A quick history....
Car is 85 Corvette, L98 TPI

Did head, intake, rocker arm, & longtube header swap.
Car should easily make 275rwhp. (It made 239 w/dyno issues highlighted below)
Car had some vacuum issues, since taken care of.
Car wouldn't go into closed loop.
Traced it to bad injector connection in header tube that had O2 bung welded into it. Fixed it.
Car ran excellent.
Took car to dyno and only change was filling up gas tank for the drive.
Find 40hp in my buddy's blower tune, can't wait to caveman tune mine (FP and timing).
6 passes on dyno, all of them at mid to high 12:1;s, but then with severe misfire/very erratic AF readings at 3300 rpm's, then by 4000, over 14:1 AF and no power. Car does NOT lose fuel pressure when it goes/shows lean.
I had some brand new injectors anyway (24's), so I swapped them out. No difference.

Even though most items were changed during head swap, I swapped (again) the plug wires, cap, rotor, coil, fuel filter. I even re-adjusted valvetrain. Originally at 1/8 turn past zero, now 1/2 turn past zero. (Hydraulic flat tappet stock cam w/1.6's)
Car still misfired and wound up melting catalytic converter guts.
Opened exhaust up, car screamed...for a day or two...again.

Swapped TPS for known good one, no change.
Tried advancing and retarding base timing, no change.
Upped and downed base fuel pressure, no change.

When it misfires, it backfires and/or pops through intake and exhaust. I can best describe it as the sound of a blower car getting spark blown out, or a wiped cam. If I ease into throttle, up to 5800 rpm's, it doesn't misfire. If I rapidly open throttle, it misfires at least half the time, maybe more since it seems to be getting worse.

Charging system is fine. (14.2 volts at idle)
Fuel pressure doesn't fluctuate at all at WOT (38psi)
Timing at stock 6*
Misfires only at WOT (or at least that's when I can feel/hear it.

Any help greatly appreciated.

 

hook up a vacuum gauge, see if it is fluctuating rapidly, or perform a cylinder leak down test make sure you don't have valves hanging open.

 

Vacuum, at idle, is just shy of 19".

I haven't tried leaving it hooked up while driving though.

A leakdown test isn't a bad idea at this point. Thanks.

 

i doubt a leak down test will tell you anything as the car will run okay

6 passes with a 12:1 a/f is proof of that

is this missfire just a couple of cylinders or all of them?

 

I honestly have no idea. When it misfires, it acts like it loses voltage to the plugs, if that makes sense? But even though it is backfiring through the intake and exhaust when it acts up, it still revs up and has more power than stock. When it doesn't misfire at all, the car pulls and revs amazingly well for a TPI car. It seems like if I ease into throttle, then it doesn't misfire at all, or nearly as bad.

FWIW, Fuel Pressure changes mad eno difference, nor did base ignition timing changes.

I did notice that the ends of the plugs, the end of the threads to be exact, seem to be "glazed". A darker color, maybe brown, shiny look to them.

 

well if your plugs look glazed you should go atleast one step colder i think that might solve some of your problems...

what CR are you running, what heads, what cam?

 

CR is 10.7:1
Summit 170 heads
Stock flat tappet cam

 

yeah try going down a heat range...you are using regular sparkplugs right. nothing fancy?


if you spent more than 1.99 you got the wrong plugs!

 

What is timing set at now?

 

Nothing fancy here.

Autolite 3924 is what Summit's instructions recommended, so I can try a 3923 or 3922 if either is available.
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Set at stock setting of 6 degrees initial with EST unplugged. Seemed to more misfire with it bumped up to 12.

 

I had the same problem with my 406 after I did a head swap. The timing was called for 35 degrees I got some misfires and backfires. Set it to 38 and its perfect. I would would play with the timing a little, just becauise it feels smmoth while idling and driving doesnt mean you cant go more.

 

Can you guys tell me if this scan tool would work OK for what I need to do?

http://cgi.ebay.com/Actron-9150-Supe...QQcmdZViewItem

 

You can actually buy those at Auto Zone for alot less. I have a scan tool for OBD1 and no longer need or use it since I have swapper my TPI over to a Carbed motor. Let me look for it and I will PM you.

The only time that scan tool will work is if you have a check engine light on, that will only tell you what the code is and gives you a option to erase it.

 

i beleive some scan tools can actually give your real time data reporting which would be good for your situation or a laptop and a aldl cable which ever is easier

 

Yes some scan tools do, but not this one. The scan tools I believe you are speaking of are well over $1,000.

 

Are you sure?

It says:

Built-in help screens
View live engine data parameters (PIDs)
View freeze frame data

 

I have the OBD11 scanner and it only lets me read and erase codes form my camaro. I havent seen a kit like this, so Im not 100% sure on what it can do. But the scan tool that I am talking baout costs about $2,400.

 

I decided to use WinALDL and a USB/ALDL connector so I can actually datalog what the car is doing. Waiting on cable now.

 

There's a ground strap inside the distributor from the coil to the connector on the side of the dist... is that still there?
Still sounds like an ignition problem to me. I had a similar issue on another car, after much diagnosis and guesses by people way smarter than me, my initial instinct was right... ignition issue. If you can run the RPM's all the way up without it breaking up at anything other than WOT, same problem. Whats the current plug gap? I wouldnt go more than .035 just in case. High compression and weak spark + large gap= misfire at high cylinder pressure.

 

That's been my theory all along, so I hope we're right.

Yes, the small silver colored ground strap is still there under the coil mounting screws. Current plug gap is .035".

Just for kicks, I'm going to throw in a set of 1 step colder plugs. I noticed what looks like a "glaze" around the ends of the plug threads and "maybe" it is arcing out on that? I have a day or so wait on the connector to datalog, so this will be today's guess, lol.

FWIW, I also bought a new Accel super coil off of Ebay this past weekend. Figured it can't hurt.

 

what do you mean a glaze. is it a different color??

 

I'll try to capture it in a picture and post it asap.

 

I tried to datalog today using the WinALDL program. Laptop kept shutting itself off for some unknown reason...

Here's what it sounds like...does it now even sitting still just revving it up:

http://www.youtube.com/watch?v=hD4R08CVQDw

 

Thanks for the replies guys.

Cam is wiped....#7 exhaust rocker has maybe half the movement of all the others.

 

aww that sucks, was it a fairly new cam?

 

Well that was unexpected. Guess you just never know whats going to fail next... Just out of possible weird luck maybe the lifter is bad? Hopefully?

Time for a matched cam for those new parts you got?

 

No, it was the stocker w/1.6's....car just clicked 77,000 miles.

 

thats good in a way though, at least u didnt wipe a new one out... and now theres a opportunity to get one matched perfectly to the powerband you have now compared to the stocker

 

You got that right. I guess I jinxed myself by saying it sounded like a wiped cam, lol.

Whether it's lifter or cam, at this point if I have to take it all apart again so soon...it's getting a new cam. Just need to think about what I want to do and where I want to head with the combo now. I purposely chose all "smaller" parts (170cc chamber heads, stock style TPI intake, 1-5/8" headers, etc...) to compliment the stock cam, so now I'm kind of screwed in that respect.
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That's true.

The ironic thing is that even though the cam is/was wiped, it still did/does outpull the stock heads/intake/exhaust by a ton....

If I could just zone out the intake popping, I'd be good to go.

 

lol i wish myn didnt either, who knows my cam is prolby flat 2 but i wouldnt have it any other way its transportation lol, Still go's good 2

Wish you luck with your Future cam swap

 

Thanks. I appreciate it.