audi1996

I recently took off the heads from my 1996, A6 wagon (2.8 L, V6) and had them rebuilt. I reinstalled them , replaced the timing belt (with the special tools for camshaft alignment with crankshaft), replaced thermostat, replaced timing belt idler pulley and timing belt tensioner, replaced water pump, replaced accessory belt tensioner, replaced accessory belt, and both oxygen sensors (each one is located on exhaust manifold coming out of heads).

I now have a rough idle and you can hear the engine missing. I have tried pulling spark plug wires one at a time but no detectable difference. I get a malfunction indicator light if I drive at highway speeds and the code translates to misfires on cylinders 4,6 and multiple misfires. Also accompanying oxygen sensor errors.

I have been trying to troubleshoot the rough idle/misfires by doing the following: replaced all spark plugs, checked spark plug wires for continuity (good), checked coil (good), cleaned out idle stabilizer valve, sprayed brake cleaner at base of each fuel injector to check for bad gasket (all were good).

Does anyone have any suggestion on what to try next or what might be wrong?

Brdman*

check all rubber and braided hoses for leaks, clean MAF (check FAQ), and replace or check the rear/post-cat 02 sensors.

audiqv8

recheck the firing order, perhaps you have two wire reversed. Also check the coil pak.

pgoyer

I can tell you what your problem is. You have not set the basic timing correct. I know you mention the proper tools, but we all make mistakes. If this were my motor, I would go right back down to the timing belt and reset the motor, making sure I knew exactly where TDC was on cylinder no 1 and I would make sure that I followed the Bentley.

Here's the rub, if you try to set the tension on the timing belt with the cam sprockets tight, you will never get it right. Both cam sprockets must be loose when you set the tension of the belt, otherwise the timing will be altered slightly and the engine will run, but rough. I would bet money that your timing is off just a tad.

Good Luck

Muhammad

Also have a GOOD fuel injector cleaning done.

If those two don't correct it, your next options are new spark plugs and/or O2 sensors if they are old.

O2 codes can be set off by a number of things, usually fuel-system related. Poor injector spray pattern would probably do it...

Could also be ignition wires.

Start with the cheapest repair, and go up from there until it runs good. =)

audi1996

Thanks for the advice. I have already replaced my O2 sensors and I replaced most of the vacuum lines that I had to remove in order to pull the heads. I have already checked the coil pack. I have not cleaned the MAF sensor but it wasn't running rough before I pulled the heads.

I am going to look at the timing belt again and reset it. I have the tools to lock the cranskshaft at TDC and also hold the camshafts in place so I can pop the cam sprockets off. Can't hurt. I will let you know.

audi1996

Also, I have cylinder #3 being at the top of its stroke when the crankshaft is at TDC. Is that right or should cylinder #1 be at the top of its stroke?

(cylinder #3 is closest to firewall on passenger's side of engine).

audi1996

Cleaned out the EGR passageway that connects the EGR to the exhaust manifold. This was not the problem.

I was then playing with the vacuum lines and found that the vacuum line to the EGR valve is active when I am at idle. This should not be the case. This indicates that my EGR sensor is bad, the solenoid for the vacuum relay is bad, or the electronic control for the EGR is on the fritz. I will keep you posted.

In the meantime, anyone know how to test/find the EGR sensor?

whilst04

Go back and check your basic timing.
Could have slipped a tooth when releasing tensioner.
Just a thought.

audi1996

It turns out that there is vacuum going to the EGR valve when the car is at idle. This should not be the case. I am tracing the vacuum back to the purge valve (? is that what it is called ?) and I will try to clean out the purge valve and see if that fixes the problem.

FYI. My understanding is that that EGR valve should only open at constant, high RPM (i.e., constant freeway speeds). This allows for a backfill of "inert" gases that lower the combustion temperature. The lower combustion temperature reduces the amount of NOx produced which means less hazardous pollutants. The EGR sensor attached to the passageway between the EGR valve and the intake plenum. When the computer tells the EGR valve to open it expects the EGR sensor to undergo a change in voltage due to a change in temperature (exhaust gas is really hot!) which is a feedback to the computer to verify that the valve opened. If the valve is always open then it causes a rough idle and the Malfunction Indicator Light (MIL or check engine light on the dash) will come on when you drive on the freeway since there will be no detected temperature drop at the EGR sensor (it will always be hot since the valve is always open).

Want to see if this is the cause of your rough idle? Disconnect the vacuum hose from your EGR valve while the engine is running and put your finger over the end of the hose to feel for vacuum. If there is vacuum and your idle smooths out then chances are that your EGR valve is fine and you need to look up the vacuum line for the cause.

I will keep you posted on what I find. Any suggestions on what else to check?