Low compression > runs like crap - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• Low compression > runs like crap by belchfire on 29 Sep, 2008 21:30
• This is a continuation of the "destroy all lifters" post.
    Got it all together but it's impossible to start and runs real rough.
   I thought that maybe I had smoked the injectors when I fried the engine so I got a set of Bosch rebuilt ones. Same thing.  Everything is dead nuts-cam, IP pump, etc.  Did a compression test and it was 265-280 psi. Did a cyl leakdown test and the valves are tight, the rings are OK, and the head gasket isn't leaking between cylinders.   The book says that comp should be somewhere around 450-500 with a minimum of 375 just to fire.
    What's going on here? I replaced a 1 hole gasket with what I believe was a 1 hole gasket. I didn't mic it because everything is same same. The head is flat so I didn't install a big chamber head in it.
    Is there something weird with what might be a Chinese head? Do Hydraulic head gaskets have different coding? Does anyone know the proportional ratio of thickness to compression ratio?
• #1 by jtanguay on 29 Sep, 2008 21:51
• what is your timing at?  does the idle smooth out when the motor is hot?  even with the compression you have, the motor should idle just fine when the motor is hot.
• #2 by gigaz2 on 30 Sep, 2008 04:20
• still sounds like IP is 180º out to me...
  
  all the cylinders have the same compression? that's weird  :?
• #3 by belchfire on 30 Sep, 2008 18:14
• Timing was set @.040"  like the book says. I inserted the IP lock pin between the gear hole and the plate hole. There's another hole 108 deg out but that one's sort of oval and doesn't look like the right one.
  That wouldn't affect the compression anyway as it's merely a fuel delivery function. What could affect things is if the cam were off. I made sure that the flywheel mark truly did correspond with piston TDC and it does. The cam slot will only work one way so that's correct.
    I did a cyl leakdown test and all cylinders were tight at TDC. I thought that maybe there was leakage between two but not so. It runs better when warmed up but that might be due to better fuel atomization. Still idles like crap and I have to pull out the advance to keep it from jumping out of the frame. It runs a little better at speed I think because there is around 5 lbs. boost and that brings the compression up to normal.
     Some thoughts:  Could the gasket have been mislabeled and is actually thicker that the previous one? Are the valves set deeper and therefore increasing the chamber volume? Is the pre-chamber volumes the same between old and new heads?
    There seems to be only .004" difference between one gasket and the next. That's pretty small and it seems that a variety of factors could influence the final CR.
• #4 by monomer on 30 Sep, 2008 18:40
• compression seems to low, way to low
  
  
  My 1.6 mech. was 180-230psi. It never *wanted* to start. The PO is to blame on mine, I'm sure nothing was timed right, and after taking the block apart, I'm surprised it ran to begin with
• #5 by burn_your_money on 01 Oct, 2008 05:00
• I have 250 PSI and it runs just fine.
  
  Take the nut off the end of your IP pump sprocket and make sure that the key is at the 11 o clock position at TDC. It it's at 5 o clock, your pump is 180 out.
  While you had the head off, did you verify that the mark on the flywheel is infact TDC?
• #6 by gigaz2 on 01 Oct, 2008 07:59
• mine was 180 off because I forgot to take note on the key position when disassembled the pump.
  
  what do you have to loose by trying it the other way around?
• #7 by Smokey Eddy on 01 Oct, 2008 17:10
• Are we talking new rings or ... honed or ... bored?
  does it have wrecked rings?
  The wrong gasket could drop your comp by that much too. If you hadn't said the comp was so low and you just put it back together i'd say you have the pump 180 out too.
  

  Quote

  450-500 psi

  Holy Crrrapppp. If mine had that kind of compression... frick. 500 psi... would you need glow plugs at 500 psi!? thats rediculous. My starter would melt if it had to produde 500psi per cylinder. Am i nuts for being so shocked at 500 psi??
  Perhaps the TDI's have 500psi brand new?
• #8 by burn_your_money on 01 Oct, 2008 17:21
• 500 psi is normal for a new engine
• #9 by Smokey Eddy on 01 Oct, 2008 19:08
• it's lots!
• #10 by zukgod1 on 01 Oct, 2008 20:33
• I wonder if your gauge is off, all the cyl measure the sam tells me your gauge is off. That only answers the compression#'s though.