Intermittent white exhaust smoke in 6th gear

[dcdoug]

I've been more of a lurker on this forum and have solved most questions I had through reading the posts of others. This is for a bone stock (mostly, no engine mods) 2007 GT500. Over the past week I have noticed white exhaust smoke when in 6th gear and either going up a long grade or when off throttle or lightly on the throttle. It only happens occasionally and only lasts for 5-10 sec and then clears up. I haven't seen it happen in any other gear or when pushing it hard. I plugged in my code scan tool (INNOVA 3100) and there weren't any codes, although since it happened during a 100 mile trip home yesterday, it's possible they were no longer in memory I guess.

Any thoughts as to what this could be?

 

[1st usa car]

i thought white smoke meant burnt coolant... but i am no expert.

 

[dcdoug]

i thought white smoke meant burnt coolant... but i am no expert.

Me too - usually head gasket. But wouldn't that happen a lot more often and be more likely to happen under higher RPMs? I don't see how there can be coolant leaking into the oil and then only blowing white smoke intermittently. But that's why I'm posting.

 

[ZOMBEAST]

Check your IC reservoir too. Could have a leak in the intercooler under the S/C.

 

[nxhappy]

white smoke would be burning coolant, you would notice low water levels in 1 of the tanks...also smells sweet like burning coolant ...blueish smoke would be burning oil, which is more likely. Sounds like the latter because you are noticing the smoke under load. That motor is known to get some oil in the intake tract, so a catch can is a good idea.

 

[MastaAce03]

Gotta down shift buddy. Going uphill while under throttle shouldn't be happening in 6th gear unless you're doing 170mph.


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[Papaw]

I'll second the catch cans. They are a good idea even if that isn't the problem to your smoke. If they don't clear up the smoke, then a compression / leak down test is next.

 

[dcdoug]

OK, thanks for the suggestions. I'll check the IC reservoir levels and also for leaks. I'll also look into catch cans. If I don't find anything then will move on to a leak down test.

Weird that it only does it occasionally in 6th gear at highway speeds when engine RPMs are low. I'd think the opposite would be true.

 

[rotor_powerd]

OK, thanks for the suggestions. I'll check the IC reservoir levels and also for leaks. I'll also look into catch cans. If I don't find anything then will move on to a leak down test.

Weird that it only does it occasionally in 6th gear at highway speeds when engine RPMs are low. I'd think the opposite would be true.

Applying load at low RPM in an overdrive (6th is a double overdrive) gear sends cylinder pressures through the roof. Nothing about it is good for the engine. With high cylinder pressures comes higher crankcase pressure. Under boost the crankcase vent system can only vent pressure, there is no longer a scavenging effect with the intake pulling a slight vacuum across the motor. Combine all of that, and you get excess blowby/oil pushing into the intake - most likely the cause of the smoke you are seeing.

 

[dcdoug]

Applying load at low RPM in an overdrive (6th is a double overdrive) gear sends cylinder pressures through the roof. Nothing about it is good for the engine. With high cylinder pressures comes higher crankcase pressure. Under boost the crankcase vent system can only vent pressure, there is no longer a scavenging effect with the intake pulling a slight vacuum across the motor. Combine all of that, and you get excess blowby/oil pushing into the intake - most likely the cause of the smoke you are seeing.

Ok, that makes sense and would support why it stops after it burns off the blowby. I didn't find any drop in coolant levels in either reservoir or any leaks around the IC that I could see and the oil on the dipstick at least is normal looking.

So sounds like adding catch cans and shifting to 5th for any long hill grades is the solution to my problem.