Engine miss fire

[romfordray]

Engine light came on and engine appears to run rough.

Had major service done with new spark plugs and new coil on cylinder 1.

CEL light swere reset.

Engine ran fine for aout 1 week (les than 200 miles) but CEL light came back on.

Went back to service centre and they said something wrong with cylinder 1, possibely need new coil. Pointed out coil and plug already changed, they checked out teh coil was ok and changed the plu. They say it is unusual for the plug to go, and I should check if miss fire continues.

1. Can the ngine miss fire and the engine light not come on.

2. If it's not the coil or plug it must be the cylinder (piston rings, valves ?) is that normal on a 70k miles car and what would a rebuild of he cylinder cost /take.

[Loren]

1. Yes, if it corrects itself within a period of time.

2. We would need the fault codes in order further diagnose the error and potential problem area.

[romfordray]

1998 Boxster 2.5 Manual.

The engine light came back on so I went back to service centre.

They put the diagnostic tool back on which indicated mis-fire on cylinder 1.

Plug had been changed and ignition coil changed, so they swapped over the fuel injectors on cylinder 1 and 3. They also did a compression check on cyclinder 1 which was fine, reset warning light and cleared all faults.

Car ran good for about 1 day when light came back on, returned to the garage and they hooked up the diagnostics again which reported mis-fire on cylinder 1 again. They did a temperature check and cylinder 1 was cooler than the other cyclinders.

Garage have advised that all external parts have now been checked and appear to work ok, only option now is to take the head off and check the valves. They believe that problem may be caused by a broken/weak valve spring. Taking the head off is around 18 hours labour!

Is this possible/has anyone heard of anything similar And is there any other checks that should be made before the head comes off?

[ar38070]

When they changed the coil does that also change all of the wiring? If not maybe there is something wrong with the wiring from the ECU to the coil pack. Thinking about it I believe the wire is separate.

I had this problem once myself (1998 2.5 too) after a plug change. Turned out to be a loose connection between the coil pack and the wiring harness.

[romfordray]

It doesn't appear to be a wiring problem. The problem existed before they fitted new plugs and coil. And since being fitted they replaced the plug on cylinder 1 and moved the ignition coils around, checked all connections and made sure it was sparking correctly.

They can't think of anything else external to check and only option seems to be to take the head off!

[ar38070]

The fact that it runs okay for a while then acts up seems to me to point to an intermittent electrical problem. If it were a head problem you would not think that it would take a while for it to show up.

[romfordray]

I don't know anything about teh engine/electrics, but why does it only mis fire on cylinder 1 when the plugs are changed and the ignition coils changed round. There would need to be another connection that is unique to cylinder 1 which is failing. What could that be?

Surely a sticking valve can be intermittent?

[Guest Sir.Rhosisoftheliver]

Had this happens as well and it was just the connection to the coil its self. Cleaned it all up, used some contact enhancer from Wurth and all was well again.

[romfordray]

The connectors were swapped around when they moved the coils.

So it can't really be the connectors and there doesn't seem to be anyother wires/connections that can be checked out.

A weak valve seems to be the only diagnosis. The missfire is worse if I have a lot of stop/start operation. Normally if I leave it for a few hours after it has been running rough it runs fine. It's normally ok on a long run as well.

[ar38070]

There is a wire from the ecu to the coil pack. I do not know if there is a connector between the wire and the ecu but there probably is. There is a connector between the wire and the coil pack. Either of these connectors could be at fault.

Also the wire itself could be at fault. This wire would not be moved from cylinder to cylinder unless it was disconnected from the ecu. If they actually disconnected the wires from the ecu and swapped the wires and the problem did not change cylinders then okay it is something wrong with the cylinder. It seems unlikely to me that they would do that. Most times (I am not sure about the boxster) the wires are different length. They usually can only reach one particular cylinder so swapping them between cylinders is physically impossible.

When I had my misfire (loose connection) the engine ran fine until I pushed it hard. If I took it to redline and held it there for 30 seconds or so then it would start misfiring. If I never took it to redline it would run for hours without any problem. After the CEL came on if I shut the car down and let it cool off (15-30 minutes), disconnected the battery to clear the CEL, it was fine again.

[romfordray]

Thanks for this advise, I'll go back to the garage and get them to check the connectors and wires from the ecu. I want to make **** sure taking the head off is a last resort, 18hrs labour is a lot of money!