Hey everybody, I'm in need of some major help. My car is a 1998 A4 2.8 front wheel drive. At idle the engine is miss firing, cold or hot, and only at idle. The car drives great, no miss when driving it. No fault codes are stored. The miss is random from watching the VAGCOM, its in cylinder 1,2,3 and 5, sometimes just one cylinder and sometimes two or more.

This is what I have done to the car: just rebuild the engine(due to no compression because of carbon on the valves) so compression is good. The throttle body is clean and working correctly. Spark plugs were replace, check the plug wires, they are good. Swapped ignition module with another one, no change still had miss.

Checked fuel pressure, it is 64psi with vacuum hose on and 72psi with it off. Audi's spec. is 55.1 to 60.9psi with the vacuum hose off!?! I think this is where the problem is but I'm not sure and here is why. The fuel pressure regulator is set at 4 bar (58psi). So I replace my regulator but no change to the pressure. Checked the return fuel line for issues, none found. So I remove the return hoses off of the fuel rail and put a 5 feet hose on and put the other end into a empty gas tank and run the car, still no change in the fuel pressure. Next I checked the voltage going to the fuel pump, it was 13.9 volts with the car running. The only thing left is the fuel pump, but do to cost I'm trying to verify that it is bad first before replacing it. Why would the fuel pump pump more than it should? Can it go bad like that?

I thought that a fuel injector maybe leaking causing the miss at idle. So I checked the fuel injectors for leaks by putting the fuel rail with injectors in a tank of water with air pressure on them, no leaks. Also performed a fuel injector clean but still have a miss fire.

Please help, any info or ideas will help.
Thanks

 

Your pressure doesnt seem too much higher than spec though, I wouldnt say, I wouldnt think that would cause a miss. Try a different gap on the plugs maybe. If your injectors were leaking than maybe you could blame it on the fuel pressure but if they arent I dont think its that.

 

New plugs? correct plugs gapped correctly? You sure your timing isn't off? What if you swap injectors between cylinders?

 

youre sure the compression is good? id check it again... also are you sure you put the head gaskets on perfectly and the head(s) werent warped? I wouldnt think both banks would misfire if one was wrong, but not sure. definitly do another leak down on it

 

reading your post again i see it only happens at idle, that sound more like an ignition problem. Id check the gap on the plugs like dragon said.

 

The spark plugs that are in the vehicle are NGK's BKR6EKUB (they are factory plugs)and they have two electrode that are pre-gapped. I have installed thousands of pre-gapped spark plugs in BMW, Mercedes, Audi's and never ones gapped a pre-gapped spark plug and they never coming back with a misfire because of the gap.

But there is a first time for everything. Found Audi's spec for the gap: 1.6 mm (0.063 in). Question do you gap a two electrode spark plug the same way as a single electrode.

dragonrage you ask: [QUOTE]You sure your timing isn't off? What if you swap injectors between cylinders?[QUOTE]

Will if the timing was off the engine would be running rough thru the whole rpm range not just at idle and could also bend a valve. And plus I checked it three times before I installed the covers. As for the injector swapping, the vehicle is misfiring in random cylinders, never just one.

[QUOTE]FasterA4ThanUrs you say:[QUOTE] reading your post again i see it only happens at idle, that sound more like an ignition problem. [QUOTE]

But if it is an ignition issues wouldn't the engine misfire at all rpm ranges? not just between 700 and 800rpms?

I'm not at all knocking yall's ideas so please don't take it the wrong way. Because I think yall are correct about the ignition system being the problem. Is there a test from the ignition module?

O and I have to ask if anyone with a 2.8 liter engine has put a fuel pressure gauge on it and what did you get for a reading? I know that the high fuel pressure may not be causing any issues right now, but it could cause one in the future. I think there is a clog in the fuel feed line or in the fuel rail.

Thanks for all yall's help,

 

No, actually, it wouldn't necessarily run rough, and timing would have to be pretty off to bend any valves.