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relative compression test tolerance on 5.4L engine
What is the allowed tolerance of relative compression test? My 08 F250 5.4L has intermittent rough idle trigger by code P0012 and P0022. I took to a ford dealer and they told me cylinder 2 has 9% lower than other on relative compression test. They advised to replace engine with new harness. Is 9% low compression out of spec tolerance. I thought typical error is 10% but I am not sure why they advised me to replace new engine with 9% lower on compression
Yeah 9% doesn't seem like enough to warrant a new engine. I've driven plenty of (pushrod) V8's that were, say, 105 and 125 on different cylinders for years and no problems but a theoretical (and likely practical) loss of power.
Here is the invoice and their recommendations. Replacing engine with new harness is the only option they will do. No other repair they will perform. I did measure compression on 3 cylinders and below is the result
#1: 210 (dry), 245(wet)
#2: 125( dry), 130 (wet)
#4: 195 (dry, 225(wet)
any expert here can make comments whether the engine is a concern?
Not sure where the 9% value comes from but the problem appears more severe reading their description and looking at the relative compression pdf. Thing is, everything they suspect is almost surely true -- it's a 3V
I was faced with a similar situation with my '08 F350 V10 and made the best decision-- walk away and buy an '11+ with the 6.2.
That said, since it's still running you might get away with a timing set only if the roller followers and cams are not yet damaged as they suspect they are.
Worst case, I'd get a second opinion from a reputable shop familiar with Ford 3V's
A new harness isn't going to resolve a low compression issue. Maybe the mechanic misspoke because your compression test results are way higher than 9% low! Make sure the cylinder wasn't washed down with fuel and do a second compression test.
I retested Cylinder 2 and it was 90 dry instead 125. It appears this cylinder compression is not stable. I first tested as 90, retested twice as 125. Next day, tested again twice and both were 90
