free to move in the AC duct-work. It is vacuum operated. If there is a cracked hose going from the engine to the outside air actuator would this cause the tubo boost to go too high and reduce engine power.
No idea how the vacuum line is run behind the dashboard.
Still no new codes being thrown.

 

[KLXD]

Boost control being pressure operated and AC being vacuum, they are different, independent systems. So no, a cracked hose on the AC control wouldn't affect boost.

 

[Rapidrob]

I thought the N75 valve was vaccum controlled?

 

[gboss]

I thought the N75 valve was vaccum controlled?

A3/B4 are pressure-operated wastegate.

 

[Vince Waldon]

It's pretty common for there to be no codes thrown... the ECU is mostly worried about monitoring emission-related systems.

A loss of power that comes back once the engine is restarted is actually the very definition of limp-mode... good step-by-step in my sig for tracking it down one step at a time.

Which steps in the HOW-TO have you tried?