volsfan32119

So I started digging deeper to figure out my blue smoke and oil consumption issue. I did a bunch of research and decided a compression test and leak down test were in order. When I pulled the plugs 7 had a ton of oil on it so I knew that would be the problem. Sure enough 7 was an issue. I only had time to do the compression test today on the odd cylinders. 1, 3, 5 were all around 220 psi but 7 was only 60. A couple squirts of oil and the number stayed at 60 so I'm thinking my leak down will reveal a valve issue. My question is why is it always 7? When I was researching I found people said 7 is always the issue. Why is that?

mk3cn4

iTrader: (3)

Someone may correct me, and this might not specifically relate to your apparent valve problem, but my understanding is that #7 generally gets slightly more air due to the manifold design or something because the air shoots straight back that direction, and tends to make it the leanest cylinder, so that's why you always see the broken ringland or whatever in that cylinder. A good practice is supposedly to bench flow the injectors and put the fattest one in that bank on #7.

I'm not a pro so someone correct me if I'm wrong on that.

99 Black Bird T/A

iTrader: (4)

The rear cylinders are the last cylinders to receive cooling and tend to runner a little hotter as a result.

When fuel injectors are flow matched the injector that flows the most is often put on #7, 2nd most on number #8 and so on so the rear cylinders will be slightly richer.

Thats my understanding..

Paul Bell

iTrader: (9)

This is usually reduced by having four corner cooling like the LS1 & 6 had. But, some people will argue against it. I say it can't hurt and it can help.

truckdoug

iTrader: (25)

do people running carb style intakes go lean in the back too? are we talking forced induction only?

I kept it very safe fuel wise on my previous build, but this next one I'm gonna turn it up a few clicks. I had my 80's flowed thinking i'd put the fattest one in #7 as mentioned

They were all w/in 2% of each other so there goes that plan, lol

I switched to the NNBS intake after reading some BS on performance trucks about how they never get lean looking plugs now they run the NNBS intake.

Also rustled up a 4-corner steam vent thingy too.

we'll see in a coupla months.

elliss12

iTrader: (1)

It sucks the plug is soaked, wouldve been nice to see its color. If a ringland is broken / scored then a little oil wont bring up your compression results id imagine. Do a leakdown and verify the source. Oily cylinder plus low compression sounds like ringland versus stuck open valve in my opinion. I had bent valves that still gave 210psi cranking compression. My ls1 had a very oily cylinder 7, it fouled the plug in like 30 miles. 8 was also bad. I was lucky that a marvel mystery oil soak in the cylinder fixed my issue.

volsfan32119

Thanks for the replies guys. I'll post the results of the leak down test when I do it.

pantera_efi

iTrader: (1)

Hi, I too would like to know if the # 7 myth is true ?

My facts :
My more common LS engine is the BAJA race engine, MOST ALL with "reversed" intake's, mostly OEM's.
The water flow is direct by the head gasket with results of LOWER rear cylinder temperatures.
This FACT is simple to observe, with empirical data, as is often the case with a "buggy" rear engine application, the rear CLT sensor location is most common.
I too agree that a plugged steam vent could cause a problem, too easy to find, thus "ruled out".
I worked with PML when we created the LS-1 "carbe" intake.
Then the GM engineers created their version of the LS-3 Carb Intake.
We, at Weger's, found their center four cylinders where lean with results of great power drop.
They were asked if we could change their core box, "no" was stated, which we did on our own, the improvement changes.

A BROKEN piston cause is too much advance in that cylinder, NOT A FUEL PROBLEM.

My question, the #7 question, is "could this be related the the use of a 24xe TW".
I know the 58x is a more accurate method of advance specification.

The answer could be know by the difference in TW tooth count with frequency matching of broken #7's in early engines vs late engines ?

Lance

joecar

Hi Lance,
Are you saying detonation from over-advanced ignition...? Wouldn't under-fueling also cause the same situation...?

truckdoug

iTrader: (25)

I have heard about the precision of the signal from the different 60-2 (58x) and the 24x tone wheel types...but never thought it could lead to enough variance in ignition to cause a problem, especially one that manifests in mostly the rear cylinders

pantera_efi

iTrader: (1)

There is little chance to damage an engine when the correct amount of Spark Advance is specified.
The chance is MUCH GREATER with an "over fueled" engine (11:1 AFR) and too low of a Spark Instant.
That is a case of MELTED piston crowns in very little time.

The OEM GM Advance Strategy is a "low" advance value with a near "stoike" AFR at WOT.

The chemistry of combustion states max combustion temp is greater at a point below Stoichiometric then as above "stoike" the combustion temperature become less.

My method for Knock Detection IS Crankshaft Acceleration Measurement.
We DUE NOT use a Piezo Cristal Microphone for timing retard Knock Detection with the ECU-882C.

My GUI allows for a graph to be created when measurement of the Target Wheel is needed.
This graph, when viewed, can observe changes in crankshaft position related to engine operation.
I do not often use the 24ex TW, thus my experience with that crankshaft "flex"/"harmonic" is limited.

Thus observation a #7 problem is low if not for this thread it would be unknown by myself.
The truth is I have no engine failures related to engine control by the ECU.

Lance

RockinWs6

I lost number 7, broken ring land....................7 runs lean because its at the back of the intake. Gets a bit more air jammed into it. The problem isn't too bad on the stock tune, GM knew this and tuned for it. Still a big problem, I, seen a bunch toss number 7 into the exhaust.

When mine broke number 7 the fuel filter needed changing. Fuel pressure was a little low(55psi) @WOT and it cooked number 7. Of course I didn't know the fuel filter was dirty, I didn't even hear it ping or anything. It happens pretty quick in these engines.

When I pulled the spark plug the porcelain around the electrode was damaged and of course it was soaked with oil.

volsfan32119

Results are in and it's not good. Leak down confirmed I have problems in the lower end. Motor will be out soon and I will build it better, stronger, faster!

So I was trying to figure out when this might have happened. I go back the the last time I got gas and I think I might have accidently put 87 octane in. Could that have caused the problem?

ddnspider

iTrader: (26)

it has ZERO to do with the intake manifold. Plenty of data shows when the intake is turned around it will still hurt number 7 first.

truckdoug

iTrader: (25)

so its a timing issue like lance said?

does it happen on 58x cars as often as 24x cars?

ddnspider

iTrader: (26)

it's a lack of coolant circulation to the rear cylinders which make them run hotter, which means they can tolerate less advance. So technically people blow them up from too much advance but that's not the true root cause.

truckdoug

iTrader: (25)

so its steam pockets/hot spots in the head causing pre-ignition?

is that due to the water circulation system or the steam ports?

SB2 engines used external coolant tubes between the heads when a similar problem arose in early testing

ddnspider

iTrader: (26)

since more seem to have issues when the steam ports are blocked a la ls6, it seems like the steam ports being blocked is the issue.

truckdoug

iTrader: (25)

well sheeit. i switched to a NNBS intake on the turbo truck for nothin i guess

kinda looks cool i guess

ddnspider

iTrader: (26)

what's an NNBS intake? Why did you swap, though it was the stock intakes fault?