Nvmedic

626 2.0 low compression

5 posts in this topic

Hi guys. Been reading a lot of threads but never posted. I have an issue now and im looking for ideas (confermation really) my 97 626 2.0 blew a lower rad. Hose and over heated. I replaced the head gasket due to low/no compression. Now engine is very hard to start. Worse when cold. Once its started it drives good. Did a compression test and this is what i got. 
#1=90 PSI
#2=40 PSI
#3=40 PSI
#4=90 PSI

WET TEST 
#1=120 PSI
#2=50 PSI
#3=40 PSI
#4=100 PSI

My question is rings? Or warped head? Maybe both?

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You can continue guessing as to what the cause is, or you can continue to diagnose, your choice.  Get a cylinder leak down tester, or search online one of the many ways you can rig up your air compressor and compression tester to keep a constant pressure on a given cylinder.

 

Starting with it at top dead center, rotate the engine by hand with a breaker bar (all other plugs out for ease).  Oil cap should be off, radiator cap, throttle plate open, and ideally a second person to listen to the exhaust.  As you rotate the engine, it should be easy to figure out where the leak is

  • if it's coming out the oil cap, you have a lower-end problem (rings, cracked ring land, hole in piston, crack into an oil galley, etc.) that is allowing excess pressure to escape into the crankcase.  This will happen best in the positions where both valves are closed, but may also happen slightly in other positions if it's bad enough.
  • if it's coming out of the coolant or bubbling the coolant, you know you have either a bad head gasket, warped head, improperly torqued head fasteners, cracked coolant jacket, etc.  again, this should happen worst in positions where both valves are closed (compression stroke and ignition stroke).
  • if you have an intake valve issue (bent valve, broken spring, damaged seat, broken off valve head, badly enough worn guide, etc.), you should hear air leaking out of the throttle plate in positions OTHER than the intake stroke.  Additionally, the intake stroke may exhibit poor flow as the intake valve may also not be opening correctly on that stroke.  the intake tube should be off and throttle plate pinned open for this exercise.
  • if you have an exhaust valve issue (bent valve, broken spring, damaged seat, broken off valve head, badly enough worn guide, etc.), you should hear air leaking out of the throttle plate in positions OTHER than the exhaust stroke.  Additionally, the exhaust stroke may exhibit poor flow as the exhaust valve may also not be opening correctly on that stroke.  this may be difficult to hear or feel all the way at the tailpipe, I advise disconnecting the exhaust after the exhaust manifold to observe.
  • you can simply rotate the cylinder to TDC with both valves closed (end compression/beginning ignition stroke) and put 150psi or so on the leak down tester and try to listen, but the above method of rotating the engine through each stroke is the absolute gold standard definitive diagnostic method.

 

Since you JUST did a head gasket, the absolute first thing I would check based on your symptoms is your timing belt alignment marks.  The timing being off could be a likely explanation for your low compression and hard start.  Often numbers will rise on a wet test simply because putting some oil directly into the cylinder like that is going to help ANY car seal the rings better, it is not a normal condition for that volume of oil to be on the cylinder walls above the level of the first ring.  If you are doing the above cylinder leak down test, a timing issue will exhibit no leaks into the coolant or oil, but leaks into the intake and exhaust at inappropriate times of rotation (usually 45 degrees or less of ratchet rotation before/after you are expecting TDC/BDC based on the fact that it DOES start it can't be that far off).  It is very easy with a belt to set the slack to the wrong side and look like you're on at first, and end up being 1 tooth off.  And with hydraulic tensioners, there is always the possibility of it skipping a tooth on start up due to low oil pressure.  Always double check (and this takes even less time than above).

Hope this helps, sorry if it seemed a bit harsh.  But continuing to guess at your problem is either going to lead to a junkyard trip to get a head and BINGO! or you back on here in a few days, even madder at the car, more frustrated, and a couple hundred less in your wallet.  In my personal experience of 10 years in an aftermarket shop and my own side customers, I have never seen a 2.0 Mazda warped head, or many aluminum warped heads for that matter.  It is true they can overheat and distort, but in my experience as they cool down they tend to find straight again if they were torqued correctly (ask anyone with a 302 Mustang at the track with aftermarket aluminum heads...throw head gaskets at it in the pits, let it warm up and they seal right back up).  Older iron/cast heads, those can definitely warp and tend to hold the warp.  Even the Hyundai 4cyls that are infamous for taking out head gaskets, never had one warp a head.  Not saying that it's not possible, just that I haven't run across it.  I don't claim to have seen everything, or even much for that matter, hell I'm an ER RN now so what do I know?  :-D

Good luck, Chris

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Thanks. That sounds like some good advice. Ill find some time in the next few weeks to test it the way you described. Like you said. Its better than throwing parts and money at it will no good direction. I did key in on what you said about the timing marks. I cant tell you how many times i went back and checked the marks and all look good, but when you mentioned the slack... if i put the belt on from the intake cam gear first, then the exhaust cam gear, would that put the slack on the wrong side and cause the issue? The marks all line up when the engine is off, but would it slack up too much and cause a valve timing issue? It dont sound logical but its a thought.

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