Hello! My audi a4 b5 1.8t AEB engine is misfiring badly when maintaining a steady speed, but when I accelerate harder than average it is good. And if I let go of the gas pedal while driving it will stop misfiring. This occured after I took out my sparkplugs and put them in again after checking if they looked ok. The obvious answer here would be that I somehow managed to break the plugs while doing this, but open for suggestions.

Scanned with vagcom, no fault codes are showing.

If anyone has any tips for fixing this I'd be very happy

 

Double check the leads to your coils, what gap are you running on the sparkplugs?

 

If a random misfire happens enough, it will throw a P0300 code. Take the car to any repair shop or a auto parts store. They can read the codes and some scanners tell you pending codes. Write the exact number down and go from that point.

Since it clears up when you give it gas, I would suspect a vacuum leak causing a lean condition. Being an Audi, it could be some high tech frammis that's cracked.

If it was a spark plug or coil, it would misfire all the time.

 

I too have a random misfire problem with a 98 A4 1.8T AEB head. It's weird as it will 1st throw code p0300 which is a random misfire then eventually throw a misfire code for each of the 4 cylinders. It runs fine once the RPM is above about ~1300 rpm. I finally gave up trying to locate a phantom air leak. I paid a professional to diagnose it. After $500 I was told the issue was internal to the engine. What could be going on internally that would cause it misfire only at idle? I tried seafoam, evap valve, all new EGR hoses and all new vacuum lines. I paid for a smoke test on the vacuum and pressure tested the intake plus good even readings on the compression test. I'm stumped.

 

I had a misfire on my B5 that wasn't doing it all the time and the issue was that my plugs were not gapped properly, since this issue only started after you took your sparks out, try an old set of plugs that were not completely worn and see if there is any difference and maybe that might solve your issue. Tenspeed is correct in saying that your car can misfire if you have a vacuum leak but that usually happens when the car is under load and after driving the car for a while the car will adapt to the leak until the counters are cleared which is usually done by clearing errors found in the system.

 

Oh and another thing, misfires can be difficult to pick up when scanning the car for errors even on new Ecu's, so if you scan your car and do not find any errors, don't be surprised.

 

make sure you didn't break any vacuum hoses...
just swap the plugs for new coppers to rule them out.
You might have broken your suction jet pump... the black plastic thing at the back of the head.
Most of that stuff is brittle. just a thought.

 

Misfires can be anything from improperly gaped plugs, to a coil pack, connector boot, wiring harnress, ICM, leaky valve stem, vacuum leak, or even a leaky injector o-ring.

Start with the basics and work your way down the list.

Jason

 

I still have a rough idle with random misfires. I've gone over every inch of the intake system and vacuum lines. Paid big bucks to two mechanics just to tell me they can't find the source of the problem either. I can see the fuel maps changing as the ECU tries to correct the fuel/air ratio. So air is getting in somewhere. I know this sounds odd, but I believe the misfire issue came with the replacement engine. Original 1.8T ruint from lack of oil but never had idle/misfire issues. Replacement engine from 99 Passat. Same block and engine codes as original. Replacement went well except for this nagging rough idle/misfire. The replacement engine had the intake and exhaust with turbo all in tact. Thinking it better to simply put it in as is trying to minimize the dis-assembly requiring more parts/gaskets. In retrospect, if I had it to do over, I should have removed the head and had it rebuilt and removed the exhaust and inspected the turbo and all the lines associated with it. In trying to fix it, the entire intake has been reworked. hence, my thinking to look at the exhaust side before moving on to head removal. Any ideas? I'm still stumped. What's been done so far:
1-Replaced spark plugs with NGK iridiums (Gapped from 028 up to 032 with no change)
2-Replaced all vacuum lines and EGR system with 034 silicone hoses
3-Swapped intake manifold from original engine with new gasket
4-Swapped fuel rail with new seals and checked fuel pressure (all good)
5-Replaced CTS sensor with Green square top (I have two blue CTS)
6-Replaced Ignition Module on top of air box.
7-Replace that N80 valve (evaporative purge valve?)
8-Swapped Cam and Crank position sensors
9-Swapped Air Temp Sensor

I have not replaced the MAF as it does not improve idle when disconnected. Could it be the MAF from the original setup which had no misfire issues?

 

have you scanned the car to see what error it is throwing, and have you changed your coils?