18RG Mystery Misfire issue...

[shelldrake]

Hey Guys,

Helping a friend out and trying to get a resolve with a misfire issue on an 18RG in her celica. Not blowing smoke. The oil isn't too black. No apparent oil/water contamination.

The symptoms: Misfire. rough running, lack of power, will not rev out

The diagnostics: Pulling the plug leads individually revealed the cylinder one makes no difference to the idle. Lead number four makes a very minor difference when disconnected. Two and three makes an obvious difference. The spark at the leads is strong. Plugs were changed. Coil was changed. New dissy cap and rotor. Ignition is aftermarket fireball ignition. Has an optical pickup and slotted disc inside dissy. Checked, cleaned. Swapped leads to different cylinders. Checked ignition timing. Issue remained on problem cylinders.

Compression is low, but within one psi - Approx. 120 psi on all four.

We dismantled carbies and inspected. All looked cleaned, but I rebuilt, blew out all channels with compressed air. Cleaned very thoroughly. Set floats again. Issue remained on problem cylinders, although sounded better. Switched the carby positions. (Back to front, front to back cylinders) No change.

Exhaust temperatures indicate no event in cylinder one (approx. 150 degrees C) I believe it was just localised temperature. Cylinder two and three were both between 250-350 degrees C. Cylinder four was around 200 degrees C. After about 5 minutes of running.

The cam timing appeared perfect when I turned it over by hand. Having said that, they are welded then reground cams. I have not checked lobes with a dial indicator, but they measure very close from base to peak with a set of verniers. Valve clearances are definitely tighter than spec and indicate an issue.

Intake (Spec- .009-.013) Exhaust (Spec- .011-.015)
1 .007 .009
2 .007 .009
3 .006 .010
4 ZERO! .011

The cam cover has grooves from contacting the timing chain. See pic below. The chain also looks polished on the top side, so it's been caused by the current chain. The upper tensioner is tight. I checked and adjusted.Any thoughts before I pull the head off?

[river]

Hi,

Can't borrow a set of stock cams from someone?

How long has the engine had this problem? Did one day it just start doing this?

seeyuzz
river

[shelldrake]

The motor was purchased out of a write-off. Apparently the car was running fine for months after the transplant, then after a decent drive it became difficult to start. It was limped home and when I had a look at it a month or so later it was found to be only running on 2.5 cylinders.

The cams are a regrind to the most aggresive "stock" profile. But yes - This may be an option. At TDC the slots in the cams sit dead vertical with the lobes on cylinder one pointing outwards.

[George]

Considering the grooves it can be easily out of timing, for example timing chain jumped a tooth or two. Besides maybe the re-grinded cams should be installed in different way than stock ones? I mean different position of marks at timing gears.

There's yet one stupid reason for misfiring, extra air into intake manifold after carb. If the engine runs well at full throttle/full load this is ultimately the case, check for vacuum leaks.

One more pain in the arse - head gasket could be broken in a number of fancy ways. So for example adjacent cylinders may get connected and no water/oil loss will occur at the same time. Or water could leak into some cylinder but don't spoil the pan. Check for happy bubbles in the radiator.

[S. Lowlux]

KISS, Air leak on intake manifold to head. Check all the vacuum hoses for splits & leaks including the brake booster. PCV valve & plumbing.
Why pull the head off when all your compressions are good.

[w810sc]

The symptoms: Misfire. rough running, lack of power, will not rev out

Compression is low, but within one psi - Approx. 120 psi on all four.

Intake (Spec- .009-.013) Exhaust (Spec- .011-.015)
1 .007 .009
2 .007 .009
3 .006 .010
4 ZERO! .011

Hi Eddie
there's 2 of your problems there^^ 120psi compression (minimum is like 140 psi right? anything lower and I would say it needs new rings) and zero valve clearance on intake valve 4, the cam could be keeping the valve open at all the times - but in saying that, you got 120psi in all cylinders so the valve might be seated.