NEED HELP Hesitation to Accelerate

[omer]

hi,
I have Trailblazer 2005 LT 4.2L 6L and i am facing problem while accelerating in between when it reaches to 60 km/h till it reaches 90 km/h and after that it runs fine and until 60 km/h also runs fine but if i press the accelerator it hesitate or fluctuation type of feeling but to get out of this situation i used to press harder and reach above 90km/h and its fine after that.
I went to many mechanics but they couldn't figure it out exactly and told me several things some told me to change
1) coils
2) fuel filter
3) injectors
4) catalytic converter

i already did complete engine overhauling and cleaned injectors ,fuel filter cleaned but i didn't change coils and about the cat its not there the previous owner removed it .
things to be noted there are no any codes coming and i checked with several mechanics and they told me the Transmission is ok there is no issue with it.
i am facing this problem before overhauling and still its there.
now iam confused ........i don't know what to do.....need help please
one more thing i noted is the temperature here in our country is nearly 40-47 c and if i travel to some places where the temp is 18c it seems a bit ok but still have issue .

 

[Mooseman]

Let's go through that list:

1) Not likely. Coils either work or they don't. You would get a misfire code and CEL
2)Fuel filter cannot be cleaned. Must be replaced. Being in a dusty area, it wouldn't be a bad idea.
3) Injectors are also like the coils but could be clogged. If getting full power on acceleration, this might not be the problem.
4) Since you don't have a cat, this would not affect anything. How did the previous owner bypass the error codes? Use the spark plug defouler trick on the rear O2 sensor?

How many KM on the truck? What about maintenance? Fluids and spark plugs done lately? Use only ACDelco 41-103 plugs? Have you cleaned the throttle body and disconnect the battery for 30 minutes?

 

[Matt]

Does the 05 still have the fuel filter on the outside or is it in the fuel tank and n/s?

 

[omer]

Thank's for the reply
@Matt : in 05 its inside the tank
@Mooseman : regarding the cat when i told my mechanic he told me the cat is missing and there is no o2 sensor , and i have a doubt that is this the thing which is effecting .
I bought it 280000 km on it and the head gasket blown ,after that we overhauled it completely with new piston and all the o-rings almost every thing with new oil pump.and cleaned the throttle body and changed the air temp sensor also .and the spark plugs are new from acdelco.
the main thing is that the problem was there before overhauling and still remains ....after overhauling its done 8000 km only.

 

[Matt]

@omer that's what I thought. A pain to drop the tank to change the filter. Doesn't make sense to me why they changed it from where it is on my 02.

 

[budwich]

Do you get a different response if you select 3rd on the transmission instead of "D"? Note that things should "shift" properly all the way up but you should see higher rpms once in 3rd and in torque lockup than with "D".

Get data logging thru app like torque looking at things like timing advance, air flow / mass.

 

[Mooseman]

Should also track misfires.

 

[mrrsm]

If the previous suggestions work... Okay!... But on the outside chance they don't solve the problem...my guess is that since this is miss is happening at highway speeds... that once again...the CPS (Cam Position Sensor) and the CPAS (Cam Position Actuator Sensor) might be the two likely culprits... especially if the PCM is NOT Throwing any DTCs. These three videos show where these are...and what to do to R&R them both:

First the easy (and least expensive one) to start with...

This one is probably the Bad Guy... Because it can get clogged and cause misses and NOT throw any codes:

Un-Boxing with GOOD Information on why to USE THE OEM vs After-Market

This one show the actual R&R of the CPAS and DO AN OIL AND OIL FILTER CHANGE!:

 

[omer]

THANK'S i will check it and will update...........

 

[omer]

@budwich can i use the ELM327 OBD2 device to do that. because i don't have this device and in market i didn't find the original device.only duplicate one.

 

[budwich]

I think that's how a lot of people use it, I do with an android tablet and bluetooth elm327. Yes some of the "east asia" devices don't work but for their cost, you can try a few without breaking the bank. There are posts on here of people's suggestions of which are "better".