sk8ercbs

My car has a strange problem i cant seem to figure out. When i first start it, it will idle fine, at around 14.7 to 15 afr, but as the car warms up it start to get rich, normally stopping at around 12.5ish. I have tested the water temp sensor it is fine and the o2 sensor doesn't seem to be changing anything when its plugged in or not. but that wouldn't explain why its starts getting rich as it warms up since the o2 sensor doesn't do anything till around 160 degrees. Has anyone had similar problems or have any ideas?

One side note. If i blip the throttle itll go rich but then jump up to 14.7 to 15 and stay there for around 10 seconds , maybe 15.

OnTheChip

You might try cleaning the MAF sensor.

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sk8ercbs

Ive tried 2 different mafs

brndck

you may have a leaking injector or torn o-ring.

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sk8ercbs

Wouldnt that cause it to lean out rather than get rich?

IvoryWhiteS13

A leaky injector or bad oring would cause a richer reading because it is unburnt unmetered fuel entering the cylinder. Also check for boost leaks/exhaust leaks

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sk8ercbs

Okay, thanks. It would most likely be an injector problem or something else, this is all happening at idle so a boost leak wouldnt be the cause. Also when i create vacuum leaks it leans it out.

Kingtal0n

probably a boost leak

do a pressure test from the compressor through to the engine
try to include the intake air pipe but it isn't totally necessary if you can inspect it properly

You are wrong if you think boost leaks don't screw things at idle... thats actually quite common

sk8ercbs

Okay ill check it out but what im saying is wouldnt the boos leak cause it to be messed up from the moment of starting the car. i dont understand how a boost leak could gradually make the car run rich. It seems like it would just be rich from the beginning.

Kingtal0n

thanks for asking,

When you first start the engine, exhaust gas is very cold. That means it takes up very little space. So it does not spin the turbine very fast.

That means when first started the compressor is also spinning "slowly". I use the term loosely (stock turbos still spin pretty fast when 'cold').
Anyways, when you first start because the wheel is so slow at first, there is very little airflow kinetic energy in the intake pipes.

A turbocharger is an airflow kinetic energy device, not really a "positive displacement forced induction", its more like a baseball throwing machine that speeds up as the exhaust gas warms and expands, throwing more and more balls faster and faster over time.

So imagine you are chucking really slow microscopic baseballs through the intercooler tubes at first. They sort of bumble around, but they aren't moving fast enough yet to 'break through' the cracks/leaks in the tubes. They can barely break through the resistance of a typical large intercooler, the engine mostly breaths "on it's own" at the other end. With the engine running and pulling a slight vacuum that helps 'absorb' any pressure the compressor would have created at first. You can visually verify this state by looking at a factory bypass valve, which will hang closed at first start because of the above reasons about slow moving baseballs(air molecules) and engine breathing.

As the engine (exhaust) warms the bypass gradually will hang open at idle (as intended) to allow the high kinetic energy intake air molecules to recirculate back to the pre-compressor inlet, to prevent them from flowing backwards through the compressor (surge) which slows the compressor down as it happens, leading to a rubber banding effect where the compressor speeds up, flows too much for the engine to use, surges and slows back down (screwing the maf and running rich) then finally is going slow enough that the attempted positive pressure dissipates and the plumbing goes into a slight vacuum, which allows the compressor to start speeding up again, and the process repeats over and over forever as a dynamical system without a point of stability (it never settles). This is why a bypass with typical (smallish) size turbochargers is absolutely a requirement when using a maf sensor, and why a bypass helps to eliminate turbo 'lag' (because it allows a compressor wheel to speed up faster than the engine's breathing would allow, by recirculating excess air molecules back to the 'starting line' where they assist with the inlet pressure differential and reduce lag). Also note that either of these situations (Boost leaking from a hole, or not having a bypass and creating an unstable dynamic) will cause the maf sensor to read incorrectly and dump fuel into the engine. So not having a recirculated bypass can make engine run just as rich at idle as a boost leak will. Even with no boost leak, the condition will just alternate, running fine, then rich, stumble, fine again... smooths out for a moment... then rich again, stumbles, cough. Then running fine again... etc....



Anyways the point was, as the turbine warms or if you started to drive the turbine 'working harder' now is able to throw those baseballs much harder, so they start to break through the plumbing's leaking areas easier. If you drive for 5 minutes then stop at idle again the turbo is flowing WAY more air than it did when you first started the car.

it can't be seen on a boost gauge because the difference in pressure from one situation to the other is less than 1psi, and you'd need to be measuring pressure between the compressor and before the engine to see it happening. A compressor wheel speed monitor would also show the 'lack of bypass' issue. It isn't a problem for huge turbines because they typically won't move a compressor fast enough at idle, but the situation will still arise at part throttle if there is no bypass and compressor flow exceeds engine flow- the air has to go somewhere. It flows backwards through the compressor (chu chu chu surge noises) slowing down the wheel and putting abnormal stress on the blades, ultimately bending/breaking the tips of the wheel is the outcome generally (look at high mileage turbos that ran without a bypass).

FWIW the super AFC has an airflow 'clamping' feature that I found extremely useful back in the day. When tps is less than X (closed throttle) it clamps the maf voltage to a set value, allowing the engine to leak as much air as it wants, the compressor can surge all it wants, and the a/f will remain unaffected.
The reason I mention this is because I have several friends with cars I've tuned, that don't want to recirculate their bypass valves, and this is the ONLY way to prevent the above from happening if you are using a MAF sensor and don't want to recirculate with a "loose" (open at idle) bypass.

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sk8ercbs

so if i just plug my megasquirt in ill be able to tell it that whole thing you explained is the problem or not? It would still do it for the boost leak but not the whole not having recirculation thing since id be using a map sensor?

Kingtal0n

Not sure I understand your question,

If there is a boost leak, in any turbo system, it will create major problems. Even small leaks are significant.

For every molecule the compressor "throws" it must use some energy which is given through the turbine, energy from exhaust.
So if you start throwing 'away' compressor driven molecules (from a LEAK ANYWHERE) the compressor will now demand more energy from the turbine, thus increasing turbine exhaust gas pressure, which increases the temperature of the turbine and reduces the efficiency of the engine (high EGP), overall it will reduce performance for small leaks,...

If you create a large enough boost leak the engine will detonate because exhaust gas pressure/temperature can get very high.

boost leaks are also known to 'overspin' the turbo which is another disaster.

If your question was "will a map sensor tell me if I have a boost leak" the answer is NO, only a MAF can do that. Which is why I believe they like to use maf sensors on (all) turbo cars that 'normal' people drive (people that don't pressure test their setups frequently, like I will do when using a MAP sensor)