1. Z20let Bad Knock At 5k Rpm Calculator

• 99 silverado 5.3 has a noticable knock at about 1500 - 2000 rpm. You can actually hear it while driving. I have 17.5k miles on mine and it had this detonation last Summer as well. As soon as the temps got cool, the detontaion stopped. If the octane is less than 87, you may get a heavy knocking noise when you drive. If it's bad enough.
• Turbo losing boost at around 4000 rpm. Ask Question 8. I have a 2005 Opel Astra Turbo Coupe. I would imagine most of you guys are more familiar with the Vauxhall or Holden badge, but it's the same car with the same Z20LET Turbo Ecotec engine. Anyways, when driving, the turbo kicks in at 2500 rpm, as usual, but when I reach 3500 rpm, I hear.

This isn't a general question regarding air/fuel ratios. I want to know why Subaru specifically insists on making their cars run ridiculously rich under boost. For instance, my 2007 Forester XT has an AFR of 10.5 under full boost (hard acceleration). ANd it's not just mine, all turbo Subaru's run very rich. My previous car rarely went below 11.5. As far as I was aware, the best AFR for heavy loads is around 11.5 to 12.5. 10.5 just seems a bit dangerous (Yes, there is such a thing as too rich). I have a small-ish TD04 turbo on it and it doesn't boost more than 0.8bar/11.5PSI and quickly tapers off to 0.6bar/9PSI so there's not much heat.

Hello, this question pertains to my wife's Oldsmobile Intrigue with the 3.5L 'Shortstar' V6. Saturday, when returning home from a party, the engine began to make a tapping noise. I don't know if it started suddenly or if it just slowly became loud enough to hear. It's now tapping constantly while running at any RPM. It seems to do it on accel and decel.

I should probably point out that under normal driving conditions, the AFR hovers around a pretty healthy 14.7.

Captain KenpachiCaptain Kenpachi

2 Answers

Actually running rich is less dangerous than running lean. I don't know specifically about Subaru, but in general, running lean can cause detonation more easily. And yes, while 14.7:1 is the stoichiometric ratio deemed as perfect for an air fuel mixture, running rich has the big advantage of reducing the chances of pre-ignition or pinging. In a turbo car, this could be considered death rattle because it can destroy the engine quickly. I found this entry on the interwebz which sums things up pretty well in concern of boosted engines:

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.

The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.

Mind you, while we say 14.7:1 is the perfect air fuel ratio for a complete burn of the mixture, most vehicle manufacturers run their tunes a bit rich in the first place straight out of the box. This holds true whether the vehicle uses forced induction or is naturally aspirated. Because of this, I was a bit surprised you said your car runs at 14.7:1 when not under load as it doesn't seem right ... that is unless you've had some tuning done on it.

Pᴀᴜʟsᴛᴇʀ2♦Pᴀᴜʟsᴛᴇʀ2

So after LOTS of research, it turns out that my Subaru Forester XT is running at 10.5 AFR at WOT because Subaru's don't like our 95RON fuel very much and it's a safety precaution to add an additional margin for error to prevent knocking if I ever tried filling up at a dodgy fuel pump in rural Africa.

Captain KenpachiCaptain Kenpachi

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The symptoms of a car not accelerating like it used to can occur on many high mileage vehicles. A driver might not notice these signs in normal day to day driving but they become abundantly clear when driving up a steep hill or when quickly trying to accelerate into fast moving traffic.

Z20let Bad Knock At 5k Rpm Calculator

It is in those situations that drivers will come to notice the obvious slow acceleration and the fact that the engine of their vehicle is struggling to keep up.

Read also: Symptoms of a Faulty Speed Sensor in Your Vehicle

Common Causes of Car That Won’t Properly Accelerate

This is a common problem in high mileage vehicles but there are no serious or immediate concerns regarding major engine problems. Some of the causes are actually minor and should be checked first in order to locate the issue and possibly fix it as well. Here are some of the most common causes:

#1 – Mass Air Flow Sensor Clogged or Malfunctioned

The mass air flow sensor (or meter) is located and attached to the inlet air cleaner. Since the function of the air flow sensor is to measure air mass that is flowing into air intake, a clogged or bad mass air flow sensor could send the wrong data to the engine ECU for calculating the air fuel mixture. A symptom of an air flow meter malfunction is car that won’t accelerate correctly.

#2 – Oxygen Sensor Malfunction

An oxygen sensor is device whose function is to monitor the exhaust emissions of the vehicle so that it can analyze the air-fuel ratio going through the engine of that vehicle. To make a long story short, a car needs a proper amount of fuel in order for the fuel to burn properly in the combustion cylinders so that it may run smoothly and may accelerate as required.

This sensor sends the information about the amount of fuel being used to the computer unit of the engine and if this sensor get damaged, then the engine of that vehicle will have no idea what ratio of air-fuel mixture to use which may result in a fuel rich mixture. This can cause the slow acceleration of the vehicle even with the accelerator pedal completely pressed making it unreliable when it’s needed.

#3 – Malfunction of TPS

The basic principle of the throttle position sensor (TPS) is to detect the throttle valve opening angle which is controlled by accelerator pedal. Then the TPS will send this data to the ECU. If the TPS malfunctions, the engine speed cannot be controlled by the accelerator pedal and engine speed will increase or decrease without any press or depress the pedal.

#4 – Dirty or Clogged Fuel Filters

A dirty or clogged fuel filter is another reason for a car not accelerating like it’s supposed to when required to do so. With a dirty fuel filter, the engine won’t be getting enough fuel which means that the vehicle won’t be giving the acceleration performance that it should. Replace the fuel filter as soon as possible.

#5 – Clogged or Dirty Air Filters

Just like a fuel filter provides the engine with the clean fuel, an air filter provides the engine of a vehicle with clean air to be used in the air-fuel mixture which will be sent to the combustion chambers to burn. If an air filter is clogged, then the engine won’t get the right air-fuel mixture resulting in slow acceleration. Replace the air filter as soon as possible.

#6 – Timing Belt

This component is as it sounds. The timing belt is something like a VIP in the list key of components of an engine. If this belt is off even by 1 tooth, it may cause some pretty notable acceleration problems.

However the list of causes of poor acceleration does not end here. Other culprits behind slow acceleration may include a slipping clutch, the transmission system itself, or an unexpected or unrelated problem which may surprise even veteran mechanics.

If you’re not comfortable troubleshooting a car that won’t accelerate, I recommend taking the vehicle to a reliable mechanic and let him give it a full checkup. This will allow him to properly diagnose the issue and recommend a suggested fix.