ThirdGenZ28 92

So I'm finally throwing more money and time into my 92 Z28 L98 and the one main problem still remains, poor MPH. At Etown I cant run anything but 94 MPH in the 1/4. It doesnt matter if I cut a 1.76 or a 2.1 it runs 94 MPH. I ran it last week after a year and half hiatus and the car ran a 14.3 and 2 14.1s all at 94 MPH BUT my times were off due to a possible broken cat and 80+ degree temps(the exhaust is slated to be redone June 27th to take care of that). My question is what would cause me to run that exact MPH each and every single time even tho I could be .6+ off my best ET? It's like I run into a brick wall and under WOT you can hear the car bog and it actually drops back 500 rpms, recovers and continues on. Any ideas?

As for "mods" I have: Holley AFPR set at 47 psi, MSD-6AL, SLP converter(cooler and B&M shift kit), Hypertech chip(Ive run with and w/o with the same results just cooler engine temps), SLP 1 3/4 headers, Catco dual cats, Flowmaster cat back, MAT relocate, airfoil, crank pulley, K&Ns in gutted boxes and that's about it other than suspension pieces and an alum DS. The only wild car was I ran at Atco on street tires(normally I run DRs) and I managed a 13.758 at 99.x MPH. I would run street tires now at Etown but I run 17s now and sold the 16s. Also the TPI is dead stock as is the injectors and what not, completely stock L98 with 109k miles.

I just replaced the stock fuel pump with a 255lph in tank and Ill be running a completely new exhaust (SLP y-pipe, single 3" cat and SLP Loudmouth as of June 27th). I'll get more runs after that to see if anything is different but is there anything I may be over looking here?? I've had some good suggestions in the past but nothing has worked yet. Any info or suggestions would be appreciated as it's been plagued with the problem for years now, thanks!

Here's a video of a 13.70 run and 94 MPH traps... 13.709 ET

My best slip... 13.521 ET

That street tire run from Atco(a year after my best ET)... 13.758 ET

RBob

iTrader: (1)

Have you disabled the speed limiter in the ECM?

RBob.

Ed Maher

I've said it before iand i'll say it again, seems like the TCC is locking up and killing you just after that 2-3 shift.

If you're not going to burn a chip for it, the least you can do is try a run with the TCC unplugged

ThirdGenZ28 92

Ed I've taken your suggestion and run with it unplugged and it did no good. I actually have a piece I bought back then that goes between the tranny and the harness to keep it from locking up. I'll try it again next time out but that was one of the good suggestions I followed but it didn't turn out to be the cause. Also I wasnt aware there was a speed limiter in the ECM? I havent gotten into burning my own chips yet but Ill certainly look into it thanks.

1bad91Z

iTrader: (1)

Turn that fuel pressure down a few lbs. That will take care off your bog. You probably need to lean it out some.

Ed Maher

So i am perfectly clear on this, you did try to run with the TCC completely unplugged. A delay box alone isn't going to fix it necessarily.

Beyond that, and maybe it's been covered, but have you ever had a scan tool on it? That should tell the entire story really. Something happens to make it nose over like that and odds are the ECM knows exactly what it is.

ThirdGenZ28 92

Yes I've run with it completely off and I ran it with the delay harness in place and it's all the same result. As luck would have it I recently borrowed a scantool just for this very reason and I'm quickly trying to read up on exactly what to look for. Any general parameters to look for that would raise an eyebrow about whats going on?? The car does not throw any codes either BTW. I WILL get this thing figured out, I was close to quitting but I refuse. Thanks again for the help!

PLANT PROTECTION

iTrader: (6)

At what speed does it bog and the RPM drop?

Ed Maher

Depending on what scan tool you are using, ideally you'll want to be looking at say:

MPH
RPM
O2
MAP
Injector pulsewidth
spark advance (total)
knock counts
knock retard

Also couldn't hurt to make sure the TPS stays constant and that the EGR and CCP stay off during the run although they sholdn't be ble to actually hurt you.


Most likely between that stuff something should jump out at you. Once you have a log of something i'd be happy to look over it for you if you need some help figuring it out, just shoot me an email or PM.

ThirdGenZ28 92

It bogs shortly after the 2nd to 3rd shift, RPMs drop off and it recovers. I have some footage of it let me dust it off and I'll post it again in a bit.

Ed it's an auto Xray and I'm some what familiar with it. I guess capturing run time at the strip would be ideal since I cant really get it near 94MPH safely anywhere else. This coming Fri my exhaust gets done and maybe Sat or the following weekend Ill take it with to me to the track. For now I'll see what's up and see if anything obvious jumps out at me. I do have a cheap *** Pep Boys distrib. when mine crapped out YEARS ago. I had to replace a module but other than that it's still the cheap replacement, worth getting a a nicer one?? I'll get some info in the next few days and Ill take this to Emails if you dont mind. Solve this problem and you get a great big thank you on my site and I'll owe you a beer or ten some day. Thanks again.

ThirdGenZ28 92

I had footage of the tach under WOT but lost it. Here's is some lesser quality footage of a 13.553 sec run. 1st and 2nd pull real nice but listen as soon as it shifts into 3rd. It'll bog/stumble then recover. Been that way for a few years now(1.5 year lay off from the track).

13.5 Run From Inside

56k friendly only 1.2 MB.

RBob

iTrader: (1)

Listened to the run a few times. To me it sounds like the engine starts to misfire in the top of second gear, shifts into third, and slowly gains power back.

There could be a couple of things going on. A top of 2nd misfire can easily be a lean out. This can happen from fuel slosh, or a weak pump (which you will soon find out as you have just replaced the pump).

The misfiring is causing knock retard to happen which is why 3rd is weak for a short time.

If it truely is in third gear when the stumble occurs then it points more toward an ignition problem. The shift could be causing knock retard which can cause the stumble and loss of power.

You said the tach drops off when the problem happens? I would return the ignition system back to stock GM as much as possible. Don't use the MSD and try to get a good known distributor and swap it in. Even swap over a known good coil, cap and wires if you can. This is the easiest method of eliminating problems.

Check the wiring harness connectors at the power leads and distributor/coil. For the distributor connectors look into the end of the connector and eye-ball whether any of the pins don't look quite right (hard to describe). With the engine running at idle wiggle, push and pull (firm but not over-zealous) on the connectors. Watch for a stumble.

Check that the harness wires are long enough where a torqued-over engine won't tug them loose. Or that the distributor connections don't hit the firewall.

Getting a scan of a run will also help. Just be sure that the scan tool is in open mode and not one of the diagnostic modes.

RBob.

ThirdGenZ28 92

Rbob...thanks for the suggestions sounds like some good advice. I just ordered new wires and I plan on doing a full tune up and I'm all but set on a real distrib instead of that Pep Boys get me by in there now. Hopefully over the weekend Ill look into this more and put it on the scan tool and see what's up. Ill post back as soon as I find something out, thanks!

j91z28d1

check your injector plugs

I had a problum with mind and it would only do it at the track when i lunched hard

somewhere in the harness there must have been a break so when i shifted something moved.

mine was 6 and 8 i finaley burnt both pistons on the spray racing a CRX with a big *** turbo.

just a guess

JAYDUBB

When you shifted into third, did you let off or did your car just die and come back to life? Do you have a fuel pressure gauge mounted where you can see it?

ThirdGenZ28 92

Nope I know better than to let off before I complete the run, the car is dieing on it's own and recovering. I've done a few WOT passes locally with the fuel pressure gauge taped to the winshield and I did not notice any drop in pressure. Then again I can't really take it up to 100 MPH and not exepect anything bad to happen. Recently I added a new fuel pump, I replaced the fuel filter, it has an adj FPR so the only week link next in line would be injectors. I may just order up a set since these have 110k miles on them anyways.

vwdave

In that run, what were your shift points? Sounds like you shifted into 2nd at 3000.

ThirdGenZ28 92

I don't shift manually, the car shifts on it's own(in D) and produces better ETs this way. What you are hearing is 5-5.5k rpm shifts last I checked.

vwdave

Your RPM's sounded low, my car is screaming at 4k, yours barely sounds like its going.

Not like this has any relavence on your problem.

mystikkal_69

iTrader: (2)

the 1-2 shift sounds like it shifting @ 4-4.5K or so?????

ThirdGenZ28 92

I think it's the video that makes it sound that way. Believe me I've looked at everything I can possibly look at. I just reset my detent cable and I know for sure now it's shifting at 5-5500rpms. Ive been playing with a scan tool and FP gauge today. Once I go closed loop my BLMs were 108 when I began and I got them as good as 124 and integ 128. Im trying to learn about this part of TPI as quickly as possible, there's a wealth of knowledge on here it's just a matter of sorting it out. My stock FP was 47psi as it sits now I have it dialed in at 49, close to 50 and I got the above readings and the car "seems" to run better and Im seeing better 02 readings. If anyone has any pointers they can shoot me by Email about what I'm trying to do Id appreciate it. I think if Etown runs this Fri(July 4th) I may try and run to see if the new fuel pump, exhaust and playing with settings has helped any. If not back to the drawing board and maybe some Paypal inspired prom begging

ThirdGenZ28 92

Here's that video I shot of the tach when I 1st started having this problem or atleast noticed it. You'll see the tach climb like it should, shift, then fall back to 4k RPMs and strugle and if you look closer it actually falls back another 500 RPMs or so before I get out of it. I was bored and came across it going thru some old video and figured Id throw it out there.

3rd Gear Bog <-- 2MB but larger screen shot to help you see better.

vwdave

Damn, that is weird as hell.

I donno where to even begin to look.

Black 91 Z28

I know how you feel. I ran last night at the track and was very dissatisfied. The weather was crummy 90F temp and very high humidity. I think my car is having the same problem. I take off fine, into second seems on but going into third ipower falls off for a sec and then finally the RPMS start to come up. They don't drop like yourd though. I looked at my scantool (Moates) data and I had a lot of knock retard. I'm running 8* BTDC and I had 5.5* of retard on the 2-3 shift. So I'd say look and see if you are running with retard. Never the less I've never broken 94 mph either.

David

ThirdGenZ28 92

It's plagued the car for a long time now, ran a 13.5 this way so I'm wondering if there isnt more left in it. Sounds like there MIGHT be. I just replaced the stock fuel pump with a 255lph and I plan on a full tune up. I also recently added a 52mm TB not that it makes a difference but hopefully next time I run I either eliminate the bog OR Ive eliminated a few items it may be. I finally have a scantool to borrow for a bit so my next run down the track Ill see what's going on with any knock retard. After that I plan on looking into injectors and a distrib as they are two of the few unknowns.

vwdave

Why dont you have the car dyno'ed with a wideband O2 sensor?

ThirdGenZ28 92

I guess I could but that tends to be pricey. Then again so does replacing parts to eliminate problems lol I might have to look into that.

Ed Maher

Knock retard on the shift is a very likely candiate. You could rule that out by looking for it on the scan tool, or just bypassing the knock sensor with a 3.9k ohm resistor to ground.

ThirdGenZ28 92

I should be headed to the track this coming week with scantool in hand. Hopefully it'll be something simple like that but if not atleast we can rule it out. Why wouldn't it happen after the 1-2 shift though?

PLANT PROTECTION

iTrader: (6)

It almost sounds like a 2-3 shift or a torque converter problem. I doubt it is the motor if your 1st gear runs strong.

vwdave

Wouldn't the car be under more of a load in 3rd gear as it would in 2nd? If so, the car being under a load is a reason for detonation.