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TPI mods, then MISFIRE that won't quit

R

rel3rd

Guest
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
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
I'm chasing a simular problem on one of my projects, it will run fine for a while then barely run. The scanner shows that the computer is taking 20 degrees timing out when it goes bad. I was told check knock sensor and look for a vacuum leak. I have had a code 43 since start up and replaced all sensors except the one under the plenum.
Since that is the last sensor to replace I will pull the intake and check the gaskets and see how the manifold fits on the heads which were bought on Ebay and may have been cut.
I actually have had to walk away from this project it has frustrated me so much.:W :W
 
R

rel3rd

Guest
I can relate. Mine gets no trouble codes, but makes me look at it in a whole new perspective since it's been mean to me, lol...
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
do you have a scanner that will read in real time while you are driving, I have an Actron Scanner that shows the computer pull 20 degrees retard when it acts up; now all I have to do is find out why.;LOL
 
R

rel3rd

Guest
do you have a scanner that will read in real time while you are driving, I have an Actron Scanner that shows the computer pull 20 degrees retard when it acts up; now all I have to do is find out why.;LOL

No, I don't. Everyone says the 85's are very slow computer-wise, and that some scanners don't read accurately with them. My buddy has a Snap_on scanner I may be able to borrow. Hopefully, that will interact OK with my car.
 

bossvette

Gone but not forgotten
Joined
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Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
I have an Actron that I bought from E-bay it has cartridges for GM, Ford and Chrysler. I didn't see any today but they show up from time to time. The one I have is an older model I can try to find out the sellers ID he sells test equipment and gages if you like.
 
R

rel3rd

Guest
I have an Actron that I bought from E-bay it has cartridges for GM, Ford and Chrysler. I didn't see any today but they show up from time to time. The one I have is an older model I can try to find out the sellers ID he sells test equipment and gages if you like.

If you could provide me with a Part# that would be great. I'll probably just bite the bullet and buy new...unless it isn't a bother finding the Ebay seller's name. Thank you.
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
Actron ScanTool model CP 9110 the Ebay seller http://myworld.ebay.com/sandyarogera
I don't know if this model is still availabe new, it may have been obsolete since the newer OBD2 and 3 stuff came out
send him an e-mail he may have some left or know what will do the same thing you want it to read while your are driving .
 

vetteboy86

Well-known member
Joined
May 26, 2003
Messages
2,760
Location
IN
Corvette
1986 Black "Indy 500 Pace car replica"
If you could provide me with a Part# that would be great. I'll probably just bite the bullet and buy new...unless it isn't a bother finding the Ebay seller's name. Thank you.

Do you have a laptop? If you do you can download a free version of datamaster. You get 20 scans free, then you pay. The program provides graphs and charts, and also real time readouts. I found that it helps greatly, and I still have not used up the 20 free scans. All you need is the program, and a cable that cost around 30 bucks if I remember right. If you require more info let me know.

I would unhook your knock sensor for a pass. If no problems, try and replace it.
 
S

speedmaster4

Guest
went thru same senario with backfiring, rich (super) exhaust, etc a few months ago...finally traced to an early style alternator (same type as used on 85 vette) that put out great voltage,etc but also had excess ''ripple'' which was confusing the ecm...that was a fresh rebuilt alt...changed to later style cs-130 alt (everything else had already been changed at least once) and prob was GONE.

bad ignition module in dist can also exhibit similar symptoms.

shots in the dark, jus' tryin to help.

my sympathies, been there.
 
R

rel3rd

Guest
Actron ScanTool model CP 9110 the Ebay seller http://myworld.ebay.com/sandyarogera
I don't know if this model is still availabe new, it may have been obsolete since the newer OBD2 and 3 stuff came out
send him an e-mail he may have some left or know what will do the same thing you want it to read while your are driving .

Thank you. Please post back if you hear back from him. :beer
 
R

rel3rd

Guest
Do you have a laptop? If you do you can download a free version of datamaster. You get 20 scans free, then you pay. The program provides graphs and charts, and also real time readouts. I found that it helps greatly, and I still have not used up the 20 free scans. All you need is the program, and a cable that cost around 30 bucks if I remember right. If you require more info let me know.

I would unhook your knock sensor for a pass. If no problems, try and replace it.

My son has an almost new one I'm sure I can use. Is it datamaster.com ?

I thought about unhooking the knock sensor, just as a "I didn't try that yet" thing, lol....I'll give it a shot tomorrow, since I'm off work.
 
R

rel3rd

Guest
went thru same senario with backfiring, rich (super) exhaust, etc a few months ago...finally traced to an early style alternator (same type as used on 85 vette) that put out great voltage,etc but also had excess ''ripple'' which was confusing the ecm...that was a fresh rebuilt alt...changed to later style cs-130 alt (everything else had already been changed at least once) and prob was GONE.

bad ignition module in dist can also exhibit similar symptoms.

shots in the dark, jus' tryin to help.

my sympathies, been there.

ANY help is greatly appreciated as I am running out of patience and ideas.

My alternator is newly rebuilt, but it was done before the mods, and it was fine then. I heard AutoZone can check the module for free, so that's an option too.
 
R

rel3rd

Guest
Just went to Datamaster listings. Unfortunately, they do not list 85 Corvette. :mad

86 and up only :ugh
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
This one sounds like an engine mechanical or *maybe* an electrical issue, though with the MIL not on, if it *is* electrical, it might not be something the engine controls disgnostics will pick up.

First, I'd crack open the FSM, look at the engine wiring diagram then tighten every ground connection. On an L98, the main engine controls ground is a big-assed bolt near the oil filter. I'd remove the bolt, clean the block and the connectors then put the bolt back in then tighten the snot out of it. If there are other grounds, tighten them.

Since you've swapped all the ign components that pretty much rules that out.

I'd run a compression test or better yet a leakage test.

Now, you said:
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.

Do you mean you ran 6 passes and all was well but on the 7th pass it was FUBAR or...do you mean you ran 6 passes and all was well to the mid-range and than past that, it went lean? Also, how did you determine that the fuel pressure at WOT/high rpm was adequate?
 

bossvette

Gone but not forgotten
Joined
May 19, 2003
Messages
3,233
Location
West Unity Ohio
Corvette
1968 1997
Thats probably the one (does OBD1 and 2) that replaced the one I have. If you do your own work it will pay for itself in no time. I have used mine on several other vehicles, it is OBD-1 only. I don't know what dealers/shops charge for a scan nowadays but it wouldn't take too many trips to add up to the price of a scanner.
 
R

rel3rd

Guest
Do you mean you ran 6 passes and all was well but on the 7th pass it was FUBAR or...do you mean you ran 6 passes and all was well to the mid-range and than past that, it went lean? Also, how did you determine that the fuel pressure at WOT/high rpm was adequate?

No, 6 passes ALL did the same exact thing. I only verified that fuel pressure was not fluctuating and/or dropping at anytime during a WOT run, whether on the dyno or on the street. When it misfires, fuel pressure doesn't change.

Here's the dyno passes on one graph...You can see HP was definitely still on the way up, right until 3300 or so when the misfire screwed up the entire remainder of the runs.

site1420.jpg
 

G Winter

Well-known member
Joined
Aug 21, 2004
Messages
2,177
Location
NW Iowa
Corvette
1990 red convert 6 sp
Did both of you with this problem have valve guide work done?
Does anything seem to relate to engine heat ?
Many yrs ago,befor ECMs, I worked on LP engines. I had one case on a truck with a rebuilt engine. After it was loaded heavily and extra warm it would begin to backfire and miss. The problem turned out to be valve guides that had been fit just a little to tight and the valves were hanging. The way I found it was to take a compression check very quickly after the problem occured before it could cool down. After it cooled it didn't show up. I might be way out in left field here but thought I would mention it.

Glenn
:w
 

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