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350matt
350matt, Engineer
Category: Hyundai
Satisfied Customers: 20577
Experience:  Mechanical Engineer BEng Hons 22 years experience in car engine industry, 10 years in formula 1
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I have a 05 Chevy Aveo. I 't ask Chevrolet people because it

Customer Question

I have a 05 Chevy Aveo. I don't ask Chevrolet people because it is a korean car. I have this missing like shaking at idle speed and I have changed more than 10 new or used sensors and I tested compression of each cylinder which they all around 125psi. The shaking is caused by timing constantly jumping between -2 to 5 . I tried disconnect different sensors and only got a stable -5 after I disconnect idle control valve.
Submitted: 9 months ago.
Category: Hyundai
Expert:  350matt replied 9 months ago.

Hello

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth cleaning out the idle speed control motor / valve as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile

Customer: replied 9 months ago.
Hi. I am a mechanic myself and this car is really tough for me . It's not lean condition. It's rich. I checked fuel pressure it is normal at 65psi.I added propane gas to vavuum port there was no change on idle speed. OBD live data show long term -17 so engine is rich. also I can smell the rich exhaust. This engine doesn't have an air flow meter. it's only have a MAP sensor. The idle is so bad and I can't sell it. it drives fine . no lost power anything. I changed used ECM and throttle body with idle control and TPS. I changed new crank sensor, cam sensor,air tem sensor, o2 sensor. new coil pack , new wires and spark plugs. The shaking was caused by unstable timing which will stable after idle speed reach 1200rpm or above . Do you know what sensor input can makes computer constantly adjust timing at idle speed? I changed a used ECM and there's no change in problem so the ECM is not at fault.
Customer: replied 9 months ago.
I checked compression and all 4 cylinder at 120 psi. vacuum gauge reading is low but not fluctuate. There is no OBD code except P0300. There is no missing code such as p0301, p0302 ,3030,304.I also took fuel rail out and tested by crank engine. there is no leaks and at 4 injectors have same spree patterns.
Expert:  350matt replied 9 months ago.

Hello

the idle speed ignition timing will move around quite a bit as the ECU uses ignition timing to control the idle as well as moving the throttle variation of about 8-10 ° is normal

I'd check over the MAP sensor connection and wiring and also if its connected to the inlet manifold with some rubber hose then replace this too

Customer: replied 9 months ago.
The wires ok. if the wires is bad then there would be a code. also if the MAP sensor has problem it will have bad acceleration which this car doesn't have.
Expert:  350matt replied 9 months ago.

Hello

is this the version with the MAP sensor on the bulkhead and joined to the manifold by a rubber tube ?

if it is then check the condition of the tube for leaks, cracks and blockages as these can all create this issue

if not and its the one where the sensor is directly on the manifold then that won't be the issue

Customer: replied 9 months ago.
I just checked the vacuum hose for the MAP sensor by blowing it use my mouth and NO restriction. No cracks in good condition. Since the car don't have air flow meter so the MAP is very important. The engine will stall immediately if I disconnect either the hose or wires. This proves nothing wrong with MAP sensor.
Expert:  350matt replied 9 months ago.

Hello

OK in that case have you tried replacing / checking over the coolant temp sensor?

as this has a strong effect on the fuel mixture strength

if you were to disconnect it the ECU should revert to using a default value of a warm engine so I'd try that next

Customer: replied 9 months ago.
The live data OBD scanner tells the engine coolant tempreture. I observed coolant tempreture before I started the engine and it match ambient tempreture after an overnight cooling down. I will give it a try anyway.
Expert:  350matt replied 9 months ago.

understood

good luck

Customer: replied 9 months ago.
Disconnected the coolant sensor and it didn't make any different . Also when coolant tem sensor disconnected live data show -40F so is not warm engine.
It is random miss fire at idle only. ECM never set up any cylinder miss code. It drives fine.engine mechanical should be ok otherwise it won't accelerate.
I re-checked vacuum leaks with a stethoscope and no leaks. very small intake manifold .
I switched new and old o2 sensor and they make no different. the old o2 is only has bad heater
Expert:  350matt replied 9 months ago.

Hello

Ok is this still on a manual / cable throttle?

if so they I'd suggest to clean out the throttle body with some brake cleaner and a lint free rag and also disconnect the ISCV and set the closed position of the throttle so you can get 650-700rpm

the reason for doing this is to try and incrase the flwo of ait into the inlet manifold to try and improve combustion.

Customer: replied 9 months ago.
None of your suggestion worked. You said at beginning erratic timing is normal and I think you are wrong. The erratic timing causing the missfire. I don't now why and that is why I am spending money here to find the answer but you have not give me a answer yet. Can you transfer me to other expert because I am going to rate you 1 star since your answers have not helped solve the problem.
Expert:  350matt replied 9 months ago.

understood

good luck

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