2003 Audi A4 1.8T Quattro Sporadic Deceleration and Loss of Power

[TMeschke]

Hello all,
I'm having quite an odd problem with my car that has been going on for quite some time now. While driving my 2003 Audi A4 1.8T Quattro 5mt, there will be times when it feels like the brakes come on for a second, and the car decelerates briefly. This will happen very randomly throughout a drive, and seems unrelated to any driving condition. Sometimes it will happen a few times in a row and sometimes will just happen once. It's not a sharp deceleration, like a misfire, it just feels like the brake is lightly tapped. While it could be entirely unrelated, there have also been a few instances lately where the car will randomly feel very sluggish, come onto boost very slowly, and peak at only around 10 psi (when it should peak at about 21 psi). Then out of nowhere the power will return after a couple minutes, and the car will feel normal again. I've had a coilpack fail before, and it is definitely a different feeling.

Ever since I got the car, it has had a lot of odd quirks. One of the previous owners put a very horrible (possibly homebrew) tune on the car. The car will spike from vac to 21 psi in an instant with no middle ground, and only seems to come onto boost when I don't want it to. It makes it extremely difficult to start from a stop, without launching off the line . But I digress. The real issue is that the car doesn't seem to post CELs at times. There have been several instances where a coilpack has failed, and no code was posted for the misfire. So even though my VCDS scan comes back clean when checking the engine, I have a hard time believing that nothing is throwing a code.

While I would be surprised if this issue was indeed related to the brakes, it should probably be noted that I have Brembo 17Zs installed in the front, using a stock brake booster. I also have the 2.0 TFSI coilpack conversion (since I've had issues with coils in the past).

I'm sure I left out some important details somewhere, since I'm terrible at putting my thoughts to words. Let me know if there are any other diagnostics I could do to narrow down the problem. Unfortunately I only have VCDS-Lite, so I'm a bit limited in that regard. Thanks!

[BARRY]

this is odd... have you run a code reader to read for any trouble codes?

[TMeschke]

Yes, checking for codes in VCDS-Lite (for the engine at least) comes back clean. As I mentioned in my post though, I don't entirely trust this since I've had an obvious misfire before, and no codes where posted.

[BARRY]

well we need to isolate the issue. it is not a braking issue, as our cars do not have active braking, or electronically controlled brakes.

so moving along it would seem it is a power issue. sounds like your engine is dropping fuel, then fuel is returning. or maybe dropping spark entirely. this could mean anything...if i were to call something out directly i'd say, your crank angle sensor. crank angle sensors fail anywhere between 50-170k...and they do fail quite often. thankfully its a cheap part and easy to change, even for the mechanically challenged.

until we've ruled that out, its really hard to determine, based off your explanation.

[TMeschke]

Thanks for the advice. I don't think it's a loss of spark, since it's not a complete loss of power, but more so a small, sudden resistance. My main focus is fuel delivery, since that seems to be the most likely cause. I'll have to look more into the crank angle sensor, to see if it matches my problem. I'll keep scanning for codes as well to see if anything comes up.

I did have one code that came up a while back for my engine cooling thermostat, but it didn't come back after I cleared it out. I really can't imagine that would cause my problem though.


17700 - Map Controlled Engine Cooling Thermostat (F265): Open Circuit

P1292 - 004 - No Signal/Communication - Intermittent

[SJorge3442]

Perhaps the brake pedal switch is bad? If you drive with your foot on the gas (think highway driving) and then also use your other foot to hit the brake, the car will cut the throttle and slow the car down as it believes the throttle/pedal is sticking and its trying to stop you from having a runaway car. Try doing a test of driving down the road at 45+ (it might only work at highway speeds) and also ride your brake while on the gas. See if it feels the same.

[MetalMan]

Couple of things I would check if my car was behaving like this:

• Log throttle plate position. See if it fluctates at the time you're seeing the odd behavior. The ECU could be closing the throttle due to the unexpected / rapid boost onset.
• Check the diverter valve.

[TMeschke]

The brake pedal switch was another one I was slightly suspicious of, since the power cuts I'm having feel exactly like the brakes are being applied. I'll have to give that a try. Thanks!

[MetalMan]

Perhaps the brake pedal switch is bad? If you drive with your foot on the gas (think highway driving) and then also use your other foot to hit the brake, the car will cut the throttle and slow the car down as it believes the throttle/pedal is sticking and its trying to stop you from having a runaway car. Try doing a test of driving down the road at 45+ (it might only work at highway speeds) and also ride your brake while on the gas. See if it feels the same.

Solid suggestion. One easy way to rule out brake switch (clutch switch too) is if cruise control works. If CC works and works consistently, then there's a good chance brake and clutch switches are fine.

Coincidentally I think my brake switch has the opposite problem, where it doesn't engage soon enough. The result is that I can have slight braking force before my brake lights turn on, and I believe my ABS module's intermittent fault is because it detects pressure in the brake system while the brake switch appears disengaged.

[TMeschke]

Couple of things I would check if my car was behaving like this:

• Log throttle plate position. See if it fluctates at the time you're seeing the odd behavior. The ECU could be closing the throttle due to the unexpected / rapid boost onset.
• Check the diverter valve.

Sounds good, I'll give that a try. Which log number will give me a readout for the throttle plate position?

I've wanted to replace the DV as well. The previous owner installed some DV that is "rated for 700 hp", so I'm curious if the spring in the DV is too stiff.

[MetalMan]

Sounds good, I'll give that a try. Which log number will give me a readout for the throttle plate position?

I've wanted to replace the DV as well. The previous owner installed some DV that is "rated for 700 hp", so I'm curious if the spring in the DV is too stiff.

Not sure which measuring block it is, but I would bet it's within the first 20 measuring blocks. Shouldn't take but a minute to cycle through them and find which measuring block it is.

I hear you on the DV... on my car I just switched to a 710N. With the 710N, it either works or it doesn't. Takes the guesswork out of piston-style DVs.

[TMeschke]

Solid suggestion. One easy way to rule out brake switch (clutch switch too) is if cruise control works. If CC works and works consistently, then there's a good chance brake and clutch switches are fine.

Coincidentally I think my brake switch has the opposite problem, where it doesn't engage soon enough. The result is that I can have slight braking force before my brake lights turn on, and I believe my ABS module's intermittent fault is because it detects pressure in the brake system while the brake switch appears disengaged.

That's a good point. I haven't had any issues with my cruise control turning off. I'm fairly sure I've had this issue while my cruise control is engaged.

[MetalMan]

There is a caveat... I meant that it means the brake and clutch switches are fine for this problem if CC works. The switches can still be bad and have other effects (like my ABS issue as an example).