engine not very responsive 40-50 mph

[signalnc]

OK- I usually only drive this '03 ext on the weekends. My wife drives it during the week. I noticed if I try to accelerate while going about 40-50 mph, like when I try to merge onto the highway from the ramp, the truck seem unresponsive and very slow to gain speed. If I punch it from a stop it goes fine, or from other speeds. Just about 40 mph up to about 50-55 it's seems sluggish. I just added Bg44k 2 tanks ago. It doesn't seem normal to me. No SES light and it's been running great otherwise.
Throttle body is clean, new ac delco plugs, coil packs. Fuel filter is new, but I did not install it. I had it done at a shop. I'm questioning the fuel filter because I have a feeling the use the cheapest parts they can get. Not sure if that would make a difference. They installed an idler pulley 2 months ago on my Jeep and i ripped it out last week because it was screaming from the Chinese bearings.

thanks

 

[gmcman]

When you are accelerating at the troubled speed, are you at WOT (floored) or part-throttle?

How many miles are you at?

Did you perform the plug change?

 

[signalnc]

floored and about 3/4 pedal...If I remember right if you are below half pedal it's less noticeable.

131k and yes I did the plugs myself

 

[gmcman]

Do you know your gear ratio? I'm assuming 3:42 or 3:73 leaning on 3:42. Generally right after 1st gear or near the end, if you floor it you won't get a downshift to 2nd( edited below) and will lug 2nd for a short while right when you need it.

Did you unhook your battery after the plug change? I like to do this so the ECM can relearn the change in fresh plugs.

Have you replaced your upstream O2 sensor? If over 80K on yours it's going to start to get lazy and if it's original could very well be your problem.

I would disconnect your battery first for about 30 min and see what happens, O2 sensor if it's original.

Purchase a GM sensor as well, no aftermarket.

Edit.....you wont get a downshift back to first.

 

[signalnc]

I replaced the o2 sensor on the left side of the motor about one month ago. not sure if it's the upstream or downstream. I got a bosch. I guess I could get a delco and see what happenes

 

[gmcman]

I replaced the o2 sensor on the left side of the motor

Actually it's the right side for sake of argument, always in relation to being in the drivers seat. Against the exhaust manifold it's the upstream sensor.

I can't say anything for the Bosch sensors, they make great products but this engine likes factory sensors to feed the ECM. This could be a suspect.

 

[gmcman]

Did you perform a TB relearn after the cleaning? This generally affects the idle but always something to do.

 

[Chickenhawk]

Fuel filters are almost always the first on the suspect list, and the last to ever cause a problem. They rarely need replacing beyond their scheduled maintenance interval and they are almost never the cause of problems. They are the most oversold item in garages today. In all the years I have been driving, writing about driving and working on cars, I have never seen a single instance where it solved anything.

On the other hand, your vehicle and mileage indicates that your cat converter is suspect. They rarely throw codes, even when clogged so much it barely accelerates on the highway. You should get an exhaust backpressure test done at any garage or muffler shop. Let us know the readings. They will test the pressure at idle and at 2500 RPM. (Pressures should be near 0 at idle and no more than 2 or 3 PSI at 2500 RPM.)

Have you ever had a flashing check engine light that indicates a misfire condition? Is your engine coolant temp gauge needle pointing straight up at normal, slightly to the left or slightly to the right?

Do you feel this is a new condition or are you just noticing this now? That is an awkward speed and unless you really put your foot down to force a downshift, the engine is running below its power band and it will always feel a bit sluggish in that area. (This is one of the reasons why so many manufacturers are going to six-speed and more transmissions.)

 

[signalnc]

I did perform a relearn after the throttle body cleaning...although that was story in itself. I was having idling issues. I installed the o2 sensor and did the relearn and it cleared up.

this is a new condition.

the temperaure gauge has been running at just a hair past 210 for the past 2 or 3 weeks. No misfires.

I will have the back pressure test done, just not sure when. I will post results when i get it done.

thanks for the input

 

[gmcman]

When you changed the plugs, did you apply the dielectric grease to the plug tip?

 

[signalnc]

yes dielectric grease applied

I was thinking I either have a clogged cat or something wrong with the transmission. I have to call around and find someone to do the back pressure test. hopefully I can get someone to do it tomorrow...does anyone know what a fair price is?

 

[Chickenhawk]

They are usually free or very low cost at muffler shops because they want to sell you a new cat converter.

I bought my own backpressure gauge for $80. (We have two Trailblazers in the family but thankfully, haven't had to use it yet.)

When they do the test, note those two readings and let us know. This helps our own database of knowledge for future members.

Cat converters are running on ever-narrowing margins of air/fuel ratios and they are far more vulnerable to day to misfires or temp problems than even 10 years ago. This is why we tend to suspect cat converters more and more as our platform ages. It may not be the cause of your problems but at least it is quick, simple and inexpensive to eliminate as a possible cause.

Certainly much cheaper than a transmission fluid change ... which might be your next step.

 

[signalnc]

I talked to the muffler shop. He said he'd gladly take my $50 for a backpressure test, but all I need to do is take out the upstream o2 sensor and drive it. If the power comes back it's the cat.

just thinking out loud--taking it out would also make the o2 sensor suspect-if it runs better...I replaced the o2 sensor with a bosch 2 or 3 weeks ago...i think an ac delco o2 sensor is in my future

 

[floridarv]

I think he meant with the open o2 hole it would releave the back pressure if the cat is bad. IMO

 

[gmcman]

but all I need to do is take out the upstream o2 sensor and drive it. If the power comes back it's the cat.

just thinking out loud--taking it out would also make the o2 sensor suspect-if it runs better...I replaced the o2 sensor with a bosch 2 or 3 weeks ago...i think an ac delco o2 sensor is in my future

Here's my thoughts on this......you say it's sluggish or down on power when under a good load from 40-50, right at the beginning of 2nd gear. Again, what gear ratio do you have? Open the glove box and on the sticker inside there will be many 3-digit codes, look for either:

GU6 = 3.42
GT4 = 3.73
GT5 = 4.10

You may have a bad CPAS or possibly a dirty one.

I don't think it's the cat, if it was, it would be a turd all the way to the redline...you would feel it.

If you pull the O2 sensor and drive it, then the ECM won't see any fuel and try to add more richening the mixture further adding to sluggishness.

Could you post a vid of the acceleration?

Do you have a vacuum gauge, vacuum pump, digital volt meter?


Do this:

Remove the CPAS wire harness and see if there is oil soaked in the connector.

I really would like to know what the MAP sensor is reading with the engine at idle and what the vacuum reading is.

To measure MAP voltage, check the middle light-green wire with the POS probe and attach the NEG to the battery. Be very careful not to ground the light green wire, you are only checking voltage.

Unhook the battery NEG cable for 30 min and clear the ECM.

I think you may have a bad or dirty CPAS and maybe a bad O2 sensor. You are asking the engine to accelerate at the beginning of a gear and if you have the 3.42 ratio then you will feel that even more.

Are you sure the trans is in 2nd gear when you are experiencing this loss of power and not 3rd? Just ensuring the trans is downshifting and you are in 2nd.

 

[CaptainXL]

I talked to the muffler shop. He said he'd gladly take my $50 for a backpressure test

So will I if you want to pay me $50! Especially if YOU take out the O2 sensor. That's all the work. lol. If they don't have an exhaust pressure gauge then sadly that's not the shop I would want to get work done at.

Honestly, if it revs in park close to 4000 rpm with it floored then it's not a clogged cat. My suspicion would be that you aren't getting enough fuel or the truck isn't downshifting or torque converter isn't unlocking.

But first you want to disconnect the O2 sensor and drive it that way and see if there is any difference. At WOT there shouldn't be because the engine bypasses the O2 at WOT and goes into open loop. Then get a fuel pressure gauge on it and see what you got. Also, see what the vacuum is reading as well.

What was the last thing worked on? That would be highly suspect.

 

[signalnc]

the last thing I did was replace the upstream o2 sensor with a Bosch-so I ordered an AC Delco sensor from Amazon last night, and I'm going to return the Bosch to the store. I'll see if that makes a difference.

The muffler shop has a backpressure gauge, he was tring to save me the trip and the money...he was knowledgable about trailblazers and was asking me if I put the delco iridiums in, and other relevant things.

I will get see if I can get a fuel pressure test done, or just buy one and do it myself if they are not too much money. I also need to drive it more myself to check the symptoms. When I drove it Monday, i was merging onto a highway, it didn't seem to have any power, and when I floored it dropped 2 gears, when it felt like it should have just dropped one. So maybe the torque converter isn't functioning properly. I will have to take it out and drive it around after work. It's my wife's vehicle (mom's taxi), and my commute to work is only 1.8 miles at max posted speed 35 mph.

I just had the all the fluids changed about 2 months ago, including trans fluid.

thanks for everyone's input

 

[gmcman]

Downshifting 2 gears is fine. Make sure you check the CPAS and its connector. If you could post a vid it would really help.

 

[CaptainXL]

I just had the all the fluids changed about 2 months ago, including trans fluid.

See, that's the kind of stuff I am referring too. It all helps.

 

[signalnc]

I want to do a fuel pressure test. can anyone suggest a pressure test kit? there are too many to choose from online. I have a harbor freight and a northern tool about 30 minutes away if they sell anything decent. I don't want to spend too much on it. I just want something that will work. any suggestions welcome.

I replaced the camshaft position sensor about a month ago, but not the crankshaft sensor. I was looking on alldata and it looks like you need a scan tool to do a relearn when you replace the crankshaft position sensor. I don't have a scan tool. I will get one if it's cheaper to buy one than have a shop do the relearn. Anyone have a suggestion for a scan tool? thanks

should i remove the crankshaft position sensor, take it out and inspect it to see if it's dirty--then clean it? without doing the relearn?

 

[Phantom]

I want to do a fuel pressure test. can anyone suggest a pressure test kit? there are too many to choose from online. I have a harbor freight and a northern tool about 30 minutes away if they sell anything decent. I don't want to spend too much on it. I just want something that will work. any suggestions welcome.

I replaced the camshaft position sensor about a month ago, but not the crankshaft sensor. I was looking on alldata and it looks like you need a scan tool to do a relearn when you replace the crankshaft position sensor. I don't have a scan tool. I will get one if it's cheaper to buy one than have a shop do the relearn. Anyone have a suggestion for a scan tool? thanks

you could always go to the parts store and borrow one of theirs. granted you have to put money down but you get it all back when you bring the tool back. i did that when i did the fuel pressure test on my wifes envoy.

 

[signalnc]

thanks Phantom, I think I'll go that route.

I took it out for a drive tonight and it seemed to shift ok. I could not get it to duplicate what I felt Monday with regards to the shifting. It does seem sluggish-- when you ask for throttle it bogs down. more than I've ever noticed. it eventually catches up. I watched the tach while driving and it was getting up over 5k. I didn't try WOT in park.

 

[gmcman]

There has been times when mine just acted like I attached 1000 pounds of trailer to it and I couldn't figure out why. Perhaps being your wifes vehicle, maybe she drives it easy and after you cleared the codes the ECM hasn't seen any WOT parameters to store so when you were getting on it there was a sluggishness and now the ECM is fine tuning the hard acceleration and it feels better....dunno.

There was one time an older MDX was next to me and the wifey was with me and I told her I think he wants to go. She said, yeah right.....that's an MDX. So it was on and when we took off, the voy sounded like an old quadrajet bogging it's way through 2nd gear and I couldn't believe it.

I think the CPAS was on it's way out at the time but nonetheless it just fell on it's face.

I tried my local Harbor Freight recently and I don't think they carry fuel pressure gauges. Sears and probably Northern Tool might but Phantom had a good point on the parts stores.

 

[CaptainXL]

The CPAS IMHO is a critical wear item and should be replaced ever 4 or so years regardless of condition. If the CPAS has never been replaced I would do an engine flush prior to changing it. Otherwise you risk contaminating the new one. Failure to replace a malfunctioning CPAS can result in valvetrain anomalies and excessive wear/burnt exhaust valves.

 

[signalnc]

just to be on the same page as you--the CPAS is the crankshaft position actuator sensor?..on the left (driver) side of engine. Just a few questions.

if i remove it will oil come out? can I remove it, clean it and re-install? Alldata shows a relearn if it is replaced. Is the relearn necessary if I clean it? and is the relearn necessary if I replace it?

I already replaced the camshaft position sensor which was on the right side of engine


thanks

 

[gmcman]

The CPAS is on the right side (passenger) side in the front of the motor. About the size of 2 D-size batteries and shiny.

 

[gmcman]

Sorry I misread your post. The CPAS, is the CAMSHAFT Position Actuator Solenoid.

 

[signalnc]

I replaced the camshaft sensor sensor already...I was talking about the crankshaft sensor-alldata shows a relearn using a scan tool after replacement.

captain did say to replace the CPAS ever 4 years or so, but I was confused if that was the camshaft sensor or crankshaft sensor.

 

[gmcman]

I can't imagine the crank sensor is an issue, I guess I would go ahead and swap O2 sensors since you already have that on the way, and change the fuel filter to eliminate that possibility.

When you change the fuel filter, wear safety glasses. I learned the hard way and what I did wrong is when I de-energized the system, let it run out of fuel, but before I removed the fuel lines I didn't bleed down the system from the pressure test port at the filter.

This gets rid of some residual fuel in the lines leading up to the fuel rail and I received a splash of fuel in my right ear and my eyes. Let me tell you that fuel in the ear is almost the mother of all pains....landed me in the ER.

When you change the O2 sensor, be sure to clear all the ECM again.

 

[signalnc]

this is the crankshaft relearn process from alldata--

1.Install a scan tool.
2.With a scan tool, monitor the powertrain control module for DTCs. If other DTCs are set, except DTC P1336, refer to Diagnostic Trouble Code (DTC) List for the applicable DTC that set.
3.With a scan tool, select the crankshaft position variation learn procedure.
4.The scan tool instructs you to perform the following items:
•Accelerate to wide open throttle (WOT).
•Release the throttle when fuel cut-off occurs.
•Observe the fuel cut-off for the engine that you are performing the learn procedure on.
•The engine should not accelerate beyond calibrated RPM value.
•Release the throttle immediately if value is exceeded.
•Block the drive wheels.
•Set the parking brake.
•DO NOT apply the brake pedal.
•Cycle the ignition from OFF to ON.
•Apply and hold the brake pedal.
•Start and idle the engine.
•Turn the A/C OFF.
•The vehicle must remain in Park or Neutral.
•The scan tool monitors certain component signals to determine if all the conditions are met to continue with the procedure. The scan tool only displays the condition that inhibits the procedure. The scan tool monitors the following components:
•Crankshaft position (CKP) sensors activity-If there is a CKP sensor condition, refer to the applicable DTC that set.
•Camshaft position (CMP) sensor activity-If there is a CMP sensor condition, refer to the applicable DTC that set.
•Engine coolant temperature (ECT)-If the engine coolant temperature is not warm enough, idle the engine until the engine coolant temperature reaches the correct temperature.
5.With the scan tool, enable the crankshaft position system variation learn procedure.
6.Accelerate to WOT.
IMPORTANT: While the learn procedure is in progress, release the throttle immediately when the engine starts to decelerate. The engine control is returned to the operator and the engine responds to throttle position after the learn procedure is complete.



7.Release the throttle when the fuel cut-off occurs.
8.The scan tool displays Learn Status-Learned this ignition:
•If the scan tool indicates that DTC P1336 ran and passed, the CKP variation learn procedure is complete.
•If the scan tool indicates DTC P1336 failed or did not run, refer to DTC P1336.
•If any other DTCs set, refer to Diagnostic Trouble Code (DTC) List for the applicable DTC that set.
9.Turn OFF the ignition for 30 seconds after the learn procedure is completed successfully.
10.The crankshaft position system variation learn procedure is also required when the following service procedures have been performed, regardless of whether DTC P1336 sets:
•Engine replacement
•PCM replacement
•A harmonic balancer replacement
•Crankshaft replacement
•Crankshaft position sensor replacement
•Any engine repairs which disturb the crankshaft to crankshaft position sensor relationship.

 

[CaptainXL]

I received a splash of fuel in my right ear and my eyes. Let me tell you that fuel in the ear is almost the mother of all pains....landed me in the ER.

It burns like a motha. Same thing happened to me 10 years ago working on my brothers ford ranger. Had to go to the er and flush out my ear. Very bad pain. When working directly underneath fluids (especially fuels, alcohols and the like) use proper ppe and stay out of direct line of anything that could drip straight down onto you.

 

[BRomanJr]

I replaced the camshaft sensor sensor already...I was talking about the crankshaft sensor-alldata shows a relearn using a scan tool after replacement.

captain did say to replace the CPAS ever 4 years or so, but I was confused if that was the camshaft sensor or crankshaft sensor.

The CPAS IMHO is a critical wear item and should be replaced ever 4 or so years regardless of condition. If the CPAS has never been replaced I would do an engine flush prior to changing it. Otherwise you risk contaminating the new one. Failure to replace a malfunctioning CPAS can result in valvetrain anomalies and excessive wear/burnt exhaust valves.

Just for clarification:
CaptainXL was talking about the CPAS Camshaft Position Actuator Solenoid, it is not a sensor. Only the Crankshaft Position Sensor requires a relearn as your Alldata states.
You can replace the Camshaft Position Sensor or the CPAS without requiring any adjustments.

 

[CaptainXL]

Yeah, I haven't seen many changing crank sensors . Usually a part like that would be replaced only after being towed to a shop and being told by a mechanic. The crank sensor is more of a go/no go part due to the fact that it is required for the engine to run. I wouldnt suspect it.

 

[signalnc]

ok thanks for the clarification....I did change the camshaft position actuator solenoid last month. I did not flush the motor. I guess I should take it off and see if it's fouled up.

I went down to the auto parts store and they do have a loaner fuel pressure test kit. I know there is a test port on the '03 right at the fuel filter where it hooks into. I'm just not familiar with the test. Can anyone tell me how it's done? I was reading alldata and it says you need a scan tool to "command" the fuel pump on. Is there a way to do this without a scan tool? Also the guy at the store said it comes with a multi meter to test the voltage at the injectors. It looks like the injectors are under a black cover to the right of the throttle body.

 

[gmcman]

The crank sensor is going to be the more expensive route since you need a high-dollar scan tool or pay a shop to perform the relearn. It would be later on my list since it hasn't been disturbed, no codes, and stock PCM.

 

[gmcman]

MAY03LT has a good YouTube vid on testing fuel pressure, pain to link with this phone....sorry.

 

[signalnc]

thanks. I'll look at his youtube channel

 

[gmcman]

It burns like a motha. Same thing happened to me 10 years ago working on my brothers ford ranger. Had to go to the er and flush out my ear. Very bad pain. When working directly underneath fluids (especially fuels, alcohols and the like) use proper ppe and stay out of direct line of anything that could drip straight down onto you.

I change my fuel filter with expanding foam earplugs now...ha. I forgot how long i was rinsing in the sink and i do know when i was finished with the shower I ran the water heater down....was an eternity.

Made it to the ER, drove myself, of course there was some hottie at the registration desk, with her hottie friend visiting.

"What are you here for?"

" I have gas in my ear"

Needless to say it was somewhat comical.