explain this picture
 

So after a rebuild, the car wouldn't make as much power as it did before the rebuild. Bigger cams and everything else basically the same.

Leakdown test showed 8%, 10%, 17%, 10%. Although we put wisecos in it and they expand more with heat than the supertechs, number three was obviously a concern. Very difficult to tell where #3 was leaking as the air movement noise seemed to resonate to everything, but it seamed loudest from the dipstick tube.

The car ran absolutely fine, just low on power. No missing at all.

Looking at the exhaust ports of number three I thought it looked like maybe raw fuel was exiting the chamber and cleaning the runner. Yet the car wasn't missing so that didn't make sense.

So then the thought was maybe it was just running really really lean in that cylinder. Pulling the head revealed that although there is some slight evidence of minor detonation at some point, all the pistons look the same and all have been on the rich side for sure.

Really at a loss as to why the exhaust runner looks like it does. As for leakdown, we didn't check piston to bore clearance when assembling and #3 seems to be much greater than the rest. Gonna call Wiseco tomorrow for the specs and try to come up with an inside mic to check bore size and roundness.

Any sign of a head gasket leak? That exhaust runner looks steam cleaned.

Keith

93 views and only one reply. Looks like I'm not the only one stumped.

Head gasket looked fine btw.

Trying to think outside the box, and I'm still waking up. I doubt this is the case with megasquirt/e85/injectors, and everything else you've done.

It says it's a '90, so are the injectors still batch fire? If that is still the case, perhaps your number three intake valve isn't seating all the way (leak down number), and you're actually getting fuel washing through after it's getting past the intake valve after the combustion stroke. Then what ever unburnt fuel that didn't get burned with the combustion stroke is getting pushed out of the exhaust.

Maybe it's far fetched, but I'm on a my first cup o' coffee and you haven't had many other ideas.

It also says you're water cooled. Is this an individual jet per cylinder set-up, or one jet near the throttle?

Also just waking up, but IMO that's gotta be fuel or coolant. My vote is for fuel. I don't think lack of missing is confirmation otherwise

That's where I was heading.

See comments in quote

I'm not a mechanic, I sell insurance. Can and do I work on cars? Yes. But an expert I'm not.

I can't figure out how enough fuel to wash the port almost clean could be exiting the chamber without lack of spark.

Lack of spark = engine misfire.

Wrongly times spark = backfire.

I suppose the injector could be sticking open, but it seems there would be evidence of that?

Of course my two man machine shop is on vacation this week, so I can't even pick his brain.

Actually the top of the runners are a little black. That would probably also point toward it being fuel vs water. Maybe. At this point I'm not to confident in anything I thought I knew. :bang:

How do the cam lobes look on number 3?

Sub'd out of curiosity.

Cams look fine. Yes, SUB and they look fine as well.

Looks ported. Any chance you have gone too close to coolant passage in the port and there is a pin hole that when coolant is pressurized leaks in there. Had a similar issue on a Toyota years ago and took us forever to figure it out.

This really sounds like a winner...

Keith

Hmmm...., now that's an interesting thought. I had ruled out coolent since there was never a hint of white smoke from the exhaust at all. However, in this scenario if it was post combustion chamber I wonder if it would smoke.

You could do a leak down test to test this therory and move the piston, I saw that somewhere on youtube...

EDIT : Go at 7 min
<iframe width="420" height="315" src="//www.youtube.com/embed/Ku8tRBv8rH4" frameborder="0" allowfullscreen></iframe>

[QUOTE=joyrider;1156924]You could do a leak down test to test this therory and move the piston, I saw that somewhere on youtube...

EDIT : Start at 7 min

It would barely smoke as the flow typically is pretty small and it is not as hot. Definitely something to check for.

I may be wrong, but this is how my mind was picturing it.

In batch fire, the injector for #3 would fire for it's intake stroke (intake valves open, sucking fuel/air in), and then fire again (against the back of the closed intake valves) during cylinder 1's intake stroke.

During that time it's cylinder 3's combustion/exhaust stroke, so as it moves downward it might pull that second pulse of fuel past a poorly sealing valve. Some of it would get burned, but running big injectors and E85 levels of fuel load might mean not all of it would get burned. Leaving unburnt fuel to wash through the exhaust port.

Like I said, it's probably far fetched.

If you're talking about a pin hole past the valves it would explain the steam cleaning, but not the leak down variance. At least probably not. It's a very good theory, but we had a car like that, and the compression was obviously fine. It was just mysteriously loosing coolant. It wasn't even a ported head, just a bad/rough casting that eventually corroded through on cylinder 4.

Maybe two different problems. I was trying to theorize something that would explain both issues at once.

How do you really know you don't have a misfire?

This looks like either an injector is not working right and dumping fuel, or you have no/little spark.

Do you have the oil from the engine? you could do an oil analysis and see if there is more fuel than normal.

Batch fire doesn't mean all for injectors fire on every stroke. It means 2&3 fire together on the compression stroke and 1&4 fire together rather than each injector firing individually. #3 isn't going to fire on the exhaust stroke.