I apologize if not allowed, but I believe I need a performance engine builders input to resolve the fault on an industrial engine at work.

I’m purposely not providing specific engine make/model as I don’t believe it is pertinent and would only colour input. It is a 9.5:1 spark ignited carbureted engine with electronic emissions feedback - a fuel system familiar at the very least to many of us.

Customer complaint is hard start mostly no start, engine commonly stalls when accelerator is depressed. Owner states the symptoms came almost overnight.

In my diagnosis I found:

- noted engine starts / runs easily with additional fuel, still struggles to accelerate - requires additional fuel to stay running in open loop

Have repeatedly checked for intake leaks with carb clean, brake clean, spray bottle water without resolution.

Engine will idle closed loop with 15-17 % fuel correction, makes steady 20inHg in this state. Depress the accelerator and it runs horrifically lean & stalls or stumbles through and runs lean at steady rpm with max 35% correction.

Valve lash, Spark timing and valve timing have been confirmed. I’m a little embarrassed to say that nearly EVERY component in the fuel system and emissions system has been swapped with known good without improvement. The only untouched component is the intake manifold.

The only oddity I’ve noted during my diagnosis:

Compression test executed as per service manual, 220 - 240 PSI. Spec is 167 psi +\- 5.

60 to 70 psi above standard, confirmed on two different test kits.

I removed the 02 sensor in the manifold and repeated the compression test with the same results.

I took a picture of the cylinders with my borescope. Pistons do show some carbon buildup.

I’m buffaloed.

Is it at all plausible that the high compression from carbon build up would cause the engine to run this lean / hard start / stall during unloaded rev?

 

Does the engine in question have VVT?

 

No it does not. It’s based on a conventional late 80s overhead cam 16V 4 cylinder.

 

I dont have an answer to your problem, but this is your biggest clue. Carbon doesn't occur overnight so not your problem. Suddenly occurring is telltale of a component quickly failing.
Also, that picture doesn't look like much carbon to me. I think you have already touched the issue and missed seeing it somehow.

 

Agreed that the input from the operator doesn’t line up - however this engine does have adaptive fueling, allowing it to make corrections as required without throwing a fault code until it is in closed loop.

I suspect that I have missed something myself which is what brought me to pose the question here. I’m not playing games, the customer is looking at a significant invoice and I have naught to show for it.