Trying to find an engine miss -- compression & leakdown test...

FixedWing · 11-01-2005, 03:11 PM

I have had an occasional miss at idle for a while now. From VAG-COM I know that the miss is mostly on cylinder #5 and only really during the warm-up phase. I also get occasional random missing on other cylinders. The missing is only present at idle and usually vanishes once the car is warmed up.

So today I pulled the plugs and did a compression and leakdown test on the car. These are the results:

Cyl, compression (psi), leakdown (psi)
1, 190, 79/80
2, 190, 79/80
3, 188, 78/80
4, 180, 72/80
5, 175, 69/80
6, 192, 79/80

So what are people's thoughts? My feeling is that the lower figures on cyl. #5 are not low enough to explain the problem. On an aircraft engine, 69/80 would not be unreasonably low. But I think they might partially explain it.

The engine has 144,000 miles on it.

I've decided that a leakdown test is preferable to a compression test. It is much more accurate and repeatable. It also isn't that difficult to perform. Just put the car in gear and push it a bit at a time until the pressure starts to build and reaches maximum.

Next I'm going to get my fuel injectors cleaned and see if that makes a difference.

The compression test was done on the power of my Odyssey PC680. I ended up cranking the engine 13 times for about 6 seconds each. It was a 15 C day and the engine was still warm. The battery had no trouble doing this. Of course, the plugs were out but still, I thought the battery did very well. The engine was cranking as fast at the end as it was at the beginning. I was pleased! :-)

Stephen

Mike2kS4 · 11-01-2005, 03:22 PM

FixedWing · 11-01-2005, 03:29 PM

Already swapped plugs, coilpack and the two electronic thingies (obviously don't remember the name) on top of the airbox.

I've also replaced the MAF, the coolant temp sending unit and used a tank full of injector cleaner (which did improve mileage).

I So just checked the cylinders (rings & valves) to see how they are. Last thing to try is sending out the injectors for cleaning and testin.

Stephen

RKA · 11-01-2005, 03:49 PM

FixedWing · 11-01-2005, 03:59 PM

surfnturf · 11-01-2005, 04:00 PM

Does the car run normally but miss a little at idle only? On my dad's TT the intake manifold came loose and caused it to miss on #1 and #4 but it ran almost normally at speed, although leaking some boost. If you hear a hissing sound at idle it might also be the manifold.

Mike2kS4 · 11-01-2005, 04:01 PM

FixedWing · 11-01-2005, 04:01 PM

FixedWing · 11-01-2005, 04:02 PM

andrew20195 · 11-01-2005, 04:03 PM

Oh, and was your engine warm or cool when you did the test? I was thinking if you have a cold misfire, but you did the test at operating temp., that 13% leakdown might really be more when the problem occurs.

11-01-2005, 03:22 PM	#2
Mike2kS4 Elder Member Account #: 38083 Join Date: Jun 2002 Posts: 12,105	I'd try swapping coils and injectors around too see if the problem moves. __________________ 2002 S4 Avant, 6spd. Bone stock. <a href="http://www.autospeed.us"> </a> Caveat: Any advice I express regarding potential tuning and such is purely informational. If your knowledge and abilities are not up to the task, no amount of advice can assist.

11-01-2005, 03:29 PM	#3
FixedWing Senior Member Account #: 66531 Join Date: Dec 2003 Posts: 7,155	I did that first... Already swapped plugs, coilpack and the two electronic thingies (obviously don't remember the name) on top of the airbox. I've also replaced the MAF, the coolant temp sending unit and used a tank full of injector cleaner (which did improve mileage). I So just checked the cylinders (rings & valves) to see how they are. Last thing to try is sending out the injectors for cleaning and testin. Stephen __________________ ================================================== ======================

11-01-2005, 04:00 PM	#6
surfnturf New Member Account #: 87881 Join Date: Feb 2005 Posts: 262	Re: Trying to find an engine miss -- compression & leakdown test... Does the car run normally but miss a little at idle only? On my dad's TT the intake manifold came loose and caused it to miss on #1 and #4 but it ran almost normally at speed, although leaking some boost. If you hear a hissing sound at idle it might also be the manifold.

11-01-2005, 04:01 PM	#7
Mike2kS4 Elder Member Account #: 38083 Join Date: Jun 2002 Posts: 12,105	You can just lift the rail and slip the squirters out, it's pretty easy. __________________ 2002 S4 Avant, 6spd. Bone stock. <a href="http://www.autospeed.us"> </a> Caveat: Any advice I express regarding potential tuning and such is purely informational. If your knowledge and abilities are not up to the task, no amount of advice can assist.

11-01-2005, 04:01 PM	#8
FixedWing Senior Member Account #: 66531 Join Date: Dec 2003 Posts: 7,155	Yes. I've done an intake leak pressure test and there is no intake leak. __________________ ================================================== ======================

11-01-2005, 03:11 PM	#1
FixedWing Senior Member Account #: 66531 Join Date: Dec 2003 Posts: 7,155	Trying to find an engine miss -- compression & leakdown test... I have had an occasional miss at idle for a while now. From VAG-COM I know that the miss is mostly on cylinder #5 and only really during the warm-up phase. I also get occasional random missing on other cylinders. The missing is only present at idle and usually vanishes once the car is warmed up. So today I pulled the plugs and did a compression and leakdown test on the car. These are the results: Cyl, compression (psi), leakdown (psi) 1, 190, 79/80 2, 190, 79/80 3, 188, 78/80 4, 180, 72/80 5, 175, 69/80 6, 192, 79/80 So what are people's thoughts? My feeling is that the lower figures on cyl. #5 are not low enough to explain the problem. On an aircraft engine, 69/80 would not be unreasonably low. But I think they might partially explain it. The engine has 144,000 miles on it. I've decided that a leakdown test is preferable to a compression test. It is much more accurate and repeatable. It also isn't that difficult to perform. Just put the car in gear and push it a bit at a time until the pressure starts to build and reaches maximum. Next I'm going to get my fuel injectors cleaned and see if that makes a difference. The compression test was done on the power of my Odyssey PC680. I ended up cranking the engine 13 times for about 6 seconds each. It was a 15 C day and the engine was still warm. The battery had no trouble doing this. Of course, the plugs were out but still, I thought the battery did very well. The engine was cranking as fast at the end as it was at the beginning. I was pleased! :-) Stephen __________________ ================================================== ======================

11-01-2005, 03:49 PM	#4
RKA Member Account #: 2062 Join Date: Mar 2000 Posts: 67,038	Just swap two of the injectors. __________________ 2005.5 A4 Avant 2.0T Quattro 6MT APR 91/93/100 \| APR Exhaust \| 19" BBS RS-GT's \| Carbonio \| Hotchkis \| Partridge-in-a-pear-tree Others: 2006 TL on the block 2001 S4 sold 2000 A4 sold * September 11th, 2001 - Never Forget *

11-01-2005, 03:59 PM	#5
FixedWing Senior Member Account #: 66531 Join Date: Dec 2003 Posts: 7,155	How easy is this to do? I thought the fuel rail had to be taken off. __________________ ================================================== ======================

11-01-2005, 04:02 PM	#9
FixedWing Senior Member Account #: 66531 Join Date: Dec 2003 Posts: 7,155	Ok, then I'll try that next. If it isn't an injector, then I'm out of ideas. __________________ ================================================== ======================

11-01-2005, 04:03 PM	#10
andrew20195 Audiworld Junior Member Account #: 84711 Join Date: Dec 2004 Posts: 618	Could you hear air escaping anywhere during your leakdown test? (edit) Oh, and was your engine warm or cool when you did the test? I was thinking if you have a cold misfire, but you did the test at operating temp., that 13% leakdown might really be more when the problem occurs. __________________ --------------------------------------- 2001 Laser Red/Onyx Leather S4 AWE DTS R.I.P. 2005 Evo Nothing yet. ---------------------------------------

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