'Occasional' loping idle...

[Schrade]

Oh the irony.

How many times I quoted myself on vac. leak detection, and I now have AN INTERMITTENT one.

Every so often, it lopes about 300 rpm's at idle. But not always??? When not, it's smooth as can be.

Is there a common place to start? Like plenum / runners, on L98?

Happens only in Closed Loop. Got the can of started fluid just earlier...


Vacuum leak.

Detection process follow to a 'T':

EVERYone should bookmark this post too.
Originally Posted by Blade_1 View Post
Idle to warm motor. MUST BE CLOSED LOOP WARM.

Brick on GAS pedal, block TB linkage, anything to get a STEADY idle - about 2k rpms. MUST BE STEADY.

Spray starting fluid everywhere air can get sucked in, A LITTLE AT A TIME, until the motor surges. That's the leak. If it's in a tight spot, light a book of matches, blow them out quickly, and feed the smoke to the draw. Don't stop when you find one leak either. Plenum-to-runners, runners-to-intake, emissions hoses (EVAP cannister, EGR) valve covers, brake booster, EVERYWHERE. Vacuum/intake leak will make a 2012 car lope idle, and also will make a 50 year old lawnmower lope idle.

Bookmark this page!!!


This ain't hard. But I know of only 1 PERSON who ever got it right, and found his intake leak.

Even mechanics don't know how to get it right...

[scottfab]

Oh the irony.

How many times I quoted myself on vac. leak detection, and I now have AN INTERMITTENT one.

Every so often, it lopes about 300 rpm's at idle. But not always??? When not, it's smooth as can be.

Is there a common place to start? Like plenum / runners, on L98?

Happens only in Closed Loop. Got the can of started fluid just earlier...

L98 I have no idea. On an LT5 I do.

[XfireZ51]

Sticking throttle?

[mike100]

tank purge solenoid leak near throttle body? or failing injectors- that's pretty common on the LT5.

what is your warm idle speed?

[Schrade]

tank purge solenoid leak near throttle body? or failing injectors- that's pretty common on the LT5.

what is your warm idle speed?
Holds 650 steady, when not loping...





L98 I have no idea. On an LT5 I do. Hmm... ok - let me 'clarify':
Oh the irony.

How many times I quoted myself on vac. leak detection, and I now have AN INTERMITTENT one.

Every so often, it lopes about 300 rpm's at idle. But not always??? When not, it's smooth as can be.

Is there a common place to start on LT5? Like plenum / runners, on L98?

Happens only in Closed Loop. Got the can of started fluid just earlier...
Of course, if it's your secret, maybe I can PayPal some $$ for the info?

And then again, maybe I can't. ;)

Smartass. :rolleyes:

Rest assured Sir; you are NOWHERE near as smart as yours [not so] truly......

[sammy]

just a thought , what kind of connection do you have on the map sensor ,at the back of the manifold , if it is just a rubber hose that could cause it ,if it is either too long or if the rubber is getting weak .

[Paul Workman]

Some thoughts...

Dropping RPM is not what usually happens with a leak (in closed loop): just the OPPOSITE.

Is typical of an injector failing -the loping idle, drop in RPM; affects specific cylinder(s).

Also typical of ingnition: start with the easy stuff - change the plugs first

Fuel pressure? Leak-down test results?

Just sayin...loping, low idle, affecting specific cylinders...an air leak isn't on the top of my list (nor is casting dispersions)

P.

[Schrade]

Some thoughts...

Dropping RPM is not what usually happens with a leak (in closed loop): just the OPPOSITE.

Is typical of an injector failing -the loping idle, drop in RPM; affects specific cylinder(s).

Also typical of ingnition: start with the easy stuff - change the plugs first

Fuel pressure? Leak-down test results?

Just sayin...loping, low idle, affecting specific cylinders...an air leak isn't on the top of my list (nor is casting dispersions)

P.



just a thought , what kind of connection do you have on the map sensor ,at the back of the manifold , if it is just a rubber hose that could cause it ,if it is either too long or if the rubber is getting weak .

So such a symptom CAN be generated electronically?

With the tranny controller module and speedo module giving occasional flashing / trouble LED's, and nasty new static on the radio, I suppose that's suspect as well...

Static FP tests consistently holds 47-48 PSI; leakdown to 45 after :15. Dynamic FP not yet checked yet.

[scottfab]

Oh the irony.

How many times I quoted myself on vac. leak detection, and I now have AN INTERMITTENT one.

Every so often, it lopes about 300 rpm's at idle. But not always??? When not, it's smooth as can be.

Is there a common place to start? Like plenum / runners, on L98?

Happens only in Closed Loop. Got the can of started fluid just earlier...

I didn't get the indirect reference to LT5 above. It's clear now that there is such a reference.

I can only speak from my own experience. Start by draining any oil build up there is in that little hose going to the MAP (as has been mentioned) then clean out the carbon in the IAC tunnel and the IAC itself. It's easy to get at and causes variation in idle if crud builds up there AND it is intermittent. IAC build up can also cause high idle.

The permanent fix for the oil in the little hose is to angle the center part of the hose higher than the other ends (tie wrap).
The fix for the IAC is to run the car hard now and then. Short trips tend to build carbon up in it.

Every car, no matter what it is, has it's quirks and reoccurring issues. Years ago I started a "sound library" to catalog stuff such as piston slap, bad water pump, etc. (it never caught on) I never did catalog the loping idle issue I had since I began collecting sounds after that. Too bad else you could compare what you're hearing to it.

I believe in sharing freely what I learn (no Paypal) and what I learn from others. The only exception is when I have to physically provide hardware and hours of work :neutral:
Well I guess there was two other exceptions. One was when I sold and donated all profits to "mom" from making CDs of each spring gathering at the NCM by the then ZR-1 Registry AND the other was a "How to Rebuild the ZF6" CD in 1999.

Good luck finding the source of the loping.

[Schrade]

I didn't get the indirect reference to LT5 above. It's clear now that there is such a reference.

I can only speak from my own experience. Start by draining any oil build up there is in that little hose going to the MAP (as has been mentioned) then clean out the carbon in the IAC tunnel and the IAC itself. It's easy to get at and causes variation in idle if crud builds up there AND it is intermittent. IAC build up can also cause high idle.

The permanent fix for the oil in the little hose is to angle the center part of the hose higher than the other ends (tie wrap).
The fix for the IAC is to run the car hard now and then. Short trips tend to build carbon up in it.

Every car, no matter what it is, has it's quirks and reoccurring issues. Years ago I started a "sound library" to catalog stuff such as piston slap, bad water pump, etc. (it never caught on) I never did catalog the loping idle issue I had since I began collecting sounds after that. Too bad else you could compare what you're hearing to it.

I believe in sharing freely what I learn (no Paypal) and what I learn from others. The only exception is when I have to physically provide hardware and hours of work :neutral:
Well I guess there was two other exceptions. One was when I sold and donated all profits to "mom" from making CDs of each spring gathering at the NCM by the then ZR-1 Registry AND the other was a "How to Rebuild the ZF6" CD in 1999.

Good luck finding the source of the loping.

Thank you very much Sir...

[VetteVet]

Schrade,

One other thought is an air leak in the exhaust. Negative pressure pulses can draw "false air" through an exhaust leak upstream of the O2 sensors and can cause the symptoms that you are describing. I would especially look in that direction if the loping that you are experiencing comes when coming down to idle after running the car at highway speeds (heat in the exhaust system opening up the leak).

Jep

[Schrade]

Schrade,

One other thought is an air leak in the exhaust. Negative pressure pulses can draw "false air" through an exhaust leak upstream of the O2 sensors and can cause the symptoms that you are describing. I would especially look in that direction if the loping that you are experiencing comes when coming down to idle after running the car at highway speeds (heat in the exhaust system opening up the leak).

Jep

This one sounds pretty good too there Sir ...

The shop that did the tranny swap did have the exhaust down, and I do swear it sounds different (although I hadn't done that many miles on the car before). Doesn't SOUND too much like a leak, but will keep this is mind.

Will try to get a data log somewhere here as well.