While doing some street tuning last week, I was netting 13 psi through my D1SC on a 383. Tuning was cut short due to a poly lock coming loose on one cylinder and the stall converter blowing through. Since I have replaced the stall and replaced the poly lock on the #2 cylinder...Now I can only net 9psi and the car is pig rich on the in-car wideband. It was tuned at 10.8-11.2 at WOT and now slams <10 at WOT.

With the down cylinder, did this increase my boost PSI??

What has screwed up my A/F ratio?

 

Your saying your psi was up though and then you lost the poly lock, right? Thats what it sounds like. I wouldn't that would cause a boost increase. I'd be more concerned about the psi decrease "since" you lost the poly lock. Bent valve or pushrod maybe? You never know what that valve could do losing a poly lock. A compression or leakdown test should verify that.

Ken R.

 

I agree with rogue. It's not a loose rocker that caused increased manifold pressure - it's a loose rocker that potentially damaged you're valvetrain. The wide band now is indicating rich because you have a cylinder that is squirting fuel - but not flowing air due to a bent pushrod or something else in the valvetrain.

Are you running a turbo? If so, probably why you are seeing less boost now. You could have a dead cylinder, less energy.......

I know this doesn't sound real nice but it's just my initial thoughts.

I don't think a leak down will indicate valve train condition, only how a cylinder is sealing at TDC. So unless a valvehead/seat has been compromised (or maybe a bent valve not seating) this might not reveal pushrod/camlobe condition.

EDIT - Just read you are running a blower, not turbo. Sorry about that.

Lets us know what you find.

-Scott.

 

Positive manifold pressure or boost is relative and is a measurement of restriction inside an engine. If by saying you had a poly lock come loose, you mean that rocker arm adjustment was changed and resulted in the intake and/or exhaust valve opening partially or not at all, this could very well be the reason you were seeing an increase in boost on your gauge. How long was this occuring? More information would be helpful. Not sure what you mean when you say the "stall convertor was blowing through".

 

BoostedZ28, This only occured for about 3-4 WOT pulls on the interstate during a street tuning session. The poly lock came completly off, but the stud was still in the rocker, so the valve was only opening partially.

At WOT when the car downshifted to 2nd at 60-70 it would bounce off of the Rev limiter and eventually shift with a slight lift of the throttle and would stay on the Rev limiter when shifted to 3rd. The converter couldn't hold the power.

Rogue/LT1 the stud that the push rod rides in, only backed out a few turns, just enough to not let the valve open completly. Do you think that this possibly bent the push rod??
Thanks!

 

I would double check it. Pull the pushrod and see if its bent. Do the leakdown or compression test also. If your good on all fronts, then no worries, but at least you'd know. I'd be worried about bouncing it off the limiter also. It can only take so much of that.

Ken R.

 

Was the loose poly on the exhaust valve? If so MAYBE when you reinstalled it you over tightened the valve. That would cause the valve not to fully close which would cause boost to drop and a rich condition. Just something to think about.

 

SC/TA I was thinking that also, along with I am hoping that I might have fouled that plug with all the added fuel sitting in the cylinder and not being pushed out. I am going to replace the plugs and see if that helps, if not you wanna help pulling the covers and looking at the pushrods??

 

Other than rich, how does the car run now? If there was damage done to the valvetrain, it would be fairly obvious based on how the motor is currently performing and how it sounds. Other than additional tuning, are you having an issue with the car currently?

 

Give me a call. I would be glad to help.

 

Boosted, the car doesn't run no where near what it did before. It also sounds just a little differnt on the exhaust note (open cutout). Plus now it blows black smoke like crazy at WOT. The valve train in as quiet as the day I first fired her. The only other thing is it is hard to start when cold, before she would fire instantly.

 

Maybe go back and re-adjust the rockers with the car running and see how she runs.

-Scott.

 

Well, SC/TA and I pulled the valve covers and checked all the rockers. WE did find one that seemed a little tight and one that was just a tad loose. We even fired her up and listened with the covers off and everything sounds good now, even the exhaust note is back to normal. While working on the rockers, I found that I had a small air leak around the throttle body coupling. So with all that fixed, I gained back 2 psi, but car is still PIG rich and makes no power.

So I am still missing 2psi and the car is very hard to start when cold. Originally the car would fire up just like it was a new one. What am I missing here? IS the tune that far off? :Another leak somewhere??

 

As far as the reduction in boost that you are seeing, the only real way to check for leaks is an old fashioned thorough, physical inspection of the intake system. Loosen the clamps and pull the couplings off and re-install them, checking to ensure they are properly installed and seated. If you are not using t-bolt style clamps, now would be an ideal time to "upgrade".

Your "rich" condition can really only truely be measured with an A/F gauge (and/or dyno). Times being what they are today, it's not easy to drop a couple of hundred dollars arbitrarily, but consider the purchase of a wideband A/F ratio gauge. It would help to answer some of your current questions as well as give you information/peace of mind later.

How do the plugs look? Do they all look the same???