Rpm drop when stopping.

[Hodgeee]

Ill try tomorrow to redo all the grounds, cant hurt.

 

[Doubeleive]

Ill try tomorrow to redo all the grounds, cant hurt.

it's possible but you would think after some time it would just flat out fail and / or throw a code, if this was something new happening then it would seem more likely.

 

[Hodgeee]

Been doing it for about 6000 miles. All the work was completed about a month ago to no avail.

Tried the braking from speed, same thing.

Noticed something i have never seen before. When coasting, between 5 to 8mph at the walmart grocery, the idle would do the same thing continously. It did it in nuetral also. Any faster or slower it would idle perfectly. I did not redo the grounds but did look at them, they all appeared super clean. This truck has never seen offroad before, or towed anything by the new and clean appearance of the entire underside and tow hitch. There appears to be 6 major grounds, from battery, engine block, interior fuse block, main wiring harness to the injectors and computer, and under the A pillar. looked good.

I was wondering, ive read about vapor lock and it was 101 out today, but in the last couple of weeks, with all the rain, its been around 70 degrees out, and it still did it. Just thinking the new thing was vapor lock.

 

[Hodgeee]

Checked fuel pressure, its the same as before the new fuel pump 43 to 48. It settles at 48psi, but with a lot of stop and go traffic, it goes up to 55 to 58psi which is high for the Flex motor. When this goes up, then my idle swings up and down at parkin lot speeds, and sometimes at idle. If i just drive a mile at normal pace, the fuel pressure goes back to regular. My chevy mechanic friend says its vapor lock which he says is common in Vegas during the summer, and a lot of customers bring theirs in to get checked out. This doesnt fix my brake issue, but what also can be done with the vapor lock?

I notice that the fuel lines are right next to the hot transmission, and go right by the exhaust. Surprising they routed it that way. Anyone else have this issue? Can I heat wrap my fuel lines, and will that fix it?

 

[Marky Dissod]

... notice that the fuel lines are right next to the hot transmission, and go right by the exhaust.
Surprising they routed it that way. Anyone else have this issue? Can I heat wrap my fuel lines, and will that fix it?

You could, not sure it'll address this symptom specifically, but can't possibly hurt.
I'm wondering if your alternator is just beginning to show the very first signs of aging past its prime?

 

[Hodgeee]

Tested, seems fine, charges at 14.4volts, plus new battery.

 

[Hodgeee]

I think i might have figured it out. Was going to redo the grounds since its only 91 out, mild for our summers. Decided to remove the throttle body for a deep clean, saw it was a Dorman. I know when you put in a new throttle body accordimg to my service manual, your supposed to do a relearn. So before anything else i followed the steps, and immediately the slow speed idle fluctuation almost dissappeared, and the rpm drop at braking only drops about 100rpms, and a couple times not at all. That is almost normal. The more i played around, the more the idle and braking acted up and got worse.

Its one of the few things i wouldnt normally replace, especially when tested good. I think the throttle position sensor in the new body might be bad or set incorrectly. I ordered a new ACDelco body and will get it next week. Ill report my findings after install and relearn. Worst case scenario it will be all gm original parts.

 

[Doubeleive]

for $36 I would roll the dice just to rule it out

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[Hodgeee]

$138, Acdelco, cheapest i could find.

 

[Doubeleive]

$138, Acdelco, cheapest i could find.

I don't know if I would drop that much just to see if it is the problem, if you buy it off ebay and it's not the problem you can return it and get your money back.

 

[Hodgeee]

Thanks for the tip.

 

[Hodgeee]

Canceled throttlebody order. This morning a friend of mine let me put his 2008 Avalanch tb on. Relearned it, went and drove, does the same thing. Put mune back on, relearned and same thing. I do notice that the idle thing starts when its 100 degrees or higher. Being annoyed, took it to a second dealership, explained it, and they tested it, said they have no idea, and they said they have a handfull of customers Tahoes with the same braking problem. They said mine was the worst yet, same problems, no idea. They did also state a lot of customers have come in and suffer from vapor lock.

At least they didnt charge me the $189 diagnostic fee that the other Chevy dealership did.

So back to square one, i guess ill just live with it.

 

[Dustin Jackson]

Sometimes you just gotta pick your battles for your own sanity

 

[Foggy]

When the ambient temps are up - 100* like you mentioned, the intake air
temps will be 10-20* higher..
At those temps the timing tables pull quite a bit of timing advance out to
protect the engine. I would put money on it that is the issue. When you pull out
timing advance at idle type rpms, the engine runs ****** and can even stall a bit.
If you don't experience this at lower temps, then this is the issue and there's nothing
that can be done ... The only cure would be to set the idle up about 50-100 rpm which
is what I've done anyway as these things idle a little too low in gear

 

[Doubeleive]

maybe find a local tuner and have them look at it and try bumping the idle up 100rpm, some people do that to smooth out the idle

 

[Marky Dissod]

You could, not sure it'll address this symptom specifically, but can't possibly hurt.
I'm wondering if your alternator is just beginning to show the very first signs of aging past its prime?

Tested, seems fine, charges at 14.4volts, plus new battery.

When you tested it, was everything else in the car already ON? That's a bit more of a test, especially at the low-as-possible OE idle.
Which brings me to ...

find a local tuner and have them look at it and try bumping the idle up 100RpM, some people do that to smooth out the idle

Most ten-year-old vehicles appreciate an ever-so-slightly raised idle; among the MANY benefits of getting tuned.

 

[swathdiver]

This is almost always a throttle body issue. What is the percent at idle? Even after cleaning and old worn out throttle body can still do this.

Anyway, if you don't want a new throttle body, have a tuner raise the idle 50 rpms.

It is SOP for @BlackBearPerf to raise the idle speed 50 rpms to eliminate this condition before it even starts as the cars age.

On another note, are any of your engine vent or PCV hoses damaged or kinked? Any issues with the exhaust system? Any engine driven accessories causing a high load? What's the load % or Torque on the engine at idle?

 

[Hodgeee]

I have a tuner that i can look at load, thought about that but never tested it, will check it out tomorrow. They said too late to cancel throttle body, but i can return it once i get it. Might just keep it, especially i trust an AcDelco unit better than the Doorman. Had issues in the past with Doorman items.

Thanks for all the great input! And yes, checked for any kind of vacuum leaks, and replaced the vacuum lines, checked the intake gaskets also.

 

[SNES Avalanche]

Uneven Idle is getting progressively worse but cannot find the cause (Fuel Pump / Injectors?)

Hey all, I drive a 2008 Chevrolet Avalanche (Body has 150k Miles Engine has 40k Miles...AFM sure is great) 6.0 L L76 V8 (gasoline) with a GM 4L70 transmission. I have an issue where the truck's idle has been getting progressively worse over the last few months. Issue: When at a stop the idle...

www.tahoeyukonforum.com

For whatever reason some of us are unlucky and our trucks just don't enjoy idling. I tried almost everything you can think of and to this day I still get the shakes at stoplights.

Tuning my idle RPM helped as it no longer bogs down but at idle the truck vibrates.

 

[Hodgeee]

Ok, bumped the idle from the factory 550 in gear, and 500 in park or nuetral up 50rpms on both. So rpms are still dropping down to 350 when braking, and now its starting to do it even at low speeds, but i think its the temperatures causing it. When it was cooler out it only did it on medium to hard braking. It did make it a little peppier off the line, so will keep the new settings, or do you guys recommend setting it to 650, which seems alot.