I’ve recently been having some rough running on start up which lasts for about a minute or two. The head temperature is noticeably lower than the others during start up, again recovering after about two minutes.
Upon investigation it would appear there is no compression when cold on No. 4 cylinder (all other cylinders ok) however when preforming leak downs with a hot engine only a few hours ago they were all ok, in fact number 4 was the best with only 15% loss.
If I look under the rocker cover on no. 4 it appears the exhaust valve is not fully closing, by measuring the end of the rocker arm (valve side) to the side of the engine case, it appears to be 2mm lower than the others. This is also confirmed by visual comparison of the valve washer to the rocker case to other valve washers vs the rocker case.
The roller lifers have been replaced with new on that cylinder and still the same problem (I thought it could be a clogged up lifter not bleeding down)
The engine is 3.3l gen 3 roller cam, with hollow push rods.
The engine has been running perfectly well up to some recent head work after low compression was noticed on no. 4 about 5 hours ago.
It would appear that the push rod has grown but how is that possible? The engine temps are very stable (always below 180degrees CHT and 700degrees EGT)
I've noticed that the new hollow push rods are 205mm where mine has 207mm, have you changed the length to compensate for a reduced cylinder length and maybe my push rods got forgotten about?
But why would that only affect one valve?
Any help on this matter would be appreciated.
Hi,
I've consulted with the engine experts on this one...
Original hollow pushrods for early Gen3 roller lifter engines were 207mm. They were progressively shortened to 206mm, then 205.6mm. Most engines were fine running the longer pushrods, but some were not (valves being held open) – hence the shortening of the rods. The change of lengths didn’t have any relation to cylinder length as this generation engine has only been built with 107mm pots.
I’m not 100% sure why after the headwork performed this one valve is being held open. Possible reasons could be:
- Lifter movement restricted by the manifold retainer or the lifter slot itself. Retainer may possibly be too tight against the lifter, preventing the lifter from riding hard on the cam lobe. This could explain the difference between hot and cold leakdowns.
- Head recession on the side of the offending valve effectively making the pushrod ‘longer’.
Suggest we measure where the top valve spring washer is located when the head is installed to indicate the load on the valve while installed. Uninstall the head and remeasure the washer location. If the washer is in the same position (no relief/same load), the problem must be in the head (sticky valve guide, problem with valve spring, valve seat, etc). If washer indicates less load (as would be expected), problem is not in the head and possibly the first suggestion above.
Otherwise, it's not unknown for lifters to get a little grumpy for no good reason... swapping the lifter out is a relatively cheap / simple thing to try.
Apologies for the slow reply, things have been too busy lately.
Regards,
Doug.
