engine trouble, need help

03, C4S, 80000km. First problem was a single misfire on #4 cyl. CEL reset and car drove well for a day and a half, then continuous misfire and shaking, not accelerating properly under load. Car taken to shop, replaced plugs and swapped coils, problem continued. Compression checked, came back slightly low, leak down check completed but was OK. Slight hiss observed starting engine, now thinking broken valve spring-but no continues sounds/ticking etc observed. Injector cleaner added, car sits for a while, runs fine! Test drive all ok throughout whole rev range. Comes back to shop, "hesitation" rough idle starts again at 4500 rpm....#4 cyl. Any ideas? Just bad injector? What and how to proceed? Thanks

Prob worth swapping, prof. cleaning, or replacing that injector...

Spoke to my mechanic today and injector is coming out. Hopefully the replacement will cure the problem. He's still a bit uncertain on what could have caused a low compression number, but good leakdown check (possible sticky valve-unrelated). Hope to know more soon.

Valve seat could cause this problem. Before my engine let go I lost power once and the car stalled. I restarted it and the car was fine for a couple weeks before the seat failed completely and ruined a cylinder. Good luck finding the issue, hope its something that can be found and repaired easily.

If you added an fuel additive (injector cleaner) and it helped I'd suspect you have water in the fuel system. Look for Dry Gas. You probably got a bad tank of gas. I had an old Nissan that would occasionally behave exactly as you described after a ton of rain or if I washed the engine. I thought the engine was toast...instead I add some dry gas it it clears right up. It's a cheap option to try. -Shawn

Just curious what the compression and leak down numbers were and whether it was leaking through the valves or the rings?

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#4 was down to 150, but leak down came back "perfect", compression was thus not checked again Idling car at around 3000 for a bit. then going higher revs, cures the problem and engine runs well keeping revs in higher range, when letting it idle again for a bit, problem returns, again only to #4. Would expect dirty/water fuel to cause issues across the range and not isolated to one cyl. When the car runs well, there are absolutely no odd noises coming from the engine which excludes any permanent problem with valves etc. I'll post results when new injector goes in. fingers crossed. In the mean time still welcome any ideas.

What were the other cylinders at compression wise? What does the plug look like in #4 compared with the others? Were these tests performed on a warm engine? Leak down is a static test. The theory goes, if the leak down comes out good but the moving test (compression) comes out low, that can point to an issue with ring seal. Conventionally, one way to check for this is to squirt a little oil into the cylinder and repeat the compression test. If the comp numbers go up this can help prove that. This is not a conventional engine though, and we're being told not to expect conventional results... so take the above info for "normal" cars. Hopefully a new injector will wake it up.

Not trying to alarm you -- just keep informed - hope this is not the case. Possible coolant in #4? Cracked cylinder liners have behaved similarly. washes out oil on rings if it sits and drops the compression? check plugs for evidence of coolant. maybe boroscope some have had issues with starting, i.e. it sits and coolant pools in the chamber others have had issue after its been running, heats up, opens crack and coolant enters chamber.

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I'm with others wanting to know what the plugs looked like. Could be an injector. Maybe plugged or not opening completely. The short/long term fuel trims can help one know if an injector is suspect. The DME will attempt to adjust the fueling to all cylinders of a bank even though only one injector of one cylinder is bad. Thus if an injector was not opening properly or injecting a full dose of fuel the short/longer trims on that bank might be on the rich side and the DME added more fuel to every cylinder of that bank. With the proper diagnostics equipment a tech can kill spark/injection to each cylinder in turn and id the weak cylinder. It is the cylinder that causes the least amount of rpm drop when the cylinder's fuel/spark is disabled. It has this effect because being weak the cylinder contributes less to the engine's total output than the other cylinders. However, you already have a misfire code for the #4 cylinder and the misfire code says the DME has already id'd a weak cylinder. You can try a new injector, I guess. Were this my car, I think I'd prefer a more definitive diagnosis made though but as long as your ok with the injector replacement idea. Bad fuel or water in the gas might not affect all cylinders the same. The #4 cylinder (in this case) might be the weakest of the 6 cylinders and bad/watery gas pushes it over the misfire edge. Additionally, when the CEL comes on we tend to alter drastically how we drive the car, run the engine, and the other cylinders thus do not get a chance to manifest misfires. But see what the new injector does for the symptom. I assume the coolant level is ok? No signs of any oil droplets in the coolant tank? Have you considered removing the oil filter housing and pouring this oil and the element in to a *clean* drain pan just to see what you can see? What you see or do not see will determine what you do next. Sincerely, Macster.

It's also possible that the injector gets stuck open and drowns the cylinder, washes the cylinder walls down with gas and the oil with it -- lowering the compression. You'd think you see this with leak down test, but maybe during your compression test the oil gets back into the cylinder rings and forms enough sealing not to have a leak during the leak down test? keep us posted. Mike I'm pulling for a FI issue -- way cheaper and easier to fix.

Thanks for all the replies. The injector is on it's way and should go in tomorrow if all goes well. I'll get back in touch with mechanic and ask some off the questions posted and will pass on some of the recommendations too in case they were missed. Keeping fingers crossed....

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Were there any other codes with the misfire codes? Were there any pending codes? Some OBD scan tools might have an upper limit on the number of codes they can read. IIRC the OBD spec allows for 64 codes per controller but a scan tool maker might not be able to read/process more than a handful of codes. (The most number of codes I've seen reported by one vehicle is 6.) Sincerely, Macster.

As far as I know, there were no other codes. Haven't been able to talk to mechanic yet to ask more questions. Thanks again for your feedback, it's much appreciated.

Welllllllllll wanna race? If it is something bad then maybe it would surface like Mobi Dick or Jaws in the process.

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sure...what's the worst that can happen? At least give me a car length so you can enjoy the show when the engine sh.ts itself. As for update: Porsche in their infinite wisdom sent the wrong injector so car will be in shop a bit longer. Should know more towards the weekend.