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Discussion Starter · #1 ·
Background: My '08 smart is at 91k miles. I've been noticing poor gas mileage (about 20% less than normal) for about a year now since I moved from high altitude to sea level, about 2k miles ago (might be a coincidence). My driving style I regularly get 45mpg, but easily push it 50 or higher on highway drives. At high speeds (80+) it appears to get normal fuel economy (around 35mpg); but I never stay there long. The car has run rough at idle since birth, so I haven't been able to tell a significant difference lately; although I did just have to get the exhaust flex pipe repaired a little less than a year ago, so that might be an indicator of running rougher or just the bad climate I moved from. No codes except for an intermittent EVAP system code; & that hasn't reappeared in a while.

I saw Torque report normal fuel economy on 2 recent trips (less than a week ago; verified in torque logs), so was hopeful; but it's returned to crummy again this week. Anyway, I decided to swap the plugs (still original) along with some other routine maintenance (tranny fluid, coolant flush, etc.) on this long weekend since I'm due for the oil change; and after doing a temp-run up on the new coolant I did a compression test while it was still pretty warm (almost too hot to touch).

From left to right: 170 145 170 Clearly I've found the culprit

The plugs all look great, a slight brownish color for all 3 of them; but all widened out to 0.045 gap or a smidge more (that's the biggest gauge I have). Sure, a little oil in the threads; but could be my fault from DIY oil changes & doesn't look like any burned oil on the plug tips. Because the plugs all look great I'm not sure a wet compression test will tell me much. I can post pictures if it'll help diagnose. I don't have a leakdown tester.

The only other possibly relevant tidbit is this car slowly eats coolant since birth. I mean super slow, about 2-4 oz every oil change. Engine temp has never gone above 210°F (that was on a long haul through imperial valley at 125+°F outside). Never any obvious leaks, no skimming in the oil, & no oil in the coolant so either seeping into a cylinder or a hidden leak in the undercovers. Supposedly MB did a compression test at 30k miles under warranty & all came out fine; could never find the "leak".

So I'm wondering if I've already got a burned valve or if there's some hope. I'm thinking of dumping a bottle of sea foam into the cylinder & letting it soak overnight since I'll be changing the oil anyway. I suppose a compression test after that shouldn't hurt; but I'd be afraid to actually run the engine until I changed the oil -- thoughts?

But am I just wasting my time?
If it's a burned valve I probably don't have the time for a rebuild, so it may be time to trade it in or get someone to do a restomod & make it electric for me (with reasonable range -- I can't deal with a 90 mile range!!!). If I can recover the cylinder I'm probably up for the valve clearance adjustment; and I've got a motorcycle I can ride in the meantime to avoid further damage (although that's a bit of a gamble itself here in Los Angeles).

Any helpful thoughts would be appreciated.
 

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Discussion Starter · #3 ·
jetfuel..luk….
Agree - I've had the new Denso's waiting for this since 25k 😅. I also see a EV conversion is too much hassle. If it ain't recoverable then onto the sale-block -- too bad, I tweaked everything to fit me perfectly over the years.
 

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…you’re in the right path…seafoam or Marvel Mystery oil…I’ll probably go as far as to flood that cylinder with injector cleaner in the compression stroke…let it soak and drain (blow out)…
…carbon build up is nasty and keeps valve from sealing…
….borescope inspection can tell a lot

jetfuel…but that’s just me
 

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Discussion Starter · #5 ·
…let it soak and drain (blow out)…
1 bottle soaking in there now; didn't think about the compression stroke so it took all 16oz to fill to the top of the plug hole. Oh well, $10 well spent if it works. Figure I'll let it go overnight or maybe even a couple days. Looking into a borescope so I can try to see if a valve is actually burnt; fingers crossed.

How would you drain? I was thinking of pumping out what I could, then letting it turn over a few times w/o the plug to push the residual out? Then I can take a compressor to it from the plug hole & pray I push out any debris. Clearly I didn't think through the "incompressible liquid" 😔 before starting down this rabbit hole.

BTW, I see you all over this forum all the time - thanks for being a rockstar & so helpful!
 

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…in the compression stroke to use the least amount…a small hose (3/8) probably will do and a syringe or turkey baster to suck it up…with the coils disconnected and all plugs out a few cranks will finish the job…let dry…

jetfuel…roaming the asylum since ’08
 

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Discussion Starter · #7 · (Edited)
Interesting, looks like the seafoam has already drained at least all the way out of the plug hole -- I'll get out my pump to check the cylinder shortly. It also appears to be in the oil now (went from golden brown / ready to be changed, mobile 1 btw, to 100% clear & smells like watered down seafoam). Presuming that's what happened (I don't think it would evaporate that fast, but will test a capful); that means it slipped past the rings? It's light stuff, but pretty sure the rings are supposed to hold gas :confused:. Maybe that wet compression test is in order :unsure:

Update: I pumped out about 8oz, so it hadn't all leaked out. The fluid was now yellow vs. clear & there was a few decent sized black carbon chunks my pump picked up, so on a whim I went ahead and turned it over a few -- more dirt came out the plug hole than I could imagine (along with the seafoam I could not get w/the pump)! It's like the whole cylinder must have been shellacked with carbon. Re-ran the compression test & it now reads 170; but that could be just the seafoam sealing the rings :unsure:. Hmm, how to safely "wash out" the rest of the carbon...

Now I'm considering doing the same cleaning on the other 2 cylinders if the compression on #2 still holds in the morning.
 

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Discussion Starter · #9 · (Edited)
The dreaded number two cylinder.
I did a quick search - have you seen other posts with low compression on #2?
Possibly relevant:
Most helpful, also interesting side note in posts 43/69/81 RE Intake: Surprise! My Car is about to Die (NOT)
Suggestions to use MMO/etc in gas, no report if it worked: FAQ: - Number 2 cylinder low compression
#2 out of whack a bit, no report if MMO worked: Cylinder head, valve and guide need replacement?

Likely Irrelevant:
Engine overheat (all 3 low): Dealer repair, part rebreaks, compression low.
Blown HG: $1200 passion update number 1
Reverse problem, but much worse & gave up: Compression Test Results
 

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Discussion Starter · #11 ·
Type in : burnt valve
Ah, thanks - search terms are key. Borescope on the way so I can check without tearing it down; $30 for HD 5.5mm model from Amazon. Compression dropped a tad this morning (to 160); may not be relevant / measurement error or could indicate rings are the culprit. Still soaking in Seafoam for now; decided to soak all 3 cylinders with as much junk as I saw out of the one already.
 

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Discussion Starter · #13 · (Edited)
Update: scotch laiden, I decided to pump out the seafoam & re-test the compression in all 3 cylinders. Learned a LOT!

Results: 192 192 194

Pump Carbon results: Large chunks observed only for #3

Paper-towel carbon results cranking 3x (from plug holes): #1 produced what I'd expect from head buildup, basically a small smattering; #2 still produced more than #1 despite the previous cleaning; #3 produced zero? perhaps because a valve was open when pouring in seafoam & the plug hole didn't fill with seafoam like #1 & #2

I'd consider this a "wet" compression test, since I'd just pumped out the seafoam & cranked it 3 times to spit out what I could. I've left it to "air" overnight since that clearly reduced compression results on #2 over last night, we'll see how it does in the a.m. Since my initial test, I learned "wet" compression test is relatively agnostic of engine temperature, as the "wetting" is occurring from inside the cylinder vs. outside. Essentially the incompressible liquid "leaks" through the rings slower than the test, so you get a "near perfect" reading if the leakage is caused by the rings.

I also learned that you must pay very close attention to how the compression gauge is attached. In this test I'd initially received pressures as low as 120 psi!!!! Turns out the tester hose was cross threaded! I received intermittent values of 120, 143, 165 & 180 while re-seating the hose. The key was to count the number of turns needed to "screw it in". If it felt off, high or low, then be ready to repeat the test by re-seating the hose from the beginning as it was likely cross-threaded or otherwise "hung up". I got lucky as I tested #2 first & read 192; as the best # so far I knew anything lower on #1 or #3 was probably bogus (despite the scotch). It's also possible carbon was still propping the valves open -- unless someone has experience here we will likely never know 😋. Oh, and ensure you have a good battery charger hooked up; so the "cranks" don't slow down due to the charge level!!!

Results? #2 still experienced significant carbon buildup; perhaps a "feature" of the engine design? It's possible the fuel economy drop had nothing to do with the compression b/c of the inconsistent readings; we will know more in 12-24 hours. At the end of the day, I might have cleaned a bunch of carbon buildup that was causing higher compression & frankly a fuel economy boost because my initial compression reading could have been inaccurate -- we shall see once I test tomorrow & ultimately get it back on the road. Clearly my 1.5 mile commute to work is killing the car 😏.

Anyway, oil dipstick is now reading .25 quarts above max; presumably from the seafoam. Curious of any thoughts on doing a warmup & round-trip short 1.5 mile in this condition or if I should just stop being lazy & drain a bit :oops:?
 

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Discussion Starter · #14 ·
Update: 194 182 187; but I no longer trust these compression readings or the original ones. It seems any little speck of carbon or dirt in the threads or o-ring seal & the numbers are way low. Perfectly threading the hose I got results as low as 98! Over 20 attempts with tons of compressed air to get those readings 😕

Definitely got some of the seafoam into the exhaust b/c it kept spewing white smoke for 30 minutes at idle; so had to take it up on the freeway to clear it out. Went ahead and drained a bit of oil b4 doing that just to be safe.

Car still runs as rough as ever at idle, maybe worse, but perhaps smoother above 1500 rpm. I was happy to see 45-55mpg popping up on Torque driving city streets back; not definitive of course, so will hopefully get a longer test in this weekend.

Still to come: Borescope (pretty sure valves are fine based on compression tests, but hey it's fun); Dry Compression Test (final say on a ring problem in #2, if I can get consistent test results); & fuel economy check
 

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...I'll take it you put the new plugs in...right???...
...other stuff that "I" will try but you don't have to...
...with the throttle body intake hose removed...key on...full throttle...key off...the butterfly should stay open....pressurized cylinder with valves closed and listen for leaks at intake and exhaust side with some home made stethoscope ( hose and foam cup)...
...rough idle?? ... OK at high rev........vacuum leak??? can of starting fluid will find the leak..
...warm engine and running rough?.......at idle..pull one coil plug...stop running?? ...good coil
...next coil plug...same treatment until it keeps on running with the suspected coil pack plug removed ....if a bad coil pack acting up...

Jetfuel...please read disclaimer and assumption of all risk that comes with performing any kind of work associated with maintaining any and all high performance engines in the back of this page...
 

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Discussion Starter · #16 ·
...I'll take it you put the new plugs in...right???...
. . .
Jetfuel...please read disclaimer . . .
Yeppers 😏. Came gapped at 0.040, so re-gapped to 0.035

Good suggestions

Never knew you could "pin" the butterfly open, & I have an auto stethoscope I can try - haven't used it in decades. Those high compression #s do lead me to believe the valves are probably good; I shouldn't be able to get a bad "high" reading unless I completely misunderstand how a compression test works. A good next step if something looks fishy on the borescope & doubles as an excuse to clean the throttle body. (y)

Hmm, it is possible the system had a vacuum leak since birth. Might even explain the occasional EVAP system code. Where do you "inject" the starting fluid? Intake or on the hoses / joints? Mist everywhere & hope to see a draw? I should have thought of that today when the seafoam filled my garage up with thick white smoke o_O

I might try the coilpack test for sure; easy enough when I have to uninstall plugs anyway. I was always suspect of coilpacks since my mustang had a huge coil; seemed like it was "designed on the edge" with little margin. Works for everyone now, so can't really argue I suppose :censored:

Honestly happy if I just get back to average 42-45mpg & know it won't quit for another 50k or so. Might have to do the valve clearance check/adjust regardless per what I'm reading on here :unsure:

Oh, and I love it; almost as as good as Smart's "crush warning" anytime you jack/lift :LOL:
 

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…a very light spray on the hoses..fittings..and unions one at a time will make the rpm go up if not properly seated or hole worn through…
…which Mustang???

jetfuel…nothing here…too early
 

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Discussion Starter · #20 ·
still got my 65 tho…
Sweet ride! :cool:

Driving Test Update: Merely 2mpg improvement; 40mpg at 60mph over 20 miles. It's possible torque's calibration was off, but this was a relative test to the prior best 38 mpg result. I'm used to 43-45 at 60mph.
Test Update: Compression test in same conditions as first (hot engine): 171 172 170; note: new spark plugs did have some oil in the threads - not unexpected since I filled the plug holes up with seafoam; but could be some other leak :cautious:.

Conclusions: Either initial test was incorrect or debris blocking valve was successfully removed; no collapsed ring & valves good for the moment. Cause of mpg reduction not fully identified :confused:

I started using the borescope, but the thing's battery died. I can tell you it wasn't pretty; lots of rust, carbon & even oil on the face of all pistons & 1 valve I saw 😬. Will re-attack with full battery tomorrow; but maybe I don't want to know :rolleyes:; will also do the coilpack test. Tons of junk in the spark plug hole; not sure how to clean it out as compressed air has obviously done what it could; probably mineral oil is making it stick -- thoughts?

Clearly need to do the vacuum leak test as well; might wait for the serpentine belts that are on order to come in...
 
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