My 04 ENVOY 4.2 sounds like an old man getting up in the morning

jhenry999

Original poster
Member
Jun 28, 2015
27
My 2004 ENVOY 187K mile 4.2 sounds like an old man getting up in the morning. Races up to 1800 then drops off a cliff and slows till its about to stall with loud knocking, then races up again to 1800 with lots of sputtering and coughing in between, then stabilizes around 1000 slowly lowering to it's purring idle speed of 625. All this drama takes about 1 minute.

Its as if the PCM is getting some very bad info from the rest of the team and is tinkering with the timing while trying its best to juggle the air/fuel mixture to please its owner, (me).

No codes. Runs very well the rest of the day. Coils are all firing. Pulled 2 plugs which are tan and dry although the pin the spark jumps to seems very pointy. VVTS connector has no oil in it. No vacuum noises. I spray MAF sensor at each oil change. Tried a 5 minute MOTOR MEDIC Motor Flush before last oil change. Didnt help.

Dont want to start throwing parts at it but wondering about:
VVTS, maybe its clogged. FUEL PRESSURE REGULATOR, its the original. TEMP SENSOR, but its giving normal temp readings to dash cluster.

My 03 Trailblazer did the EXACT SAME THING at 275K before I sold it but also ran great after the first start up of the day
Outside temperature is in the 60s, this is Florida.

With no codes I'm stumped.

Some of you must have had this problem at high mileage. What was the fix ?
 
Dec 5, 2011
574
Central Pennsylvania
I'm an old man.... and I wish I could get up in the morning....

It seems you've done some reading and know a little about this platform - good.

Let's go down the list:
Plugs - "pointy" - probably what they should look like. HOWEVER, if they have over 100,000 miles on them, replace them. ONLY use 41-103 ACDelco GENUINE plugs, and verify they are genuine before installing them. If they are genuine they will not need gapped. Checking 2 is good, replacing all is better. RockAuto is your friend here. There are often rebates on these plugs that will bring the price down to "reasonable" as they're like $8 apiece.
FPR - this doesn't sound like an FPR issue.
VVTS (actually CPAS) - no oil in connector is obviously good. PULL IT and CLEAN IT, just in case. If it spins while installed, REPLACE IT. You may need to loosen the power steering pump to do so.
TEMP SENSOR - if the gauge is reading STRAIGHT UP while at operating temp (not a "tick" to the right or left), then the sensor is good. If it's a "tick" or more to the right or left (left, usually) then you may have a stuck thermostat. These are a PITA to change, but can drastically effect performance and mileage. Many also change the sensor while they are there. The biggest difficulty is that you have to drain coolant to change the sensor. Then it's a matter of getting access to it. I've done 2 and they weren't "terrible", but time consuming. I also reused my coolant which most people will tell you not to.
I used to be a big fan of motor flushes.... until after using them for years I couldn't recall a single instance where they had any noticeable positive impact. I also hesitate doing flushes on this engine as loosening any crud may cause more problems than it solves.

But wait, there's more!

Something you didn't mention and that's very close to "free". CLEAN YOUR THROTTLEBODY. There are several posts/threads on here illustrating how. It should be removed from the intake and thoroughly cleaned, including all the vacuum passages on it (I think there are 2). I usually use TB cleaner and a toothbrush and rag. While cleaning the throttlebody your battery cable should be disconnected altogether. This will cause the PCM to "relearn" information regarding the throttlebody. You may choose to pull PCM fuses instead to avoid having the HVAC solenoids go through their learning procedure which can sometimes break them in their old age. I try to clean my throttlebody every oil change. It's cheap, easy, and fun to do... ok, not fun but cheap and easy.

Air filter - If you can't see light through it, replace it. I blow mine out with compressed air every oil change. Not likely to be causing this issue, though.

Fuel filter - I'm not sure if 2004 has an in-the-tank mounted filter or a frame mounted filter. If it's frame mounted, replace it - there's likely a how-to on this site but above all, relieve the pressure in the fuel lines before disconnecting any of them. You can do this by removing the fuel pump solenoid and trying to start the car. If it starts, let it run until it dies. You will then have zero fuel pressure.

Take a close look at your exhaust. It may be necessary to do a backpressure test. This can be a huge PITA as the oxygen sensor is often impossible to remove. What we're looking for is if the catalytic converter is plugged.

I'm sure there's more to investigate at this basic level that I'm forgetting. Someone will likely be along soon to comment further.
 

rchalmers3

Member
Jul 11, 2020
207
Irmo, SC
Well said @TequilaWarrior However I'd place the Throttle Body cleaning at the front of the preventative maintenance suggestions and monitor the results.

It may be that the T/B has a mechanical issue. Removing it for a good cleaning won't fix a broken mechanical issue, but you may notice a change in symptoms that will allow you to determine if the T/B is the source of your troubles.

Rick
 
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jhenry999

Original poster
Member
Jun 28, 2015
27
Thanks guys. All good advice. You've covered all the basics that I've done before.
Was hoping someone would dazzle me with a magic bullet fix that I'd never heard of before and the car and I would live happily ever after.

I forgot...Here's something I did that I'd like comments on:

I removed the big air intake box so I could watch the butterfly in the throttle body during the ridiculous warm up show. I thought the PCM would be jacking the butterfly around trying to get the mixture right. NOTHING ! It did not move at all.

Is this normal? Or, what would this indicate?
 

budwich

Member
Jun 16, 2013
2,027
kanata
you could be helped by getting some real time data at the start of the exercise and then again once things get "smooth". My initial guess is a "vacuum issue".... note necessarily a leak but a bad sensor or wiring. That would be my first data read along with fuel trims.
 

Mektek

Member
May 2, 2017
656
FL
I just replaced a fuel pressure regulator in an engine that when started fast idled briefly and then died. Pull the vacuum fitting and check if there's fuel in the line........
I also replaced a map sensor in a different vehicle that had exactly the same symptoms.
 
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Some have reported improved performance when replacing the fuel rail mounted fuel pressure regulator. Expensive little bugger so worth it to check your pressure first and for leaking gas, which could explain the idle speed changes.
 

jhenry999

Original poster
Member
Jun 28, 2015
27
Cleaned throttle body and MAP sensor. Cleared codes. A little rough startup and NOW I get a PO455 code. The code says GROSS LEAK...NO FLOW. Finally a clue !

My guess is a BAD PURGE VALVE. I know the PCM plays with it to test it at the start up. It would act just like a vacuum leak. Also the EVAP CANISTER VENT VALVE could be the culprit too.
 

NJTB

Member
Aug 27, 2012
612
Flemington, NJ
Just did the same thing with a P0455, the code was intermittent, maybe once a month. Started by tightening, then replacing the fuel cap. Worked for a while, then the code came back.
Since it's been warmer, I crawled under the truck (2004 TB) and inspected all the lines to the cannister purge and vent valve, and the cannister, everything looked good, nothing worn or loose, and decided to replace both. Relativley cheap, took about an hour. After all, it's 17 years old, right?
No joy, code came back after about the third startup.
Now I'm going to have to get it smoke tested and hope I missed something.
Killer is, when I hook the Tech 2 up, it shows the evap system passed. Go figure.
 

budwich

Member
Jun 16, 2013
2,027
kanata
not sure but I think the system relies somewhat on knowing somewhat about atmospheric pressures which I think come "by" the MAP (calculated or otherwise). If it can't readily determine the manifold vacuum, then it may impact its "work" on other things besides engine running.

Quote: "This diagnosis detects leaks in the Evaporative Emission System (EVAP) purge line using engine intake manifold vacuum"

Anyway, either way, hope you find the "root cause".
 

Mike534x

Member
Apr 9, 2012
894
Just did the same thing with a P0455, the code was intermittent, maybe once a month. Started by tightening, then replacing the fuel cap. Worked for a while, then the code came back.
Since it's been warmer, I crawled under the truck (2004 TB) and inspected all the lines to the cannister purge and vent valve, and the cannister, everything looked good, nothing worn or loose, and decided to replace both. Relativley cheap, took about an hour. After all, it's 17 years old, right?
No joy, code came back after about the third startup.
Now I'm going to have to get it smoke tested and hope I missed something.
Killer is, when I hook the Tech 2 up, it shows the evap system passed. Go figure.

When you replaced the canister, did you blow out of the lines leading from the canister to the purge valve? they usually get dust and other crap stuck in them and a little hit from the air hose will blow it out. My Envoy had it, but it was caused by a hair line crack in the purge valve, and the canister up in the engine bay (V8). When my Avalanche threw it, it was because of the filler neck having a kink in it from one of the tank straps breaking and causing a 1/2 inch dip.
 

NJTB

Member
Aug 27, 2012
612
Flemington, NJ
@Mike534x
I didn't replace the cannister, just the purge valve near it, and inspected the hoses for holes, abrasion, and gave them a tug test to be sure they were on tight.
When I replaced both valves, I was looking for pieces of charcoal and found none.
Nice pick up on the hairline crack, I probably would have missed that.
 

psa55

Member
Oct 29, 2014
116
New Brunwick, Canada
I've had this rough startup for about 18 months and assumed it was related to an intermittent misfire I've got. Mine would rev to about 1400 and down to 500 and almost stall, especially on cold starts in winter. Not as bad at the OP. After reading this thread, I cleaned my throttle body and MAF and I replaced the MAP sensor and Intake Purge Valve. It now starts like days gone by. Nice smooth and consistent idle on start up. Its been fantastic on the cold mornings.

I have developed a vacuum leak however... I am going to test the old purge valve to make sure its not the new valve I put in that may have a crack, but it appears to be the vacuum line from the purge valve to the throttle body. I looked on RA and see that line is not available at the moment. Is this a dealer item or is there any specific vacuum line diameter that I can buy to replace it with? Can I just slide the hose on the fittings at each end or do I need the specific clips on each like the factory hoses?
 
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jhenry999

Original poster
Member
Jun 28, 2015
27
It's me again...the original writer of this thread. I FOUND THE PROBLEM !

I read somewhere that the PCM turns the PURGE and VENT valves in the EVAP system on and off, AT START UP, just to test it...sort of like the AC system tests all the actuater vent doors AT START UP, just to test them. Thats the click, click, click you hear when the AC tests a broken one and cant get past it until you change the MODE button.

I just didn't know WHICH Evap valve it was. So I went to my local Pik & Pull yard, found the newest 4.2, and grabbed both for $12.

When I installed the rear CANNISTER VENT VALVE (the easy one) there was no change.

But, when I put the PURGE VALVE (mounted on the block...the hard one) in...it started up perfectly !

Its like having a new car !
 

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