Symptoms: Engine runs rough at idle/fast idle when cold. Feels like it has dropped a cylinder, but at above 1500rpm feels normal. Did the normal check for boost leaks, error codes, and faulty connectors. Found pulling the connector off #6 coil pack had zero effect to idle. Removed #6 plug, looks normal and not wet, so does not appear to be spark problem. Swapped plug and coilpack with #4 to test. Problem remains at #6 cylinder. So, because plug is dry, but no rev drop on #6 when disconnecting spark, I assume injector #6 is not firing at low revs when cold. Once engine is warm, all is normal. Car is still running original injectors, were cleaned and flow tested 40k back (5yrs ago). Assume injectors have done 160-180k. Apart from swapping injectors, is there any other testing I can do to confirm? Any flaws in my theory?
Cold compression test, Pex's old zed was doing the same thing and that was put down to a sticky valve.
Worn inlet valves will do the same thing, may have the crappy old 30P inlet valves. As Chrispy says do a compression test when cold, then confirm with a leak down test to see where the problem is - my bet is on the inlet valves if you have an 89 model.
My money is on low compression caused by worn rings. Very predominant when cold. But as said leak down test to be sure.
Not discounting what the other guys are suggesting re compression issues (which may well be the end-result) but exhaust all other options before you rip into the engine. It could also be just a dud injector and/or spark plug. I have seen leaky injectors cause strange effects and spark plugs that were useless when cold but came good when hot. Check the injector resistance when stone cold - and also when hot. Should see no more than about 12-13Ohms under both temps. Also pull that spark plug. See if its rooted or just fouled. If the injector was leaking slightly, the spark plug could be getting fouled whilst cold but with a bit of heat once up to operating temps, the fuel burns off and the plug comes good. Stranger things have happened ... and it costs you nothing but a bit of time to eliminate the simple causes before assuming the worst case scenario.
my zed was dropping a cylinder on idle when cold, then would fire normally when I revved it. changed the plugs and problem solved. you might get lucky.
get a long screw driver and put the metal end on top of the injector holder and the other end against your ear to see if you can here it ticking do this to a couple of the others first so you know what you are listening for
I never got to the bottom of mine, but through a process of elimination, I'd say low compression was my issue, but only when cold. Valve or rings, unknown
Did Compression test on cold engine today. Results (psi) are pretty sad. #1 - 82 #2 - 110 #3 - 50 #4 - 80 #5 - 50 #6 - 78 Considering the engine still runs well when hot, I guess the turbos are doing a lot of compensating. Will leave the leakdown test for another weekend. Time to start stockpiling for the engine rebuild.
I am suprised it even runs with those numbers, are you sure your compression guage is correct and did you do it correctly?
Hot Test Results Any experts please - what do the plugs indicate #6 on left thru to #1 on right Did another compression test this morning, this time engine was at normal operating temp. - all at 150 or 155psi. Cyl Cold Hot #1 82 150 #2 110 155 #3 50 155 #4 80 150 #5 50 150 #6 78 155 Any suggestions as to what may be going on when engine is cold?
Hot compression test results are good, cold results are very odd. I would say re-do the cold test, extremely unlikely the compression pressure will double cold to hot. Hot results indicate all is ok with the piston/rings/bore. Get a hold of a leak down tester and check the valves, although valve leaks are usually more predominate when hot. Have you actually checked the spark to that cylinder both hot and cold? Maybe a high resistance in the PTU or circuit when cold.
Old style valves (30P) from the first batch of VG30DE engine had issues and wore quickly, the valve face wears away to the point where the cam follower runs out of travel due to the valve stem being so high. What happens when the engine is cold is you get piss poor compression, as the engine warms up the cylinder heads expand ever so slightly more than the valves & the valves begin to seat correctly allowing the compression to return. I've seen this at least half a dozen times normally at around 160000Kms on the odometer, car will run like a bag of shite for the first three or four minutes from cold & gradually pick up cylinders as the temp increases. It could be another problem but this is a known common problem & from what I've seen so far in this thread......
Yep. costs little to eliminate these lesser causes first. Of cause this still wouldn't account for the cold low compression results unless the comp tester is inaccurate. It would be interesting to run another cold test to see if the results are the same? At the end of the day, you also have to assess your expectations of the engine. You aren't likely to be expecting power when cold so the results don't really matter until the hot results follow suit ... if you have a bit of misfire when cold but it disappears when hot does that really matter? That is unless you are looking to extract max horsepower or looking for an excuse to rebuild... Eric does a very competitive rebuild price which would be worth investigating if you do consider rebuilding at some stage.
The risk from continued use is that at some point the valves will fail completely, normally chipping a section from the side of the valve off dropping it into the combustion chamber. It depends on how much of a rebuild you want to do, if you are just looking at sorting the top end then better to do it sooner rather than later. If you want to do a full rebuild then you could just keep running it & risk piston, head & bore damage (even turbine damage). Obviously if you build up a spare motor this may be of little concern I know of a few of zeds running around in a similar state, missing when cold then running fine hot, it will run for a few thousand more Kms before any serious trouble & with a VG30DETT the performance will be virtually the same. If you leave it long enough the tune goes all pear shaped resulting in large flames from the exh while being driven hard I was disappointed to loose that feature when I replaced my first bad engine
This is all assuming there is excessive internal wear. Until a 2nd cold comp test is done verifying the 1st results, it could still be spark/injector related cold misfires...
