dragonZ28

iTrader: (32)

It looks like my car might be down for a few days - maybe even longer. Here's the problem:

I was on my way to school and I had to slow down in traffic. When I began to slow down, the idle dropped drastically - below 400 RPM. The car died at the stop light. I started it up again, it idled low and died again. I started it up again, gave it some gas to keep the idle up and then I drove it onto a liquor store parking lot where it died again.

I tried letting it sit for 10 minutes, but it did the same thing on startup. At this point, I popped the hood and adjusted the idle screw. The idle did go up - above 1000. So, I tried to back it up. I put the clutch pedal down, put the shifter in reverse and began to slowly let up on the clutch pedal. As soon as the clutch disk began to grab, it died again - instantly. It did this several times in both forward and reverse. I let the car sit for about 20 minutes and tried again. It started up and the idle was high. I attempted to drive the car home, and I successfully did so, but it wasn't fun.

Everytime the car slowed down, the idle would begin to drop. This time it went down to about 650RPM. Common sense says that if it started at over 1000RPM, then it should be around 1000RPM at a stand still in traffic. Something just doesn't add up. I had to keep giving it gas to keep the engine running.

I just did a 408 swap, and I'm thinking it's an IAC problem or a vacuum leak. Does anyone have any clues? BTW, I have put 450 miles on the engine without any problems. This came out of nowhere.

wait4me

iTrader: (10)

HMMM,, Fuel pressure is?

dragonZ28

iTrader: (32)

Nope. I have the Racetronix kit, and 48lb injectors. Once the car is driving all is fine. It is not a fuel issue. I have an LSX intake manifold I think may be leaking air through. Like I said, the car has driven over 400 miles. If it was a fuel issue it would have been noticed long ago.

jimmyblue

iTrader: (11)

What else went along with this 408 swap, that is new?

You really want to hook up some kind of logging, the
possibilities are too many to just guess (usefully).

The fuel pressure question shouldn't be blown off, but
checked. Too much down low could expose a crusty
injector with poor narrow-pulse response, etc.

You're looking for one of many possible component
failures that cause an abrupt degradation. Something
as simple as a plug wire come loose. I would not epect
a vacuum leak to develop 450 miles later but one
abrupt maneuver could (and has) g-torqued an intake
just enough to slide open a gap. Easy enough to check
with the propane or carb cleaner (if you can hold an
idle; I'd pull the IAC connector and set it up with the
stop-screw). And on the IAC side, it sounds like it's
not trying very hard. You might need to reprogram
that, and in fact I hope you have the PCM educated
about the different cylinder volume and such... the
idle depending quite a bit on the speed density tune.

But a little logging beats a whole sh!tload of Internet
speculation.

dragonZ28

iTrader: (32)

The car has a LS1edit tune from the previous heads and cam 346 setup. All I did for the new setup was give it a baseline injector tune until I got it to the 500 mile mark. It looks like it will go on the dyno next week for a full blown tune to get everything working as it should, but I didn't want to put a new engine on the dyno. If the car was driving fine for over 400 miles with this tune and holding a good idle previously to this circumstance, I have to rule out a tuning "problem". Even though the tune is not perfect, it won't all go bad at once - there would have been these problems from the get go with 0 miles on the new engine.

Anyway, I pulled the IAC and cleaned it up. It was gunked up, so I hit it with some carb cleaner. Then I pulled the intake. I dismantled this piece of junk today, and that's exactly what it is - a piece of junk. It was leaking around where the top piece meets the base because the gasket was bad, and it was cracked at the base!!! I did a complete repair and service on this thing today, and the quality is much higher now. I tore the entire one piece gasket out, and RTV'ed the entire intake to make a form matching seal - everywhere! The crack was epoxied over from the inside. If anyone has any intake gaskets lying around, I'd be glad to have them. Just PM me with a price. It looks as though mine weren't doing their job. Here are some pics:

Could this be the culprit behind my problem? Looks like a stress crack to me. I think this just might cause a vacuum leak, lol.


The Tri-fecta


A better look at the runners


The shell


Clamping it down to make a good seal

SmokingWS6

iTrader: (1)

That sucks. Glad you got it figured out. Have you fired it back up since trying to reseal the manifold?

dragonZ28

iTrader: (32)

Nope. I'm going to let it all set overnight before I snug it down. Once it's snugged down the stress may break a weak seal, so I'm giving it a chance to set for a day or so. I need the intake gaskets before I put it back on also. They aren't sealing properly because they have been snugged on and off about 10 times. They are flat as can be, so they aren't sealing very well. They have no body left to them so to speak. I hope that this is the problem and I have corrected it, but it may just be a small part of a bigger problem.

The composite material that these LSX manifolds are made out of is pure crap - nothing else. Just by holding it in your hand, you fell like you can twist it in half and shatter it all to hell. I guess I would call it "brittle". The base has no webbing on it, so it has 0 structural support across the base. When tightening the piece down, you are putting stress across the entire piece, and this stress is all pushed toward the center of the intake. I am willing to bet I am not the only one who has had this problem. The factory GM manifolds are much stronger, and MUCH higher quality. The 3 piece design is just friggin stupid. There is no reason behind this that would be worth doing this. It is a low quality piece that is built with cheap material and sold at a high price. Yes it makes power, but that does not make it a quality part. I'm thinking about getting rid of it for an LS6 intake and some cash. I like having confidence that **** won't break - especially an intake manifold. Intake manifolds should NEVER break - period! That might make for a nice vacuum leak wouldn't you say?