LT1 Killing Question

[ekiernan]

Let me begin by giving thanks to all of you who have taken time to post questions and responses on this site! Your input has been ENORMOUSLY helpful to many of us who are not as automotively gifted as many of you are. Without some of this information available through your writings, I would not have a drivable LT1 equipped Z. It is far from finished, but it runs and I have put about 200 miles on it to date.

 

I am hoping some of you gurus could speculate on what might be my LT1 killing problem. What I have been able to determine so far is the following:

 

I can be cruising at any speed (10 mph-65 mph+)and I can take the T56 out of gear and coast without a problem. I have the computer set for an idle RPM of 800. The engine, while coasting, will oscillate between 800 and 1000 RPMs about once every three seconds. Fine. I don't like how it does that, but I will problem-solve that later. At least it stays running.

 

When I coast to a stop either with brakes or just a stop through running out of forward progress, the engine with sputter and kill. It appears to start having trouble running at 5 mph and less. The interesting thing is that this happens if I come to a stop in reverse too! It does not matter whether I have the car in gear or not with the clutch depressed. Whenenver I come to a stop, I have to feather the throttle to keep the motor running.

 

After the engine kills, I can restart the engine and it fires right off and runs like a champ (until the next stop sign or light)! Man, I love the power, the sound, and the performance of this V8 Z!!!

 

I am running without a MAF and using speed-density tune. This thing runs perfect in every other way! The motor will idle just fine when first started or restarted after a kill event.

 

Do I suspect a questionable IAC? MAP? IAT? It is a '95 LT1. I have no SES installed yet. I have a USB cable to a 16-pin OBDI connector but have not had success getting my computer to recognize the USB connection on COM1 to determine what or if I am throwing a code(s).

 

Please let me know if I need to throw out more knowns for anyone to take a guess at what might be my problem.

 

Thanks in advance for any and all suggestions!!!

[GOTHALOSISM]

Tripple check all your vaccuum lines. I had that issue when my car had the L28ET setup and everytime I came to a stop the car stalled out. It ended up being a vaccuum line that went to my boost gauge that had a leak in it.

 

By the way beautiful looking Z!!!

[kamikaZeS30]

Idle Air Control Valve, should be somewhere around or on your upper intake manifold or at the throttle body (I don't know where specifically on the LT1), this is a general rule-of-thumb for most fuel injected vehicles.

 

By the way, "bitchin" ride man. Best 2+2, hands-down. I don't normally like them, but yours makes me want one, now.

Edited September 21, 2010 by kamikaZeS30

[Moltar]

Tripple check all your vaccuum lines. I had that issue when my car had the L28ET setup and everytime I came to a stop the car stalled out. It ended up being a vaccuum line that went to my boost gauge that had a leak in it.

 

By the way beautiful looking Z!!!

 

I'm with gothalosism on this one, sounds exactly like a vacuum leak (surging idle, but car runs fine under load).

 

Buy a can of brake/carb cleaner and start spraying parts of your engine that you think might be the cause of a vacuum leak (brake booster, etc), and listen to the engine.

 

If you spray somewhere and the engine chokes and wants to die, that is where your vacuum leak is coming from.

 

Good luck!

Edited September 21, 2010 by Moltar

[ekiernan]

Idle Air Control Valve, should be somewhere around or on your upper intake manifold or at the throttle body (I don't know where specifically on the LT1), this is a general rule-of-thumb for most fuel injected vehicles.

 

By the way, "bitchin" ride man. Best 2+2, hands-down. I don't normally like them, but yours makes me want one, now.

It is a surging idle ONLY when the car is rolling (and not under load) and above 5 or 10 MPH. At a standstill, it is purrrfect! I can definitely hear the LT4 cam I installed. Nice and pleasantly lumpy. That's what leads me to beleive it has something to do with the VSS input into the ECU. It knows for sure how fast (slow) the car is moving. I'm guessing the IAC is affected/controlled somehow by the VSS. I'm almost positive the vacuum lines are tidied up. And yes, the IAC is located just under the throttle body in front of the air intake.

 

Thanks for the compliment on the ride. I know most people don't care for the 2+2's but when you remove the bumpers, they have an eye-catching look. This one is nearly body perfect and VERY rare in Wisconsin as most of them rusted away and broke in half. I wondered at times why I went through the pain and expense of V8-ing a 2+2, but man, I LOVE it!!! The car was my Dad's and has been in the family for 30 years! I'm 42.

 

The attached pic shows what the car looked like for 25 of those 30 years. Of course it was a LOT cleaner. This was a shot I took after I removed it from 10 years of storage.