Reason for my car going into Limp Mode?
01-15-2005, 06:45 PM
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Reason for my car going into Limp Mode?
I've posted a few weeks back about my 97 1.8TQ bucking often under boost. Well I've realized what is going on... it's overboosting and then going into limp mode. When in limp mode it won't go over 14-15psi and then the engine bucks. This will continue to happen until I clear the "charge pressure maximum limit" code.
I have a APR chip, hyperboost PBV, intake, exhaust, (and more mods that don't matter now) new front O2 sensor, cleaned BPV, replaced spark plugs, cheaked for leaks, checks ohms on N75 valve and it still happens.
If I punch it at the right speed in 3rd gear (works the best to overboost) it will pop up to 23 psi and hold there for a few seconds before it goes into the first limp mode. This has happened since it started to get cold, but I don't know if that is it.
Is there anything else i could log with my VAG tool to find out what could be causing it? You would think it would be the N75, but I checked the ohms and they came within spec with the engine off of course.
I guess if I find out why it is overboosting I've solved the problem. It holds boost without a problem if i keep it below 15 psi.
Chris
I have a APR chip, hyperboost PBV, intake, exhaust, (and more mods that don't matter now) new front O2 sensor, cleaned BPV, replaced spark plugs, cheaked for leaks, checks ohms on N75 valve and it still happens.
If I punch it at the right speed in 3rd gear (works the best to overboost) it will pop up to 23 psi and hold there for a few seconds before it goes into the first limp mode. This has happened since it started to get cold, but I don't know if that is it.
Is there anything else i could log with my VAG tool to find out what could be causing it? You would think it would be the N75, but I checked the ohms and they came within spec with the engine off of course.
I guess if I find out why it is overboosting I've solved the problem. It holds boost without a problem if i keep it below 15 psi.
Chris
01-15-2005, 07:37 PM
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turbo 101...
Forgive me...I don't know how much you know, so I'll briefy go through it all.
1. The turbo runs off the exhaust gas flow from the engine.
2. There is a bypass for the exhaust gases called the wastegate. If no boost is needed, the wastegate is open, and if full boost is needed, the wastegate is closed.
3. The wastegate is actuated by the N75 valve. A vacuum hose runs to the wastegate (from the N75), and when the ECU determines it's got enough boost, pressure is sent through the vac line which triggers the wastegate to open (thereby bleeding off exhaust gases, and tapering boost).
4. There is a feedback loop from the turbo to the N75 via a vac line. This line carries the actual boost the turbo is producing. If there isn't enough boost, the N75 will keep the wastegate closed. If there is too much boost, the N75 will direct pressure to open the wastegate.
If your N75 tested out fine, I would suspect one of the two vac lines leading to the turbo is pinched off, or has a hole in it. You can remove the headlight to get a better look in there. Just remember, by moving things out of the way to see what's going on, you may be removing an object that's causing the line to be pinched. Considering you mentioned the issue is somewhat weather dependent, I would suspect the issue is a kink in one of the lines. As the hose warms up, it may have enough flexibility to allow some enough air through, but as it gets cold, the hose may harden, preventing pressure from pushing through the kink. So, if you inspect everything when it's cold, odds are you'll see the kink, since the hose will be "frozen" in place. Good luck.
1. The turbo runs off the exhaust gas flow from the engine.
2. There is a bypass for the exhaust gases called the wastegate. If no boost is needed, the wastegate is open, and if full boost is needed, the wastegate is closed.
3. The wastegate is actuated by the N75 valve. A vacuum hose runs to the wastegate (from the N75), and when the ECU determines it's got enough boost, pressure is sent through the vac line which triggers the wastegate to open (thereby bleeding off exhaust gases, and tapering boost).
4. There is a feedback loop from the turbo to the N75 via a vac line. This line carries the actual boost the turbo is producing. If there isn't enough boost, the N75 will keep the wastegate closed. If there is too much boost, the N75 will direct pressure to open the wastegate.
If your N75 tested out fine, I would suspect one of the two vac lines leading to the turbo is pinched off, or has a hole in it. You can remove the headlight to get a better look in there. Just remember, by moving things out of the way to see what's going on, you may be removing an object that's causing the line to be pinched. Considering you mentioned the issue is somewhat weather dependent, I would suspect the issue is a kink in one of the lines. As the hose warms up, it may have enough flexibility to allow some enough air through, but as it gets cold, the hose may harden, preventing pressure from pushing through the kink. So, if you inspect everything when it's cold, odds are you'll see the kink, since the hose will be "frozen" in place. Good luck.
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