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Discussion Starter #1
Hello All, I am new to this site, and also have only owned my 2000 Beetle TDI for a little over a year. I am from Jacksonville Florida, but I live in Guatemala Central America. I purchased my car in Alexandria, VA and drove it down to Guate. About 6 months ago my car went into the infamous "limpmode" for the first time. I was ascending a mountain at about 55 mph when a Semi puled out in front of me. I down shifted, dumped the clutch and hammered the brakes, at which time the turbo quit. The very next time I started the car the problem was gone. I have a very similar situation happen a few months later and after a hard down shift, and hard braking the car entered limpmode. In the following months my symptoms changed from occasional limpmode to constant low power with rough idling and rough acceleration up to about 1500 rpm's.

Since the above described incidents, I have replaced the N75 valve, cleaned and serviced the injectors and inter cooler. There was no change, so I took the car to a highly recommended "racer / mechanic", he scanned the car and reported only 4 lb of boost coming from the turbo and told me I needed a new turbo. I did not agree, so I removed and thoroughly cleaned the EGR, intake, EGR cooler (which were all almost totally clogged), and turbo (which also was very dirty, but after cleaning appeared in very good condition as per inspection by the same racer / mechanic). Upon removal of the turbo I found the VNT actuator had a hole rusted through it, and was replaced. I have also replaced all the vacuum lines (in this order).

This Sunday I finished re assembling the car. I also replaced all the required gaskets, and added some high temp silicone for good measure. Sunday the car ran better than ever, since I purchased it. It ran very strong, and the "check engine" light went out for the first time since I have owned the car. Turbo performed exceptionally well, and no rough idling or acceleration. I did notice something odd. After turning the car off I hear a low pitch buzz or hum that sounds like a closed valve leaking vacuum or pressure. The first time I can remember hearing this noise was the day I replaced the N75 valve. I have no idea where the sound is generating from because it only occurs for a few seconds and I can only hear it from inside the car. It is clearly coming from the engine compartment, but I do not know from where.

HOWEVER, Monday morning the car began to idle rough again, and rough acceleration up to 1500 RPM, and the check engine light has returned. The car is entering limpmode once again. The only component I have not replaced is the MAF sensor, because both mechanics who have worked on the car told me it was fine and did not need replacing. Can the MAF be tested? HELP!!!!
 

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Because you say it sounds like leaking vacuum, it probably is. Use a vacuum tester and a pressure tester to test vac and pressure on the lines, even the ones you didn't replace. It's possible the big black ball, the vacuum reservoir is leaking. It has lines going to the brake booster in addition to vacuum pump and other places.

The MAF can be tested, look through the FAQ article on testing the mk4 MAF. For fine testing you need a VCDS cable to measure what the MAF is reading. I wouldn't replace it unless I knew for sure that it was bad or else you're just throwing money at the problem.
 

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Discussion Starter #3
Thank you for the reply, how do I test the vaccume lines? If it is the Reservoir would that cause limpmode?
 

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Remove one end and put vacuum on the other end. Maybe the reservoir is leaking or the main metal hose coming off it is loose. If there isn't vacuum the N75 solenoid and the VNT actuator can't work. This is just going off your hint that there's a sound like leaking vacuum after you shut the car off.
 

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Discussion Starter #5
Remove one end and put vacuum on the other end. Maybe the reservoir is leaking or the main metal hose coming off it is loose. If there isn't vacuum the N75 solenoid and the VNT actuator can't work. This is just going off your hint that there's a sound like leaking vacuum after you shut the car off.
I just want to clarify, the reservoir is the black plastic ball on the front of the motor, correct? mine does not have a metal hose, it only has rubber. I know because I already replaced it. Also, forgive my stupidity, but what is the round metal thing on the side of the valve cover with a hose that runs between the N75 and the brake booster? Could this be contributing to the problem?

Can I disconnect the line that plugs into the air box and connect vacuum to there? That to me seems to be the very beginning of the vacuum cycle. Also what should I look for once I have the vacuum hooked up? I would assume I listen for a hissing?

Can you think of a reason why all this started with downshifting and heavy braking?

I tested the MAF when the car was not in limp mode. I did not notice much difference, only that the motor would not rev over 4000 rpm. It would red line before I disconnected it. Then it went into limp mode when I reconnected the MAF and I did not notice any difference at all.
 

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Yes, that is the reservoir. Not sure what thing you're talking about on the valve cover, pics would help.

Look through the FAQ article on boost leaks. It will give you some ideas and sounds. Hissing. The point of checking the vacuum lines is to make sure they aren't broken. One person reported that they had a chunk of carbon stuck in the line which acted like a little valve. That could happen too.

I assume you've already done a boost leak check. Before you start chasing things, also find out where the hissing noise is coming from. You've already found something strange, fix it before chasing other things. It could be from the EGR system or lines as well.
 

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Discussion Starter #7
Update

OK I found where the buzzing noise was coming from at shutdown. It was the EGR solenoid. I think it may have had a damaged diaphragm, or just sticking. I shot some carb/choke cleaner inside it and no dirt came out, so I dont think it was a question of cleanness. I connected the origional N75 I removed and the noise has stopped.

Question: What is the difference between the EGR solenoid and the N75 solenoid? I compaired the numbers and markings on the two solenoids and with the exception of one number found at near the top, both are identical. Also the obvious, one is black and one is white.

Question: Should there be a big, noticeable difference in the way the car runs with the MAF disconnected? I truly do not notice any difference. I have removed it and am going to clean it right now to see if anything changes.

Thank you for your help!
 

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EGR<->N75 can be swapped. I'm not sure if there's an internal difference. The car will run without the MAF because the car substitutes a default value. However, if power is unchanged with it plugged in vs. not plugged in, the MAF, MAF plug, or wiring is bad. This is because the default value is low compared to a working MAF. Use VCDS to confirm the MAF value that the computer sees.
 

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Discussion Starter #9
update

Well thank the Lord we have found the problem! I also identified the round thing on the side of the valve cover. Ok, round thing is the vacuum pump, and my problem was the check valve between the vacuum pump and the N75 / EGR solenoid. I think one of the mechanics who replaced some of the vacuum lines put the check in backwards. Subsequently it was damaged, so it has been removed and the car runs great! Thank you again for your help!
 

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Ah... I am happy to know that you had a good resolution. I will update the FAQ to include this problem.

In the picture below, was it #7 that caused the problem?
 

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A couple of quick questions about that check-valve: (I've checked many places for a definitive identification and orientation of the valve) Which side is the outflow? (black/white) & The valve flow is towards the vac pump to allow evacuation of the reservoir and then hold vacuum?
 
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