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I have a 1999 Audi A4 1.8T Quattro with 100K miles, manual trans. It’s been running for most of it’s life with a GIAC performance chip (ECU) which basically boosts the turbo pressure from stock 0.5 to 1.1 bar. I have never had issues as a result of this upgrade. Everything else in the car is stock except the stereo.
For the past 1.5-2 years I’ve been dealing with a somewhat minor problem. When traveling at freeway speeds (60-70 mph), the engine will intermittently stutter. It feels like something is pinching the fuel line for a half second, then releasing, then pinching, etc. To make things worse is I never get a check-engine light (CEL).
If I lay off the accelerator, the problem goes away. If I can keep a constant speed on level roads, it typically does not occur. The easiest way to reproduce the problem is:
1. Set cruise control at 65 mph
2. Go slightly downhillIt seems like the computer is adjusting for going downhill resulting in slightly less fuel, and ends up over-compensating (the “pinch” or stutter), then realizes it’s going too slow, then adds throttle and over-compensates, then back and forth.
Reading the Audi forums (audiworld, audizine, etc.), this appears to be a common misfiring problem – the coil packs are notorious for going bad. HOWEVER, every post I’ve read where coil packs or spark plugs have been the problem, there is always an associated CEL. Also, owners report this most often when under full throttle or idle – and I never see my problem during those conditions.
What I’ve tried so far:
1. Replaced fuel filter at 95K. It was on the original from factory and it was time. No affect.
2. Replaced spark plugs. I needed these anyway and bought the recommended NGK’s from the dealer. No affect.The next recommended troubleshooting step is to replace the coil packs, but these are fairly expensive in my engine: $50-90 each. Other possibilities:
– MAF or other sensor (dirty?): I’d expect a CEL if this were the case
– Vacuum leaks
– Other hose leaks?Any recommendations on where to look next? I’m caught the car repair bug which is why I’ve become such a ETCG fan and not afraid to get dirty! 😛 Any advice is greatly appreciated!
Thanks!
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seeing it is happening under load. could be fuel or spark
related. I’m surprised no cel. You could try cleaning the MAF.https://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Sorry if I wasn’t clear – it actually does not happen under load which is why it’s weird. I’ll clean out the MAF next, and if that doesn’t help I’ll go for new coil packs.
Thanks!
The GIAC chips is that a replacement chip for the one that came stock with the car or is it a piggyback system that basically piggybacks itself unto the cars original EFI/ECU system?
I don’t know the Audi very well, but I’m assuming in 99, that was an OBDII computer . . . why not beg borrow, or buy a data logging scanner, and take it for a ride to see if you can see what is happening when that stutter occurs.
Thank you for all the suggestions. I’ll reply to each in this response:
- wysetech: According to my maintenance records, the O2 sensor was never replaced. I would think it would cause a CEL if bad or lazy, no? I’ll keep this in the back of my mind.
- TehTDK: I’m not sure, but I think it was the original ECU + Flash. I would have had to remove my ECU and mail it to GIAC, have them flash it and mail it back, or have the work done in a GIAC shop. I went to a local shop because I couldn’t be without my car for the week or so for the shipping.
- Tomh: That is a great idea. I did buy a Bluetooth OBD II unit a couple of weeks ago to work with my phone’s Torque app, primarily to pull codes but I think it has the ability to perform data logging. I only have the free version of Torque but will pay the $5 for the pro version.
I’ll try to grab OBD II data after work today and post any findings.
Thanks again!
That is indeed a strange issue. The ignition system will typically misfire under load, unless a coil or plug is so shot that it’s just a dead miss.
Without faults or data you are kinda boned as this is not a known issue on your car. If possible, get your hands on a Ross-Tech scan tool or something similar so you can view value blocks 15 and 16. These will display which, if any cylinders are misfiring when the problem occurs.
You can remove the MAF sensor and make sure there is nothing on or near the screen that could affect the air flow. Even a tiny leaf or bug on the screen can skew the readings. This is unlikely.
The only other thing i can think of is the cam or crank sensor. The crank position sensors can become dirty and cause ‘hick ups’. You can usually resolve that by removing the sensor and wiping off any debris on the tip. Check the sensor wiring by jiggling it and seeing if the engine stumbles. Cam sensors have caused this as well, but i have only seen that on the 3.2 FSI engines. You can start your car and unplug the cam sensor and take it for a road test. Once it’s started, it will run without it. For that matter, it will also run in default mode with the MAF unplugged.
Hey just put my two cent forward, could be coil packs as suggested or could be a ignition control unit problem aswell, thought I would bring that up as no one else has
Thanks again for all the suggestions.
Based on FourRings’ reply, I took the A4 out this morning but only on local roads – no freeway speeds – I just wanted to double-check that the stuttering did NOT occur under WOT. Floored it several times up to 6000 RPM (redline is around 7K) and definitely did not misfire, stutter, or lose power. Steady power all the way up.
I had the OBD II unit plugged in but stupid me forgot to start data logging. I’l be sure to turn it on the next time I hit the freeway.
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