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King of Smart Gadgetry
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Discussion Starter #1
When you run a compression test on a healthy 451 engine, what would the readings be? 150 pounds? 160? I haven't been able to find the answer anywhere on the internet. Surely the specs for a new healthy engine could be found somehwere.
Here on the forum a search of compression test gives a whole bunch of scenarios with readings ranging from 125 to 200 lbs for reading that people got when testing their engines, so I haven't a clue as to what it should be. DCO
 

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That 15 bar converts to 217 psi, which sounds high to me....:shrug:
That sounds a little high to me as well, but remember, it's a 10:1 compression engine. At the lower limit it's 159 psi.

I know on my Dodge turbo motors we like to see 150-160 psi, and they are only 8.4:1 compression, so 217 isn't really out of the question...
 

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Discussion Starter #5
I wanted to get to the bottom of it so I bit the bullet and called my "not so local" Smart center and they connected me with their head Smart mechanic and I asked him the same question. He quoted me exactly what JWIGHT said, but he too said that seemed a little high.
So now I'm gonna spill the beans......Before I tore my 451 down to get the valves and guides replaced my commpression readings from cyl 1 to 3 were 155-150-105. So I had the head done with all new guides, valves and seals. I assembled it and it seemed to run okay. I haven't driven it much yet. Now just for the heck of it I thought I'm gonna pull out those plugs and see what the compression readings are now. So I did and I got 120-120-120. I was flabbergasted. Now I re-tested again with another compression gauge just to be sure and I got 120-125-130. So since they are pretty consistent I would "assume" the valves are right and adjusted properly. So with 36,000 miles and the cylinder look great with no scarring and there's no smoking and no noticable blow by of any kind. So that's why I asked because I figured 150-160 was par for the course, but after reserching it here nobody quotes a compression test reading of 217 pounds. Now my original 155-150-105 was on a hot engine and my latest test was on a cold one,but I doubt hot to cold would cause 30 pounds difference. So I'm disappointed and anxious and don't know what to do next. If I drive it like this and the readings are indeed acurate the power is bound to be lower. And secondly I don't like running something that's not right. So maybe I'll just go and buy a new compression gauge and test it warm just for my peace of mind.
 

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Leakdown test?

Before the head was replaced on our 2011 the technician found cylinders 1 and 3 leaking 5%, but cylinder 2 leaking 75% - hence the P0302 code. Might want to check for leaks as well. :)
 

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Just my 2 cents if the valve seats were reground this could create a compression lose, and it has been my experience that a warm engine does relate to higher compression.Smart should advise you what is proper procedure to checking compression.
 

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What about carbon deposits within the combustion chamber? If there were deposits when you tore the engine down, this would cause the reading to be higher. I wouldn't worry about the readings that you recorded. If the car runs well, drive it and enjoy it.
 

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If you just put the engine back together then I would not get too wound up about the compression readings. I would drive the car awhile and see how it runs. After a few hundreds of miles to get all the new parts familiar with each other, then test the compression with the engine warm and the throttle fully open.
 

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Discussion Starter #10
Thank you Rusted Wrench. I was waiting to hear from you. Smart Bob said the same thing. I'll take your advice and finish buttoning it up and drive and see how it goes. The underneath is so clean and shiney that I hate like hell to hit that first mudhole or drive it on a rainy day. You might read in the paper that I hit myself in the head with my crutches......twice. Thanks everybody !
 

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Just got back from a 1400 mi. round trip to Austin, down I35. Traffic really moves on that road and my cruise was set at 73 with frequent passing runs up to 80. My plugs had over 50k on them, so when I got back I decided to change them. They looked perfect and I hated to throw them away. Decided to check the compression while I was at it and all three checked at 160 psi. This is on an engine with over 60k on it and absolutely zero oil comsumption. It's a great little engine!
 

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Discussion Starter #12
I buttoned it up tonight. I starts instantly and has no leaks. It idles smooth and the engine is noticably quieter and is so smooth idling I find it hard to believe it's the same engine. The smoothness reminded me of the old packard 6's. I thought it was noisy when I first started it, but it was the air pump.....DUH.....Since I had the tailgate off I decided to go ahead and install the LED turn signal bulbs and resisters that I've been sitting on for a while. Also had some wiring to fix that was damaged in the ECU compartment while at the Smart Center.
So if it quits raining I pull it out tommorrow and drive it, no need to dirty up the underneath until I just have to......right? DCO.
 

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I did my Smart with 80K miles and got 170-172-180 on compression cold.

Leak down was set to 100 psi and held for 20 minutes.

I still get constant P303 fault codes.

Changed plugs numerous times, changed the gap .028, .030, and .035, tried Iridium's, changed coil packs, cam sensor, crank sensor, checked all grounds, plugged and unplugged the ECU, disconnected the battery for an hour. All three injectors tested and serviced by RC Engineering tested perfect flow patterns. Air filter new. Always Mobil 1 0-30 oil. Been using Amzoil PI fuel injector and valve cleaner for almost a year.

I find it hard to believe that at 170 compression would trip the P303 code so easily.

I also noticed my engine idles a bit rougher than on my 20143 which has 155K miles on it.

Any suggestions??
 
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