Background: 2007 S ~56K miles. Bought used with ~19K miles back in 2009. Suspect the PO was an autocrosser because the car has the limited slip differential and the run flats were beaten when I bought it . Had to change the clutch already at ~33K, which was earlier than I had expected.

Car surges or misses under moderate to hard acceleration/boost, but runs fine at steady throttle and speed. No problem noted on very mild acceleration. Starts fine, idles fine, but does consume oil like its an RX-8. I've already replaced the spark plugs in an effort to solve the missing under acceleration, but that didn't fix the problem. Thanks to the fine folks here who pointed me to Harbor Freight, I was able to score a compression tester with the proper Mini spark plug thread for about $22. Got the following compression readings:
#1 - 115
#2 - 110
#3 - 105
#4 - 108

First rotation typically brought the reading to ~60 psi and then subsequent cranks brought it to the max value.

The kit has a rather long tube leading to the gauge and so it seems the motor turns over about 8-10 time before reaching the max value. No telling what the uncertainty in the gauge is, but the max reading is 300 psi, so a 2% error on full scale could have it off by as much as 6 psi. Haynes manual says min compression reading should be 116 psi - can't find compression values in the Bentley.

Suggestions? Is it time for a rebuild or do I let it limp along until something major happens? The car is a hoot to drive and a keeper.

 

Compression is very similar on all cylinders so I don't think I'd be concerned. But...have you checked for carbon build up on the intake and valves? I walnut blasted my valves at app. 70k miles and now runs great @ 83k miles. Most folks clean their valves around 50K miles.

 

Someone either abused that engine or ran it low on oil for a time, 2007 S had 45K when I purchased it just over a year ago. Perform compression test recently. I have 53k on it at time of compression test, have those valves walnut blasted every 3k0-40k!
#1 - 155
#2 - 151
#3 - 151
#4 - 153

 

Given the sad state of the original tires and the short life of the original clutch, I think the original owner didn't treat it very well. My thinking is that its such a fun car, I'm going to rebuild the motor and give it a second chance at life. I haven't looked at potential carbon buildup, but that is on my mind as well. I wouldn't want to take apart the short block if I didn't have too, but I'm thinking the rings have seen better days. Anyone else have any suggestions before I yank the motor? Thanks!

 

I'm with gregsmini, the values are all similar. I would suspect the compression tester might not be all that accurate. Before all the hassle of a rebuild, I would have the intakes walnut blasted. You're at the right mileage for needing it, and could explain the surging/miss you're seeing..

 

I had similar readings using my harbor freight equipment --- 95 - 110. Shortened the long hose to a minimum length and increased the readings by about 5 psi. Someone suggested that the hose threads are quite a bit shorter than the spark plug threads, so the hose doesn't go into the chamber enough, causing a larger combustion area. Shouldn't make a big difference, but maybe?

I'm in the middle of replacing pistons / rods (for strength, not oil burning) and found no abnormal wear on the rings or cylinder walls. Gotta agree with gregsmini and azdsrt, walnut blast before anything else. Lower end engine work is painful and needs lotsa special tools.

 

Hi Folks, Thank for the helpful responses. I thought about all the replies here, the large volume inside the pressure testing hose from Harbor Freight, and watched some compression testing videos on youtube. So I re-did the compression test and simply let the motor turn over until the pressure stopped rising on the gauge, this should address the volume issue. Here is what I got dry/wet, not a big change from the first time:
#1 118/137 16% improvement
#2 112/133 19%
#3 109/130 19%
#4 110/134 22%

Typically on the first stroke the pressure only went up to about 60 psi. I rather not simply try to clean up the head and still have a ring problem. Given the difference between dry/wet readings, I suspect there is a ring problem and as some of you have suggested perhaps a head scrubbing is in order as well. So I'll yank the motor out and see what is going on. I'll post some pics here as I make progress. I use the opportunity to put the motor back to "new as stock" with any recommended improvements I can glean from here, such as timing chain update and valve cover update. Its too fun a car not to see what it would be like in tip top shape.

 

Before you yank out the motor, why not:

1.) Do a leakdown test, this will at least tell you if it is leaking past the intake valves, the exhaust valves or the rings.

2.) Remove the intake and see how much carbon buildup you have. Especially of the leakdown test shows suspect intake valves.

These are both rather quick to do.

 

This here gives you an idea of what's needed to perform a leak down test.

 

I think the OP REALLY wants to tear that engine apart. Get back that new mini feeling...

 

It's just a valve cover, it doesn't bite.

 

???. I think the OP has more than the valve cover in mind..

 

You don't have to remove the valve cover to do a leak down test.

 

I really think you're jumping the gun a little. I would try some other things before I'd go tearing into the motor. The missing is likely carbon deposits like others have said. You could always go throw it on a dyno somewhere for $75. That would give you an idea how much power the motor is making and whether something is wrong or not. Compression numbers can be difficult to compare since they can vary greatly depending on whether the motor is hot/cold/wet/dry/etc.

 

I was only referring to a leak down test, it's easier to manually turn the cams/crankshaft in order to get the pistons to TDC.

 

Definitely test with plugs out & throttle plate open.

Make sure battery is fully charged