Low Compression = Depression

[Laband]

Hello fellow Fiero people,
My Fiero mis-fired on over-run slowing to exit a freeway after a 30min high speed run with a toll booth stop half way where there were no problems. It has never before mis-fired. On the short exit up ramp I think there was a drop off in power but the engine continued to run and dropped to idle OK before rounding a traffic island. Very shortly afterwards the motor back-fired and so I entered a parking lot. Now dropping to idle speed again the motor stopped after a few secs, something it has never done before. After turning the motor several times it started and I backed into a parking space. Normally the motor fires and starts imediately. The motor idled for about 30 secs and died with an aweful knocking sound like a run-on pinking knock. It sounded the same when it stopped from idle speed the first time after entering the car park. It has not started since and unavoidably of course with the motor cold has 50% lower compression on 4 cylinders relative to the other 2. You've guessed it's a V6 and the low comp cyls are middle + right on the front row and middle + left on the back. The middle back has very low comp ie <50lbs. The concensus of opinion is blown headgaskets. But, there have not been other symptoms of this except for the backfire maybe. No indicated overheating, no loss or pressurised coolant, no oil in coolant or visa versa, no smoking or steaming exhaust. And would it not be very unusual for BOTH gaskets to blow? The motor has only covered a genuine 120k kilometers and has never been stripped for any repair. It has in the past overheated a couple of times and recently been run for I'd say about 150kms with a leaking EGR tube. Could it be piston rings or burnt exhaust valves? But that must be too much of a coincidence on two independant sets of adjacent cylinders, isn't it? I welcome any help or comments and please ask any questions you wish in that regard.
I have to at least take off the heads and I've a Haynes workshop manual and full engine gasket set ordered but the xmas post looks right now to have me scuppered. Tomorrow I have to buy a torque wrench and valve spring compresser locally before the outlet closes until Jan 10th. I need to know now what the torque wrench settings are for the cyl heads bolts so that I can get a torque wrench that has the correct range of settings available. Can anyone help?

[aaron88]

If your head gasket is that bad then you should have oil in your coolant.  I'm with you, I don't think both could go bad all at once just like that.

I’m thinking that if there is a lot of trouble finding out what the problem is it might be cheaper to just change the motor with a used one, maybe $300 or so.

Did you add oil to the cylinder and check again (adding about 15ml of oil).  If the compression is then okay it’s likely your rings.

If compression is low on first stroke and doesn’t build much after that and you have basically the same result after adding oil, usually it’s your valves.  Perhaps your timing chain skipped a link or two because of a bad tensioner or something and you bent some valves.  Did you hear any weird ticking before your motor quit?  It could have been your timing chain bouncing around a bit.

From what I can see browsing through my manual you need from 15 ft-lb to 66 ft-lb.  I have seen a torque at home depot from about 20 ft-lb to 150 or so, and I thought I had seen one somewhere that was from 2 ft-lb to 100 ft-lb but I can’t seem to find it anywhere anymore (I thought it was at Home Depot too).  You must remember to zero your torque wrench after you use it so that it keeps it’s calibration.

Welcome to the forum.  We are a cool bunch here and between us have a lot of know how and knowledge (I’m more on the know-how and not so much on the knowledge).

Where from; are you.


Aaron

.

[dguy]

Ditto to what Aaron said.  Almost. 

Unless something very unusual has happend, there should be no worry of bent valves.  The 2.8 is not an interference engine to my knowledge.

Everything you describe has me wanting to check the cam/crank timing before messing with the head gaskets, even though getting behind the timing cover is far less pleasant than pulling the heads.   

[fiero308]

I hope you won't take this the wrong way, but have you done compression checks before?  What type of a compression checker did you use; ie was it a 'push-in' or a thread-in type?
Aaron is right; with a cold motor a bit of oil will make a BIG diff; and you have to do exactly the same thing to each cylinder of course. 
Misfires always make me think electrical first and other things second; of course if you DO have a compression prob that is an obvious concern. 

But anything is possible; an elect problem can cause a bad backfire - (injector squirts; no (or weak) spark; several cycles; then BANG!!) and THAT can make a timing chain jump; esp on a bit older chain that is stretched a bit.  Then of course you have a bunch of issues.   I have no idea about the 2.8 as far as interference; I thought that most motors were the interference type (when the valves are not in proper synch with the pistons, that is) ... so maybe maybe.......

But develop a list and work from easiest to check to hardest; always do the easiest thing first.  If there is ANY room for being unsure about the compression check, then do it again and be really fussy about it.  If it gives the same numbers then you have a mechanical issue.  Too bad, but not much way around it.
If for some reason it comes back a bit better, then you may have elect and/or fuel delivery problems.
And how long have you had/driven/known the car?  That may or may not give you some background or hints.  Maintenance?  when were things changed or serviced or fixed or whatever?  Stuff like that; think back thru it.  Ignition control modules are FAMOUS for problems.......  bad plugs; bad wires... bad connections almost ANYwhere......
Keep us posted and good luck!
gp

[2ML67]

Odds of both heads having problems is rare I would definetely look into a timing chain first. If it has jumped a couple of teeth it would cause valves to stay open when they shouldn't and this can mess with your compression check. Have seen very similair things happen with 2.8's in Cavaliers before. When gearing down or slowing off the highway a loose chain gets even slacker and jumps or breaks. Quite often a chain goes when shuting the engine off then just won't restart. Most times the engine is ok just needs new timing chain, unfortunately its not an easy job in the car but it can be done that way if necessary. Dan

[Laband]

A million thanks to aaron 88, Don Guy, fiero308 and 2ML67 for responding to my call for help and welcoming me to the forum. I'm pleased to have gone only so far as taking off the top part of the inlet plenum before taking advice from your goodselves. I'm using a screw in tester and having checked 3 times with similar results tried with oil. Unfortunately starting with the lowest comp pot with <50lbs I put in way too much oil and even with a fully charged battery and no plugs in the other pots 2/3 revs put the needle on the gauge over on the stop at over 300lbs and stopped the engine dead with too much comp for the starter to overcome. Does this tell us anything? Releasing the pressure sprayed me in oil. It was late and so I packed in for the day. I will check the other pots with oil and post the results for your valuable opinions. Thank you aaron88 for the torque info. I got one 20lb-150lb. Your replacement engine is an idea but later I'll give you some details of my situation and you'll see there are significant complications. The low comp and therefore no start is the theory of a well paid diagnostic mechanic who first looked at the Fiero in his workshop. He had the car for a week and so I assume he ruled out spark and fuel. At the point of breakdown I myself checked before calling the tow truck, there was a spark at the plugs and despite the gauge showing quarter tank I put in a gallon from a can. His report on the varying comp matches what I've found up to now. He made no mention of a test with oil in the pots. That would likely have cost me an extra wad of currency! We all agree it just doesn't feel right. That's why I stopped at the inlet plenum, bought a comp tester and etc etc. Seems to me, considering all your posts, the best thing to do is wait for a manual to arrive (Both Chilton + Haynes ordered on E bay) and then go down the timing chain path before anything else. You indicate it's a difficult job with motor in the car. Years ago I fitted a new water pump with the motor in car. Is it worse? It would be good to find out if the motor is or not an interference engine. It certainly turns over with no rattles or knocks etc. I've got nowhere on the GM websight. Any ideas?
Thanks again guys! Please post any more views, info and ideas you come up with.
I'll keep you posted from my end.

[fiero308]

the idea of putting oil in is simply to ensure a ring seal; it should be in the amount of about a teaspoon.......so if you put in TOO much; ie so much that the engine wouldn't crank (?!?!!?) I hope you didn't 'fill' the combustion chamber; oil and liquid of course doesn't compress and hydro locking the cylinder can bend and break very expensive parts......
so I HOPE that hasn't happened.
If you have so much that it sprayed out maybe take all the spark plugs out (if they aren't already; you should have them out for cranking the engine anyway) and crank it a number of times to try to get as much oil as possible out.

Fuel pump: it should run so you can hear it; if you are careful you could disconnect a fuel line and run it into a container and test the fuel pump; !BE CAREFUL!; the gas will be under about 35 psi which is a LOT more than you think!!
but you need to make sure it is running reliably.  It would be better if you could test it against some kind of flow restriction which is how it normally operates (the injectors and rail) and that would simulate an on-car situation.  Fuel filter should give a less intermittent problem; it would load up and restrict flow to everything so the car would generally lose power; not act like this.

The ignition module is a cheap and easy thing to fix and is WELL known for failing; the best advice is to get the GM Delco one; and be aware that they can have sporadic problems; they might not act up all the time.

But this all sounds like something else; I was just covering the easier things.
Let us know how it goes; I am sure it will be an education for everyone.
good luck
gp