Is low compression cause for no start?

[AntiScope]

So my headgasket blew a few weeks ago. I replaced it the next day, hoping the head wouldn't warp too much after taking it off. I used a little copper spray on the new gasket, but to to avail. I should have gotten the head shaved obviously. It will crank but not start. I have Spark/fuel/air and the timing is correct. Compression test came out something like 50/120/120/120/50/50 while cranking.

-Is it a pretty safe guess the low compression was the cause for it not turning over?

-Assuming I have to get the head shaved, what is needed for a good head refresh while it's out? I'm also switching to ARP studs while I can.


Thanks guys, I'm looking forward to learning some more while I do this.

[spdracrm3]

"hoping the head wouldn't warp too much after taking it off" head dosen't warp when its off ,warping occurs during overheating

best to figure out the low compression before removing the head again by doing a leakdown test (doubt its a headgasket sealing issue unless the head is warped beyond repair), but even with those compression numbers if fuel and spark are present it should try to start ,and if it ran it would be rough for sure.

first add some engine oil to the bores (couple squirts out an oil can in each)and turn over a couple times then let it sit 1/2 hr and retry your compression test ,if numbers improve its a piston/rings sealing problem ,if number stay the same its a valve issue (you did adjust the valves right?)

then do a leakdown test to confirm where leakage is...

[AntiScope]

Forgive me if I'm wrong, but I was told that when taking the head off, the aluminum can "relax" resulting in an uneven surface. It was attempting to start at one point, barely. Now it wont even try. I added oil and it still didn't try to start but I can try another compression test to see if anything changed, and perform a leakdown test. Thank you very much for your advice

[spdracrm3]

"Forgive me if I'm wrong, but I was told that when taking the head off, the aluminum can "relax" resulting in an uneven surface"

if this were the case aluminum couldnt be used in making cyl heads

[E30NJ]

If the head was removed hot (which I know it couldn't have been) then it would have warped the head but in the time it takes to get the head off, the engine should have cooled down. My guess would be valve seals or piston rings. What do you mean "Now, it won't even try"? Did you burn out the starter?

[AntiScope]

If the head was removed hot (which I know it couldn't have been) then it would have warped the head but in the time it takes to get the head off, the engine should have cooled down. My guess would be valve seals or piston rings. What do you mean "Now, it won't even try"? Did you burn out the starter?

No, its cranking but it won't start or even get close. At first it was attempting because it would be firing, although shitty. It never really sustained itself though. Now it's just not doing anything when cranking it.

[AndrewBird]

A head will DEFINITELY warp slightly just from removing it. Stress is built up in the head from heat cycling and the only thing holding it flat are the head bolts. When you pull the bolts out, the head will warp a few thousandths. This is why you should always surface the head, regardless if it was overheated or not. Sometimes it won't warp or won't warp enough to matter, but you don't want to have to pull the head again to redo it if it is warped.

[AntiScope]

A head will DEFINITELY warp slightly just from removing it. Stress is built up in the head from heat cycling and the only thing holding it flat are the head bolts. When you pull the bolts out, the head will warp a few thousandths. This is why you should always surface the head, regardless if it was overheated or not. Sometimes it won't warp or won't warp enough to matter, but you don't want to have to pull the head again to redo it if it is warped.

Thank you for the info, I knew I had heard this somewhere. I was hoping some copper spray would save me but you're right, I should have surfaced it.

[Hooffenstein HD]

I doubt low compression would stop it from starting. M20's will run with a completely blown head gasket, just not very well.

If you removed the head I assume you had to disconnect the C191 plug? It's common for the wires going in and, or out to break. Pull back the boots and check there.

[whodwho]

A slightly warped head wont give you low compression with a new gasket(it would have to be SO bad you could probably see it warped...)

With those compression figures it should start, not run the best but start...there are many reasons for a no start when you have R&R the head. The common ones are CPS / pulse wires switched(late model) switched fuel hoses(late model) bad connections at any of the connectors, miss timed timing belt

If you have the valve timing correct, those figures are just bad and most likely you fried that thing, did it blow the gasket from overheating? It most likely overheated the pistons and is pinching the rings.

You should have a leak-down test done and see where the loss is from before you pull the head

[95BMWIC]

When you removed the head, did you follow this sequence of loosening the head-bolts? See pic from Bentley.


Some "relaxing" may occur during the removal - but if done properly - there should be no warping. Even if there is - a good headgasket should seal with some warping..

You have 3 cylinders with 50 - (Cylinders 1, 2 and 6 - if I am reading your numbers correctly ) and the others at 120. (All cylinders are out of spec if you are using PSI?) All cylinders should be at 142-156 psi - So this leads me to believe their may be an issue with your testing. Being from Germany - not sure if their is a conversion or not.

Just to double check - pull all plugs out, remove relay, make sure the gauge is threaded all the way in (this can cause a misread) and crank 4 to 5 revolutions.

If you still get the numbers you did - then I would perform a wet compression test (with oil) for rings and then I would put a vacuum gauge on the intake - this will tell you a great many things (I can send you a link for this). It could be that you have carbon built up on your valves - you didnt mention whether the head was cleaned - so maybe your valves aren't seating properly when closed..

Finally - I would perform a leak down test - although you need a compressor and special tester to locate the leak.

[AntiScope]

Is low compression cause for no start?

Wow, thanks for all the information guys! To be clear, yes I followed the torque specs from my bentley manual. I'm very familiar with torquing do to the nature of my job, so I'm absolutely sure its torqued correctly. Perhaps its not a warped head after all. If I have time today I will recheck everything whodwho suggested and see if it will attempt to start. After that I will do another compression test wet and a leakdown and report back with my findings. An employee at the shop was helping with the compression test, perhaps it was not done properly. Hoping for better numbers this time! Thanks again everyone.

[Kershaw]

switched pulse wires have gotten me more times than i care to admit. however, if they were switched the car would not crank at all, since the cps is not getting a signal through. definitely check though and check the ohm spec on the cps.

there are lots of things to go wrong with this resulting in your situation as stated already. go through and triple check everything. how about the crankcase ventilation tube? is that seated properly? if it's not, you have a massive vacuum leak.

[AntiScope]

I will check the tube this sunday when I go through everything again, thats surely a possibility. I don't think the pulse wires could be switched because I never unplugged them in the first place, but just took that bracket off entirely. So the CPS must be working, because the car is cranking?

[Hooffenstein HD]

Have you run other diagnostics? Fuel, air, spark?

Have you checked the wires on both sides of the C191 plug? This plug can cause lots of issues.