• TomB
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  • From: Austin, TX
  • Registered: 2004-03-27
  • Posts: 1,095

Topic: MexiBeetle Compression Test 56,000 miles

Some months back I started to change plugs on my 2000 MexiBeetle, and didn't do the left side because it's just hard to get at the plugs there with all the Mexi fuel-injection hoses, rails, wiring, and the A/C compressor and hoses adding to it on that side. I thought maybe I'd wait until I dropped the engine. However, with a little free time today, and realizing these plugs have never been removed from the engine for 30,000 miles I realized I shouldn't wait. The plugs would be wearing larger gaps, which stresses the wire insulation, etc.
Part of the problem with this setup is that the FI hoses sit just in the wrong place for my short socket extension, and the longer one is too long. But I found the factory cheapo plug wrench really fits better - so I used it. You may say this is a poor design - making it hard to get at the plugs; but on the other hand the FI means you don't NEED to touch the plugs for 25K or so. Here's what all the plugs look like after 30K use:
http://i110.photobucket.com/albums/n106/TomB_TX/Plugafter30K.jpg
While I had the plugs out I decided to test compression, since I've never tested this engine, and I've heard the stories claiming the Mexi engines wear out in 50K. I have an unusual system for testing compression, based on an industrial data acquisition system I designed. I put adapters made from old spark plugs in all cylinders at once, with 1/8" instrument tubing going to the pressure inputs of my data system, then record all the pressure signals at once as the engine is cranked. Here's how the Mexi looks:
http://i110.photobucket.com/albums/n106/TomB_TX/Mexi56KCompressionTest.jpg
With this method you see the full pressure profile as the engine cranks. You don't get the final reading building up after several revs, but if you have sticky valves it will show up as differences between peaks of the same cylinder over several cycles. This plot starts with cyl 1 in Blue, then 4 in Cyan, 3 in green, and 2 in red. So cyl 3 is a bit lower than the others, but still within new spec limits. (For the Mexi, new compression is spec'd as 116 to 145 psig. These range 140 to 151.
This is after 56K in Texas heat as my daily driver, with lots of 70 mph freeway, mixed city, with AC running most of the summer, with common temps 90-100F. It still uses no measureable oil, doesn't leak, starts instantly, runs great. If you get a chance to get a Mexi - DO IT!

Last edited by TomB (2009-04-11 13:34:40)

  • From: Cincinnati, Ohio
  • Registered: 2005-09-10
  • Posts: 2,336

Re: MexiBeetle Compression Test 56,000 miles

Tom,

Thanks for sharing all this info...looks like you really gave us some great "long-term" diagnostic data.

Your troubles getting to those plugs due to the fuel injection reminds me of how hard it was to get to my plugs on my old Type III Squareback.  With the fuel rails, the injectors, and the wiring going to each injector it was pretty tight.  Plus unlike a Beetle, with the Type III Squareback you don't have much height to deal with either, due to the floor in the rear cargo area being just inches above the "pancake" engine.

Your engines compression numbers look pretty good with 140psi for the weakest cylinder.  Those "new" Mexi compression specs seem sort of wide (116-145psi)...116psi would seem sort of low to me for a cylinder on a new engine.

Yeah that plug looks a bit worn (the electrode is pretty well rounded & stubby).  Did you happen to measure the gap on that old plug to see how far out of spec it was (or maybe it wasn't)?

Thanks again,

- Nick

Last edited by Bug In My Nose (2009-04-11 14:55:26)

1979 Super Beetle Convertible
  • TomB
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  • From: Austin, TX
  • Registered: 2004-03-27
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Re: MexiBeetle Compression Test 56,000 miles

Plug gap is specified as .028, and after 30K were about .042. The ignition on these cars is triggered by the ECU, and has no problem with the larger gap - but it does stress the wires and such. This car still has the original wires, dizzy cap and rotor after 8 years and 56K!
These are just plain NGK copper core plugs. Modern cars (like my Touareg) go 30-40 K on plugs, but use Iridium plugs for wear resistance - at $13-18 per plug!
These plugs did show signs of blow-by past the ceramic - but so do the Iridium plugs after 30K.

  • From: Cincinnati, Ohio
  • Registered: 2005-09-10
  • Posts: 2,336

Re: MexiBeetle Compression Test 56,000 miles

TomB wrote:

Plug gap is specified as .028, and after 30K were about .042.

I figured that the gap would be pretty large...not sure I figured it would be that much!  Good thing the ECU can compensate for the change in gap.

Just imagine how rough a carb. engine would be running (if it would run) with a spark plug gap .014 away from spec.!

- Nick

1979 Super Beetle Convertible