Across the board drop in compression - possible from cleaning cylinder deposits ?

[va3ux]

During the past 4 months, I've seen an across the board drop in compression in my engine. Also for the first time in my engines history, beginning 4 months ago, I started using fuel additives. In May I started using FP60 and used it constantly until July when the 90 deg hot weather hit. Pinging was bad enough in July that I switched to 89 octane for the first time ever and that stopped the pinging. Then, based on posts here, I went on a bit of binge using fuel system cleaners : Redline SI-1 (two full bottles at shock dosage stretched over 3 tank fulls), then Shell Top Tier gas for several tanks here and there, then two full treatments of Techron (shock dosage), and finally back onto 87 octane and FP60. Which brings us up to today. No pinging and the engine runs great, although it always did run great anyway.

Concurrent with all this nonsense, I've been on an Auto-RX cleaning program that started in May and is still under way now (ARX Rinse #3 currently in progress). Because of that, I've been doing compression tests every 4 - 6 weeks just out of curiosity. When I started in May, compression readings were essentially 175 - 185 for all 6 cylinders. Now : 160 - 175 for all 6. They all seem to have dropped the about the same amount. I haven't ruled out that my old compression tester isn't the cause of the drop since it is common to all testing (and ti got dropped once). The other possibility is that the rings in all cylinders have deteriorated equally during this period, but I feel that this is unlikely (probably).

But it also possible to remove enough combustion chamber deposit from the cleaning programs I used, that the compression would actually drop as much as I'm seeing ? I can say this : when I started in May and looked down the spark plug holes, the piston tops looked all rough and irregular. During this week's conmpression test I was surprised to see several pistons with smooth flat tops, and I could read numbers and letters stamped into the tops that I'd never seen before.

Engine : 2001 Chrysler 2.7L V6 w/ 157,000 miles. Compression ratio essentially 10:1. Factory compression "specs" : minimum 100 psi, 25% variation maximum !!!

Thoughts appreciated.

Phil

 

[ebaker]

I'd guess the motor's fine and the guage is off. Maybe the valve on the compression tester is leaking down a little.

 

[JAG]

I'd borrow a different tester because I'd be freaking out if I were you to know what's really going on. Usually deposits reduce ring sealing, not improve them. Ask yourself if the deposits on the piston tops were thick enough before to increase the compression ratio and therefore the compression readings. They'd have to be quite thick to do that but you did say it was pinging a lot before so it's possible. In the meantime, go find another tester.

 

[va3ux]

quote:

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Originally posted by JAG:
I'd borrow a different tester because I'd be freaking out if I were you to know what's really going on. Usually deposits reduce ring sealing, not improve them. Ask yourself if the deposits on the piston tops were thick enough before to increase the compression ratio and therefore the compression readings. They'd have to be quite thick to do that but you did say it was pinging a lot before so it's possible. In the meantime, go find another tester.

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Well, I think I will get another tester just to confirm. I'm not freaking out but I definitely would like to know what's going on.

Were the deposits thick enough ? Who could tell ? There's no perception of depth when looking straight down on a deposit. But technically any deposit at all reduces the CC volume at TDC, so the compression ratio would be increased to some extent.

 

[cfromc]

We're talking cc's here so some deposits can raise the CR. My last set of AFR heads were spec'd for 64 cc chambers. A reduction to 58 cc chambers would have given me too much compression while an increase to 72 would have given me too little. Just a change of 8 cc's either way would have led to undesireable results. For example: a 383 Chevrolet Stoker motor could yield CR of 9:1 with a 68cc head (depending on gasket thickness and piston +/- cc) yet yield a CR of 9.8:1 with a 60cc head (and the same gasket/piston). Big difference. If you're already at 10:1, just a little bit of deposits would be too much.

 

[va3ux]

Excellent. Thankyou. I had no feel for what sort of CC volume might be typical. In round numbers we'll say my compression dropped 6% overall average. If I had say 65cc CC volume in my little engine and we therefore say there has been a 6% increase in volume due to deposit removal, that would only be around 4cc volume increase. Not very much when you consider the deposits are on the underside of the 4 valves plus the CC roof plus the piston top. So it is possible !

quote:

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Originally posted by cfromc:
We're talking cc's here so some deposits can raise the CR. My last set of AFR heads were spec'd for 64 cc chambers. A reduction to 58 cc chambers would have given me too much compression while an increase to 72 would have given me too little. Just a change of 8 cc's either way would have led to undesireable results. For example: a 383 Chevrolet Stoker motor could yield CR of 9:1 with a 68cc head (depending on gasket thickness and piston +/- cc) yet yield a CR of 9.8:1 with a 60cc head (and the same gasket/piston). Big difference. If you're already at 10:1, just a little bit of deposits would be too much.

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[vizvo]

One clue would be to compare "before and after" emissions results. If NOX is down, then cleaning deposits probably lowered compression ratio. Also HC should be a bit lower too (carbon deposits absorb raw gas, and keep it from vaporizing and burning thoroughly).

By the way, your compression results are just fine, and absolutely nothing to fret over.

 

[Gary Allan]

Well, I doubt that you're destroying your engine with accellerated wear in that short amount of time. Since your compression is fairly even ..at least within an acceptable range ..then I don't see the problem. That just tells me that any compression numbers were not from poor ring seal or coaked/stuck rings ..but that you did have deposits. I'd also doubt that all your valves decided to lose their seal at the same time either

 

[mechtech2]

It makes sense that removing the deposits would have this effect on cranking compression.
Or, you wore out your gauge from over use!

 

[andyd]

it pinged before,you gave a dose of cleaners. Now it doesnt ping. As you said, there was an across the board 6% drop in compression. I'd say the cleaners worked. fine, and that the engine is prolly in better shape. Pinging will hole pistons if it is severe enough. The engine should also pass NOx emissions better too as a result. good job.

 

[va3ux]

quote:

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Originally posted by Gary Allan:
Well, I doubt that you're destroying your engine with accellerated wear in that short amount of time. Since your compression is fairly even ..at least within an acceptable range ..then I don't see the problem. That just tells me that any compression numbers were not from poor ring seal or coaked/stuck rings ..but that you did have deposits. I'd also doubt that all your valves decided to lose their seal at the same time either

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Yea, I agree Gary. And I do know that the "new" compression numbers are actually OK on their own. If I'd never done a compression test before this most recent one, I would still be satisfied with the numbers.

The whole thing would be quite different if one or two cylinders dropped but the others all stayed at the original values.

 

[tom slick]

get yourself a leakdown testor, they are more accurate.

 

[Big Jim]

Try not to confuse compression ratio with cylinder pressure. A carbon build up can significanly raise the compression ratio. It may or may not increase cylinder pressure, particularly through all running conditions.

Cylinder pressure is the force that actually drives the piston down and it is dependant on many factors in addition to compression ratio. Cleaner cylinders, better ring seal, better breathing from unshrouded valves among them. If you are feeling an improvement in power then it is because you have increased cylinder pressure as a result of your cleaning. The increased cylinder pressure may not be present at cranking speeds where the added carbon may have given a boost at the expense of causing other things that cut cylinder pressure at higher engine speeds (and actual operating conditions).

 

[Steelhead]

NO pinging is a GOOD thing for long engine life... predetonation kills pistons, rings, wrist pins, rods, rod bearings, crank bearings and heads
pinging in not only causes by high compression but also red/white hot carbon deposits on top of the pistons and on the head combustion chamber

excessive carbon buildup in the ring packs raises ring to cylinder wall contact pressure pushing up friction losses and wear

careful on the carbon cleaner injector cleaner use....many do wash down the cyl walls and cause short term increased wear....Chevron has recommendations for frequency of use of their strong Techron Concentrate...
can increase injector wear also
some have components that have lubricity

water injection combustion chamber cleaning is safe on all components if done at slow rate....very effective carbon remover...low cost

 

[va3ux]

quote:

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Originally posted by Big Jim:
Try not to confuse compression ratio with cylinder pressure.

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There's no confusion with the terms Jim. We're talking about the cylinder pressure measured during a cranking test, with the engine essentially acting as an air compressor (no combustion). Under these conditions, "cylinder pressure" is proportional to compression ratio.

 

[va3ux]

quote:

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Originally posted by Steelhead:

water injection combustion chamber cleaning is safe on all components if done at slow rate....very effective carbon remover...low cost

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Steelhead - I'm aware of of the water injection cleaning method but I don't any specifics. Typically, what volume of water, at what rate and for how long ?

Thanks...Phil

 

[Big Jim]

quote:

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Originally posted by va3ux:

quote:

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Originally posted by Big Jim:
Try not to confuse compression ratio with cylinder pressure.

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There's no confusion with the terms Jim. We're talking about the cylinder pressure measured during a cranking test, with the engine essentially acting as an air compressor (no combustion). Under these conditions, "cylinder pressure" is proportional to compression ratio.

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I'm gald you understand. I often find confusion in this area. I was just trying to point out that there is more involved.

 

[Steelhead]

hi phil,
i use an 1/8" i.d. tube from a qt or L water jug/bottle into the throttle body plate hole or air gap at 2k rpm into hot engine...prevents hydrolock...takes 10>15 minutes to injest 1q/L
cleans everything from head comb. chamber to cat. converter including exhaust valves and EGR valve

cliff

quote:

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Originally posted by va3ux:

quote:

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Originally posted by Steelhead:

water injection combustion chamber cleaning is safe on all components if done at slow rate....very effective carbon remover...low cost

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Steelhead - I'm aware of of the water injection cleaning method but I don't any specifics. Typically, what volume of water, at what rate and for how long ?

Thanks...Phil

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[Louie's gone fishing]

Remember that a compression tester also tests your battery and starter. Sounds like your on the right track to figure out the readings. In other words, there is no problem, and a few more miles will prove it.

 

[va3ux]

Thanks Lou.

I'm going to re-test at the end of this ARX Rinse Cycle, and I'll use the original tester plus another one to compare readings (2 tests per cylinder). This will be interesting, and I'm fairly sure that I am not diagnosing a real problem at all.

At present, the car runs like new, it has 158,600 miles on it, and I'm getting absolutely outstanding gas mileage : 31.8 mpg from a 2.7L V6 on a full size car (2001 Intrepid), when driven steady on the highway with the cruise set for 110 Km/hr (68 mph) for hours on end. 3 tankfuls in a row with the same numbers +/- 0.2 mpg