4.6is low compression cyl 3
 

Discovered today that my intermittent misfire in cyl 3 is something serious. Low compression. Found out after changing sparks and coils and the miss got worse after being better for a day or so.

I think having plugs and coils that work at optimum has just revealed the real weakness where ever it is.

It now goes into engine fail safe mode on warm starts and I have to use inpa to reset the adaptations. The fault code is now not misfire but something else, I'll check it later. but as it's a head off job either way, I have stopped trying to diagnose it and braced myself for a big bill. Should have bought from a dealer, but you live and learn.

I'm giving to a trusted mechanic in the week but does anyone know what the possibles are?

Valves or piston rings. Valves will be the cheaper. You can perform a leak down test and isolate if it's valve or rings. Hissing sound at the intake manifold will be an intake valve, hissing sound at the tail pipe is an exhaust valve, hissing sound from the oil dip stick is a piston ring.

What were the compression readings? Dry or wet?

Add head gasket to the list.

How long has there been an intentent miss? Has the engine overheated prior to the beginning of the problem?

Did you actually take a compression reading or are you assuming it's low compression because you replaced the coil and plug. Other possible cause for a miss is a bad injector. Have you rule out that you don't have a bad injector? Computer will shut down the injector if there's a misfire to prevent damage to the Cats.

For some reason I wasn't emailed to say this thread had replies so I missed them. Sorry - I would have replied earlier.

The compression readings were taken by a local mechanic but they didn't have availability to do the repair until a few weeks. They are not my 'regular' mechanic anyway, I was just driving past when the EML came on so pulled in for a fault code read and it went from there. I took the car away and it is booked in with my regular garage.

A second garage confirmed it had low compression in that cylinder - its LPG converted and the I went back to the garage that did the conversion. It was a previous owner that had it done and the LPG installers remembered the car, they said the previous owner came back to them for a misfire and they found low compression on cyl 3. Thats as far as they went. The owner obviously sold the car on, and that person was the guy I bought it from. Private sale so no warranties.

I don't know what has happened since the fault began because it was obviously in existence when I bought the car. What I have done I have definitely exacerbated it but I drive my cars hard.

When I bought it, the misfire was undetectable - no EML, drove normally. It only appeared after 3 weeks of ownership and after I had thoroughly tested its performance for those 3 weeks.

It doesn't overheat but it does have a coolant leak from a lower rad hose. Its leaking only drips at a time but i guess if this leak existed prior, it could have caught a previous owner off guard and potentially it could have overheated in their ownership. This isn't confirmed though.

None of the mechanics think there is a problem with the block, they think its contained in the head, but who can say fro certain until they pull it apart.

Because its LPG converted, it has 2 fuel delivery systems and 2 sets of injectors - the regular petrol and the LPG, the fact it does the same exact thing on both systems means I can rule out injectors, fuel pump, filters, bad fuel etc.

are you seeing more condensation from the exhaust than usual?

No, exhaust is quite dry, and same on both sides

No white, blue or black smoke when you rev it to 4-5000 RPMs?

Nope, no smoke at all.