Causes of lean condition in one cylinder????

Kurt K

SCCoA Member
As some of you know, I've recently had a myriad of major engine problems. One of them was severally deforming the dish of one of my forged pistons in my CMRE Stage II clone (all Stage II components). I thought it might be a bad injector that caused the lean condition in my #4 cylinder, so I sent my 6000 mile 42# lucas injectors to get tested. All of the injectors flowed within 10 cc's of each other at 100% duty cycle, so that rules out the bad injector theory.

Any suggestions for possible causes of an intermittant lean condition in a single cylinder? And yes, the car was tuned.
 
Possably a vac leak on that cylinder where the intake runner goes into the head. Maybe a flow imbalance in the intake manifold itself. If you did any porting in the intake passages in the head you may have uncovered a porosity in the casting of that runner. Injector may not be getting a good electrical signal caused by a short or the output driver in the ECU acting up.
Does it have a steady miss?
Just some possability's, If I think of some of some more I'll come back and post.
 
How about?

A slightly leaky intake valve?

Are you sure the problem is with just one cylinder going lean, or the whole engine, and just #4 was the first to melt down?

Clogged fuel rail? Pressure imbalance in the rail?

Coolant flow problems around Cyl #4???

I remember a guy with a 2.2L turbo chrysler that burned up cyl #4 cause the thermostat outlet was in front of cyl #3, so the coolant jacket around #4 acted like an "appendix", just sort of holding the hot coolant in a loop. Since that cylinder got hotter first, detonation occured there.

His fix? he welded a plate over the original out let and drilled a hole and welded a new neck behind cylinder #4. He claimed he didn't have problems after that.
 
Mike 38sc: The intake and heads were both ported by Coy.
Injector may not be getting a good electrical signal caused by a short or the output driver in the ECU acting up.
Does it have a steady miss
It didn't have a steady miss, which is the wierd thing. And yes, I'm leaning toward an electrical problem at this point. I think I'm going to try and get a new injector harness.

Is there a test for the output driver in the ECU?

Randy: I did a cylinder balance test while in Carlisle which showed a problem with number 4, but that was with the electrodeless spark plug.

Deep6:
Are you sure the problem is with just one cylinder going lean
Pretty sure. I actually burnt the elctrode and most of the ceramic off the #4 spark plug. I figured the lower compression was from raw fuel washing the cylinder wall. I was hoping the compression would actually go up a little over time as the rings seated (remember this was a new rebuild).

I am going to get the head checked out.
 
Yes Kurt. You will have to backprobe the injector wiring at the point it connects to the injector, but the engine would have to be running to do that.
Hmmmmm......Let me make some calls and I'll see what I can come up with.
 
Kurt,What was the color of the top of the piston,and spark plug end.

Was the head gasket put on right with the front marking toward the
front of engine.

On the valve margin did it look like it was seated.You could try some
lapping compound on the valve.To see if it was seated on the valve
seat.And running true.

Sorry to hear your having problems.I was enjoying reading about
your performance gains.

Randy
 
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Currently the piston tops are black and carbon coated. The plug in question didn't have a tip to check the color, the electrode and ceramic were gone.

I'm using the multi-layer 4.2 L head gaskets and I assume they were put on correctly--I didn't assemble the engine, but the builder has assembled numerous engines. I doubt it was a HG problem.

I need to drop off my head and get them checked anyway and possibly decked, again.
 
Kurt are you sure the cylinder was lean?
Could it have been detonating or preignition?
The reason I ask is that a lean cylinder on a forced induction motor usually burns a hole through the piston.
A deformed piston is usually caused by detonation or preignition.

Do you have any pics of the deformed piston. I would like to see it if I could.
 
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Well, I guess I'm not 100% sure it was lean. Isn't detonation sometimes caused by a lean condition? I didn't burn a hole in the piston, but I think it was close. I will attach 2 pictures, they are hard to judge if you don't know what you're looking at. So I'll give you a brief description. The "crater" on the dish of the piston is roughly 1" accross and about 5/8" deep. The underneath side (which isn't pictured) got noticably hot and is discolored, especially where the deformation occured.
 

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Kurt I'm wondering about that greenish brown deposite on top of the piston. Was that there when you pulled the head?
What about the ring grooves, are they deformed or wavey?
I can see that the rings are not stuck in the grooves or broken, but look for shiny spots around the top and bottom of the top ring groove. You will have to remove that ring to see it.
Could you post a couple pics showing both skirts of that piston and the ring grooves.

What's the rod bearing like on that rod?
If you can remove the rod bearing, torque the rod cap back on and take a inside micrometer and measure around the big end at 12 o'clock, 1 o'clock, 2 o'clock and 3 o'clock posistions.
What readings do you get?
 
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Mike, I actually don't own (or currently have access to) an inside micrometer. I did take more pictures tough. As for the green, I think that's an irregularity caused by the digital camera. Here's 4 more pics.
 

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OK that greenish look threw me. Ive seen that green deposite before, but that doesnt pertain to this.

I do think your lean. I beleive you just came as close as could be possable to holeing the piston without actually doing it. You were fractions of a second away from burning right through.
That little dip in the piston top and the scored skirt tells it all.
The little dip is where was next going to be a hole. The skirt tells us that the piston expanded rapidly from heating up to fast and the skirts contacted the cylider walls causing the scoreing on the skirts.
If the spark plug had not melted you would have holed it.
Hows the cylinder walls look in that cylinder?
Do any of the other cylinders show ANY signs of this?
Look for the tell tale signs on the other pistons.
 
In my personal opinion, that piston looks like it was running lean.
I had an old chevy that I tore down where the pistons looked exactly like that. My machinist told me it was running lean and it had extreme cylinder heat. If I were you though I'd make sure the block is ok, I had to have my chevy bored over because the cylinders were egg shaped from the piston skirts rubbing.
 
Lean with some detination

Thing to look at.
1.injector,eletrical pulse,injector flow
2.Ported manifold.flow inbalance to that cylinder.
3.valve margin,not laped valve to see if it leaked.
4.to much od on blower pulley,
5.manifold leak.
6.head gasket leak.
7.plug heat range for that cylinder.

That is if only one cylinder was lean.

The builder would be the best source for infor.
Has he been able to veiw your pics.

Hope you can get it lined out for Wenzville.

good luck Randy
 
Mike 38sc: All of the other pistons look fine. And I know I got lucky, I drove on that cylinder for over 4000 miles after I melted the plug. The cylinder wall does show a little wear. I'm hoping I can just add a little clearance to the piston to get past it. If not, I've got another block I should be able to use. I really don't want to have to order a new set of pistons, since I think I was able to find some used pistons from another guy with a similar fate.

Randy: 1) I sent the injectors out to be flow tested and they passed with flying colors--all within 10 cc's of each other.

2) The mainfold was ported by CMREand he's ported numerous without any issues.

3) I still haven't had the heads tested.

4) I'm only running an early pulley on a S-model SC

5) Maybe, but I didn't notice anything out of the ordinary

6) There was a little evidence of a slight HG leak, but none getting to a water jacket. Just a little carbon under the HG, near the outside of the block. But I doubt I could tell if it was the cause or result of the lean condition.

7) At the time of the problem, I was running stock heat range plugs.

Yes, the builder has been seeing the pictures.
 
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