It seems to me that a lot of people are having problems with lean-out conditions on their forced induction motors. I have a theory that I wondering if anyone else has considered. A lot of vacuum is being pulled out around the #7 intake port due to the brake booster, MAP sensor and rear port that I utilize to plumb my primary vacuum source to boost my 347 forged Twin Turbo setup. I'm running a LS6 intake manifold.

I'm currently getting a minor pop at RPMS between 2,000 and 2,500 under a load generally in third or fourth gear. When the pop occurs it shows a lean spike on the #1 bank O2 reading on the HP Tuners software for a split second and then it normalizes. My tuner is perplexed as it only occurs under this condition and the car and tune runs perfectly otherwise, boost at 8lbs AFR around 11.6 and 14 degrees timing.

A buddy of mine and I were thinking that I should re-direct my primary boost vacuum source up to the front of the manifold and perhaps even redirect my brake booster vacuum line as well upfront and on the opposite side. I'm going to try this to experiment but was curious if anyone else has tried or encountered this.

 

don't know of anyone that's ever actually tried it but the lean #7 theory is not that too much vacuum is being drawn from the back, but that it's inherent to the design of the intake, it's a more of a straight shot for the air to go to the #7 port so it gets more air.

is this lean condition verified with a wide band or a stock narrow band sensor? It sounds like a tuning issue to me. Is the car running a MAF or SD?

 

There are also theories about water circ around the #7 hole as well. I'd like for you to try it and see what happens though

 

Yes, i believe i remember Kurt post up about them doing a test putting the intake on backwards and still #7 was the issue.

 

Under boost there is no vacuum being pulled.

 

Ditto to what Superman posted.
I believe the common #7 failures are the result of a cooling issue as well.
At WOT under boost the air to the vacuum booster or map sensor is stagnent and will not affect anything with the tune.
Kurt's intake manifold experiment proves this to be the only answer.
Under full power #7 must get hotter than the other cylinders which leads to detonation and eventual meltdown of that cylinder.
A safe tune and forged pistons will give you a much larger margin of safety.
This is part of the reason for going to E85 for my build, much safer power.

 

camarols1 who is doing your tune for e-85?

 

I will be doing my own tuning for the most part.
Tiago was nice enough to shoot me one of his race gas tunes for a
starting point.
I will basicially bump the fuel pressure until the low speed is happy for initail break in
then start street tuning easing into boost a little at a time.

 

Any mis-fire will show up as lean on the WBO2. For the misfire cycle(s), the unburnt air passes through into the exhaust and shows up as excess oxygen. That doesn't really mean that the tune is lean.

 

Check to see if the converter is locked when this happens.
If there is a problem with the tune at that load and RPM it will not show up on the dyno because the engine is not operating under the same conditions.
Maybe throw a little fuel at that part of the MAP and see if it fixes the problem.
Otherwise, check for bad grounds, sparkplugs, etc...
Good luck!

 

calongo SS, Verified with HP Tuners with a Wideband O2 installed, AFR remains good at 11.5 when it does it and the O2 spikes down lean for a split second. I'm running a SLP MAF, could the aftermarket MAF being contributing to this? I have not tried a stock one.

John D, this occurrence happens right at the entry point of boost or just before with the throttle at 30% or less. Any more throttle including WOT and it does not do it.

engineermike, what could be causing my mis-fire as I agree it could be one i just installed new NGK TR6 splugs gapped at .028 and new GM Packard wires.

camarols1, I'm running a 6 speed tranny and it has done it in 2nd, 3rd and 4th gears under a load (going up a hill) at part-throttle and low RPMs. we played around with richer and leaner AFR's and it nothing we did tune wise affected it, like I said the AFR's remain constant it just the driver's side O2 that shows the instant spike

I'll try my vacuum theory and go out for more testing and report back later today.

Thanks guys!!

 

I tune all forced induction cars to 0.75 lambda (11.0:1 AFR) to add safety. I've seen very little difference in HP varing the AFR from 10.5-12.0 so I usually aim for something close to the middle of the road. I don't know the cause of the #7 failures, but a little extra fuel helps keep it alive.

 

i say try out the stock MAF and see what effect it has. Many people have minor/major problems with aftermarket MAF's for various reasons (not saying your tune sucks) but the maf might be the cause of something strange...Doesnt hurt to try and its a quick swap out either way

 

I'm not terribly familiar with LS1's, but I'd try swapping the coils and injectors from one bank to the other to see if the problem swaps sides too.

 

Definitely switch to a GM maf.....if you are opposed to a SD tune for some strange reason.

^^^^Hella 1 post behind you engineermike, lol.

 

Is this a turbo car? Are you already making boost or coming out of vacuum at 2500 rpms and 30% TPS? I wouldn't suspect that you are, if not I would think that it is too rich and not enough advance in the tune.
It's more likely that the MAF is a cause of your problems though.

 

Yes Twin Turbo HP Kit with 57MM turbos tuned at 8psi front mount. Pop occurs in between3 lbs of vacuum and zero boost what I call the transition. I don't have a GM MAF lying around but should I just use a stock one for my car or what? If so, what other GM cars run the same MAF. My mom has a 2000 Tahoe with a 5.3L will that work to try?

 

I think I solved a lot of my problem in my ventilation system which I believe was creating a vacuum leak. I have the ZO6 valley pan cover with the vacuum port. I had this running to a oil/gas separator and then back up into the intake manifold port directly above on the passenger side front. Although this worked well to catch an oil spill over it must have been pushing unnecessary crankcase pressure back into my intake. I blocked the valley cover port off and ran my into over into my boost vacuum lines and I noticed an immediate difference, one my idle was smoother and the RPM's at idle dropped from 850 to 700 (almost too low), after a test drive it seems to be so much smoother throughout the RPM especially in the mid-range from 3-5K RPM in any gear.

I noticed I was getting some oil spillage from my breather that covers my oil fill hole so I decided to take the plug out of the valley cover and run a 12 inch piece of 3/8" fuel line connecting a separate breather, hopefully this will prevent some of the excessive oil that was coming through the oil filler breather. How is the ventilation system suppose to work. I have a hose/breather at the back of the driver's side valve cover and the passenger side valve covers plugged along with the throttle body vacuum port plugged. Assuming I had a vacuum leak that is now cured, what will this do to my AFR because I just had it tuned the other day. Will it now be leaner or richer or about the same. It was set between 10.5-11.5 depending on the load and gear. I would think a vacuum leak would cause it to run lean and now it may be a tad bit richer which is safer anyway right?

 

Not sure you guys are experiencing a lean condition. The corvette guys have figured out that oil from the PCV is drawn from the front tube along the bottom of the manifold to the back which blends into the number 7 intake runner. Oil increases deposits. It also has a low octane rating which can trigger detonation. I can't find any octane info for real or synthetic motor oil, but crude oil has around a 60 octane rating according to text books.

The solution is an oil catch can before the PCV inlet, and frequent emptying out of the oil can (it overflows when it's full). Most of the pics showing a dead #7 also show an oily path into #7 which is usually considered as part of the damage instead of the cause.

 

if thats were the pop is happening it dosent seem thats its matter of #7 going lean. at 0psi all the cylinders will pull in the same amount of air, as in no piston is getting more air pumped in then all the other ones