1996 passat B4 limp mode

Hi everyone,

I have been dealing with poor performance. I think I am in limp mode.

Injection pump is new and timing is perfect. Vag-com fuel delivery volumes are normal. Filter is new. Timing belt is new. Crankshaft pulley, cam pulley and injection pump pulleys all lined up by VW procedure and spot on.

Turbo is new.

Turbo Waste gate actuator rod adjusted by dealer.

No engine codes are coming up.

My vacuum pump pulls 27 psi.

Car runs the same with MAF disconnected.

Vag-com does not have a turbo boost pressure mode but does have a "Manifold air pressure" mode. In this mode the specified MAP is 1800 mbar and the actual is 1100 mbar. My guess is that the MAP specified/actual reflects boost pressure by the turbo and that a low "actual" MAP during load conditions means the boost pressure is low or there is a manifold leak.

I am trying to test the N75 valve and circuit. There are 3 lines on the N 75 valve. One is red, from turbo. Second is blue, to waste gate actuator and the 3rd is ambient air-pressure I assume. Not sure of function of red line from turbo to N75.

A lot of these N75 valves are run off vacuum but my 1996 B4 tdi does not have a vacuum line attached to it from the vacuum pump. I am not sure how the waste gate actuator works. Is there a vacuum from N75 to the waste gate actuator or positive pressure from N75 to actuator?

Can I remove the blue line from the actuator and apply a vacuum to check function or do I need compressed air?

Where does N75 get it's vacuum or pressure from and what value should it be?

The next thing I am going to try is to put a pressure meter on the blue nipple of the N 75, run the car with vag-com and step thru the N 75 activation mode and see what the meter does. It should tell me vacuum or positive pressure coming out of that nipple.

Maybe there is a simpler approach.

Thanks ,

airlenny1

Well, I found a problem to add to the above post.

I put my combination vacuum/pressure gauge on the red line to N75. Pressure was "0". I blew into my pressure gauge just to make sure it was working....and it was.

I then hooked up the blue line to the gauge and again, nothing. I did this just to make sure the lines were not crossed.

I then removed the red line from the turbo and hooked the gauge directly to the turbo. Again a reading of zero. When testing I revved the engine from 0 to 4,000 rpm. Needle on gauge did not move.

So, there is no pressure being generated by the turbo for the red line to N75.

The turbo has about 500 miles on it. All in limp mode. I put it in myself and it took quite a deal of effort and time to remove the old one and put in the new one.

So either I have a leak at the manifold, manifold turbo connection or exhaust connection or a related connection. If not, the turbo must be bad. I cannot think of any other reasons why the turbo would not give me pressure at the red-line connection.

Thanks,

Mark

Likely not limp mode if you have seen no codes, sounds more like a mechanical problem from what you are describing, not the ECU shutting things down.

Typical limp mode code on these older WG turbo TDIs would be the 00575 code which really only is set by excessive boost, in other words actual boost running above the ECU's specified value. Boost running below the requested amount doesn't usually throw a fault in these. The diagnostic monitor is pretty relaxed for underboost on the AHU/1Z, unlike on the later VNT motors.

To your question about N75 function: the N75 in the early motors like yours does indeed run off pressure, not vacuum like the later motors. The line from the turbo compressor housing (IIRC the red line as you describe) provides positive boost pressure directly from the turbo to the inlet side of the N75 valve. Meanwhile the ECU monitors charge pressure from a different, black line that attaches to the hard tubing from intercooler to intake. (ECU takes its signal from this different location, rather than the same red line from the turbo, to account for pressure drop across the IC and plumbing, etc, so that actual pressure delivered at the intake manifold is measured accurately -- MAP for shorthand.)

When the MAP measured by the ECU approaches the desired value, the ECU begins opening the N75 valve using a duty cycle to pass some of the pressure from the red line to the blue line that runs to the wastegate actuator. This opens the wastegate, using pressure from the turbo. The ECU then continues to increase or decrease the N75 valve duty cycle to regulate pressure applied to the wastegate and thereby keep the actual MAP tracking specified MAP as closely as it can.

These early pressure-actuated N75 valves are thus similar in concept to an old style ball and spring manual boost controller, like the type someone might have used to turn up the boost on an old mechanical engine. It just serves to interrupt and manage the pressure signal from the compressor housing to the wastegate, same idea, but enhanced with electronic feedback control. On a system with no leaking hoses and a good N75 valve and a freely-moving wastegate and actuator, it works very successfully.

Regarding your specific problem, lack of pressure generation to the line from the compressor housing does indeed sound like the key issue. Sounds like wastegate stuck wide open maybe due to a badly misadjusted wastegate actuator. Or a very large boost leak somewhere. What was the issue that caused you to change the turbo in the first place?

Thanks for the helpful reply.

I changed the turbo due to progressive loss of power that would come in surges. When I took it out , the shaft had quit a bit of movement in it so I replaced it.

After it was replaced it never acquired it's baseline power so was somewhat worse than before I put the new turbo in. The p1252 code then came up as a short to ground or bad cold start injector. I thought this problem should not cause a dramatic loss in power and the car was starting just fine. The code has not cleared but I have not tried to fix that problem yet. Car is quite loud on acceleration and normal sounding on deceleration.

I have been approaching the car in steps.

1) Pulled MAF sensor: car ran the same.
2) Checked vacuum to EGR=20. Disconnected EGR vacuum line: car ran the same.
3) Checked red line pressure under load= 3 psi. Should be 12 psi.
4) Checked for obvious leaks in the turbo piping. Did not see any.
5) Checked MAP on vag-com under load and specified=1800mbar, actual= 1100mbar
6) Going to see if I can manually close the wastegate and see if there is a difference in red line pressure and MAP under load.
7) The black line from the turbo piping to the ECU is new due to leak in the short segment on the ECU that triggered engine light in the past. The short segment and long segment are new.
8) Wondering if WD 40 around turbo connections would be a better way to check for leaks? What rpm is best for this?

Thanks,

Mark

airlenny1

I think I may have stumbled across something in prior forums that helps with the P 1252 DTC. And may explain a lot of my problems.

P1252 in VW discussions has 4 names for the same thing making it confusing to figure out. There may even be more names.

I found these:
1) start of injection solenoid circ open/short to ground
2) cold start solenoid circ open/short to ground
3) acceleration solenoid circ open /short to ground.
4) N108

It appears that this solenoid is attached to the injection pump and is needed to advance the timing of the injection pump, in cold conditions, to keep engine running smoothly. From other discussions , it appears that when the solenoid is not working, the timing is defaulted to "advance" and fuel is limited by the ECU. The engine can sound loud and there can be a lack of power. That there are other sensors that sync with the N108 solenoid also. They all seem to connect to the 10 pin connector. In Bently manual it notes for my B4 passat that there are 2 wires coming from the solenoid that go to the 10 pin connector in positions 9 and 10.

The solenoid circuit is grounded near the battery and others have found open wires that have the insulation worn off of them due to rubbing. Usually the problem is corrected by fixing the wire and ground connection. Others have found a solenoid that is faulty and the wiring is ok. They have to replace the solenoid.

When using Vag-com while the solenoid is in default "advance" mode, the injection pump timing cannot be assessed and it will read 255 no matter how the injection pump is positioned. So this N108 problem needs to solved before timing can be done accurately with vag-com.

It seems to follow that if the engine output is crippled by the ECU when N108 is not working, then the turbo output will be less than specified. So it also follows that the turbo should be looked at after the N108 problem is solved.

I was mislead by the name :cold start solenoid". I ignored the P1252 dtc because my car started just fine in cold weather so I thought it was not a big deal.

Will see what happens with correction of the N108 problem.

airlenny1

When looking at my B4 passat 1z engine, the 2 wires coming from the cold start solenoid (N108) go to a 3 pin connector not a 10 pin as described in my last note.

Two of the pins in the 3-pin connector go to the N108. The third goes to the injection pump solenoid fuel shutoff and is the 12v line to that solenoid.

This shutoff solenoid is activated by the famous 109 relay in the fuse panel that has a history of failing. Mine did yesterday and I was stuck with car in idle and no response to acceleration pedal. Jumpered the solenoid to the battery and got home. New 109 relay solved that problem.

Anyway, I followed the ground wire from N108 through the 3 pin connector, then through a 24 pin connector and then along a large wire bundle that coursed under the battery. The bundle went through an indent so the battery would not sit on top of it. The bundle looked crushed so I undid the protective wrap and found a corroded wire that looked like it got hot, burnt off the insulation and then corroded until it fell apart. This is the wire to the N108 ground. On inspection, the battery was put in without its clamp and has been bouncing on the wire bundle for the past 10 years.

Tomorrow I will inspect all wiring in the bundle and replace the damaged ground wire.

Update on poor running B4 Passat 1Z engine.

P1252 code : Found the ground wire (looks like black with yellow stripe). Insulation is near completely gone. Wire is corroded and the copper is oxidized to green. It has disintegrated so there is no continuity. Confirmed with 24 ohm resistance when tested at the 3 pin connector to ground. Insulation burned and has melted to adjacent wires affecting insulation on other wires.

Removed all the bundle tape and traced the bad wire all the way back to the fire wall.

Connected a good wire to ground wire remnants and the car instantly ran better and sounded better. The P1252 code eventually came up again but was off for quit a while before it came back up.

Will replace the whole wire and inspect the other 30+ wires in the bundle for damage.
Now have to get to the wiring at the fuse panel and run another heavy gauge wire through the firewall back to the 24 pin connector in the engine compartment.

Then road test.

Nice work

Still interesting that the P1252 comes up again after fixing but maybe there's more wiring problems. Where exactly did you find this wire that was the problem? I'm interested to know what system it belongs to.

Steve

Isn’t black with yellow tracer shared with a bunch of different sensors?

I’m impressed that the battery lasted 10+ years...

-Todd

The wire bringing up the P1252 was the ground wire that is black with yellow stripe, coming off the cold start/acceleration solenoid .Another discussion area noted that the ground site was near the battery. There are body ground sites there but not for this wire. This ground wire went from the solenoid to a 3 pin connector then over to a round, I believe, 24 pin connector near driver's side of the head. The wire exits out of that and joins a bundle of about 30 other wires. The bundle courses under the battery in a raceway under the battery. It then goes up and thru the firewall to fuse panel.

The bundle appeared traumatized under the battery even though it was covered. I unwrapped the bundle and immediately saw the problem. The ground wire had burned all the insulation off, likely due to high resistance, and the remnants were a combination of metal fragments and powder. This damage extended to 3 inches away from the 24 pin connector end went clear to the firewall. Multiple other wires had insulation melted by this ground wire exposing copper sporadically throughout the bundle.

This ground wire could not be found on the other side of the firewall despite extensive searching and pulling down the fuse panel. I finally figured out why. The ground wire joined about a half dozen other ground wires of the exact same color by the time it got to the firewall. This group of grounds were clamped or soldered together by a copper , rectangular fixture . This bundle of grounds were then clamped to a large red/blue wire that came through the firewall. Everything beyond the copper solder joint was intact.

So I spooned out the burned wire and replaced it with a much larger wire to reduce resistance and increase capacity.

Continuation from above...….. All the other wires in the bundle were inspected and repaired as needed.
After the fix , the car started right up and the loud diesel sound was gone .The P1252 was gone. The car ran much better and had much more power. The MAP improved from 1100 to 1700mbar. The fuel feed reached specified amounts. There was no smoke out the exhaust on acceleration. The injection timing was in the low part of the graph but easy to move the injection pump while car was running to get the timing in the upper part of the graph. After all that the car ran better than I can ever remember. The only thing left is that the MAP is 300mbar below specified between 1500-3500 rpm. When I get to 4000 rpm it shoots up to 1700.There may be a small leak somewhere or wastegate adjustment needed. Need to check the blue and red hoses to N75(N75 is new) and the turbo piping plus joints. There is an oscillation at idle when cold but I recall there is a forum fix for this.
The site has been very helpful especially the section by GoFaster on low power algorithm.
The Battery did not last 10 years. I had 2 put in over that time but never checked their work to see they never bolted the battery down. It bounced around on the wire bundle with visible trauma to the bundle and probably the reason for my problem.
Thanks to all for the great support.
Mark

You deserve a high-five and a beer. Good job working through the tricky diagnostics.

Well earned, sir.

I got hit with P1252 yesterday. The middle (ground) wire got brittle & snapped in half about 2 inches from the 3pin connector. 25 years of heat and vibration The other 2 wires were starting to lose insulation from rubbing inside the corrugated shielding. Butt connector for the broken wire & electrical tape on the others, until i find some replacement wires.

Aside from the low power, what looked odd was the ecu reporting ridiculous levels of fuel consumption, 2x 3x maybe 4x normal, peaking at 15gph (normal max is about 4gph). Idle readings were still fine, around 0.11 0.12gph. I kept checking the rearview but never saw any smoke, so it wasn't actually dumping loads of fuel in, but the ecu thought it was.