Need Help Diagnosing A Non Firing Cylinder.

[Lachlan33]

Hey guys.

So my front cylinder is not doing anything.

It started out months ago, driving along it would occasionaly drop out and I would be on 5 cylinders breifly. Over the next few months it became more and more freequent until now its never working and I have only a 5 cylinder car.

I have tried a new coil pack, I have tried a new spark plug. Yesterday I did a compression test on all 6 cylinders and got fairly consistant results across all 6. It was between 115psi and 125psi across all cylinders. Seems a tad low to me but the fact that cylinder one is not any different tells me it should still be working.

My idea is that it could be the coil loom itself (trying to get hold of a spare) or the injector perhaps?

Anyone care to offer suggestions as to what else it could be? 5 Cylinders means the car runs horribly slow

[Moodles2]

I don't know if this info is useful to you at all, when it comes to the RB26, 121psi is the absolute bottom limit according to the GTR service manual for compression in any cylinder

[Lachlan33]

Hmmm, that is a little troubling if it does apply to the RB25. Not sure if that would cause a dropped cylinder but?

[madbung]

Hola, compression specs for RB25det..

Standard value: 1.20 mpa = 174.0452 psi

Limit: .9 mpa = 130 psi

Variance limit: .10 mpa = 14.5 psi

So accectable = all cylinders over 130 psi and within a 14.5 psi variance each other.

Hope that give you an idea on where you stand.

Edited May 10, 2009 by madbung

[Lachlan33]

Ok...so all being consistant but low would be what? Head gasket? Would it explain why 1 cylinder wouldn't work but the other 5 would?

[madbung]

These are indicators of a blown h/gasket

Car overheating?

Oil in your coolant?

Coolant in your oil?

This will give an indication of whether your compression rings are doing the job or not

Is it blowing the dip stick out when revved (compression in the sump)?

Like always without seeing the car it's a hit and miss diagnosis.

But with your compression test results odds are it's time for a freshen up.

I assume you've removed the coil pack and plug then cranked the car visually inspecting the spark or lack of?

[ylwgtr2]

and we can all assume youve checked for injector pulse?

[MBS206]

Do the compression check again. This time put a few little squirts of oil down the spark plug hole.

If comp goes up, your piston rings are shot.

As to the miss fire. The top loom is a common fault.

[Lachlan33]

Car overheating? No

Oil in your coolant? No

Coolant in your oil? No

Perhaps its rebuild time....... So much for selling the car lol

[GT-RZ]

misfiring will not be directly related be due to low compression.

like said above, check your wiring loom they can get a a bit crispy and stuff up... if it's not that then make sure everything is clean and your spark cannot escape elsewhere. if all is good it's likely to be a fuel problem but i think it's wiring as my first guess would be coilpack.

[Lachlan33]

Just to bump this thread so I don't need to make another.

The update is that the non firing cylinder was related to the injector loom. I have since bought a new one and awaiting it in the mail.

What I wanted to confirm however, was that the way I have done my compression check is correct. Originally I did it with the throttle closed on a dead cold engine. Since then I read you need to do it WOT and on a warm engine. I cannot currently warm the engine up, but I did test one of the cylinders again using WOT, and also smeared a small coating of oil around the rubber ring on the hose that screws into the spark plug socket.

Cranked for motor for a good 5-8 seconds, got around 130psi.

Will doing the test again with the engine fully warmed up result in much of a difference? Is the difference between WOT and closed throttle meant to be large? (Because it wasn't as much as I was hoping)

When I get a new clutch in the coming weeks I will get the shop to perform a compression test themselves, I just want to make sure I have done it right. I guess its also possibly my guage is not very accurate but that might just be wishfull thinking.

[James_03]

What I wanted to confirm however, was that the way I have done my compression check is correct. Originally I did it with the throttle closed on a dead cold engine. Since then I read you need to do it WOT and on a warm engine. I cannot currently warm the engine up, but I did test one of the cylinders again using WOT, and also smeared a small coating of oil around the rubber ring on the hose that screws into the spark plug socket.

You don't need to put oil anywhere on the compression hose. You actually inject a small amount of oil down the spark plug hole directly into the bore and test. I usually turn it over by hand a couple of times as well before doing the first test, allows the oil to move into the rings. The idea is that the oil temporarily fills any minor gaps in bad rings raising the compression, telling you that the rings are bad. If the rings are good there should be no change in results with oil/no oil.