Zipper

I just bought a 6 spd '02 Z28 Convertible. It's my first sports car in almost 10 years - other than that - I've been driving lifted trucks or crotchrockets.

I'm a 4 generation mechanic and have been turning wrenches on and off for ~15 years - mostly building mudders - so I'm familiar with mechanics - just not this car.

It appears there has been some work done to this Z!! I don't know anything about Camaro's or the LS1 motor - but it seems (at the very least) to have aftermarket headers / exhaust system.

I don't know if the motor is an LS1 with LS6 heads and intake - or if it's an LS6 motor - I've been reading the forums - but the difference still eludes me. The heads are marked 243, the intake is 12573572 - but can't find the block number.

There is a rather large black hose running up the drivers side engine compartment that is disconnected and left open at both ends (by firewall and by breather) - any clue what this is for or why it's just hanging out? Probably something to do with EGR.

The EGR Ports on the headers have been blocked off as well ...

The car runs like a top when I'm on the gas. At idle its different - it has a real hard lobe - the car shakes / vibrates when on low idle - it feels like a friggen huge cam more than a low end miss.

My problem is, when I pull off the gas (stop sign / just coasting) the motor damn near stalls - it has yet to actually die - but it gets uncomfortably close - maybe a couple hundred rpm - then jumps up back up to low idle and just purrs waiting to go.

Any info you guys provide would be great!!

2000ultraZ1911

Deff a ls6 top end

Gunslinger09

Check all the usual stuff first, plugs, wires ect and then have the tune looked at. It may have had the heads and cam changed without ever having been tuned.

Zipper

Just went and got the kids from school - a lot of stopping and going.

It's 110 Deg outside. From a "cold" start - the "stop sign effect" was minimal. But after it "warmed up" it was more prevalent. The car idles at ~900RPM. When coming to a stop the idle drops to 4 or 500, then up to 9, then back down to ~6 then back up to 9 and stays there. Every once in a while the idle will drop a 3rd time. Obviously the oil pressure drops (to ~20psi) as well - that's the most worrying part!!

I powered up the viaduct and (clutch pressed) costing down the other side and it still produces the effect - but not nearly as bad as an actual stop. (I was eliminating the "breaking" (electrical / vacuum booster / etc) as a cause)

I even tried holding the clutch with one foot, applying the break and easing the gas with the other foot - which worked - until I let off the gas. It still dropped to ~600RPM.

I'm thinking vacuum leak - but I don't hear any obvious suction noise.

Just wondering if someone might have experienced this and have a direct cause.

garygnu

check the MAP and intake gaskets.also check the PCV system for any cracked or rotted lines,or a bad pcv valve.try cleaning the MAF and IAC .

Majestic9C1

iTrader: (19)

sounds like a major vacuum leak... try plugging that big hose on the back side of the engine (if it is even connected to anything)

Zipper

The big black hoses are part of the EGR system:

Hose #7 (top right in image) is open and not connected. I cannot see Hose #8. Hose #14 (left center) is open and not connected to anything.

#39 (bottom right - assembly) is all connected per diagram.

Missing parts #27 to #33 (both sides) - because of EGR Port block plates on headers.

or pic with parts listing here

Zipper

Update:

A couple of days ago I fired up the car, turned on the AC, and pulled out of my drive way and the car died when I clutched it and let off the gas. Fired backup up and died at the stop sign. Turned AC off and it did not die any more, but RPMs still dropped to ~500 when coming to a stop.

Yesterday I changed a PCV rubber boot (at the TB) that was almost cracked in half. Made no noticeable difference.

Today I changed the plugs, wires, MAP and MAF. I did a compression check while I had the plugs out. Each cylinder was tested twice, each test was 4 revolutions - all plugs were out. All 8 cylinders tested at 180psi - happy as **** with that!!

The idle is much smoother, but still does the 500 dip at a stop - however, I did notice something ...

If I downshift all the way to first, and clutch it above 1700 RPMs, it sometimes does "the dip", but if I wait and clutch it around 1200 RPMs, the dip is not so bad, or does not dip at all.

I checked all the PCV / vacuum lines and all seemed well. Im going to spray carb cleaner around the intake pcv to see if that has any effect.

I pulled the IAC and it looks good - but it sounds funny. Instead of a smooth sounding motion, it sounds more like a stepped motion - if you know what I mean. The plunger was full of carbon build up!! See attached pic.

Two catch cans for the PCV system should be here next week - look at that MAP sensor ...

Probably going to have to pick up an IAC ... anyone have any other insite?

Zipper

Problem fixed!!

Replaced TPS and IAC. Neither one (individually) fixed it, but both together did.

All sensors are new with a lifetime warranty ... no more worries!!

alexus6798

iTrader: (24)

Glad you found your problem and actually posted back in thread. I have noticed that alot of times people ask for help and then if they figure it out, you never hear from them again. Looks like those catch cans are needed..

Zipper

I've searched a number of threads here where the poster had a very similar issue with no definitive answer and it was aggravating. It'd probably be more helpful if I elaborated for future readers:

If I had to do it over again, I'd replace parts in this order:
1.) IAC (~$130)
2.) TPS (~$25)
3.) MAP (~$65)
4.) MAF (~$110)
5.) PCV (~$5)

Test driving between each part replacement.

Plugs, Wires, and Fuel Filter were not needed, but I figured while I was throwing money at it I might was well take care of these as well - plus I wanted a compression check.

Some things I learned about all this.

The IAC plunger should be very difficult to push in. DO NOT pull the plunger out!! It can strip the worm drive threads. If that happens, the plunger can get stuck in the TB - causing the car to not start without gas pedal assistance - when started, the RPMs are high - and when you spike the gas pedal, they do not drop - but linger and slowly fade back down to idle. You can test the IAC by unmounting it from the TB, have an assistant turn the key to the run position. The plunger should extend - but keep a hand over the plunger as it will eject from the IAC - just apply enough pressure to the top of the plunger and it will stop ejecting and retreat a little bit.

Knock off carbon debris from the plunger with finger nail and clean with brake parts cleaner.

If there is a ring or grove around the IAC plunger - toss it! The IAC functioned fine mechanically, but the ring / grove seemed to be the issue. I sanded the grove out - and it helped a lot - I probably did not need to replace it, but it made me feel better.

The TPS I took off did NOT have a return spring - the new one I put on did - A nice perk if you ask me.

If the MAP sensor is "wet" with oil (blow by) replace PCV system and/or get catch cans. Good reading HERE and diagrams here

If you are going to change your plugs - might was well replace the wires - and while the plugs are out - might was well do a compression check.

nonosv6

Thanks for posting your findings. Been having this problem myself for awhile. Will have to check out the iac.

Zipper

No worries. I marked it as a serious issue because of the oil pressure dropping and then the RPMs jumping back up - but I'm sure the is enough residual oil to prevent damage.

Zipper

It's back with a vengeance!!

I've replaced the IAC, TPS, MAP, MAF, PCV, plugs and wires. 180 PSI in all 8 cylendars. She started running pretty good - stay at idle when stopping - etc.

On Sept 7th I added two catch cans to the PCV system.
1.) I have a (new) PCV check valve on the drivers side valve cover running to a Catch Can then another (new) PCV check valve after the catch can before the intake port. (redundant I know ... )
2.) On the passengers side I plugged the rear valve cover port and put a Catch Can between the front valve cover port and the throttle body.

After adding the catch cans - I started noticing small signs of the stall coming back. It's been 340 miles and the stall is back in full force. Stop signs, drive threw, etc.

Today I went and bought (another) brand new IAC and MAP - dropped them in - and no difference. I noticed oil in the hose AFTER my Catch Can on the drivers side. The Catch Can itself has little or no oil in it (nothing on the view tube) but I noticed build up in the hose between the catch can and the Intake port.

I called Speed Daddy and they said the ports on the Catch Can are universal - there is no difference between the "In" and "Out" port.

The first attached pic is the PCV and an adapter that goes between the drivers side catch can and the intake port.
The second pic is how much oil has collected in that little adapter thing (after the Catch Can).
3rd pic shows an empty view tube - but there is residual oil on the view tube walls as if the oil level was that high. That is a band new Catch Can, and it has never been emptied. I assume under heavy vacuum the oil is sucked up the view tube a little bit - but I really don't know - never seen it happen.

I used break parts cleaner all around the front and two sides of the intake and the idle did not jump - doubtful there is a leak around the intake.
Sprayed around the Catch Can, PCV, valve covers and TB nipples and again - no change. Very doubtful it is a vacuum leak - because I replaced all the sensors and **** - and again today - doubtful it is one of those.

I'm going to disconnect the Catch Can and put the PCV system back to stock - but not going to hold my breath. I just wanted to get this posted now to see if anyone can think of something while I put the PCV system back and take it for a test drive.

No Error codes either!!

I'm getting really frustrated with this situation!! Any help is appreciated!!

Edit: cant upload the 3rd pic for some reason - residual oil is on the view tube wall from the bottom to about half way up the tube.