L28E running on 5- getting frustrated!!!

[nzdatman]

Hi all, I'm really starting to run out of ideas here, so thought it might be worth putting a post up here for help.

My 280Z is running on about 4 and a half to 5 cylinders. 1,2,3 and 6 seem to be firing as normal. 5 seems to be running, but the header pipe is noticeably cooler than the others. Cyl 4's header pipe is stone cold.

 

First I checked for spark, it seems to be OK. I have swapped plugs and leads around, and checked the cap.

Second I checked the injector wiring, all seems good, have swapped plugs from 5 and 6 onto 4, makes no difference. I removed the fuel rail and injectors and turned the engine over to check that all injectors are squirting and not blocked.

Third I suspected it might be a blown head gasket between 4 and 5, so compression tested it, results are

#1- 125psi

#2- 125psi

#3- 125psi

#4- 125psi

#5- 95-100psi

#6- 125psi

 

So obviously it has a low cylinder, but it wasn't no. 4 as I had expected. It might however expalain why the header pipe on no. 5 is cooler than the rest.

 

I haven't done a leakdown test as I don't have the equipment to do it.

 

Starting to run out of ideas I removed the rocker cover to check the valvetrain. It looks to be ok. The valve clearances on no. 4 feel a little looser than some of the others but not widly out of spec and if it was I would have expected it to show in the compression test results? I intend to set them with a feeler gauge next but the way it went from running ok to not without apparant reason has left me a bit confused. I am tempted to pull the cylinder head off to see what's going on but I want to eliminate the easy stuff first.

 

Any ideas on what it could be or what to try would be very much appreciated, thanks.

[ezzzzzzz]

If you've checked all you claim then it sounds like the distributor is failing you at the applicable cylinders. I'd move all of the plug wires over one position on the cap and reset the timing. If it is the distributor you should see failing on different cylinders depending on which way you move the wires. Could be a tooth or two broken inside the distributor.

Edited May 3, 2010 by ezzzzzzz

[PapaSmurf]

also what does the plug look like in cyl 5? Compare it to the others. Is it fouled with oil? Possibly valve stem seals or rings/bore. When was the last time you adjusted your valve lash?

[nzdatman]

thanks for the replies. Haven't adjusted lash at all but the car has hardly driven much in 10 years. I will adjust the clearances but I don't think they are wildly out.

Plug 5 looks slightly wet, black but not too oily.