check out these compression #s -- would they keep it from running?

[henschman]

i've been trying to diagnose a no-start condition for a while, ever since i parked my car and it simply wouldn't start the next day. i tried everything on the gnttype.org flowchart, and everything checks out -- the motor is getting fuel and spark. just in case the spark wasn't strong enough, i replaced the ignition module and coil pack -- still no start.

so today i checked the compression and got these numbers:

1: 105
3: 75
5: 115
2: 112
4: 125
5: not checked b/c it's a PITA

i heard compression is supposed to be around 150, so there's definately something wrong with mine. but do you think compression alone is enough to keep it from starting? and what could cause low compression that would allow the car to run fine one day and suddenly make it not start the next? i'm stumped -- please help.

 

[blown okie]

remove the heat sheild from the heat/ac box and you can get to the # 6 plug way easier, and just leave the shield off, it won't hurt a thing.

 

[henschman]

is it possible that i jumped a few teeth on timing and mashed some valves when i tried to start it? you'd think i would hear something if that happened, though. hmm, maybe it happened while it was running and it let it keep running, but wouldn't start again.

 

[SouthernCal GN]

Bunch of possibilities- the facts are you have low compression, and that's as much of a concern as the no start condition.

In your position, I'd pull the engine down to the short block, get the heads checked, and check the position of the cam/crank gears.

 

[PaCemkr86]

might have a cam going flat, bent pushrod, broken rocker arm etc.. pull the valve cover off and take a look at #3 cyclinder

 

[d0n_3d]

first of all, you don't have to remove the a/c box shield to get to the number 6 plug...just get a couple long extensions and a swivel socket and you are good

secondly, 75 psi on number 3 cylinder isn't good at all...that would make an engine not want to start...how do I know this? I once had a bent valve and was only showing 50 psi of compression on one cylinder and it wasn't starting...my sister's camaro also had 0 compression in one cylinder and wouldn't start....a friend of mine's dodge neon had a bent valve and 30 psi of compression in one cylinder and wouldn't start...4 things are needed to fire a motor, fuel air spark and COMPRESSION!

 

[my1stgn]

Well obviously 75psi compression isnt good but I dont really see how that would keep a engine from starting. I had 80 psi on 1 cylinder before and I was still driving the car (not recommended) and I blew a head gasket (cylinder 1) @ LACR but the car still ran. Just miss-fired like crazy.

Also if Im not misstaked out engines are not interferance engines so even if the timing was off, I dont think you would bend a valve. However if the timing was off then your compression would suffer accross the board. I think your problem lies elsewhere. Yes compression is low but I dont think thats the only problem you have.

d0n_3d is right. You do need all four things to run an engine. Buy yourself a inline spark tester to make sure you have spark running through your sparkplugs/wires. Low compression would make it harder to ignite the plugs so Id gap them a little closer. While cranking the engine try spraying a little carb cleaner in the intake to see if you could the car tries to fire. I would spray it into the valve block above the tb/doghouse

Other things causing it not to fire up:
Wires- firing order
Sensors- Cam, crank, MAF, tps, iac valve
fuel pressure

just to name a few (sorry but I hate trying to diag. cars over the internet)

 

[gndoug]

shoot... Ive started and DRIVEN a lot of cars that have cylinders with either little of no compression. One was a ford tempo where the head gasget blew between the two middle cylinders, and both had about 20 lbs of compression...... thats HALF the engine...I dont think one cylinder having low compression will keep it from starting. Just my .02

 

[zpd307]

my brother in law's jeep ran like crap, so i checked it out. it was 4.0 inline 6 and the number 1 cylinder ended up only having 0 compression. but it still started and ran.

 

[dentmasters01]

All the other cylinders compression #'s are fine for starting and running an engine. Up to performance level is another question, but still functional as far as being able to drive it down the road. Get a timing light and see where your at. Cylinder # 3 is your main concern. I wouldn't even be trying to start it knowing I have a problem like your seeing though. If you do get it started, its gonna run like crap so why bother. Start by finding out WHY # 3 is bad and you may find your answer your looking for. Get yourself a leak down kit. That will tell you more than a compression test. Compression test only show you the symptom, Its a good start, but leak downs show you the cause.......the air is going somewhere......find out where. And like I said, you may find more than one answer to your question.

 

[chopped39]

I would to a leak down test before teardown . that way you will know if its a valve or maybe a ring problem. I have had carbon stuck on a valve seat hold a valve open . causing 60% leak down. cleaned valve and seat then checked for leakage on bench and problem was fixed.

 

[henschman]

leak down is where you fill the cyl. up with air and see how long it holds it, right? great, more **** to buy for my car. thanks for the advice guys.

 

[Jerryl]

As stated before, car should still start with low compression in one cyl and be driveable ........... but think about this before you decide (if this is a TR)
"If" you have a HG leaking into the lifter valley, it will pressurise the block.
Go from there.

 

[dentmasters01]

You got it.....A leak down tester has two guages and a valve. Its connected to an air supply. You open the supply valve and one gage tells you how much air your putting in, and the other tells you how much air is leaking out of your cylinder. Now if you already know your cylinder is shot, then this part really doesn't matter, what does matter is you can hear air when it is leaking.......start listening for the source of the leak. OH YEA, make sure BOTH your valves are seated on which ever cylinder your checking by either rotating the engine or pulling the rockers.
so

leak down is where you fill the cyl. up with air and see how long it holds it, right? great, more **** to buy for my car. thanks for the advice guys.

 

[my1stgn]

leak down is where you fill the cyl. up with air and see how long it holds it, right? great, more **** to buy for my car. thanks for the advice guys.

I wouldnt really spend time/money getting a leakdown gauge. The only reason I use it as work is to tell the customer exactly where the low compression is coming from (ie valves, gasket, piston ring) But if you want to know without spending more money its easy.

1st. Do a compression test w/ throttle blade wide open and battery fully charged

2nd. Okay you have low compression. Now add engine oil to that cylinder (this is called a wet compression test) Compression goes up ( closer to the other cylinder levels) well you have a bottom end problem (ie piston rings, cylinder walls, ect)So you will have to bore out the engine (maybe) if you have cylinder wall with alot of damage or just hone the walls and replace the piston rings.

3rd. If compression does not go up, then you have a problem in your heads or valve trian (ie valves not seating right, bent valves, flat cam-maybe, headgasket) So in order to fix your problem you have to r&r heads, get a valve job (might as well upgrade springs, valves, ect. Make sure they measure valve height and spring tension!) And replace the head gasket.

Or you could buy a cylinder leakage gauge which will tell you "Hey your leak is from you exhuast valve. Okay so now you have to pull the heads and get a valve job and replace the head gasket. (You could have figured this out with a wet compression test)

Or you find out your leakage is in the crankcase. Okay so you have bad piston rings and worn cylinder walls. Again you could have figure that by doing a wet compression test.

So I dont think you need to do a leakage test. Just do a wet compression test and you could get an estimate as to what repairs you have to do! Finding out which valve is leaking (ie intake or exhuast ) is really irrelevant since you will have to do a complete valve job anyways! Hope this helps!