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Discussion Starter #1
A quick history....
Car is 85 Corvette, L98 TPI

Did head, intake, rocker arm, & longtube header swap.
Car should easily make 275rwhp/375rwtq, probably more.
(It made 239rwhp/359rwtq w/dyno issues highlighted below)
Car originally 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, plugs, 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.
Misfire returns...

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 at the same time. 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
 

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Discussion Starter #3
I've checked that....at least a dozen times. But since you said that, I'll probably go check again in a few minutes, LOL.:laughing:
 

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when you took off the cats was there a sensor on there maybe that went into the cats im not sure on these years so im thinking that if thats gone the computer is shittin its pants trying to keep running
 

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Did you address the chip after your mods? Are you running an adjustable fuel pressure regulator? I have an 85 with lots of mods. PM me if I can help.
 

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my cousin had the same issue with his 2000 camaro ss, all it was he had a vacuum issue, after that the car worked fine. Dont know if it would work for u since they r different engines.
 

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Discussion Starter #7
when you took off the cats was there a sensor on there maybe that went into the cats im not sure on these years so im thinking that if thats gone the computer is shittin its pants trying to keep running
No, only sensor anywhere in exhaust is the O2 sensor, and that's been changed to no avail....
 

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Discussion Starter #8
Did you address the chip after your mods? Are you running an adjustable fuel pressure regulator? I have an 85 with lots of mods. PM me if I can help.
No, I didn't worry about chip because once vacuum issues were addressed, the car originally ran great after the head swap. Alvin at PCM4less said I probably wouldn't even need a chip without swapping the camshaft since the 85's shoot for a rather rich 12:1 AF ratio anyway.

I do have an adjustable fuel pressure regulator and have tried lowering base FP to as low as 34, and as high as 45 with vacuum hose unhooked. No difference at WOT.

This severe misfire issue is intermittent, but getting to more "regular" unfortunately.

If it did it from day one, and/or all the time I'd think it may be a tuning issue, but I believe I have some sort of intermittent failure of a component due to the way it will sometimes still fly, and other times pop and backfire like an old POS...lol. Although, even when it's backfiriing and sputtering, it's still pulling harder than it did stock...just sounds horrible. Bad enough that I added mufflers...
 

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

Did head, intake, rocker arm, & longtube header swap.

Took car to dyno and only change was filling up gas tank for the drive.

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, plugs, 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.
Misfire returns...

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 at the same time. 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
It doesn't appear that you have any mechanical issues with the engine (ie, burned valve, valve guide/seal, rings, or compression).

When you removed the plugs, did they all look alike, or did one or two appear different? What DID they look like?

It sounds like the ignition system components (distributor, plugs, wires, etc) are in good working order, and aren't contributing to the problem.

The injectors shouldn't be contributing to the problem (since you've replaced them, and the problem is still there).

The part that intrigues me, and makes me believe that you have an electrical gremlin is that gradual increases in RPM are fine, but WOT (and I'm assuming a snap open WOT) cause an issue.

However, that initially led me to think that the coil might be weak, but my re-reading of the original post confirms that you've already replaced it.

Here's what I think you need to do, and it's probably a pretty "old school" approach. See if you can locate a shop with a Sun engine scanner. You could do this with an oscilloscope, but you'd have to find a really good electronics tech for that. The Sun ignition scanner will show you the actual ignition traces under both light load and WOT load conditions. I believe that you are either losing spark strength or fuel injector pulse width control under load.

Are you running dual cats, or did you run the LT's into a Y pipe to the cat? If you could narrow down a bank, it should make the troubleshooting easier.

On the off chance it might be mechanical, have you performed a compression & leak down test (particularly on 5 & 7)?

Steven
 

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Assuming the car is properly prommed (most aren't) I would say it has to be one of the following: too hot spark plug (consider one step colder),high speed misses are usually hot plugs or lean misses. It could be a vacuum leak, possible bad spring or lifter . . . however if only under hard full load I still say hot plug or vacuum leak.

Let us know.
 

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After the heads were swapped, did you check pushrod length? There is a good chance you could need longer ones if the valve stem heighth has changed. Even the different thicknesses of the heads mating ssurface can make your valvetrane geometry wrong. If you are running roller rockers, take your valve cover off and see if any have score marks on them. Will look like blued spots. Also might loosen a couple pushrods and spin them to see if they get loose then tight feeling. If they do, they are bent. If you are sure your geometry is correct, might try an MSD-6AL box to see if it helps.
 

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Black 08, A&A TI supercharger. Kooks headers. CCW wheels. Much more
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Most things have been covered here.I'm sure you've done this,but just to double check,when the car was "timed",did you disconnect the single tan wire going to the distributor?.If not the timing is off.Sounds like a timing issue,but heres a couple other things,you mentioned everything in the distributor except the ignition module,i ended up using an MSD module,and double check for cracked cap.And this pic below was a problem for me on my 89,my wires were so fried i'm suprised there was current flow at all,replace this terminal board every 75,000 miles..
Corvette Distributor Terminal Board, ACDelco, 1984-1991

Links Wiring Harness to Ignition Module

Failure of This Part Will Leave You Stranded

Replacement Suggested Every 75,000 Miles

 

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Discussion Starter #15
Yes, EST bypass wire was and has always been unplugged to set base timing. Funny you mention module as I actually went to buy one yesterday and parts store was out of it. I appreciate the tip about the harness pictured as well. Definitely worth a look.

Thanks.
 

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Discussion Starter #16
Thanks for the replies guys.

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

:(
 

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Hey bud,
How often do you drag the car.How is the 4 x 3 trans hanging in there?
 

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Congrats on the spot in the back of VETTE magazine this month. You rock!

Sounds like you found the problem with #7. The 85 Vette has a flat tappet cam that is more prone to killing a lobe than the 88 and up roller models.

When you select a new cam, be very careful. The 85 Computer is not a fast processing machine and will easily get off track and cause severe idle issues if you go too large without addressing it.

Here are a couple of things from there "Been there...done it" files on my 85 when it was still running a TPI engine. Going LS2 now...so a whole new can of worms for me.

1. KNOCK SENSOR - They do wear out over time...and or may be over tightened...or just reacting funny. I had almost the same problem out of my 383 TPI motor and chased it for a few months only to find out it had a faulty knock sensor. It took about 20 minutes to jack it up, swap it (you will drop coolant when you do this...so do it quick). It took all the high RPM missfire out.

2. EGR - The 85 Computer assumes the EGR is functioning on the stock CHIP and adjusts the fuel/timing to accomodate an automatic opening of the valve. If yours has failed - it will cause a vacuum leak...if you have removed and blocked it off...your computer will stumble and it should be addressed in the program.

3. HEATED O2 - THIS IS HUGE! The 85 has a one wire O2 sensor. Once you add headers and move it further south in the pipe, it takes longer to "heat up" and will keep the car in closed loop too long - and often cause it to cycle in and out of closed loop during normal operation. This is not a good thing - although the 85 will tend to feel more perky in open loop...you want it to go closed as soon as possible. The cure is to install a later model THREE WIRE heated O2. This is easy to do as you will hook one wire to ground, one wire to ignition on, and the third wire goes right back to your purple standard O2 sensor wire. THis alone will make the car warm up faster, and stay more stable and more predictible for drag racing or any other activity.

By the way, I have a Type II bare four bolt main ROLLER block kicking around the house - too bad we are so far from each other. This is THE way to get your 85 into the zone you want to run. The roller block will allow you to run any full roller cam from 88 - 96 including the hot LT1 cams. You get more RPM, less friction, and a virtual ZERO chance of killing a lobe.
 

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Discussion Starter #19
Hey bud,
How often do you drag the car.How is the 4 x 3 trans hanging in there?
Very rarely raced. I was supposed to be racing again this Saturday to test the new mods...but we now know that isn't happening, lol.

I actually have a complete A4 conversion for the car, but I really like the 4+3. My theory was to run it until it breaks, then do the swap, but it won't break...:smack
 

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Discussion Starter #20
Congrats on the spot in the back of VETTE magazine this month. You rock!

Sounds like you found the problem with #7. The 85 Vette has a flat tappet cam that is more prone to killing a lobe than the 88 and up roller models.

When you select a new cam, be very careful. The 85 Computer is not a fast processing machine and will easily get off track and cause severe idle issues if you go too large without addressing it.
Thanks for the compliment. I still haven't gotten my copy yet. :huh:

Anyway, I already ordered a cam, rather mild and computer friendly (so they say) as that was the intention all along, hence the keeping of the stocker originally. It's like 204/214 @ .050, 455/477 lift....112 lobe spearation if I remember right. Just a notch or two above stock.

My O2 was actually working quite well according to the WinALDL program. I welded a bung into the #7 cylinder's primary tube, about 10" from the header flange. I think I'll replace the knock sensor just as a preventative maintenance thing, maybe do the same with the EGR since it'll be out in the open again for the cam swap.

Thanks for the tips.
I'd like to go ahead and build a nice roller setup, but funds aren't going to allow it this year...maybe next? I have a local who has a complete roller shortblock for a decent price. Maybe make it a long term project.
 
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