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Compression Testing


WallerZ

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Hi All,

I'm looking to compression test my spare engine that has been sat for nearly 18 months and in the last 3 years probably moved less than a mile.

Any advice for checking the compression on an engine thats been sat so long? Unfortunately I can't get the engine warm but can get it to crank over at least to ensure consistency between cylinders and comparison to my current 3 good cylinders?

For those who are interested, the V6 in the kit currently had compression of 170, 165 and 140 on the running cylinders and 140, 125 and 120 on the ones not running. Apparently a good figure on a warm test is 190-205 psi.

Cheers,

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as an aside, did you recheck the compression with a teaspoon of oil in the bores? if you get higher readings it suggests rings, if they are the same it is likely to be a valve issue, which would be worth investigation before swapping the engine

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If you've got to do a cold test out-of-car then I would just follow the normal steps, all the plugs out, throttle bodies off or wide open, and just crank it over on the starter with the engine suitably braced. As Grim says ^^ a little bit of oil down the plug holes won't hurt. There's little point trying to run up oil pressure first because by the time you've done that you've probably cranked it over enough times to have done the compression test in the first place :) assuming it's been sitting upright with oil in the bottom, no chance of water or foreign body ingress? Might be worth draining and checking the oil first, depending on how long it's been sitting, and a visual inspection with a borescope can't hurt. You're welcome to borrow mine.

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Luckily the other engine is still in the other MX6 with the bonnet on so that will be ok. I'll drain the oil and chuck fresh in, its not like I've got loads from having sorted the oil leak lol.

Thanks for the tip about oil in the bores. I'll rerun the test to check as heads would definitely be an easier and quicker swap than an entire engine.

Dan, I'll see what the oil down the bores gives me today and go from there. Maybe the issue is with valves hence the wonderful chirp I sometimes get haha

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When you say it's only running on 3, do you mean an ignition problem ie. there is no spark or fuel in the non-working three? Or do they all fire, it's just that 3 pistons are down on compression? If they' firing, but backfiring/misfiring then yes, it could be a sticky valve, assuming all the HT leads are on the correct way... ahahahahahaha-ha-ha :rofl:

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When it was investigated at stoneleigh it had new plugs fitted and every possible combination of swapping leads etc was tested to move the problem, and yet the end result was always the same 3 cylinders that the headers stayed cold and weren't affected when the HT leads were pulled from the plug. They are also the cylinders that have shown low compression.

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Guest lotusPaul

Am i correct in assuming that fuel is getting in, but not firing. No matter what plug or lead combo?. This sounds more like a spark issue. So has anyone checked the dizzy cap and rotor arm?

Quick trip to any motor factors and for the price its a no brainer. It will give you a base to start from.

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No distributor in use in this setup. It's running wasted spark with a megasquirt using an edis6. Firing order and spark was checked multiple times by multiple people and issue remained. Also, as it's wasted spark one would expect an even number of faulty cylinders if it was purely spark related.

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Just seems odd to me, it was certainly running on all 6 when I was helping Zac with it at his place (eventually, after we fixed the HT lead order!)... so something has happened after that point in time, and it seems hard to think low compression on 3 cylinders results in no ignition whatsoever. I was thinking it more likely that the low compression on those 3 is more likely as a result of that fact they're not heating up. Don't get me wrong, I'm not denying the compression results are not good, but just wanted to make sure you wasn't chasing one rabbit while there's another one eating your carrots.... ;)

 

edit: Do Grim's suggestion as a matter of course first, anyway. Will point to a sticky valve if nothing else.

Edited by brumster
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I agree it's very odd. I was pretty certain it wasn't running on 6 before and now I think about it, it definitely was because I remember when we fired it up without manifolds on we had fire from each of the 6 ports...

 

but everything tested ok...

 

This needs more thought.

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Which 3 aren't firing? Is there any chance it's one of each coil pair and there's a chance that the coils aren't recovering for the 2nd firing - for whatever reason (EDIS issue, coil pack issue [unlikely], dwell time/configuration, wiring, etc)....?

 

Like you, just looking for some sort of pattern, that's all. From memory, #1 was front-right bank, #2 was front-left bank, #3 middle-right, #4 middle-left, #5 back-right, #6 back-left?

Edited by brumster
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Guest lotusPaul

Ah.i think i may have missed something about the spark control. My appologies.

 

So moving forward i agree theres potentialy a few issues to deal with but sparking should occour in all conditions.hence my trying to solve that first.

 

I think you need to work backwards from spark plug. You say no amount of swapping will get that plug firing, so ill assume by that comment we can determin plugs and lead work fine on every other one thus ruling that out? My next question will go to coil pack, im guessing you run a pair of 3. Can you swap them over to see if the problem still persists? My aim is to see if it moves the fault to another plug indicating a duff coilpack.

 

If you can do this let us know result please.

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Coil pack is a ford 6 way item (3 coil packs in one). Spark was checked from every output and everyone agreed every output was working.

 

Firing order etc was all double checked as well.

Edited by theduck
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