BOONE

iTrader: (4)

Ive been having this stumble or misfire at 6200 to 6500 rpm on the dyno that we can't seem to diagnose. It drops timing a little "can't remember how much" but has no detonation. A/f is dead on at 11.2. When it does it, it shoots out some black smoke for a second, which I think is weird since the a/f is at 11.2 Car has meth, but we tried it without it to see what it would do and it still did it exactly the same. Its a sd tune. I have put in a new crank sensor and had a case relearn with efi live done. Also did new plugs gapped at 32 thou, and new msd plug wires.The heads were off before the dyno appt, but I am pretty sure I was thorough with everything. Is there maybe something Ive overlooked. I would like to have my car home and tuned real bad. Its still at the dyno 60 miles away.

Blown SS

iTrader: (13)

Weak valvesprings maybe?

BOONE

iTrader: (4)

I have the same springs I had before, Thanks though.

RealQuick

iTrader: (14)

Perhaps spark blowout... thats why you see a puff of black smoke at those rpms...

BOONE

iTrader: (4)

I have the plug gap at what I had before the retune. Pulley size has not changed either. Is there something else that would cause blowout I have not thought of ???

BOONE

iTrader: (4)

Would a exhaust leak at the head give these symptoms? I can hear a leak under the hood. I guess that's my next effort.

ls1_doc

iTrader: (6)

Yeah any exhaust leak before the o2 sensor could do that. But i would think the a/f would change if it was an exhaust leak.

Slowhawk

iTrader: (12)

I'm thinking weak valve springs (it happens) or the heads are lifting, or the belt slips.

2000 Tran Zam

iTrader: (2)

how many lbs are you running? I doubt the heads would be lifting unless your pushing 20+ psi.

BOONE

iTrader: (4)

Im running 11 lbs, and I just did new mls head gaskets on a freshen of 3 thousanths and new guides, studs ECT. I have the sdce 8 rib set at 20, and its real clean and dry. No belt dust. Also no 02s since im on olsd, so I guess that rules out the exhaust leak??? Or does it??? The valve springs are the patriot gold I think. Gunnar said they are good to 600 lift, and im at I think 585 on exhaust with the 1.8s. Could this still be it?? They were working well before, to 7000 rpm at times, but I know stuff wears out. I will double check all of the suggestions to make sure Its none of the stuff everyone has mentioned. Thank you for responding everyone. I want real bad to drive my car!!

EPP

iTrader: (22)

[QUOTE=BOONE]Ive been having this stumble or misfire at 6200 to 6500 rpm on the dyno that we can't seem to diagnose. It drops timing a little "can't remember how much" but has no detonation. A/f is dead on at 11.2. When it does it, it shoots out some black smoke for a second, which I think is weird since the a/f is at 11.2 QUOTE]

The black smoke when the car stumbles is common, and what is causing it to happen can be tough to figure out. I agree with Slowhawk on what could be causing it. The first thing and the easiest to replace would be the valvesprings. The Patriot Gold springs are excellent valvesprings, but since you are reusing them, possibly one has fatigued.
Something else to look at. What is the duty cycle of the injectors when this stumble occurs? Bob

BOONE

iTrader: (4)

Hey Bob, I ordered your injectors, the 60s and they are on the car. I had 42s and they made it with my old tune since I am running a lot of meth, but they were over 100%. I think the tuner said the 60s were nearly half of the duty cycle of the old ones. And I'm good with fuel pressure it goes from 62 to 58 at rev. I'm going to unhook the belt on the blower and have the tuner tune for NA and see if it will rev clean, to rule out the springs. I want to replace them on the car if I have to do it. Can I unhook the belt or the bypass to check this way or do I need to check spring rates only. It sure would be easier.

SSprayn

I had the same problem with my car over a year ago. It spent four months in the shop trying to get figured out and never did. Mine was happening in the upper 5000's rpm range and I have a SD tune as well. Good luck

EPP

iTrader: (22)

If your tuner isn't going to charge much, you could remove the blower belt and try it to see if it gets rid of the problem. At $255.00 for Patriot springs and retainers, you might just want to swap them out first though, if the tuner is going to charge you much. It's a tough decision... Bob

BOONE

iTrader: (4)

Thought id bring this thread back from the dead. I found that I had a bad coil, and the place I am tuning at had some truck coils laying there for 3 yrs, that happened to be the Mitsubishi type. We installed them and boom instant revs. Im going to try to feel good about all the other stuff I tried first, I guess I did get some patriot golds out of the deal. And I thought I had golds before but I was mistaken, so my springs were marginal. So now I will install my new fuel system and re tune. We last hit 640 something and was running out of fuel. I snagged a new 180 amp alternator while I'm waiting for my fuel system to arrive, I think I was taxing my old one pretty bad with all the **** I have hooked up. I am hoping to get this car done before salt is put down. Boone

EPP

iTrader: (22)

A bad coil! It makes sense and none of us thought of it... I haven't had but only one or two coil related problems that I can think of, as they coils get more miles and years on them, I'm sure it will be more common.
I'm glad to see you got it resolved, lets hope the salt isn't spread too soon. Bob