Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Can (slightly to moderately) more fuel damage a new engine?

Understanding AFR

Forum Posts



Tech Articles

Discussion and questions related to the course Understanding AFR

= Resolved threads


Hey guys, I'm sorry if the question has been asked already, I tried searching the forum for similar question but found no answer.

So, to the point - I have re-drilled and rebuilt my engine and updated the head significantly.

The motor in question is m50b25 non vanos (BMW e36), straight six.

- 3219ccm achieved by using M3 pistons (84mm to 86mm)

- fully decked, ported head (new 0.1mm thicker valves, seated at a higher degree, intake ports diameter slightly increased and dimpled, exhaust port slightly increased and polished)

- factory camshafts

- factory M3 crankshaft

- M3 pinktop fuel injectors

- standard 325i air intake

- Standard 325i 403 Bosch ECU, non mapped

This is just some general info if anyone needs it.

So, factory fuel injectors for e36 325i are 18lb while M3's are 21lb. For some time initially, idle has been erratic but that has fixed itself over time.

After a car has been sitting for a while, it wants to smoke out on the start up and there's a strong smell of fuel at all times. It is not condensation in the exhaust, i know the difference. It's just black-ish smoke from excess fuel. It also only happens to be smoking like that when revved on idle, while cruising there's nothing.

Lambda at idle is 0.79 instead of ~1 so there's definitely excess of fuel at all times but especially on idle, which I am aware of, but my question is, could have this damaged the engine? I bought standalone ECU which i'll be putting in soon, but i'm in progress of upgrading the brakes, fuel pump, fuel lines etc; wanted to do it all together, so I have driven the car like this for about 2500km (1500 miles), but I have noticed loss in power of... 15% or so? Initially (when engine was at 1200km-750 miles) my 0-100kmh time was about 6.6, now it's over 7 seconds. Camshaft timing is correct. Compression pressure (done by cheap but brand new compression pressure tester, the one you plug in where the spark plug goes) are 19-17-15-15-16-17 which I think, is okay (first being at the radiator, last one closest to the transmission - not by the order of firing).

Due to not having access to the real dyno, i have used portable car dyno from MagicMotorsport which, over 3 measurements, pulled out 203, 214 and 206hp which i'm taking for granted value since i don't think it's as accurate. The measurement has been done after i've noticed loss in power.

So after all this info, back to the question - could've this excess fuel damaged the engine, thus having loss in power of around 15%?

My thought is that, since it's a factory ECU, and it has noticed there's too much fuel, it's retarding the timing. Not sure if Bosch 403 from 1992 can do this, but may be? Meaning, should I be worried? I would really dislike to have to open the freshly assembled engine so I came here to see what you guys think. Thank you for reading.

A richer mixture will tend to wash the lubricating oil film from the cylinder walls and rings, or at least reduce the lubricating properties. It will also tend to dilute the crankcase oil, but this may not be a significant in the short term.

At idle is probably the worst time to have a rich mixture as there is minimal oil being thrown off the crankshaft, which is the principle lubrication for the bore. That lambda value is the equivalent of 11.6:1 AFR - that's VERY rich for idle on a warm engine and there is absolutely NO possible justification for it! I'd be looking at around 1.0 +/- 0.05 - basically as lean as I could while keeping a smooth, stable idle and throttle response - more fuel could be added later as torque demand increases but, even then, that's richer than I'd expect for power/torque on a NA engine - anyone with specific experience with these engines?

I can't see any possible reason for ignition timing being pulled because of a rich mixture, as the excess fuel should reduce compustion temperatures and slow the flame front - anyone?

Back to your question, yes, it's definitely possible for you to have damaged the engine due to excess bore and ring wear - there may be other reasons, too - a cylinder leakage test should give more information but a general guide on a warm/hot engine is for them to be within 10% and for a freshly run in engine it's be expected to be just a few percent - your highest is ~26% higher than the lowest, or a 21% variation going the other way - that's a LOT for any engine and you'd be strongly advised to have the warm engine cylinder leakage tested. Even better if you can have the bores checked with a boroscope as I would expect there to be significant scuffing of the bores.

I hope I'm mistaken, because it's going to be an expensive hassle either way, but I would expect a minimum of a hone with fresh rings and, if the bores and/ or pistons are badly scuffed, probably a rebore with a fresh set of OS pistons.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?