Runs when #3 plug wire is disconnected
 

Hi Everyone, I am going crazy with this problem on a 1994 Volvo 940 auto. It ran fine when my son parked it and it would not start the next morning. After much trial and error, I got it to run only when the # 3 plug wire was pulled. Once it is running and I plug #3 in , #3 is seems to be fighting the engine when it has spark. Like that one cyl is firing before TDC.

I figured it jumped a tooth on the timing, timing belt looked good, but still ran bad so I removed the timing belt and re-installed it and made 100% sure it was lined up. Still it only starts with #3 plug wire disconnected. SC

Have you taken a compression test?
You need to!

Although, yes, this is strange... But bad head gaskets can cause all kinds of head scratchers besides just the "water in the oil" variety.

its a big day for me, I used a compression tester for the 1st time. I get somewhere between 125 and 150 on the cyl's. Seemed like a good woof of air came out of all the plug holes. Is that good?

Just for fun I took the valve cover off. Nothing seemed broke in there. I think I should have checked the valve gap when I had cover off.
I turned the engine so it was TDC and both the lobes for #1 cyl were up

Thanks for the input, keep it coming, my brain needs idea's, SC

Nooooo! 125-150 is not great but definitely should not be doing weird s#$%!
I expected a bad cyl compression for sure... Woof of air is fine, it's compression pushing out. I wouldn't worry about the valve gap.
Well, I am stumped...
But, hey, don't give up, it's gotta be something!
Too bad you are not closer, I'd love to see the thing.

check the spark plug firing order?

Update, I had to remove a Air cond condensor to reach the #4 with the compression tester, # 4 has 50psi. the plug was real black. So number 3 was ok, it was number #4, It still is weird that it runs better with#3 or #4 disconnected, I can't have 3 and 4 both plugged in at the same time or it runs real bad. does this point to some cure? I squirted oil in # 4 and the compresssion did not get better. SC

Inspect the inside of your distributor cap and the rotor. That, and the plug wires, is the ONLY difference between any 2 spark plugs.

anyways, this engine need at the very minimum a head gasket, and quite possibly valves and or rings-and-piston-and-bore... head gasket, easy enough, the rest are hardly worth doing, find another red block to transplant (ok, if its just valves, you could find another head to tranplant onto this block, thats a lot easier than an engine block swap).

is there a chance a valve is not closing in #4. is there a fix?

depends on why its not closing, but the affordable fix is to find a head on parts car. labor involved is pretty much equiv to a head gasket replacement.

the fact that spark in 3 is messing with 4 makes me wonder if the problem isn't the head gasket between #3 and #4, thats where one of mine blew...

http://pierce.smugmug.com/Misc/1991-...MG_8164-X3.jpg

http://pierce.smugmug.com/Misc/1991-...MG_8075-X3.jpg

[btw, oops, that engine is the one from my sons' old 940SE battle wagon, which overheated after a heater hose blew, and I think the failure was between #2 and #3, #3 had cracks between the spark plug and valves.... scored a new head from a guy on this forum, which was in perfect shape, and bolted right up, and the engine was good as new]

OK, that's better! You had me losing sleep there...
So, most likely a bad exhaust valve although strange if it happened overnight.
Anyway, the head has to come off to see what you have. Not a big job especially without turbo, even with the turbo...
I have actually done this many times myself. If you are willing to tackle it, it's easy...
If you can do a timing belt, you can do the head.
And, no you don't need another head... Unless the motor was overheated.

thx Lev. it was wacky

Does this sound reasonable, bad exhaust valve in #4

the firing order is 1342,
3 fires , 4 compresses (4 sends air gas mix into exhaust manifold thru leaky valve)
4 fires, 3 exhausts
(when 4 fires it sends flame thru exhaust valve ,ignites air/gas in
exhaust manifold does small back fire)
3 intakes 4 exhausts
3 compresses 4 intakes

That sounds interesting.


I would first check valve clearances. It is fast and easy to make sure.
But... The head might have to come off for sure.

Could have dropped a valve seat. I have had that happen to me.

Attachment 16607


Also burned a valve. Car ran, but not that great.


Attachment 16608

Well, reasonable or not, the head needs to come off... Trying to logic this won't get you anywhere...

I'm sort of in a bind car wise. How bad is it to run on 3 cly for a week?
I did not see valve adjusters when I had the valve cover off. I looks like Hydraulic lifters. SC

You can run it for a week like that. What can it hurt?
Does it run better at speed, less pronounced miss?

The valve adjustment is done by shims but I wouldn't worry about that...

When I got the car the exhaust manifold was welded to broken manifold bolts. Over time I've developed massive manifold gasket leaks, after all this recent trouble shooting I learned that manifold gaskets leaks can burn exhaust valves. any truth to that?

here is a pic of the engine with my anti exhaust leak clamps

Went around the block, up a hill, got it in 4th gear. Man, it is slow on 3 cyl's.
I pity the fools that have to follow me, no CEL light came on. I have the injector unplugged. It still runs worse when I plug the 4th sparkplug up. Seems like no gas in the the cly would make the the spark a non issue. wierd ol volvo