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Low Compression But Good Leakdown NA6

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#1
wagnerov

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  • Location:Sunnyvale, CA
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  • Car Year:1990
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To make a long story kind of short: I have an STL Miata with a 1990 1.6 engine (Megasquirt, serious Webcams, JE Pistons, etc. ...but stock HLA's that will probably be going away soon so I can raise the rev limit) that has lost compression when the engine got warm.  I cleaned up the head while it was off of the block during springtime, installed it, then cold compression this was ~200psi on cylinders 1,3,4 and ~180psi on 2.  Leakdown was between 4-6% on all cylinders using a modified HF tester (tested at 50psi).  While the head was off I let the pistons sit in Marvel Mystery oil for a month to free up the rings if need be.  The engine had sat for 3 years in a nice garage before I picked it up last summer.

 

Started the thing up today and it ran great for a few minutes with a loping idle around 1000-1200.  I set the timing with the usual Tunerstudio instructions.  It sure sounded mean compared to the stock 200k-mile engine I was running in STL last year so I was excited!  Drove it on the street to clean the cobwebs out of the car, then the idle started gradually going up to 3000.  Pull back into garage and it will barely start again.  I've fought this before.  

 

First thing I do is check compression while the water temp is 170.  It went down to 90psi on cylinders 1 & 2.  Didn't even bother checking 3&4 out of frustration.  And leakdown on those same 2 cylinders is almost identical to the cold reading above (4-6%).  

 

I guess my questions are: are these discrepancies even possible?  If so, what problem do they indicate?

 

I verified the tools worked with a stock street 1993 Miata.  I am also pretty sure I have the leakdown and compression procedures correct after many searches using google and this site.  I have tons of experience with R/C car engines and Formula 500 Rotaxes...unfortunately that's not too applicable to the 4-stroke world.  This is the first 4-stroke engine I've ever worked on myself.

 

 

Thanks in advance for any ideas.  I am completely stumped and would like to figure this out before I pay someone else to rebuild the thing for me.



#2
Kyle Freiheit

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This is only a guess. what is the lift on the webcams? Have you run them with hydraulic lifters before? Age of the lifters? 

 

My guess is that the lifters were over pumping on the big cams. After running for a bit I wonder if the valves couldn't close. When you did the compression test the lifters were still pumped up and the leakdown look like crap. Then some time later you did the leakdown and this gave the lifters time to bleed down and close the valves. Like I said. Just a guess.

 

Kyle


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#3
Ron Alan

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I think this is a good guess ^^^and related somehow.

 

Had a similar experience but different results a few weeks ago. Broke a valve coming in on a cool down lap(right at the top of the spring in a 1.6)on test day before a regional.  A SM rental shop who has about a dozen 1.6 cars had a head, intake, and throttlebody all together sitting on the shelf. The piston had hit the valve twice which bent enough to leave it wedged up in the guide but created no further damage. So I figured it was safe to just yank off my whole top end and drop this spare on.

 

When we went to start the car after reassembly it just did not want to fire(battery quickly died). It just didn't make sense it wouldn't start so I already had the compression tester out(180/180/180/0 on the head that came out)so I checked it...60/60/60/60...huh??? I went back to the guy who gave me the head(not to complain)and told him what was happening. He looked at me, pulled a can of ether off the shelf and handed it to me...if this doesnt work he said,"pull it behind a car". He said "you have no compression because the HLA's arent pumped or pumping up, it needs the air to create compression". No idea if this is accurate but...

 

I went the safer route and pulled it behind a car, dammed if it didn't fire immediately!  Imagine the worst lifter tic you've ever heard in a NA car and multiply that by 16...this is what I had! But by the time it came up to temp and we ran it a few times at 3000 rpms for a couple minutes at a time it purred like a kitten!

 

Bottom line is hydraulic lifters that are not functioning properly affect things! 


Ron

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#4
wagnerov

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I didn't hear any HLA nastiness when starting the engine for the first time.  I had also rebuilt and refilled all the HLA's before putting the head back on.  Before this, I had the head vacuum checked (which took two long months at a reputable track shop nearby) and was told it was good.  

 

This morning I did leakdown again before leaving for work, and heard obvious hissing at the dipstick tube from some of the cylinders.  That would explain a lot.  Why I didn't hear this on Saturday after running the engine, I don't know.  I did clean the Marvel oil from the piston tops before reassembly, however, the little bit left inside must have given me the good compression numbers and a false sense of confidence.  

 

I was never sure of the provenance of the bottom end as it was sold to me.  So the "new" engine comes out again and will probably be sent off to a shop, and the stocker might go back in so I can do the minimum to keep my SCCA license this year.  

 

Thanks for the ideas!






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