lazy cylinder - low compression good leakdown

[trond]

I have an issue with my engine, since I assembled engine last summer. New Webcams 464/465 with new rockers, gasflowed heads and new set of 9.5Cr 98mm JE pistons.

Car is difficult in town because the problem with a faltering #6, but rev happy once throttle is applied. It pulls strong from 2500, shines around from 4500 and pulls eagerly to 6500

Leakdown numbers are all 2-3%
Compression numbers are 175-165 across the board except #6 with only 150 psi
I checked valve lash 5 times and nothing wrong
I had the manifold off and there are no vacuum leaks. Anyway it is a plenum manifold so not relevant..
Swapped plugs and cables around and no difference
In despair I swapped the entire ignition system, every component including plugs, spark plug leads, injectors and even the ECU with harness (upgraded to MS3X) but the problem with #6 is unchanged.

As part of troubleshooting I found cylinder 6 will idle with a lot higher than normal ignition advance



Before I surrender and take apart the engine again now at peak of short and hectic driving season,, is there any advice on next steps from the forum ?
What can it be ?

[stownsen914]

Can you describe what it’s doing that makes it difficult driving around town?
150 psi is indeed notably lower than the others, but I might be impressed to learn that alone makes the car run poorly. How sure are you that the running issue is traceable to cyl #6 ?
Have you double checked your carbs for a plugged jet, etc?

[trond]

at low loads and idle #6 is not running. Plugs (twin plugs) on #6 only are wet. About at 2000 rpms cylinder#6 comes on as well. With a slightly leaner mixture than normal and more ignition advance I can bring #6 online but driveabilty suffers. I don't have carbs but fuel injection- checked function and spray pattern from injectors and even swapped them around. The ECU has a test function that allow testing each individual injector and they all work. I also tested ignition with stroboscope and all cylinders are identical and stable. So not ignition related and not fuelling related

[Black 993]

Have you looked inside #6 with a borescope?

Also, it doesn't make sense that you have low compression but the leakdown test is fine. Unless there is something wrong with the cam lobes for that cylinder, it seems impossible that your leakdown results would be 2-3% but compression is ~15% lower in that cylinder.

[trond]

so engine out then.... painful
I failed to cc all cylinders on assembly, only did two to verify CR with the new pistons and assumed the others to be identical. They usually are
If not then cam related

Originally Posted by Black 993

Have you looked inside #6 with a borescope?

Also, it doesn't make sense that you have low compression but the leakdown test is fine. Unless there is something wrong with the cam lobes for that cylinder, it seems impossible that your leakdown results would be 2-3% but compression is ~15% lower in that cylinder.

[stownsen914]

I agree with bore scoping.

If there is some issue lower in the cylinder causing poor sealing, it would show in a compression test but not necessarily in a leakdown test. I’d scope it before tearing down though. Never know what you’ll find.

[pocv0]

Lower compression on #6 but the leak down is good ? Strange. Curious - you say the JE pistons are 9.5 CR, so the compression should be roughly 140 psi. Were the cylinder heads identical ? Were the the heads machined at the seal surface ? I know this sounds crazy , but perhaps the # 6 cylinder is off CC wise. Your compression is too high in my opinion based on the JE 9.5 CR . Something is off.

[trond]

Got a new compression tester and got
#1=165psi
#2=165psi
#3=160psi
#4 180psi
#5=165psi
#6=170psi

don't understand how the numbers can be so high; 165psi/14.7=11.2. However when I assemble the engine I measured 9.5CR

I was not able to see much with my boroscope

Unfortunately the stumbling and faltering persist but issue must be elsewhere

Originally Posted by pocv0

Lower compression on #6 but the leak down is good ? Strange. Curious - you say the JE pistons are 9.5 CR, so the compression should be roughly 140 psi. Were the cylinder heads identical ? Were the the heads machined at the seal surface ? I know this sounds crazy , but perhaps the # 6 cylinder is off CC wise. Your compression is too high in my opinion based on the JE 9.5 CR . Something is off.

[PeteKz]

Those numbers look more reasonable. Compression tester gauges are frequently off on accuracy, especially the cheap ones. They are still good for checking that the compression is about the same across the cylinders, and your case they are. The one outlier (180psi) might have some combustion deposit buildup inside. If you go drive it and then retest, the numbers will be different, but should still be within 10%.

[stownsen914]

I know you did the injector test with your ECU, but I wonder if that really rules out a bad injector. For example, could your #6 injector be hanging open? That could explain the wet plugs and poor low RPM driveability.

[stownsen914]

How hard is it to swap injectors? One test that could prove revealing, would be to swap the #6 injector with another cylinder. And see if the problem follows the injector to the other cylinder.

[al lkosmal]

Originally Posted by stownsen914

I know you did the injector test with your ECU, but I wonder if that really rules out a bad injector. For example, could your #6 injector be hanging open? That could explain the wet plugs and poor low RPM driveability.

This....

[trond]

i did swap injectors between cylinders, and also bench tested them all for function and spray pattern and I flushed them with cleaner both directions (no clogging observed)
I played with dead times in the ECU with no change to behavious. As the other plugs are on the dark side, closer to black I need to check the wideband lambda too. Hard to use a lamda at all with a misfire that will screw up readings. If one cyl is misfiring then that looks like lean on the AFR readings and easy to overfuel...

But yes tune (fuel or spark) related most likely. Just can not find the problem

[mikedsilva]

check wiring and earthing to #6 injector and cop.

I had a faltering ignition issue once and even though I had paid good money for a custom harness, I found that one terminal on my injector (coincidentally also #6) was not crimped at all and made an intermittent connection.

Still, u say youve swapped all ign and still it's number 6, which suggests something wrong there. Might have to check cam timing again.

[pocv0]

Or it could be a bad injector.You have eliminated everything else swapping out the wires, etc. I've had faulty injectors fill the combustion chamber.IF the injector is sticking and pulsing intermittently it could foul the plug possibly. You said the plug is wet on number 6. Swap out the injector.

[ahh911]

Originally Posted by pocv0

Or it could be a bad injector.You have eliminated everything else swapping out the wires, etc. I've had faulty injectors fill the combustion chamber.IF the injector is sticking and pulsing intermittently it could foul the plug possibly. You said the plug is wet on number 6. Swap out the injector.

If it was me having spent so much time on this I'd move into 100% verified mode, I'd remove the injector rails each into six jars and maybe the spark plugs as well, find a way to put it into test mode, or have a separate crank pulse generator to mimic run mode and make sure spark and fuel quantities and spray are okay on all six at different rpm. I think the poster has already swapped injectors and much stuff. Also, I'd video the operation closely, looking at spark vs spray on each cylinder to review closely.

If you have a maf record the voltage at high and low rpm and feed it into ecu during above test or potentiometer if it's resistive etc.. Some work, but...

Phil

[shoooo32]

I installed a set of 9.5:1 98mm JE pistons in my SC - my compression values are 162-168psi at roughly 600ft elevation.

[Dpmulvan]

Did you try swapping spark plugs? Bad plugs do happen

[trond]

I did swap plugs yes, and I found a dead one too.

Now, last night I made a test plug, cutting away the side electrode. When testing for spark I found spark to be yellow, and not blue as anticipated.


Troubleshooting continues but slow progress as I am alone in the garage mostly so hard to move along. I somehow thing it may be related to the ignition side, as MS logs seem to be fine. Except obviously for the AFR jumping all over the place and some fluctuation in the RPM as well. No strangeness on the tooth logger, or sync losses on the composite logs. No weirdness in any of the MS logs actually

Originally Posted by Dpmulvan

Did you try swapping spark plugs? Bad plugs do happen

[Dr J]

I know it’s a pain but check that the manifold bolts are tight in cylinder 6.