Rough Idle, Misfiring?

[DrunkPenguin]

My car idles pretty rough and sounds like it misfires ever 3-4 seconds. you can hear a mild backfire in the exhaust and feel it in the car when it happens.


Things Ive done so far...

cleaned air filter
replaced spark plugs
tested spark plug wires resistance, 7K ohms
tested coil pack, primary resistance 1 ohm, secondary resistance 14K ohms
replaced PCV valve


What should I try next?



John

 

[t0taleclipse]

I had the same problem. Felt like a miss and I had a loss of power, exhaust that would get ya high with one sniff and poor gas mileage. If you having the same problems, save yourself the time in doing anymore work to your car and anymore testing and pull out the compression tester out of your tool box and do a compression test. I looked for 3 hours for my problem, testing everything and it turns out I had only 20psi on cylinder 2. I did a leak down test and found it was an exhaust valve. Pulled the head off and a cunk was out of one of my exhaust valves. Let me know what you come up with.

 

[DrunkPenguin]

I had a feeling it was time to try that. Ive never done it but it seems pretty simple. I also noticed what looked like a couple small hairline cracks on the exhaust manifold. I couldnt tell if they were cracks or deep scratches tho. Also, the left-most bolt on the manifold was missing completely. If I dont pass the compression test, could this by why?



John

 

[t0taleclipse]

oh wait i was talking about my 92 laser RS N/T. you hasve a 2g gst. well do the compression test but if your not lacking power its probally a vacuum leak. my 98 gst has a slight stumble at WOT that i probally can trace to a vacuum leak but anyway if you lack power a compression test is a must.

 

[l3LacK Tsi]

my 1g tsi has this same problem compression is fine whole way across the board 172. did plugs wires etc didnt help gotta stall at stop lights problem also. anything u tried worked??

 

[t0taleclipse]

try testing your ISC, Vacuum lines for leaks and you BISS may need adjusting.

 

[DrunkPenguin]

just tried a compression test for my first time, hopefully I did this properly. I took out all 4 plugs, the engine fuse, and unplugged the coil pack. I then tested each cylinder by cranking the engine 4 times, for about 4 seconds each.


Assuming I did this right heres what I came up with, and this seems pretty bad to me.


Cylinder 4 3 2 1

184 189 179 171




So Whats next? When I was doing this I was getting different numbers each time I cranked the engine, so I took an avg of what I was seeing. Is that normal or should it have been more consistant?

John

 

[marcmsj]

be sure when you're doing the compression test, to crank the engine at wide open throttle.

 

[DrunkPenguin]

It was at WOT. I did this while the engine was cold. Should I be doing this after its gotten hot?



John

 

[DrunkPenguin]

Ok I just did it again after driving the car hard enough to get the temp up to normal, results were about the same.


I really dont quite understand these numbers but are these numbers ok or no?


4 3 2 1

dry 184 189 197 171
wet 190 200 189 184




John

 

[Vandy4g63]

yeah those are fine--that's about what my compression is....

do you have aftermarket injectors or an safc? Is your fp rewired? diff fuel pump?

 

[DrunkPenguin]

no, the cars pretty stock except for air intake modifications and exhaust. The last couple days while working on it I have noticed the smell of gas. I think coming from the exhaust because I see no leaks anywhere.



John

 

[Tarantula]

I have to say that those numbers are a bit high for a 2G. The standard compression is 178psi hot and the service limit is 133psi with 8.5:1 compression ratio. Usually if its higher than that, its because you may have carbon buildup on the piston tops. I use a Compression tester that holds its pressure. So I just keep cranking until the needle stops going higher. I position the face of the tester so that I could see it between the hood and the car from in the drivers seat.
Heres a link for you on compression testing on our cars 1G and 2G
http://www.dsmgrrrl.com/FAQs/compression.htm

Ok I just did it again after driving the car hard enough to get the temp up to normal, results were about the same.
I really dont quite understand these numbers but are these numbers ok or no?
4 3 2 1
dry 184 189 197 171
wet 190 200 189 184
John

 

[clapbak]

bummpp

im having the same problems in my 93 tsi fwd.. suggestions? im about rdy to snap .. just bought the car..

 

[DrunkPenguin]

I'm still having the problem. Just replaced the fuel filter and no help. I'm starting to suspect timing. I'm getting close to 60,000 since the last timing belt was put on so I may have to have that done and see if it helps.



John