lazy cylinder - low compression good leakdown
 

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 ?

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?

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

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.

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

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.

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.

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

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%.

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.

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.

This....

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

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.

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

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

Did you try swapping spark plugs? Bad plugs do happen

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

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

I have still not resolved my misfiring problem on my new built 3.2SS with 9.5CR and Webcams 464/465. Dual EDIS waste spark ignition with one VR sensor to both of the EDIS ignition modules (dual spark). One ignition system runs upper plug set and the other set runs the lower plugs. PIP to ECU from one ignition module, and SAW from ECU is split to both modules. MS3X ECU batch fire injection. I still have the misfire on #6 cylinder at idle and low loads. It runs fine with more load and RPM
1) Leakdown on all cylinders 2-3%
2) Compression tests on cold engine
cylinder #1 - 165psi
cylinder #2 - 165 psi
cylinder #3 - 160psi
cylinder #4 - 180psi
cylinder #5 - 165psi
cylinder #6 - 170psi
3) Plug chop; all plugs indicating lean except both #6 plugs which are both much darker and wet
4) Checked for false air (including loose manifold and hoses)
5) All twelve plugs spark nice blueish
6) Checked opposite plug #3 that is running off same coil. #3plug has fine color (#6 is wet) - so coil pack should be fine
7) Stroboscope shows stable spark on all 12 plugs, with identical timing
8) Runs equally bad on only upper ignition circuit only and on only lower ignition circuit
9) No noise on MS log files, except for the obvious fluctuation in AFR signal caused by the misfire
10) Replaced all spark plugs - no change
11) Replaced all spark plug leads on upper plugs/ignition circuit - no change
12) Tried separate power&ground direct from battery to ignition - no change
13) Tried running fixed 10 degree advance (SAW disconnected) -no change
14) Disabled Accel enrichment entirely, to prevent fuel dumping from noise or pressure fluctuations - no change
15) Tooth logger and composite logger in Tunerstudio look fine, but tried adjusting on the pot meters anyway -no change
16) Swapped injectors around, flushed and checked spray pattern- no change
17) Tested with different injector dead times - no change
18) In ECU test mode all injectors can be tested individually and all 6 are cycling and clean opening signal on the oscilloscope - so no bad connections to injectors
19) However with the engine idling injector opening signal to cylinder #6 has noise, an additional spike at as the injector closes
20) Swapped in a set of much smaller injectors, to get longer opening times and less influence of the error - improvement and was able to go on the annual Fjordluft event after all
21) Plug chop on return 500 miles later has 5 lean running plugs and #6 still too rich

What to look for to resolve the noisy opening signal to #6 ? All injectors have common 12V supply and individual switching ground in the ECU.

You may be onto something to look at the noisy signal on your #6 injector.

One thing you can look at if you haven’t already, is what’s different about the wiring/harness for that injector. Running close to a sparkplug wire or coil, bad ground, etc. SOMETHING is different than for the other cylinders.

Also, might be worth getting on the Megasquirt forum - there should be a ton a knowledge there for random yet not uncommon issues like you’re having.

When you swapped the harness, that included all the leads to the injectors, right? And you've moved the injectors too and the problem stays with cylinder 6? And you replaced the ECU and the problem remained with cylinder 6?

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I would put another 1000 miles on the motor and make sure its broken in before I would take it apart. Maybe it will self correct.

Before you start taking apart the engine, get an inspection camera that connects to your phone, that you can insert through the spark plug hole and take some pictures.

At this point, those are cheap enough that everyone ought to have one in their tool kit. They are extremely useful for many other applications too,

I am embarrassed to say that my engine running issue seem to have been related to me not having set up the ECU correctly for the RPM signal.
When I ordered the MS3X I had it set up from DYIAutotune for VR sensor. But when I finally got around to installing it with EDIS some 4 years later I had completely forgotten, and for some reason I don't want to discuss I did not check and re-configure jumpers on the board.
Nothing wrong with engine after all. Engine now runs on CDI/distributor ignition and is smooth as silk and powerful. It idles stable at 1000 rpm and 7 degrees advance. There is still a stumbling issue between 1500 and 3000 rpms, that I am confident I will be able to iron out, but above it is smooth as silk and has good power

Thanks for the follow up.

Good to hear you got it sorted out. Lots of variables with the MS system.

Good you got it sorted